\r\n\tCongenital hearing loss means hearing loss that is present at birth. I have managed children with hearing loss for many years, and the most touching thing is the light that blooms on the face while the hearing-impaired child heard his mother's voice at first time. The scene of "happy tears" impressed me so much. To hear the voice that has not been heard is so pleasant, as if this ordinary listening experience is a supreme listening enjoyment.
\r\n\r\n\tAge-related hearing loss means a progressive loss of ability to hear high frequencies with aging, also known as presbycusis. Among them are the influence of internal and external factors such as genes, drugs and noise exposure. The studies pointed out that the brain stimulation of the hearing-impaired person is greatly reduced compared with subjects with normal hearing. The connection of auditory cortex and other brain areas has declined a lot, which is probably one of the important causes of dementia or even depression in the elderly.
\r\n\r\n\tNoise-induced hearing loss is hearing impairment resulting from exposure to loud sound. There is actually continuous and endless noise in many workplaces, which may cause chronic and cumulative damage. Some young people often work hard but easily neglect to protect themselves. In addition, in recent years, entertainment noise (such as nightclubs, concerts, and personal listening devices) has caused hearing impairment in young people. These should be avoidable and preventable.
\r\n\r\n\tHearing Science is the study of impaired auditory perception, the technologies and other rehabilitation strategies for persons with hearing loss. Public health has been defined as "the science and art of preventing disease", improving quality of life through organized efforts. To avoid the “epidemic” of hearing loss, it is necessary to promote early screening, use hearing protection, and change public attitudes toward noise.
\r\n\r\n\tBased on these concepts, the book incorporates updated developments as well as future perspectives in the ever-expanding field of hearing loss. Besides, it is also a great reference for audiologists, otolaryngologists, neurologists, specialists in public health, basic and clinical researchers.
",isbn:"978-1-83968-678-8",printIsbn:"978-1-83968-677-1",pdfIsbn:"978-1-83968-679-5",doi:null,price:0,priceEur:0,priceUsd:0,slug:null,numberOfPages:0,isOpenForSubmission:!1,hash:"a4b7dbb02ba00e7412422cd5dbffa029",bookSignature:"Dr. Tang-Chuan Wang",publishedDate:null,coverURL:"https://cdn.intechopen.com/books/images_new/10529.jpg",keywords:"Hidden Hearing Loss, Plasticity, Electrophysiology, Otoacoustic Emission, Newborn Hearing Screening, Genetics, Aging, Hearing Aids, Noise Exposure, Occupational Hearing Loss, Epidemiology, Prevention",numberOfDownloads:null,numberOfWosCitations:0,numberOfCrossrefCitations:null,numberOfDimensionsCitations:null,numberOfTotalCitations:null,isAvailableForWebshopOrdering:!0,dateEndFirstStepPublish:"September 3rd 2020",dateEndSecondStepPublish:"October 1st 2020",dateEndThirdStepPublish:"November 30th 2020",dateEndFourthStepPublish:"February 18th 2021",dateEndFifthStepPublish:"April 19th 2021",remainingDaysToSecondStep:"3 months",secondStepPassed:!0,currentStepOfPublishingProcess:4,editedByType:null,kuFlag:!1,biosketch:"Dr. Tang-Chuan Wang is an excellent otolaryngologist-head and neck surgeon in Taiwan; a research scholar of Harvard Medical School and University of Iowa Hospitals. He worked in the Hospital of the University of Pennsylvania, Boston Children's Hospital, and Massachusetts Eye and Ear. Due to his contribution to biomedical engineering, he was invited into the executive committee of HIWIN-CMU Joint R & D Center in Taiwan.",coeditorOneBiosketch:null,coeditorTwoBiosketch:null,coeditorThreeBiosketch:null,coeditorFourBiosketch:null,coeditorFiveBiosketch:null,editors:[{id:"201262",title:"Dr.",name:"Tang-Chuan",middleName:null,surname:"Wang",slug:"tang-chuan-wang",fullName:"Tang-Chuan Wang",profilePictureURL:"https://mts.intechopen.com/storage/users/201262/images/system/201262.gif",biography:'Dr. Tang-Chuan Wang is an excellent otolaryngologist – head and neck surgeon in Taiwan. He is also a research scholar of Harvard Medical School and University of Iowa Hospitals. During his substantial experience, he worked in Hospital of the University of Pennsylvania, Boston Children\'s Hospital and Massachusetts Eye and Ear. Besides, he is not only working hard on clinical & basic medicine but also launching out into public health in Taiwan. In recent years, he devotes himself to innovation. He always says that "in theoretical or practical aspects, no innovation is a step backward". Due to his contribution to biomedical engineering, he was invited into executive committee of HIWIN-CMU Joint R & D Center in Taiwan.',institutionString:"China Medical University Hospital",position:null,outsideEditionCount:0,totalCites:0,totalAuthoredChapters:"2",totalChapterViews:"0",totalEditedBooks:"2",institution:{name:"China Medical University Hospital",institutionURL:null,country:{name:"Taiwan"}}}],coeditorOne:null,coeditorTwo:null,coeditorThree:null,coeditorFour:null,coeditorFive:null,topics:[{id:"16",title:"Medicine",slug:"medicine"}],chapters:null,productType:{id:"1",title:"Edited Volume",chapterContentType:"chapter",authoredCaption:"Edited by"},personalPublishingAssistant:{id:"252211",firstName:"Sara",lastName:"Debeuc",middleName:null,title:"Ms.",imageUrl:"https://mts.intechopen.com/storage/users/252211/images/7239_n.png",email:"sara.d@intechopen.com",biography:"As an Author Service Manager my responsibilities include monitoring and facilitating all publishing activities for authors and editors. From chapter submission and review, to approval and revision, copyediting and design, until final publication, I work closely with authors and editors to ensure a simple and easy publishing process. I maintain constant and effective communication with authors, editors and reviewers, which allows for a level of personal support that enables contributors to fully commit and concentrate on the chapters they are writing, editing, or reviewing. I assist authors in the preparation of their full chapter submissions and track important deadlines and ensure they are met. I help to coordinate internal processes such as linguistic review, and monitor the technical aspects of the process. As an ASM I am also involved in the acquisition of editors. 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Venkateswarlu",coverURL:"https://cdn.intechopen.com/books/images_new/371.jpg",editedByType:"Edited by",editors:[{id:"58592",title:"Dr.",name:"Arun",surname:"Shanker",slug:"arun-shanker",fullName:"Arun Shanker"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}}]},chapter:{item:{type:"chapter",id:"18553",title:"Nutritional Therapy in Diabetes: Mediterranean Diet",doi:"10.5772/20916",slug:"nutritional-therapy-in-diabetes-mediterranean-diet",body:'Diabetes mellitus is a chronic illness which has an outstanding impact on public health due to its increasing prevalence, poor prognosis, and due to the high impact on cardiovascular health. Such is its importance, that diabetes is actually considered an independent predictor of cardiovascular disease (which includes coronary heart disease and stroke). Moreover, the cardiovascular risk of individuals with diabetes is considered to be equivalent to the risk of nondiabetic individuals with pre-existing cardiovascular diesease. Therefore, persons with diabetes mellitus have an increased susceptibility to atherosclerosis and an increased prevalence of ahterogenic risk factors, notably hypertension, obesity, and anormal lipids. This compendium of the abnormalities can be found in people with metabolic syndrome (MetS) which is now regarded as a prelude to diabetes and such as diabetes, MetS is a substantial predictor of cardiovascular disease and all-cause mortality(Ford, et al. 2002).
Morbidity and mortality from these chronic diseases in the general population have a multifactorial origin, resulting from the interaction between genetic background and environmental factors. Among the latter, diet is probably the most relevant factor in order to prevent acute complications and to reduce the risk of long-term complications. Thus, it has been demonstrated that medical nutrition therapy is important in preventing diabetes, managing existing diabetes, and preventing, or at least slowing, the rate of development of diabetes complications. The basis of what constitutes optimal nutrition has been the subject of decades of research spanning the whole range of study designs, from ecological studies to in vitro modulation of gene expression. Based on this approach, there is now sufficient evidence supporting the notion that the monounsaturated fatty acids (MUFA) as a nutrient, olive oil as a food, and the Mediterranean diet (MedDiet) as a food pattern are associated with a decreased risk of cardiovascular disease, MetS, and diabetes mellitus.
In the last decade, research in nutritional epidemiology has moved from the single food approach to the dietary pattern, which better reflects the complexity of interactive effects of multiple nutrients on health status(Hu 2002; Trichopoulou, et al. 2003; Trichopoulou, et al. 1995). Thus, the MedDiet pattern is built on the basis of a consumption of fat primarily from foods high in MUFA (olive oil as the principal source of fat), and emphasizes the consumption of fruits, vegetables, legumes, nuts, and fish, as well as a moderate consumption of alcohol. In this regard, the MedDiet pattern has been associated with higher survival due to lower all cause mortality(Knoops, et al. 2004). A recent meta-analysis of prospective studies based on 1.5 million subjects and 40,000 fatal and non-fatal events showed that a greater adherence to this dietary pattern was significantly associated with a reduction of overall mortality, cardiovascular mortality, cancer incidence and cancer mortality, and incidence of Alzheimer’s disease and Parkinson’s disease(Sofi, et al. 2008). In addition, a recently cross-sectional assessment of baseline data from a cohort of high-risk participants in the PREDIMED study, a large-scale feeding trial of primary cardiovascular prevention(Sanchez-Tainta, et al. 2008), showed that adherence to the MedDiet was inversely associated with the clustering of diabetes mellitus, obesity, hypertension and hypercholesterolemia. The follow-up of large cohorts of healthy populations living in Mediterranean countries, such as the Greek EPIC(Psaltopoulou, et al. 2004; Trichopoulou, et al. 2005a; Trichopoulou et al. 2003; Trichopoulou, et al. 2005b) and ATTICA(Panagiotakos, et al. 2009a; Panagiotakos, et al. 2006) study cohorts and the Spanish EPIC(Agudo, et al. 2007) and SUN(Nunez-Cordoba, et al. 2009) study cohorts, are providing new information suggesting that increasing adherence to the MedDiet relates to a reduced prevalence of risk phenotypes. In this regard, the MedDiet pattern is being reconsidered as the one of the more holistic approaches for the control of metabolic diseases including at the same time salutary and pleasure components.
The principal goals of medical nutrition therapy for subjects with metabolic syndrome, and/or diabetes are to attain and maintain an optimal metabolic control, including blood glucose, lipid profiles, and blood pressure; to prevent and treat obesity and cardiovascular complications; and to improve general health and well-being through food choices that take personal and cultural preferences into consideration. In this regard, pioneering nutritional strategies, such as nutraceuticals, have been developed aimed at reducing the main metabolic risk factors and promoting cardiovascular health. In this context, a growing body of clinical evidence has demonstrated positive cardiovascular effects associated with olive oil, antioxidants, and polyphenols intake.
Traditionally, many beneficial properties associated with olive oil have been ascribed to its high oleic acid content. Olive oil, however, can be considered a functional food that, besides having high-MUFA content, contains other minor components with biological properties(Perez-Jimenez, et al. 2007). Thus, phenolic compounds have shown antioxidant and antiinflammatory properties, prevent lipoperoxidation, induce favorable changes of lipid profile, improve endothelial function, and disclose antithrombotic properties(Lopez-Miranda, et al. 2007; Lopez-Miranda, et al. 2010; Perez-Jimenez, et al. 2005; Perez-Jimenez, et al. 2006; Ruano, et al. 2007; Ruano, et al. 2005). Therefore, all those evidences suggest that MedDiet could serve as an antiinflammatory dietary pattern, which could help fighting diseases that are related to chronic inflammation, such as MetS and type 2 diabetes. In this context, it has been clearly demonstrated that many components of the MedDiet have been considered to be important in the treatment and modulation of cardiometabolic diseases. In the present chapter, we review the state of the art illustrating the relationship between MedDiet rich in olive oil and metabolic diseases, including MetS and diabetes mellitus and to discuss potential mechanisms by which this food can help in disease prevention and treatment.
In the last decade the incidence of conditions associated with insulin resistance, including metabolic syndrome and diabetes mellitus, is increasing rapidly worldwide. Although pharmacological interventions are available for minimizing or delaying the comorbidities associated with insulin resistance and the metabolic syndrome, as well as diabetes, initial management for the vast majority of the affected population remains focused on lifestyle modification, consisting of sustainable changes in dietary habits and physical activity. Lifestyle modification, in particular recommendations to follow an appropriate dietary pattern, has generally been accepted as a cornerstone of treatment for people with these conditions, with the expectation that an appropriate intake of energy and nutrients will improve glycaemic control and will reduce the risk of complications. The factors that regulate body fat distribution, insulin resistance, and associated metabolic disturbances are not fully understood. Nevertheless, increasing scientific evidence suggests that dietary habits may be an important environmental factor regulating glucose and fat metabolism(Phillips, et al. 2006). Epidemiological studies indicate that Western-style dietary patterns promote the MetS, while diets rich in vegetables, fruits, grains, fish and low-fat dairy products have a protective role(Esmaillzadeh, et al. 2007; Lutsey, et al. 2008; Pereira, et al. 2005). In the same line, two studies in Southern European populations showed that a greater adherence to the MedDiet was associated with reduced prevalence (Panagiotakos, et al. 2004) and incidence (Tortosa, et al. 2007) of MetS. To date, several feeding trials have assessed the effect of dietary patterns on the metabolic syndrome status (Azadbakht, et al. 2005; Esposito, et al. 2004; Orchard, et al. 2005; Salas-Salvado, et al. 2008). These studies used a behavioral program to implement a relatively low-fat MedDiet(Esposito et al. 2004), intensive lifestyle intervention with inclusion of a vegetable-rich diet restricted in animal fat (Orchard et al. 2005), the DASH diet (Azadbakht et al. 2005), and two MedDiets supplemented with virgin olive oil or nuts(Salas-Salvado et al. 2008) in comparison with standard advices. Three studies (Azadbakht et al. 2005; Esposito et al. 2004; Orchard et al. 2005) used energy-restricted diets that led to some degree of weight loss, while one study(Salas-Salvado et al. 2008) used ad libitum diets. In all these studies, a decreased prevalence of metabolic syndrome was shown in the intervention groups. In the PREDIMED study(Salas-Salvado et al. 2008), the MedDiet with nuts significantly reduced MetS prevalence at 1 year, mostly because of increased reversion of prior MetS due to reduction in waist girth in spite of no weight loss, suggesting fat redistribution. Moreover, in a subgroup of this study including the Reus PREDIMED Centre some components of the MedDiet, such as olive oil, legumes and red wine were associated with lower prevalence of MetS (Babio, et al. 2009). On the other hand, results of a study in overweight, insulin-resistant patients also suggest that, by comparison with a low-fat diet, a MUFA-rich diet prevents the redistribution of body fat from peripheral to visceral adipose tissue without affecting total body weight(Paniagua, et al. 2007b). More recently Jimenez-Gomez et al. have demonstrated that postprandial abnormalities associated with MetS can be attenuated with high MUFA diets(Jimenez-Gomez, et al. 2010).
Because diabetes is a frequent outcome in patients with sustained MetS, it is reasonable to assume that the MedDiet might also prevent the development of diabetes in predisposed persons or beneficially influence the metabolic abnormalities associated with the diabetic status(Giugliano and Esposito 2008). In this context, two prospective studies from Southern European cohorts suggest a lower incidence of diabetes with increasing adherence to the MedDiet in previously healthy persons(Martinez-Gonzalez, et al. 2008) or survivors of a myocardial infarction(Mozaffarian, et al. 2007). In contrast, in the absence of weight loss, the low-fat diet used in the Women’s Health Initiative trial(Tinker, et al. 2008) was ineffective to prevent the development of diabetes. Furthermore, in the set of the PREDIMED study, it has been tested the effects of two MedDiet interventions versus a low-fat diet on incidence of diabetes. This was a three-arm randomized trial in 418 nondiabetic subjects aged 55–80 years where participants were randomly assigned to education on a low-fat diet (control group) or to one of two MedDiets, supplemented with either free virgin olive oil (1 liter/week) or nuts (30 g/day). Diets were ad libitum, and no advice on physical activity was given. After a median follow-up of 4.0 years, diabetes incidence was 10.1%, 11.0%, and 17.9% in the MedDiet with olive oil group, the MedDiet with nuts group, and the control group, respectively. Multivariable adjusted hazard ratios of diabetes were 0.49 (0.25– 0.97) and 0.48 (0.24–0.96) in the MedDiet supplemented with olive oil and nuts groups, respectively, compared with the control group. Interestingly, when the two MedDiet groups were pooled and compared with the control group, diabetes incidence was reduced by 52%. In all study arms, increased adherence to the MedDiet was inversely associated with diabetes incidence. It is also important to highlight that diabetes risk reduction occurred in the absence of significant changes in body weight or physical activity. These results extend those of prior studies showing that lifestyle interventions can substantially reduce the incidence of diabetes in individuals at high risk(Knowler, et al. 2002; Pan, et al. 1997; Ramachandran, et al. 2006; Tuomilehto, et al. 2001). However, in these studies, the interventions consisted of advice on a calorie-restricted diet plus physical activity and, except for one study(Ramachandran et al. 2006), weight loss was a major driving force in reducing the incidence of diabetes(Salas-Salvado, et al. 2011).
Diets high in SFA consistently impair both insulin sensitivity and blood lipids, while substituting carbohydrates or MUFA for SFA reverts these abnormalities(Riccardi, et al. 2004). Postprandial lipemia and glucose homeostasis are also improved after meals containing MUFA from olive oil compared to meals rich in SFA(Lopez, et al. 2008; Paniagua, et al. 2007a). Thus, an examination of the association of dietary and membrane fatty acids with insulin secretion in the cross-sectional Pizarra study(Rojo-Martinez, et al. 2006) showed that dietary MUFA contributed to the variability of β-cell function, with a favorable relationship of MUFA with β-cell insulin secretion, independently of the level of insulin resistance.
The question as to what was the best nutrient to replace energy sources from SFA in the diabetic diet, carbohydrates or MUFA, was also hotly debated. Since the late 1980’s, many feeding trials have compared the effects of isoenergetic high carbohydrates (CHO) and high MUFA diets on insulin sensitivity in healthy subjects and on glycemic and lipid control in diabetic patients(Garg 1998; Ros 2003). Garg’s meta-analysis(Garg 1998) favored high MUFA diets, but most of the studies reviewed therein were performed with metabolic diets having wide differences in total fat content between the two experimental diets, ranging from 15% to 25% of energy. The studies reviewed by Ros(Ros 2003) were performed on an outpatient basis with natural foods, olive oil as the main source of MUFA, and <15% energy difference in total fat content between diets; the conclusion was that both dietary approaches provided a similar degree of glycemic control. Nevertheless, high MUFA diets generally had more favorable effects on proatherogenic alterations associated with the diabetic status, such as dyslipidemia, postprandial lipemia, small LDL, lipoprotein oxidation, inflammation, thrombosis, and endothelial dysfunction(Ros 2003). Although we will discuss this point later, of particular interest is the ability of the olive oil-rich MedDiet to improve mild systemic inflammation, as shown by the reduction of C-reactive protein and inflammatory cytokines in the study of Esposito et al.(Esposito et al. 2004) in subjects with MetS and by the PREDIMED study(Estruch, et al. 2006) in diabetic patients and other subjects at high risk for coronary heart disease (CHD). In addition, in a cross-sectional analysis of a population of type 2 diabetic patients, the adherence to a MedDiet was inversely associated with glycosylated haemoglobin and postprandial glucose levels during free living conditions, independent of age, adiposity, energy intake, physical activity and other potential confounders(Esposito, et al. 2009b). This association was apparent even although no strong associations were evident for each of the components of the MedDiet score, except for a modest association with whole grains and the ratio of MUFA to saturated lipids. However its cross-sectional nature does not allow us to make inference about cause and effect. According to these findings, Itsiopoulos et al. have recently demonstrated that a traditional MedDiet improved glycemic control, glycosylated haemoglobin fell from 7.1% to 6.8%, in men and women with well-controlled type 2 diabetes, without adverse effects on weight(Itsiopoulos, et al. 2010). Furthermore a systematic review of the available studies confirmed that adopting a MedDiet may help to prevent type 2 diabetes, and also improve glycaemic control and cardiovascular risk in persons with established diabetes(Esposito, et al. 2010).
Despite the beneficial effect attributed to the MedDiet, the American Diabetes Association (ADA) recommends that patients with newly diagnosed type 2 diabetes be treated with pharmacotherapy as well as lifestyle changes(Nathan, et al. 2006). The rationale for combination therapy is presumably that each form of treatment alone is imperfect. Lifestyle changes are often inadequate because patients do not lose weight or regain weight or their diabetes worsens independent of weight. Pharmacotherapy also often fails with time(Turner, et al. 1999), and some drugs have associated cardiovascular and other risks(Goldfine 2008). In this context Esposito et al. in 2009, conducted a randomized trial to compare the effects of a low-carbohydrate Mediterranean-style or a low-fat diet on the need for antihyperglycemic drug therapy in 215 patients with newly diagnosed type 2 diabetes(Esposito, et al. 2009a). After 4 years, they found that the MedDiet delayed the need for antihyperglycemic drug therapy. There were no differences in the degree to which participants in each group increased their physical activity or decreased their caloric intake, so the effect seems specific to the MedDiet and is probably, although not exclusively, linked to its ability to induce greater weight loss, in accord with results of a recent trial(Shai, et al. 2008). The between-group difference in the proportion of people needing antihyperglycemic drug therapy increased over the course of the trial and favored the MedDiet, whereas the between-group differences in weight loss decreased. Consumption of MUFA is thought to increase insulin sensitivity(Due, et al. 2008; Esposito et al. 2004; Shai et al. 2008), and this component of the diet might explain the favorable effect of the MedDiet on the need for drug therapy. In summary, although more data are mandatory, there is good scientific support for MedDiet diets, especially those based on olive oil, as an alternative approach to low-fat diets for the medical nutritional therapy in MetS and diabetes.
Several mechanistic links offer potential explanations of protective effect of the MedDiet on type 2 diabetes. Excessive oxidative stress and inflammation are closely associated with the pathogenesis of many human diseases (such obesity, MetS, diabetes, cardiovascular diseases, neurodegenerative diseases and aging). The potential reversal of those conditions can be achieved by reducing the levels of inflammation through the consumption of an anti-inflammatory dietary pattern. Usually this may occur through the reduction of systemic vascular inflammation and endothelium dysfunction without having a drastic effect on body weight. Phenolic compounds are the focus of intense research in the last years, due to the biological properties that they have proven, mainly as potent antioxidants and anti-inflammatory agents; therefore, they can modulate signal transduction pathways to elicit their beneficial effects in human diseases. These mechanisms include modulation of pro-inflammatory gene expression such as cyclooxygenase, lipoxygenase, nitric oxide synthases
Health effects of the Mediterranean Diet rich in olive and the potential mechanisms by which this diet can help in disease prevention and treatment.
and several pivotal cytokines, mainly by acting through NF-ĸb and mitogen-activated protein kinase signaling. It is of note that phenols are not uniquely represented in olive oil in the traditional MedDiet, but also in other classic foods, such as wine, fruits and vegetables. Beyond this, epidemiological and interventional studies have revealed a protective effect of the MedDiet against mild chronic inflammation and its metabolic complications (Chrysohoou, et al. 2004; Dai, et al. 2008; Panagiotakos, et al. 2009b).
The origins of heightened inflammatory activity in diabetes are diverse. In type 1 diabetes, islet inflammation is thought to be a local phenomenon driven by a focal autoimmune attack on islet antigens. By contrast, in type 2 diabetes, activation of inflammation results from systemic etiologic factors, such as central obesity and insulin resistance. Ultimately inflammatory mediators activate a series of receptors and transcription factors such as NF-ĸb, toll-like receptors, c-Jun amino terminal kinase, and the receptor for advanced glycation end products, which lead to β-cell dysfunction and apoptosis, impaired insulin signaling in insulin-sensitive tissues, systemic endothelial dysfunction, and altered vascular flow. In this context inflammation at the cellular level can be described as an increase in the NF-κB in the nucleus and with a concomitant decrease in its inhibitors IκB-alpha and/or IκB-beta(Ghanim, et al. 2004). NF-κB is a pleiotropic transcription factor activated by low levels of reactive oxygen species (ROS) and inhibited by antioxidants(Mantena and Katiyar 2006). This factor regulates the expression of several cytokines, chemokines, cell adhesion molecules, immunoreceptors and inflammatory enzymes(Piva, et al. 2006), molecules that are involved in disease such as atherosclerosis and insulin resistance. In most cells, NF-κB (p50/p65) is present in an inactive form in the cytoplasm, bound to an inhibitor IκB. Certain stimuli result in the phosphorylation, ubiquitination and subsequent degradation of IκB proteins thereby enabling translocation of this transcription factor into the nucleus. In this context, an interesting aspect was the demonstration that supplementing an endothelial cell culture with oleic acid reduces the transcriptional activation of this factor in these cells, similar to what is done by α-linolenic acid, and the opposite of the inflammatory effect of linoleic acid. Hennig et al.(Hennig, et al. 2000) exposed porcine endothelial cells to 18-carbon fatty acids. Both linoleic and stearic fatty acids activated endothelial cells more markedly than did either oleic or linolenic fatty acids. Also, compared with control cultures, treatment with stearic and linoleic acids decreased glutathione concentrations, which suggested an increase in cellular oxidative stress. This increase in oxidative stress with the subsequent activation of NF-κB could be one of the mechanisms of the inflammatory properties of 18:0 and 18:2 fatty acids. Previous studies have confirmed that fat consumption induced the activation of inflammatory markers(Jellema, et al. 2004). In this regard, Bellido and Perez-Martinez have previously demonstrated that the MedDiet, enriched in virgin olive oil, attenuated peripheral blood mononuclear cells (PBMCs) NF-κB activation compared with a Western SFA rich diet, and the effect of an n-3 PUFA-enriched diet was intermediate in young healthy population(Bellido, et al. 2004; Perez-Martinez, et al. 2007). These findings suggest that virgin olive oil could be a possible contributor to prevent the activation of NF-κB system, within the frame of the MedDiet. Besides olive oil, the MedDiet contents other source of potentially cardioprotective nutrients from fruits and vegetables which could also enhance this beneficial effect. In contrast, the opposite effect has been observed after the chronic intake of a Western diet rich in saturated fatty acids, corroborating previous data after the acute intake of a butter meal. The effect of a high CHO diet enriched in n-3 fatty acids on the NF-κB activation was intermediate. In this sense, previous data have suggested that n-3 α-linolenic acid found mainly in plants and walnuts may reduce cardiovascular risk through a variety of biological mechanisms, including inhibiting vascular inflammation.
In the same context, Brunelleschi et al.(Brunelleschi, et al. 2007) explored the effects of an extra-virgin olive oil extract, particularly rich in minor polar compounds, on NF-κB translocation in monocytes and monocyte-derived macrophages isolated from healthy volunteers. In a concentration-dependent manner, olive oil extract inhibited p50 and p65 NF-κB translocation in both un-stimulated and phorbol-myristate acetate challenged cells, being particularly effective on the p50 subunit. Interestingly, this effect occurred at concentrations found in human plasma after nutritional ingestion of virgin olive oil and was quantitatively similar to the effect exerted by ciglitazone, a PPAR-γ ligand. However, olive oil extract did not affect PPAR-γ expression in monocytes and monocye-derived macrophages(Brunelleschi et al. 2007). These data suggest the hypothesis of a protective effect of extra-virgin olive oil by indicating its ability to inhibit NF-κB activation in human monocyte/macrophages. On the other hand, NF-κB has been shown to regulate the expression of several adhesion molecules in response to inflammatory stimuli, including P-selectin, E-selectin, intercellular adhesion molecule 1 (ICAM-1) and cell adhesion molecule 1(VCAM-1)(Ghosh, et al. 1998), all implicated in atherosclerosis development. Carluccio et al.(Carluccio, et al. 1999) observed, in an endothelial cell culture model, that the incorporation of oleic acid into cellular membrane lipids reduced the expression of VCAM-1. Furthermore, it has been observed that the expression of VCAM-1 and E-selectin in human umbilical vascular endothelial cells (HUVECs), following the addition of minimally oxidised LDL, was less with LDL obtained from persons who had followed a diet rich in olive oil than from persons whose diet was rich in saturated fat(Bellido, et al. 2006). This anti-inflammatory action of MUFA also explains the fact that the enrichment of LDL particles with oleic acid, during the consumption of different types of diet, reduces their capacity to induce monocyte chemotaxis and adhesion. In accordance with these results, a previous study has shown that LDL obtained from a MUFA-rich diet induced a lower rate of monocyte adhesion to endothelial cells(Mata, et al. 1996).
The mechanism by which LDL from carbohydrate and MedDiets induces a lower expression of VCAM-1 and E-selectin is unknown; however several hypotheses have been suggested, for instance, the interaction of mononuclear leukocytes with vascular endothelial cells is most likely mediated by a complex amalgam of interacting regulatory signals in the inflammatory response characteristic of early atherogenesis. In another study including healthy subjects, virgin olive oil reduced plasma levels of ICAM-1(Bellido et al. 2004). This anti-inflammatory effect has also been observed in MetS patients who modified their diet for two years. In the group that followed a MedDiet model, the prevalence of this syndrome was reduced, improved insulin sensitivity and lowered the levels of C-reactive protein (CRP) and interleukin 6, 7 and 18. Such findings have recently been corroborated by Estruch et al.(Estruch et al. 2006) who evaluated the short-term effects of two ad libitum MedDiets (supplemented with either 1 l/week of virgin olive oil or 30 g/day of nuts) and an ad libitum low-fat diet on intermediate markers of cardiovascular disease. Compared with participants in the low-fat group, after 3 months those in the MedDiet groups had decreased levels of C-reactive protein. In addition, olive oil consumption also reduced levels of IL-6, VCAM-1 and ICAM-1. Moreover, people who eat the MedDiet that includes virgin olive oil reduce their levels of oxidized LDL, as suggested by the results of a subgroup analysis of the PREDIMED study carried out in 372 participants at high risk for cardiovascular disease, including diabetes(Fito, et al. 2007). Furthermore new data suggest that virgin olive oil intake was associated with higher levels of plasma antioxidant capacity after 3 years of intervention(Razquin, et al. 2009). In summary, based in the above evidences presented, we could assume that the MedDiet rich in nutrients with favorable anti-inflammatory properties may protect from metabolic diseases that are related to chronic inflammation and overproduction of reactive oxygen species, such as MetS and diabetes.
When explaining possible mechanisms is important to recall that fasting is not the typical physiological state of the modern human being, which spends most the time in the postprandial state. Therefore, the assessment of the postprandial lipemic response may be more relevant to identify disturbances in metabolic pathways related to inflammation and oxidative stress than measures taken in the fasting state. With regard to the postprandial state, several previous studies have demonstrated that a breakfast enriched in saturated fat resulted in an increase in biomarkers of inflammation and oxidative stress(Cardona, et al. 2008; Devaraj, et al. 2008; Ursini, et al. 1998). In this regard, the identification of increased expression of TNF-α, a proinflammatory cytokine, in the adipose tissue of obese mice and humans has been correlated with the degree of adiposity and associated with insulin resistance. This fact is crucial given than insulin resistance will drive towards an increase in oxidative stress, endothelial dysfunction and impairments in lipoprotein metabolism and blood pressure. Therefore, targeting TNF-α and/or its receptors has been suggested as a promising treatment for insulin resistance and type 2 diabetes(Tzanavari, et al. 2010). In this scenario Jimenez-Gomez et al. observed that a breakfast rich in olive oil or walnuts decreased postprandial expression of mRNA TNF-α in PBMCs from healthy men compared with a butter-enriched breakfast(Jimenez-Gomez, et al. 2009). However, the effects of the three fatty breakfasts on the plasma concentrations of these proinflammatory parameters showed no significant differences. The fact that we only found differences in the expression of TNF-α at mRNA levels in PBMCs following the intake of the three breakfasts may be due to that the synthesis and secretion processes of these proteins do not happen simultaneously, and to the short half-life of cytokines(Futterman and Lemberg 2002; Kishimoto 2005).
In the last years another interesting observation is that dietary fat may affect the endothelium(Berry, et al. 2008; Fuentes, et al. 2008; Goode, et al. 1997), and factors related to the arterial wall(Perez-Jimenez, et al. 1999). Several studies have shown that the acute administration of a high-fat meal induces a transitory disruption of endothelial function. Moreover, the effect of chronic consumption of a high-fat diet on endothelial function has also been evaluated. One study showed that a Mediterranean-style diet administered during 28 days to healthy subjects, attenuated plasma markers of endothelial activation, suggesting an improvement in endothelial function(Perez-Jimenez et al. 1999). Similarly, the chronic consumption of low-fat diets and Mediterranean-style diets improve endothelial function compared to a high-fat Western-type diet in hypercholesterolemic patients(Fuentes, et al. 2001). In this line, Esposito et al. demonstrated that the consumption of a Mediterranean-style diet by patients with the MetS was associated with improvement of endothelial function, by assessing the vascular responses to L-arginine, the natural precursor of nitric oxide(Esposito et al. 2004). More recently Radillis et al. observed that the close adherence to a MedDiet diet improves endothelial function in subjects with abdominal obesity(Rallidis, et al. 2009). On the other hand, previous studies also demonstrated that postprandial lipemia induces endothelial dysfunction(Anderson, et al. 2001; Bae, et al. 2003). According to this fact, Fuentes et al. studied the chronic effect of three diets with different fat compositions (high-SFA; high-MUFA; and a low-fat diet enriched in alpha-linolenic acid) on postprandial endothelial function and inflammatory biomarkers in twenty healthy men. This study demonstrated that the endothelium-dependent vasodilatory response was greater after the ingestion of the high-MUFA diet. Moreover this diet also induced lower postprandial sVCAM-1 levels and higher bioavailability of NOx compared with the other two diets(Fuentes et al. 2008). In the same line, Perez-Martinez et al. have recently showed that a high-MUFA diet improves endothelial cell, improving vasomotor function, with a higher availability of nitric oxide synthase and decreasing plasma sICAM-1 levels compared with a high-SFA diet and two low-fat, high complex carbohydrate diets, supplemented with 1.24 g/day of long chain n-3 PUFA or placebo, in MetS patients(Perez-Martinez, et al. 2010b). Therefore, those data carried out in the postprandial state support previous evidences suggesting that dietary patterns similar to those of the Mediterranean-style diet exert positive effects on components of the MetS and other conditions associated with, including endothelial dysfunction(Esposito, et al. 2006; Esposito et al. 2004). In the same population, the high-MUFA diet improved postprandial oxidative stress parameters as measured by glutathione levels and the glutathione/oxidized glutathione ratio. In addition, this diet induced lower postprandial plasma levels of lipoperoxides, protein carbonyls concentration and superoxide dismutase activity compared to subjects adhering to the other three diets(Perez-Martinez, et al. 2010a). Furthermore, postprandial plasma hydrogen peroxide levels were unfavourable increased during the high-SFA diet compared to the other three diets(Perez-Martinez et al. 2010a). These findings suggest that the postprandial state is important for understanding possible cardio-protective effects associated with the MedDiet particularly in subject with the MetS. In addition, these findings support recommendations to consume a high MUFA diet as a useful tool to prevent cardiovascular diseases in MetS patients.
Both endothelial cells and macrophages contribute to the generation of altered vasoreactivity and a procoagulant state through increased expression of plasminogen activator inhibitor (PAI)-1 and tissue factor and through platelet activation and acute phase reactions that increase levels of coagulation factors such as fibrinogen and factor VIII. Many of these molecules enter the circulation at levels that correlate with the degree of inflammatory activity. It has been well stablished that consumption of MedDiet as a dietary pattern, and virgin olive oil as it main fat source, is accompanied by a decrease in thrombogenesis, combining a decrease in coagulation factors and by platelet aggregation (Delgado-Lista, et al. 2008; Lopez-Miranda et al. 2007; Cicerale, et al. 2009; De La Cruz, et al. 2010; Gonzalez-Correa, et al. 2008; Lopez-Miranda et al. 2010; Perez-Jimenez et al. 2006; Rasmussen, et al. 1994; Thomsen, et al. 1995; Tripoli, et al. 2005; Visioli, et al. 2005). To infer the great value that these facts may have on diabetes, we should have in mind that cardiovascular events are the primary cause of death in these patients, and that, a lowering in their procoagulant/prothrombotic status is, nowadays, the main prophylactic and therapeuthic weapon to avoid cardiovascular events. In other words, adhering to MedDiet may act as a prophylaxis for the appearance of thrombotic driven cardiovascular events. Reinforcing this hypothesis, some authors have recently published in vitro and animal studies in which they show antiaggregant properties of virgin olive oil comparable in efficacy to those of acetylsalicylic acid (ASA). Even more, virgin olive oil and ASA, when in combination, act synergically to further inhibit platelet activation and aggregation(6, 10). Although the cited studies have been realized mostly in healthy persons, diabetic persons may also benefit from the antithrombogenic effects of MedDiet. As an example, Rasmussen et al showed how non-insulin dependent diabetic patients who were fed a MUFA-rich diet for three weeks decreased their von Willebrand factor (an important procoagulant factor) when compared to a carbohydrate-rich diet. The same authors, in an elegant design, compared the effects of two diets similar in carbohydrate and protein content, one rich in MUFA (30 energy %) and one rich in polyunsaturated fatty acids (PUFA) (30 energy %). After three weeks, the diet rich in MUFA reduced the levels of von Willebrand factor, confirming the results of their previous study, and stating the favourable effect that MUFA have on this prothrombotic molecule.
Effects of Mediterranean diet on key features and organs affected by diabetes mellitus. MedDiet: Mediterranean diet. DM: diabetes mellitus. SFA: saturated fatty acid.
It has been recently demonstrated the effects that phenolic fraction of olive oil exert at transcriptional level in vivo. In this regard, Camargo et al.(Camargo, et al. 2010) studied postprandial gene expression on peripheral blood mononuclear cells. To this end, two virgin olive oil-based breakfasts with high and low content of phenolic compounds were administered to 20 MetS patients following a double blinded, randomized, crossover design. They demonstrated that intake of virgin olive oil rich in phenol compounds is able to repress in vivo expression of several pro-inflammatory genes, thereby switching activity of peripheral blood mononuclear cells to a less deleterious inflammatory profile. In the same context, the consumption of a MedDiet with virgin olive oil, rich in polyphenols, decreased plasma oxidative and inflammatory status and the gene expression related with both inflammation (INF-gamma, Rho GTPase-activating protein15, and interleukin-7 receptor) and oxidative stress (adrenergic beta(2)-receptor) in PBMCs from healthy volunteers(Konstantinidou, et al. 2010). Moreover the same authors demonstrated the hypothesis that 3 weeks of nutritional intervention with virgin olive oil supplementation, at doses common in the MedDiet, can alter the expression of genes related to atherosclerosis development and progression(Khymenets, et al. 2009). In the same line, Llorente-Cortés et al.(Llorente-Cortes, et al. 2010) have confirmed in a population at high cardiovascular risk, that the MedDiet rich in olive oil influences expression of key genes involved in vascular inflammation, thrombosis and, in general, on atherosclerosis susceptibility. Moreover, it has been previously demonstrated in mice that olive oil up-regulates uncoupling protein (UCP) genes in brown adipose tissue and skeletal muscle(Rodriguez, et al. 2002), which is important given that UCP have been related with the regulation of body fat in mammals across its participation on the system of thermogenesis(Cypess, et al. 2009). It is well reported that mitochondrial biogenesis could, in part, underlie the central role of adipose tissue in the control of whole-body metabolism and the actions of some insulin sensitizers and that mitochondrial dysfunction might be an important contributing the symptoms of MetS(Wilson-Fritch, et al. 2004). In a recent study Hao et al. observed that the hydroxytyrosol (HT) treatment resulted in an enhancement of mitochondrial function, including an increase in activity and protein expression of mitochondrial complexes I, II, III and V; increased oxygen consumption; and a decrease in free fatty acid contents in the adipocytes. These data suggest that HT is able to promote mitochondrial function by stimulating mitochondrial biogenesis. This mitochondrial targeting property may provide a possible mechanism for the efficacy of the MedDiet for lowering the risk of cardiovascular disease and also suggests that HT may be used as a therapeutic intervention for preventing and treating diabetes mellitus and obesity(Hao, et al. 2010).
Because unhealthy eating habits and a sedentary lifestyle are among the strongest risk factors for metabolic syndrome and type 2 diabetes, modification of eating habits and physical activity constitutes an important component of any successful management program. In this chapter we reviewed the state of the art illustrating the relationship between MedDiet rich in olive oil and metabolic diseases, and to discuss potential mechanisms by which this food can help in disease prevention and treatment. Epidemiological and intervention studies indicate that MedDiet, thanks to its set of benefits, may protect from metabolic diseases that are related to chronic inflammation and overproduction of reactive oxygen species, such as MetS and diabetes. However, despite the significant advances of the last years, the final proof about the specific mechanisms and contributing role of the different dietary models and nutrients to its beneficial effects requires further investigations. In the future, the integrated application of approaches that are becoming available in functional genomics, metabonomics, lipidomics, microbiota, cronobiology, proteomic techniques, and bioinformatics analysis, will lead to a more highly integrated understanding of its positive effects on health. In this context the recent advances in human nutrigenomics and nutrigenetics, two fields with distinct approaches to elucidate the interaction between diet and genes but with a common ultimate goal to optimize health through the personalization of diet, will provide powerful approaches to unravel the complex relationship between nutritional molecules, genetic polymorphisms, and the biological system as a whole. On the other hand efforts should be put into identifying those micronutrients in olive oil that have the greatest beneficial effects on health.
In conclusion after decades of epidemiological, clinical and experimental research, it has become clear that consumption of Mediterranean dietary patterns rich in olive oil have a profound influence on health outcomes. Thus, there is good scientific support for recommend MedDiets, especially those based on olive oil, as an alternative approach for the medical nutritional therapy in obesity, MetS and diabetes.
Ciber Fisiopatología Obesidad y Nutrición, CIBEROBN, is an initiative of ISCIII government of Spain. This study was supported in part by research grants from the Spanish Ministry of Science and Innovation (AGL2006-01979/ALI, AGL2009-12270 to JL-M, SAF2007/62005 and PI10/02412 to FP-J and PI10/01041 to PP-M), Consejería de Economía, Innovación y Ciencia, Proyectos de Investigación de Excelencia, Junta de Andalucía (CT5015 to FP-J and P06-CTS- 01425 to JL-M); Consejería de Salud, Junta de Andalucía (07/43, PI 0193/09 to JL-M, PI-0252/2009 to JD-L and PI-0058-2010 to PP-M). Also supported by Centro de Excelencia Investigadora en Aceite de Oliva y Salud (CEAS) and FEDER, Fondo Social Europeo. AG-R is supported by a research contract of ISCIII (Programa Río-Hortega). None of the authors had any conflict of interest.
Electrical stimulation of the human auditory system is generally traced back to the pioneering experiments of Alessandro Volta, inventor of the battery. When Volta applied 50 volts to his own head, he reported hearing an unpleasant boiling sound [1]. However, the forerunner of a modern CI system is just over 60 years old: opportunistic stimulation of the auditory nerve [2] of a bilaterally deaf patient receiving a facial nerve graft. During the two decades following this work, various clinical studies [3, 4, 5, 6, 7, 8, 9, 10] saw the implantation of single and then multi-channel cochlear implant (CI) systems in people suffering profound deafness. While many of these pioneers suffered ridicule at the hands of the mainstream scientific community, clinical considerations prevailed. The early devices that were produced in academic institutions were transferred to commercial organizations, these often building on prior medical device experience, for example experience gained in the pace maker field.
\nToday over half a million people, from babies under 6 months of age to adults in their late 90s, have been implanted with a CI. While it can be argued that the CI is the most successful medical device ever created, the outcomes are still highly variable (Figure 1). In the best of cases, CI users can make fluent use of a telephone, understand speech in adverse listening conditions where there is considerable competing noise and reverberation, hence enjoying independence spanning social lives and careers that would have been unimaginable without their CI device. Even where speech understanding is limited, a release from the isolation of deafness through access to environmental sounds, a reduction in the level of tinnitus and support of lip-reading with a reduction in the effort required for oral communication are all worthwhile benefits from use of a CI. It should also be noted that in many cases those most satisfied with their implant are not those who receive the highest scores on standardized tests of speech understanding.
\nPercent correct scores on the CNC word test ranked from poorest to best for 113 cochlear implant users showing a large variation in outcome reproduced from Holden et al. [79].
The following sections will describe how electrical stimulation of the auditory system is achieved, with the main focus being on CI systems. The factors that influence outcome, so far as they are known, will be described, along with the challenges in delivering clinical service, both today and into the future. With the future in mind the major research topics that are currently being addressed will be outlined.
\nFigure 2 shows the various components common to all of today’s clinically applied cochlear implant systems. Sound is typically collected from microphones housed on a behind the ear (BTE) sound processor. The sound is first “cleaned” to remove noise and then processed to create the stimulation patterns destined for the implanted electrode array. Except in the case of one-piece processors, a lead connects the sound processor output to a radio frequency (RF) transmitter coil located above and behind the ear. The external coil is held in place over the implant’s receiver coil through a pair of magnets: one external and one within the surgically implanted device under the skin. This arrangement supports reliable communication across the skin through the use of RF based telemetry. The RF signal provides both power for the implant’s electronics and the information needed to produce electrical stimulation. Hence the implant consists of: its receiver coil, a hermetic package containing electronic circuits and an electrode lead assembly connecting to the electrode array that is placed inside the cochlea (Figure 3). In some of today’s CI systems the sound processor and RF coil are a single component held in place by the magnet but having no wire or BTE part. This provides some esthetic advantage but may fall off more easily and compromises sound collection.
\nThe components of a behind the ear (BTE) model of cochlear implant showing (1) the T-mic placed in the external ear canal, (2) BTE sound processor, (3) radio frequency transmitting headpiece, (4) the implant body, (5) intra-cochlear electrode array and (6) the auditory nerve.
A cross-section through the spirally-shaped cochlea showing the various compartments, including the scala tympani with a mid-scala located electrode array.
Additionally today’s implants have the ability to make both physical and physiological measurements, using back-telemetry, to transmit these data to the sound processor. Through the use of wireless technology, information can be relayed to and from a host of devices: smartphones, tablets, laptops, remote microphones or other listening aids. Such connectivity leaves a CI user well placed to use many consumer devices to enhance their communication and support maintenance of their implant system.
\nIn the earlier chapters of this book the auditory system has been described in some detail, including pathology that can result in the most debilitating degrees of hearing loss: severe to profound deafness. Fortunately, electrical stimulation can be delivered without external, middle or indeed even an inner ear. However, in the large majority of clinical cases the auditory nerve is intact and can be accessed via a very poorly or non-functioning cochlea: it being this damage that the CI bypasses. The operating principle of a CI is that small electrical currents are able to initiate activity on the auditory nerve that crudely mimics the activity produced by a normally functioning cochlea. Taking advantage of the cochlea’s tonotopic organization, currents representing high frequency sounds are delivered to the base, while currents representing lower frequency sounds are delivered to more apical cochlear locations. This is achieved through the use of an electrode array containing multiple separate electrode contacts placed along the scala tympani (Figure 4). The number of intra-cochlear electrode contacts in clinical service varies by manufacturer between 12 and 24. In addition, either the shorting of adjacent contacts, or simultaneous delivery of synchronized stimulation patterns on multiple electrode contacts, seeks to increase the number of stimulation sites available [11, 12, 13].
\nView of how an electrode array will be positioned within the scala tympani of the cochlea, here with the electrode contacts facing towards the modiolar wall behind with the spiral ganglion cell bodies are located.
In Figure 4 an electrode array is shown placed in the scala tympani. Here the exposed electrode surface, from which stimulation current is delivered, faces towards the modiolar wall, behind which the auditory nerve’s spiral ganglion cell bodies are located in Rosenthal’s canal. The remainder of the array is composed of a soft silicone that supports the contacts and the insulated wires connecting the electrode contacts to the implant’s electronics. A modiolar facing contact surface orients the electrical stimulation towards the primary stimulation targets, the spiral ganglion cell bodies. Since the perilymph fluid in the scala tympani is electrically conductive, it allows current to flow through the cochlea and achieve stimulation of neural elements. A downside is that current also tends to spread along the scala, rather than addressing only the area local to the electrode contact where we would like it to act. If peripheral processes still remain in the cochlea, extending from the cell bodies to the organ of Corti, then these may also be targets for electrical stimulation. Unfortunately it is not currently possible to accurately know the status of any individual’s cochlea. It appears reasonable to assume that a great deal of the variability illustrated in Figure 1 comes from variations in the health of those individuals’ cochleae, this variation being part dependent on environment and disease and the individual’s particular physiology, as well as how any one individual’s immune system reacts to the insertion and presence of the electrode array itself.
\nThe most common type of stimulation paradigm used in CIs today is monopolar stimulation (Figure 5a). Here the implant introduces current into the cochlea via a relatively small electrode contact. A typical surface area might be 0.2 mm2. The density of current close to the electrode contact introduces a higher probability of activating elements close to the contact, the probability decreasing for elements at increasing distance. Monopolar stimulation current is returned to the implant using a distant extra-cochlear electrode that has at least 10 times the intra-cochlear electrode contact’s surface area. This keeps the returning current density low, avoiding stimulation at this remote site. Typically there are two return electrodes on a cochlear implant, in case anything goes wrong with one of them. One is placed on the body of the implant while the other is on a separate flying lead, or on the electrode lead assembly but located outside of the cochlea. In Figure 6 an implant can be seen with its various component parts indicated.
\nElectrical stimulation configurations: (a) monopolar where current is returned to a large distant extra-cochlear return electrode, (b) bipolar where current flows between adjacent intra-cochlear electrodes and (c) tripolar where current is returned by two adjacent contacts.
A cochlear implant showing its various components. Note the two ground or return contacts, one on the case body and one on the electrode lead assembly.
Alternative stimulation paradigms are sometimes used, but mainly for research. Figure 5b shows a bipolar stimulation paradigm. Here two intra-cochlear electrode contacts, separated by around 1 mm, operate as a pair. Stimulation is introduced by one contact and returned by the other. This in theory restricts stimulation to a small part of the cochlea, so should help with the spread of current mentioned above. However, in practice current introduced by one contact is returned to the other without activating enough neural tissue to create a loud enough hearing sensation. Hence, it is often necessary to form a bipolar pair using contacts that are not adjacent but for example are spaced 2 mm or more apart. In addition, the arrangement of contacts along the cochlea means that there will be a plane of zero field between the contacts, leading to a need to use higher currents and thus produce a wider spread in stimulation. In Figure 5c tripolar stimulation is shown. Now a group of three electrode contacts are used together. Stimulation is introduced via the middle contact and ideally half returned via the adjacent contact on either side. This avoids the zero potential plane problem of bipolar stimulation and theoretically provides a tighter containment of stimulation. Again, in practice there is a need to recruit a given number of neurones to signal sufficient loudness and this means increasing the tripolar stimulation current on the middle contact. Eventually the current on the return electrodes will become high enough to cause stimulation, resulting in a wider spread of current than intended. In many cases it is not even possible to increase the current far enough to achieve sufficient loudness for a tripolar configuration. In such cases some of the current has to be returned to the remote extra-cochlear return electrode, a configuration referred to a partial tripolar. This even further increases the current spread. So, with these practical limitations in mind it is not difficult to see why monopolar is universally used as the default stimulation paradigm, despite the apparent large current spread that this entails.
\nIn the interests of simplicity some of the more technical aspects of electrical stimulation have been avoided in the text above, allowing focus on the broader application. In this section some of these more technical issues will be discussed. Where a reader is not interested in technical detail this section can be ignored.
\nOhm’s Law states that the electrical voltage difference required to drive a given current is directly proportional to the resistance through which the current has to flow. With changes in the resistance, or more generally the impedance if we consider frequency effects, a voltage source would lead to uncontrolled changes in the current being output. As described below this could lead to uncontrolled changes in loudness over time. Most of today’s CI systems deliver electrical stimulation through one or more current sources. As the name implies, this circuit attempts to deliver the current requested of it regardless of how much electrical resistance is offered by the body. A current source is said to be in compliance when it is delivering the current requested of it. Given a finite amount of voltage being available within an implant, for example 8 volts, there will be a maximum impedance into which the implant’s maximum output current can be delivered. With a typical stated maximum output current of 2000 μA, the maximum impedance for which this could be delivered will be 4000 Ohms (8 volts divided by 2000 μA, or 8/2 × 10−3 = 4000). For higher impedances the maximum stated output current will not be available. For lower impedances the implant will be limited to its maximum output current value, ensuring safe operation.
\nSince CI systems are designed to provide stimulation for essentially all waking hours, day after day over decades, their output must not damage the neural tissue that they are intended to stimulate. One obvious source of damage is the delivery of direct current (DC). If a DC current is applied to the body it will result a process called electrolysis. Here there will be a continuous transport of ions, charged atomic particles, leading to dissolution of the platinum electrode contact and destruction of the cochlear tissue: obviously a catastrophic situation. Research in animals indicates that even very small amounts of DC, 400 nA, is enough to cause tissue damage [14]. Several mechanisms are used in a CI to prevent the delivery of DC. Firstly, a balanced stimulus waveform is used, almost always a symmetrical pulse having two complementary phases (Figure 7a), although so long as the two phases contain an equal and opposite area they need not be symmetrical (Figure 7b). At the simplest interpretation, the first phase, referred to as cathodic, will depolarize neurones hence producing the electrical stimulation that we seek to achieve. The second, anodic, phase balances the stimulation resulting in no nett current being delivered to the body, thereby avoiding DC. Even with very careful design, there is likely to be some small imbalance between the two phases. To account for any in-balance, following delivery of a stimulation pulse the electrode contact is connected to ground, ideally removing any residual DC. Finally, an electronic component called a capacitor is placed in series with the stimulating electrode contact. A capacitor does not allow DC to pass, so offers yet more protection in case some fault in the electronics interferes with either of the previous two protection mechanisms. Together these mechanisms appear successful in avoiding the delivery of DC. Devices do fail, particularly in the pediatric population, with reimplantation rates over tens of years being reported at 8% from the well-established Sydney clinic [15]. Typically half of these failures are medical issues and half device failures. However, in virtually all cases it is possible to re-implant the patient, with outcomes almost always being as good as those obtained when the original device was working well [16].
\nStimulation waveforms with balanced cathodic and anodic phases may have either symmetrical phases (a), or asymmetrical phases (b) where the area of each phases is identical.
While we speak about stimulation current it is really the electrical charge that is at issue. Charge is simply the product of current times time and has units of Coulombs, C. Electro-chemical considerations mean that an electrode has a maximum charge injection capacity, such that a given size and material will only be able to handle a given charge limit in a reversible way, so that all of the charge injected in one phase can be removed in the second phase [17]. This is necessary to avoid the DC as discussed above. A conservative value for the maximum charge density, typically 30 μC/cm2 [18], taking account of the electrode dimensions, is programmed into the implant’s controlling software, ensuring that this limit is never exceeded. Animal experiments confirm that chronic stimulation with higher charge densities, for example 400 μC/cm2, results in the dissolution of platinum but interestingly not to the loss of auditory neurones [19]. The loudness sensation produced by electrical stimulation is related to the amount of charge delivered in one phase of the stimulation waveform. There is also an effect of the rate at which stimulation pulses are delivered. However, for the stimulation rates used in clinical practice, typically 500–2000 pulses per second per channel (ppps/ch) the effect of rate is quite small and largely ignored.
\nStimulation current flows through cochlear tissue as a result of voltage differences developed along the current’s path. How these voltage differences are arranged along the length of a neurone’s peripheral or medial process, or indeed across the cell body, determines which neurones depolarize, leading to action potentials being generated. The action potential may propagate to the medial synapse with the cochlear nucleus and hence initiate activity on the auditory pathway, leading to a sense of hearing being detected in the brain. Rather than stimulating a single neuron, typically hundreds, or even thousands, of neurones in a region of the cochlea will be addressed by a single electrode contact. These patterns of stimulation are interpreted as sound input by the higher levels of the auditory system, leading to the sense of electrical hearing. The next section will describe how sounds detected by the CI system’s microphone will result in the generation of electrical stimulation patterns.
\nThe main cochlear implant system functions are shown schematically in Figure 8. Sound from the microphone is compressed by a single-channel automatic gain control (AGC) system. Compression ratios in CI systems tend to be substantially higher than those in acoustic hearing instruments: six to infinity, compared to two to three respectively. This reflects both the small electrical dynamic range of typically 10 dB [20] and the exponential like increase in loudness found for electrical hearing [21]. Both considerations require tight control of the stimulation current’s amplitude to avoid discomfort. Research with different implant types shows a consistent advantage for slow-acting AGC, the benefit being a reduced compression of the information rich temporal modulations of speech [22, 23, 24], as well as a reduced co-modulation effect [25] associated with the single channel AGC.
\nA schematic of the sound processor system where sound is collected by the microphone, compressed by an automatic gain control, broken into discrete frequency channels, which have their energy assessed and mapped to the user’s requirements. This information is then combined into a digital stream, transmitted by radio frequency to the implant where stimulation currents are generated.
Following AGC, the sound is broken into a number of frequency channels, this number varying between 12 and 24 channels, reflecting the number of intra-cochlear electrode contacts available in the implant model. In Figure 8 only four channels are used to illustrate the principle. Today a Fast Fourier Transform (FFT) algorithm is often used to separate the incoming sound into discrete frequency channels. The amount of energy in each channel is then estimated by a rectification and low-pass filtering process. While the average energy is calculated over a period of perhaps 10 ms (milliseconds) or more, stimulation pulses will be delivered much more rapidly, typically once every millisecond. Hence calculations will be made that overlap in time in an attempt to follow the changes in speech energy over time. Next the acoustic energy in each channel is mapped to an electrical current amplitude that takes account of the CI user’s sensitivity to electrical stimulation. The goal is to use smaller currents that barely produce a perception of electrical stimulation to represent low-intensity acoustic activity and larger currents that are perceived as loud to represent very intense acoustic events. This needs to be managed separately for each channel, resulting in the continuous output of a stream of stimulation amplitudes for each channel. As shown schematically, these amplitudes are then combined together for transmission by the RF signal across the skin to the implant. Electronics inside the implant extract the digitally transmitted amplitudes, convert them to analogue values and then drive the implant’s current source(s), resulting in stimulating currents being delivered by the intra-cochlear electrode contacts. For virtually all of today’s clinical systems only one channel will be stimulated at a time. This approach avoids the channel interactions that would occur were channels presented simultaneously within the conductive scala tympani [26]. The disadvantage of this approach can be seen in Figure 9 where a channel is only updated during its own time period, therefore, must wait until all the other channels have been updated until new information can be transmitted. Deliberately, very brief current pulses each of around 40–50 μs duration (20–25 μs/phase) are used, so that it is still possible to update each channel rapidly enough to keep up with the changes in acoustic energy over time. This often means stimulation at more than 1000 pps/ch. Such an approach generally leads to higher levels of speech understanding than where simultaneous stimulation is delivered [27, 28].
\nAn illustration of now non-simultaneous waveforms delivers information for each channel. Once a channel has been stimulated no more information may be delivered until the other channels have been updated.
Over the years the sound coding strategy, a software algorithm that relates audio from the sound processor microphones to the electrical patterns appearing at the electrode contacts, has changed. Initially it was believed that the damaged auditory system was not capable of transmitting much information, hence the most useful information was extracted from the speech and directly coded on sets of electrodes. For example an early feature extraction strategy F0F1F2 [29] extracted the first two formants of speech, F1 and F2, from which it is possible to estimate the vowel being articulated. Each formant range had a set of electrode contacts allocated, such that higher or lower frequencies for each formant lead to stimulation on more basal or more apical electrode contacts in that formant’s electrode set. The rate at which pulses were delivered was related to the fundamental frequency (Fo) driving the vocal tract, leading to the strategies name. Such a strategy supported only very modest levels of speech understanding, around 8% correct for monosyllabic words presented in quiet [30]. The information extracted was limited to begin with and further reduced through errors generated in real life listening situations where background noise, reverberation and intensity and frequency response variations led to the algorithm making mistakes in both the extraction of formant frequencies and in the estimation of Fo.
\nIt was eventually recognized that the brain was better at extracting information than the feature extraction algorithms and hence “whole-speech strategies” replaced feature extraction. Today’s sound coding strategies simply average the energy in each channel’s frequency range and generate levels of stimulation that represent this. In some cases a so-called n-or-m strategy will work out which subset (n) channels from the total (m) number available have the highest energy and then only stimulate this reduced set. Refinements to this may neglect adjacent channels on the basis that stimulating both will not add anything, so select a more distant lower amplitude electrode to transmit more information [31].
\nStimulation delivered by a CI system will result in the depolarization of neural elements, resulting in action potentials being generated that propagate to the next stage of the auditory system: the cochlear nucleus. With reference to the schematic of Figure 8, there is a population of spiral ganglion neurones associated with each electrode and hence each frequency channel of the CI system. As mentioned above, a channel’s stimulation current will need to recruit a certain population of neurones whose firing indicates to the brain the amount of activity in a particular frequency range. Ideally, there will be a sufficient local neural population such that progressively increasing stimulation current initiates an appropriate number of action potentials, so that the brain correctly perceives the amount of acoustic activity in the channel’s frequency range.
\nUnfortunately a discrete neural population for each channel as shown in Figure 10a is not always available. In Figure 10b only a reduced neural population is available for each channel. Hence, when there is a lot of activity in one channel, requiring recruitment of a full population of neurones, these are not available locally. It is still possible to increase the stimulation current, spread the electrical field further away from the electrode and depolarize neurones that should really be associated with another channel. While this will satisfy the perception of loudness, it generates channel interaction so that we are no longer able to deliver frequency specific information to a discrete part of the cochlea. The perception will be of a blurred or fuzzy sound, particularly a problem when trying to listen to speech in the presence of competing noise.
\nA schematic representation of three different neural populations: (a) a full population exists for each channel, (b) a depleted population results in channel interaction and (c) a dead region requires recruitment from the populations rightly belonging to adjacent channels.
An alternative situation is shown in Figure 10c where most electrodes have a sufficient local neural population but one electrode is located in a so-called dead region [32]. When electrode 2 is stimulated it can only recruit neurones from the population belonging to electrodes either side of it, delivering information about channel 2’s frequency region to other parts of the cochlea, spreading stimulation widely and interfering with the otherwise discrete frequency information being delivered by the neighboring channels.
\nUnfortunately, it is not currently possible to determine what the number and distribution of neural elements is for any individual. The literature is not always helpful in this area. As shown in Figure 1, there is great variability in outcome for the identification of monosyllabic words. Since this task involves little top-down processing, much of the variability in outcome must come from the electro-neural interface. Beyond speech understanding, examining the ability to discriminate adjacent electrodes, or intra-electrode stimulation sites [33], also showed both great variability between subjects and across the electrode array of individual subjects. This task, having no confound with cognitive processes related to speech understanding, further confirms the presence of peripheral variability and its likely contribution to variations in outcome.
\nIt is unclear to what degree a loss of spiral ganglion cells (SGC) in humans will follow, even after years of severe to profound deafness. Histological studies of humans who had used a cochlear implant sometimes show a reasonable correlation between CNC word score and SGC count: for example R = 0.62 [34], R = 0.9 [35] but for a small group of only 6. However, the variability is such that the same SGC count can show variations of between 30% and 75% for CNC words, or the same CNC word score can be associated with 3000 or 18,000 SGCs. Examining the threshold current for detecting electrical stimulation in a group of 130 lateral wall electrode array users [36] showed significant differences between four groups: the increase in group mean threshold being associated with a reduction in monosyllabic word score. This works suggests that a higher SGC population (lower electrical threshold) is associated with better speech understanding.
\nThe literature listed above indicates that there is a relationship between the number of spiral ganglion cells and the ability to identify monosyllabic words when using a cochlear implant. Contributions to speech understanding may also come from a large number of additional factors, some of which include: the distribution of SGCs, angular insertion of the electrode array, distance of the electrode contacts from the modiolar wall, presence or absence of peripheral processes, fibrous sheath formation and intrusion of new bone into the cochlea. How well a given implant user has had the parameters of their sound processor set, commonly referred to as their program, is another variable that we will examine next.
\nAs has been explained above, the small electrical dynamic range available to a CI user makes it necessary to carefully adjust the stimulation parameters to suit the requirements of each individual recipient. The most important adjustment is the amount of stimulation that will be delivered in response to acoustic activity. This must be done for each of the CI’s separate channels. Each channel has two primary parameters that control its output. One will be typically called a most comfortable level, shortened to either M-Level or C-Level. The other is a threshold control, referred to as T-Level. The main CI manufacturers use these parameters slightly differently but to a good approximation T-Level sets the minimum stimulation level that the implant will deliver and M-Level will set the maximum stimulation level that can be delivered for an individual recipient. The sound processor will then arrange for the amount of acoustic range that it handles, somewhere between 40 and 80 dB depending on the user’s setting and implant model, to be mapped to stimulation levels between T- and M-Level. In combination with the AGC of the system this will give the CI user access to their acoustic environment such that hearing levels of between 20 and 30 dB HL are achieved across the frequency range 250–8000 Hz. The combination of AGC and M-Level ensures that even high intensity sounds of 100 dB SPL do not produce uncomfortably loud sensations. Unlike acoustic hearing, it is generally possible to provide CI users with access to the full range of frequencies that are most important for speech understanding.
\nWhich channels are activated is another important adjustment to make. Most audiologists are reluctant to deactivate channels, although sometimes a reduced set of channels can give a better outcome. In some cases an electrode array is not fully inserted into the cochlea, perhaps due to the cochlea being too small, or there being fibrosis tissue, or bone, that prevents a full insertion being obtained. Alternatively, electrode arrays can sometimes extrude from the cochlea [37, 38], either shortly after implantation or months to years later. In all these cases the more basal electrode contacts will need to be deactivated. Deleting electrodes from a program will lead to the frequency range being remapped across the remaining electrode contacts. There will be a coarser representation of frequency since fewer channels are now available. However, removing electrodes that are not inside the cochlea will produce a better outcome than simply leaving these electrodes active.
\nBeyond setting T- and M-Levels and defining an appropriate set of electrode contacts, there is sometimes adjustment made to the acoustic dynamic range mapped by the sound processor. This effectively controls the compression of acoustic sounds into the electrical dynamic range. It might seem logical to use as large an acoustic or input dynamic range (IDR) as possible, since this will maximize the range of sounds available to a CI user. However, it is the discrimination of different levels of sound in each channel that carries information. An excessively large IDR may squeeze these amplitude cues, reducing the ability of an implant recipient to understand speech. There are many parameters that can be adjusted in a CI system. However, it is common for the majority to remain at their default values. This may be through an inability to obtain user feedback, for example in young children, lack of time or knowledge on the part of the clinician, or a recommendation from the CI manufacturer.
\nHow appropriate values are found for the T- and M-levels depends very much on the individual CI user. For a post-lingually deafened adult it is reasonably straightforward to find these. By presenting 200 ms bursts of stimulation and using a standard bracketing approach, the smallest detectable amount of stimulation for each channel can be found and this value set as the T-level. Similarly, progressively increasing the stimulation will allow an M-level to be found, the CI user often pointing to different categories on a loudness chart as the various levels of stimulation are presented. These measures can be made for each individual channel, channels can be programmed in groups of four, or only five or six channels across the electrode array measured with intermediate channels set to interpolated values.
\nFor babies or young children and even for some adults, objective measures are often used to help set program levels. The most common measure used is the eCAP, the electrically elicited compound action potential [39]. The ability to record eCAPs is built into the fitting systems for all of today’s major CI systems. Here masker-probe or alternate-subtraction techniques [40, 41] are used to reduce the large stimulus artifact. The amplitude of the remaining physiological signal, arising from synchronized activity on the auditory nerve, is then graphed against the stimulation level. A regression line extrapolates to intersect the stimulation axis which would correspond to a zero amplitude of eCAP. The stimulation value for which this occurs is then used as a guide for setting programming levels. Avoiding stimulus artifact and allowing sufficient neural synchronization, means that much lower stimulation rates are used when measuring eCAPs than for actual everyday stimulation. The means that the absolute eCAP values can fall at various parts of an individual’s electrical range. Fortunately, it is the profile of values across the electrode array that it is important to determine. Once this is estimated a global change in level can be made to obtain appropriate loudness. In many cases the T-levels are set to 10% of the M-level since this is almost certainly not going to leave them set too high. Typically T-levels are measured at something like 25% of M-level [42]. When they can be measured and hence individually set, T-levels will tend to improve access to low intensity sounds. Often in clinical practice T-levels are set at a percentage of M-level even where they could be individually set: the additional benefit not being considered worth the additional effort needed for measurement.
\nOther objective measures are used to assist with programming, although less often due to these requiring additional equipment to be used in collaboration with the CI fitting system. There is a reasonable correlation between an electrically elicited stapedius reflex threshold (eSRT) and M-level [43]. Unlike eCAPs here the same stimulation rate can be used to measure eSRT as will be used in the everyday program. This simplifies the setting of levels and is partly behind why there is such a good correlation with M-level. Less commonly the electrically elicited auditory brainstem response (eABR) is used [44]. Again, eABR will require a lower stimulation rate to be used, so that the characteristic waveforms can be seen in up to 5 or 6 ms following stimulation. This tends to produce an extrapolated threshold for eABR quite high in an individual’s electrical dynamic range. As with the eCAP and eSRT measures, it is the relative levels across channels that are important, the profile then being globally adjusted to determine the M-levels that will be used in the program.
\nLess frequently, some statistically based approaches are used for programming. Simple so-called “flat maps” are used where the T- and M-level is the same on each channel. These are justified by the spread of monopolar stimulation recruiting neurones form a larger section of the cochlea than associated with an individual electrode contact thus tending to produce a spatial averaging. Other approaches might use a template based on the statistical average of levels previously measured for earlier CI recipients. Approaches such as FOX [45] extend this technique, recommending a sequence of programs with progressively increasing levels that are used from the very beginning. For many CI users these techniques can work quite well, although numbers of outliers will require individually tailored programs to realize their potential outcome with comfortable stimulation and reasonable access to their acoustic environment.
\nPlasticity in the auditory system means that over time the M-levels will usually increase. The longer term M-levels might be typically double those that can be tolerated during the initial fitting. After the first 2 months of device use, neither T- or M-levels tend to change significantly over time [46]. Change in levels is highly individual requiring the initial program levels to be revised numbers of times during the first few months of implant use. Where a second (or third) fitting session is planned within around 2 weeks of the first fitting, most of the change can already be accommodated. Looking across large numbers of adult CI users, program levels will be stable by between 3 and 9 months following first fitting. Individual practice can result in pediatric levels being more slowly increased, leading to 6–12 months being needed to see stable levels.
\nCochlear implants were originally designed to help those suffering bilateral profound deafness who could not benefit from acoustic hearing aids. Traditionally candidacy would have required a loss of at least 90 dB HL across all of the audiometric frequencies from 125 to 8000 Hz. Over the past 30 years we have seen, improvements in outcome (speech understanding) through better sound coding strategies and electrode arrays, improvements in esthetics as the external equipment has moved from body worn to behind the ear or single piece processors, improvements in surgery with a skin to skin operating times of well under 1 hour, as well as much smaller incisions not requiring hair shaving and at least in some cases, the preservation of residual hearing. These developments have meant that a CI can now be considered for much more than the 0.2% of the population who suffer profound bilateral sensorineural deafness [47].
\nIt is becoming more common for ears with useful low-frequency residual hearing to receive a CI. Candidacy can now include those with severe to profound levels of hearing loss above 1000–2000 Hz, but normal to moderate hearing loss for lower frequencies [48, 49, 50]: a group sometimes referred to as suffering partial deafness. Where the residual hearing can be preserved to within 10–20 dB of the pre-operative levels, many of these recipients use a combination of electrical and acoustic stimulation (EAS) in the same ear. Most CI manufacturers now make EAS processors so that a single instrument supports both modalities, offering comfort, convenience and allowing an EAS fitting to be made using a single piece of software.
\nWhere there is some asymmetry in hearing, recent practice has seen only the poorer ear being implanted, while an acoustic hearing instrument is fitted to the contralateral ear. This is often referred to as bimodal hearing. Dedicated hearing instruments (HI) have been developed that match the compression characteristics and sound cleaning operations between the CI and HI, as well as offering wireless sharing of microphone and control signals. Such systems offer convenience for the user and can combine the natural acoustic low-frequency sound in the HI ear, with the high frequency information supporting speech understanding in the implanted ear.
\nBilateral CI provision is now the standard of care in many healthcare systems, at least for children. Receiving two implants, either simultaneously or within a few months of each other, provides the best chance for the brain to have both sides work together. Redundancy, the countering of head shadow and a fuller sense of hearing are all advantages of bilateral implantation. It tends to be only considerations of cost that prevent bilateral CIs being offered universally to all those who could benefit from them. Again with wireless technology developing rapidly, the use of algorithms that combine microphones between the two CI sound processors can offer large improvements for listening in noise when beam formers are used to attenuate noise coming from directions other than directly ahead, particularly useful then the CI user is in a one-to-one conversation in a noisy location.
\nWhere a second CI is not available and there is no aidable hearing on the contra-lateral ear a CROS, or strictly bi-CROS device can be used. Wireless CROS devices are available that essentially have their microphone pick up sound from the non-implanted side and wirelessly route it to the CI processor on the other side where it is mixed with the CI processor’s microphone signal. This approach can reduce head shadow, although with stimulation only being delivered to one ear there is little ability to use the CROS device for localization. With the combination of HI and CI companies, for example Phonak and Advanced Bionics within the Sonova company, the migration of HI technology such as the ear-to-ear wireless technology has begun and will likely be more common in future.
\nIn the German healthcare system, a CI is now available to those who suffer from single-sided deafness (SSD). Typically there may be also be some hearing loss on the better hearing side, making this a highly asymmetrical loss rather than a pure SSD. Those suffering with SSD would usually explore a CROS device and a bone conduction hearing aid before considering a CI. In the end around one third of SSD cases seen will elect to get used to hearing with only one ear, one third will use a bone conduction device and one third will receive a CI [51].
\nTinnitus is another consideration that can influence treatment options, for SSD and beyond. Where the SSD is accompanied by intractable levels of tinnitus, a CI may provide relief [52]. The restoration of some input to the deafened ear can allow the tinnitus to either effectively disappear or at least be substantially reduced. In some cases, SSD in particular, the relief from tinnitus is found to be of much greater benefit than any hearing sensation arising from the implanted ear. The large majority of CI recipients report reduced amounts of tinnitus although in very rare cases tinnitus can be worsened through implantation.
\nThe cochlea is an attractive site for electrical stimulation, given that it presents tonotopic access to auditory nerve fibers with reasonably straightforward surgical access. However, where the cochlea has not formed properly or at all, due to some extreme malformation, a properly formed cochlea has been filled with bone or tissue, for example following bacterial meningitis, preventing all but minimal surgical access, or the auditory nerve is not available, either through malformation or following trauma, stimulation of the auditory system via the cochlea is not possible. In such cases alternative sites of stimulation may be used.
\nAuditory brainstem implants (ABIs) bypass the auditory nerve, targeting the next station of the auditory pathway: the cochlear nucleus located in the brainstem [53]. There is a tonotopic structure within the cochlear nucleus, although it is organized in the dimension of depth, so is not easy to access. Attempts to use a penetrating electrode array with a number of discrete needles has not been able to make better use of this tonotopic organization than a pad of flat electrodes placed on the surface of the cochlear nucleus [54]. Programming of an ABI device tends to be more difficult than that of a CI. Bone surrounding the cochlea usually keeps the CI’s stimulation contained to auditory fibers. Only occasionally non-auditory stimulation of, for example, the facial nerve can be seen in muscle twitching around the mouth or eye. This is generally programmed around by deactivating electrode contacts or reducing stimulation levels. However, in the brain stem, functions such as respiration can be adversely effected by an ABI device. This calls for much more care when programming, leading to ABI devices being offered only by specialist centers. The surgery required to place an ABI is more invasive than that required for a CI, for example, requiring lifting of the cerebellum to gain sufficient surgical access. With ABIs being placed following removal of tumors there can be some distortion of brain structures. Some surgeons prefer to remove what are often sizable tumors, allow the brainstem and brain structures to settle into place again and then perform a second surgery during which the ABI is put into place. This two-stage approach is believed to provide less chance of the ABI’s electrode pad moving out of position, risking substantial non-auditory stimulation. Outcomes with ABI devices are generally substantially poorer than with CIs. In some series there is essentially no open-set speech understanding possible [55], while in others the speech understanding is limited, with only occasional high levels of speech understanding [56]. The reasons for poor performance with ABIs are not fully understood. Beyond potential movements of the electrode pad, there are specialized auditory functions being carried out in the cochlear nucleus, meaning that simply assuming raw tonotopic stimulation patterns may not be sufficient. Additionally, those receiving ABI devices may have many other issues beyond deafness and these could also explain some of the difference in outcome.
\nStimulation of even higher structures in the auditory system has been attempted through an auditory mid-brain implant (AMBI), where the electrode array is inserted into the inferior colliculus. Currently this is restricted to a pure research device [57]. Within the inferior colliculus it is possible to access a tonotopic organization, using a shortened version of a traditional CI electrode array, 10 mm long as opposed to 20–30 mm for most CI arrays. However, when accessing the auditory system at an even higher level than with an ABI device, the amount of pre-processing that should have already been done leaves a crude CI type coding strategy only able to support very limited outcomes. Already at this level higher stimulation rates are inappropriate, leaving limited sound coding strategy options [58].
\nWhile placement of an electrode array in the scala tympani, or where necessary in the scala vestibuli, leaves the electrode contacts quite close to their target neurones they are still some 1–3 mm away. This separation prevents discrete stimulation of local neural populations as discussed above. It has been proposed that an electrode array could be inserted directly into the auditory nerve, or failing this inside the modiolus. Promising results have been shown from acute experiments in cats [59]. Recording form electrodes placed in the inferior colliculus indicates that intra-neural simulation is more localized than stimulation using an electrode array placed in the scala tympani. There are considerable challenges to overcome before intra-neural stimulation could be considered for humans. The surgical access is not straightforward, risking losing all residual hearing. The human auditory nerve being in the order of 1 mm diameter would require very small stimulating contacts. While the stimulation currents required would be smaller than those needed for a traditional CI, charge density considerations require careful consideration. Also, how well an electrode array can be placed and be tolerated in the auditory nerve, without the destruction of auditory fibers or the formation of granulation tissue will need to be carefully studied. Finally, the structure of the auditory nerve is complex, with axons from different parts of the cochlea rolling into the tubular nerve, making tonotopic targeting an additional challenge.
\nWith the CI field involving a wide range of professionals including, surgeons, nurses, audiologists, engineers, physicists, speech and language therapists, teachers of the deaf, hearing therapists, rehabilitation specialists, psychologists and health economists, research related to the field can cover a very wide range. Here some of the key topics that are most closely connected to extending current practice will be reviewed.
\nIt is clear that when less trauma is inflicted during surgery that outcomes are better [60], this being the case whether or not there is residual hearing at risk [61, 62]. Hearing preservation is thus a key topic for surgeons. The design of less traumatic electrode arrays [48, 49, 50, 63] and the development of less traumatic surgical techniques [64], including the use of robot assisted insertion [65], are factors that can lead to reduced trauma. Providing real time feedback to the surgeon during electrode array insertion is a hotly researched area. Electrocochleography (ECochG), where acoustic stimulation of the ear produces a cochlear microphonic signal [66, 67, 68] that the surgeon can use to gauge proximity to structures, such as the basilar membrane, appears promising with clinical systems due to launch in 2019.
\nThe use of drugs to reduce the body’s reaction to implantation is also an area with some connection to minimizing trauma. Steroids such as Dexamethasone or antimitotic drugs [69] have been applied to suppress a fibrotic reaction during and immediately after surgery. Some longer term benefits have been shown but mainly in lower electrode contact impedances rather than significant outcome advantage [70]. Longer term deployment of steroids and other drugs has been proposed for some time [71] but has not yet seen clinical practice. Drugs such as neurotrophins [72] have been proposed to enhance the spiral ganglion but carry considerable risk of uncontrolled sprouting of new fibers that may not lead to any improvement in electrical stimulation [73]. Likewise the regrowth of hair cells or other cochlear structures [74] is an extremely challenging problem, although simply reconnecting peripheral processes that have been damaged while leaving intact hair cells [75] may be more manageable in the foreseeable future. While not a drug, near infrared light has been shown to promote tissue healing [76], helping reduce the extent of hearing loss following cochlear stress [77] and has been proposed as an approach that might also enhance the cochlea’s ability to survive the traumas of electrode array insertion.
\nDeployment, development and assessment of sound coding strategies continues with a variety of goals. Optimizing compression parameters to maximize speech understanding [78], reviewing the effect of various parameters as well as individualized fitting approaches [79, 80] all promise improvements for strategies that are already available but could be fitted better. Likewise, tools to guide clinicians in fitting for performance, rather than simply for comfort [45] should also lead to substantial improvement in outcomes. Seeking improvement via limiting current spread, through tripolar stimulation [81, 82], phased array [83] or manipulation of field interactions [84] have so far not shown a general improvement, although benefits for some recipients have been demonstrated. New sound coding strategies that attempt to improve temporal information such as FSP [85], or reduce masking effects such as MP3000 [31] have been developed and introduced into clinical practice. There may be more benefit in reducing battery power from the likes of MP3000 or Optima than in any improvement in outcome. Connected to improving outcome, research into the listening effort required to understand speech has been increasing [86, 87].
\nThe very nature of the speech tests used to evaluate CI systems is an active area of research. Standardization across languages has involved the use of matrix tests [88]. These tests involve a fixed syntax and a closed set of keywords so can be self-administered and used essentially indefinitely. However, the sentences are not fully representative of natural sentences. Avoiding fixed presentation levels, something necessary to evaluate the function of AGC has led to development of roving presentation level tests [89, 90, 91]. These tests provide insights into where a particular subject may have problems, so could be useful in supporting programming. As with the STARR tests, multiple speakers have been included in tests such as the AzBio [92] and taken much further with the coordinate response matrix test that can run on a multiple loudspeaker array and hence mimic a more realistic test environment [93].
\nImproved outcomes have been shown thorough use of the recently developed wireless technology supporting integrated bimodal [94, 95] and CROS [96]. EAS has also been shown to produce substantial improvements in outcome [97] although the test methodology used involves a questionable comparison of conditions.
\nWith increasing numbers of CI recipients needing management by financially constrained health care systems, methods of improving patient management are being developed. These include the use of consumer devices, smart phones and tablets, to run Apps that can evaluate a CI user’s speech understanding and qualitative condition as well as remotely analyzing the status of their implant and sound processor hardware [98] and delivering rehabilitation material usable by adult CI recipients and the families of young children. This whole area will necessarily see much development in the coming years.
\nThe pressures on CI manufacturers to reduce size and improve comfort and ease of use continue, with cosmetic considerations playing a large role in the choice of which CI systems are selected. Reliability of both the implanted and external parts of the CI system needs to be continuously improved, underpinning consistent device use and reducing costs inherent in managing failures. Further pressures on cost are also critical to address so that the enormous unmet need for cochlear implants in developing countries can also be met.
\n\n a method of removing stimulus artifact that relies on delivering alternate phase stimulation pulses that are added to ideally cancel artifact and reinforce response refers to the positive going phase of a stimulation pulse that is generally assumed to provide change balance, hence avoiding the delivery of potentially harmful direct current a circuit that compresses the large acoustic dynamic range into a range that is more manageable for the restricted electrical dynamic range an electrical component that does not allow direct current to pass so offers an additional level of protection to the body in the case of a fault occurring in an implant refers to the negative going phase of a stimulation pulse that is generally assumed to depolarize neurones hence leading to electrical stimulation an electrical measure formed by the product (strictly the integral) of current and time with units of Coulombs. The charge delivered determines the loudness perceived a surgically implantable prosthesis that bypasses a damaged cochlea providing hearing through direct electrical stimulation of the auditory nerve an electrical circuit that delivers a programmed current, varying the amount of voltage necessary to achieve this depending on the electrical resistance offered by the body an electrical current that flows in one direction only that can be harmful to the body the measurement of electrical signals produced by the cochlea in response to acoustic stimulation that indicates cochlear health an electro-chemical process whereby charged particles may be drawn towards an electrode with opposite charge, leading to electrode contact dissolution or tissue damage an efficient software algorithm that evaluates the amount of sound energy at regularly spaced frequencies, so allows the channels to be calculated a measure of resistance to the flow of electrical current that takes account of frequency so is more general than resistance which strictly only applies to direct current a method of stimulus artifact removal relying on introducing refraction to identify artifact and then using multiple stages of subtraction to isolate the neural response a fluid contained within much of the cochlea that is electrically conductive a measure of how much the body will resist the flow of electrical current, strictly only considering direct current, impedance being the a more general factor the name given to hearing nerve neurones having a cell body in the cochlea’s modiolus, axon in the auditory nerve and dendrites innervating the inner hair cells an electrical circuit that delivers a programmed voltage but that will allow current to vary depending on the resistance (impedance) offered by the body
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