Visual Disability and Causes of Preventable Blindness

The World Health Organization (WHO) estimates, worldwide, approximately 1300 million people with a form of visual impairment. More than 314 million have a severe visual impairment. Of these, 37 million are blind and 124 million suffer from low vision. Eighty percent of all these cases are considered avoidable. The main causes of blindness from one country to another, in order of frequency, are cataract (39%), uncorrected refractive errors (18%), and glaucoma (10%). In Latin America and the Caribbean, the loss of vision in adults continues to be a public health problem. Blindness and visual impairment tombs have a remarkable impact on the socioeconomic development of individuals and societies and the prevention of avoidable visual disabilities in the long term in terms of attention to health care and social expenditures. Of all the organs of the body, the eye is more accessible to direct examination. The visual function can be evaluated by means of simple subjective tests. The same can be taken care of from an adequate primary care service.


Introduction
The universal aging of the population is a global concern because of its association with degenerative diseases, which can cause disabilities in humans, limit their productivity in society, and negatively affect their quality of life [1][2][3][4][5].
In all societies, blindness has profound human and socioeconomic consequences. The costs of loss of productivity, rehabilitation, and education for the blind constitute a significant economic burden for the individual, the family, and society [6].
Thus, it is interesting to know that total or partial opacification of the lens is the main cause of bilateral blindness and severe visual impairment represents about 48% of cases of visual impairment in the world and is known to have a multifactorial cause. In addition, it incapacitates the individual and increases their dependence and early retirement from life [7].
In Latin America, cataract without correction represents by far the main cause of blindness in adults over 50 years, followed by glaucoma, diabetic retinopathy (DR), age-related macular degeneration, and uncorrected refractive errors [23], see Table 3.

Etiology
The causes from one country to another. The visual impairment attributable to cataract is greater in low-and middle-income countries than in high-income countries. In high-income countries, diseases such as diabetic retinopathy, glaucoma, and macular degeneration are related to the most frequent age [24].
Johns Hopkins University has found that the main cause of blindness in white people is macular degeneration associated with age, while in the black population, it is due to glaucoma or cataract. In addition, in the elderly, blindness is three times more frequent in blacks than in whites [25].   The main causes of blindness in order of frequency are cataract (39%), uncorrected refractive errors (18%), glaucoma (10%), macular degeneration related to age (7%), corneal opacities (4%), diabetic retinopathy (4%), trachoma (3%), childhood eye diseases (3%), and onchocerciasis (0.7%). Cataract is the leading cause of easily curable blindness [7,26].

WHO subregion
For this reason, a description of the main causes is made, see Table 4.

Risk factors
The different etiologies are known to have multifactorial causes; in cataract modifiable risk factors are identified as exposure to ultraviolet rays, mainly UV-B, deficiency in the diet of antioxidants and proteins, smoking, diabetes mellitus, the use of corticosteroids, and severe dehydration. And non-modifiable risk factors are genetic, with a probability three times higher in relatives of people with the disease [37][38][39][40][41].
In glaucoma it is said that age increases the probability of suffering ocular hypertension. A glaucoma can evolve of various etiologies. The prevalence increases from 4 to 10 times in people older than 60 years [42]. Other risk factors are myopia, inheritance, African-American race, exfoliation, and pigmentary dispersion [43].
Refractive errors are associated with racial factors, the myopia and astigmatism are more prevalent in the Chinese population and hyperopia is the most common in the Hispanic population [44].
Regarding diabetic retinopathy (DR), it corresponds to one of the microvascular complications of diabetes mellitus.
In Figure 1, the relationship between glycemic control and the duration of diabetes with diabetic retinopathy is shown as a function of the time of follow-up, with different curves for the different HbA1C values [50].
Between 80 and 95% corresponds to simple or nonproliferative DR and the remaining 5-10% to proliferative DR [51].

Causes Definition
Cataract It is the opacity of the lens, which is understood as the passage of light to the retina, causes a slow and progressive loss of vision, and can appear at any stage of life, from birth to older age than being human [27][28][29][30] Glaucoma It is an optic neuropathy that presents with a characteristic structural damage, associated with the progressive death of retinal ganglion cells, loss of nerve fibers, and loss of pathognomonic visual field [31][32][33][34] Uncorrected refraction In myopia, the point of focus is in front of the retina, because the cornea has too much curvature or the axial length of the eye is excessive In hyperopia, the focus point is behind the retina because the cornea has too flat a curvature or the axial length is too short Astigmatism, a non-spherical (variable) curvature of the cornea or lens, causes light rays of different orientations (e.g., vertical, oblique, horizontal) to focus on different points [35] Presbyopia is a clinical loss of the amplitude of accommodation or, in other words, the loss of the ability to change the shape of the lens to focus on nearby objects [35,36]

Differential diagnosis
Basic knowledge of ocular symptoms is required to perform an adequate ophthalmological evaluation. It is always necessary to completely define the characteristics of the symptoms and discomforts.
The ocular symptoms can be classified in the fundamental categories: abnormalities of the eyes, anomalies of the ocular aspect and abnormalities of the sensations, pains, and ocular discomforts.
Therefore, it can be useful to make one order and ask the following questions: 1. Has the disorder started gradually, quickly, or asymptomatically? For example, was blurred vision in one eye discovered until the other eye was inadvertently covered?
2. Was the duration of the problem brief, or did the symptom persist until the current consultation? If the symptom has been intermittent, how has been its frequency?
3. Is the location of the disorder focal or diffuse, and is it a unilateral or bilateral condition? Finally, does the patient rate the degree of the disorder as mild, moderate, or severe?
In Table 5, some of the main characteristics of the most common pathologies associated with preventable blindness are observed.

Physical exam
Of all the organs of the body, the eye is the most accessible to direct examination. The visual function can be evaluated by means of simple subjective tests. The same can be taken care of from an adequate primary care service.
The clinic plays a decisive role in the initial evaluation of any patient, so the correct approach orderly and systematic is essential in the sequence of subsequent processes for the consolidation of a presumptive diagnosis and complementary tests and to achieve an accurate diagnosis and therapeutic approach suitable. Therefore, an adequate anamnesis that includes general, sociodemographic data, personal and family pathological history, and ocular and non-ocular as well as traumatic surgical history is imperative and invariably its symptomatology. The external anatomy of the eye is visible and can be examined with the naked eye or with simple instruments.
The eye is the only part of the body where you have direct vision to blood vessels and tissues of the central nervous system (retina and optic nerve). Thanks to this it is possible to identify, by ocular examination, important systemic effects of infectious, autoimmune, neoplastic, and vascular diseases.
And so, a basic physical examination aimed at the search of suspected abnormalities that merit its timely reference to a higher level of specialty is essential in the primary care physician, see Figure 2.
And it is in this sense that it can describe certain characteristic findings of these pathologies, the same ones that can guide our mental scheme to achieve a presumptive diagnosis; some of them are described in Table 6.

Prevention
The Latin America and the Caribbean region are considered one of the most inequitable in the world in terms of the distribution of goods and services, social determinants, and health.
Using a standard methodology for international use [58], it has been possible to determine the prevalence of blindness and visual impairment, coverage, and the quality of cataract services and barriers to access them in several countries [56].

Physical examination
Cataract [56] Opacification of the lens Pupillary reflexes may be slowed down, but they do not disappear  Table 6.

Author details
Mercedes Libeth Ramos Amador and Javier Eduardo Escober Torres* Doctor of Medicine and Surgery National Autonomous University of Honduras, Tegucigalpa, Honduras *Address all correspondence to: jetorres.9307@gmail.com The Vision 2020 program in Latin America, with the participation of PAHO/ WHO, the Christian Blind Mission, and the International Agency for the Prevention of Blindness, has proposed to document the problem of blindness and visual impairment in people over 50 years of age and designed a statistical instrument called "rapid assessment of avoidable blindness" (ERCE). To date, ERCE activities have been conducted to determine the prevalence of blindness [16,49,[50][51][52][53][54][55][56][57][58][59][60][61][62][63].
And it is through this instrument that in nine countries of Latin America, the following barriers have been determined for cataract surgery: lack of knowledge about the existence of a treatment, it is thought to be the destination, there is no availability of surgery services or very distant, fear of the operation or loss of vision, cannot pay for the operation, indication of waiting until it "matures" (possible waiting list), no one can accompany to the ophthalmological care, and other diseases and contraindications for surgery [64][65][66].
All are surmountable through information and education campaigns.

Conclusions
To control blindness and visual impairment, it is essential to implement plans to (a) detect cases of low vision and operate cataract cases, (b) detect and give optical correction to cases with refractive errors and presbyopia, and (c) integrate eye care in primary health care. These three interventions could solve around 67% of cases of blindness and could help detect people with glaucoma, in order to treat them in early stages.
Eighty percent of all these cases are considered avoidable. Therefore, the exhaustive evaluation in the patient with determining risks plays a key role, together with the fact that the visual function can be evaluated by means of simple subjective tests and be attended to by an adequate primary care service.
The difficulties in the supply of surgeries vary according to the country, being very available for most of the population in developed countries and becoming the most performed surgery in the elderly. In developing countries, the situation varies according to the regions or countries.
Visual disability and blindness correspond to one of the microvascular complications of diabetes mellitus, where the relationship that keeps the glycemic control in function of the time of presenting the disease is widely demonstrated. Between 80 and 95% corresponds to nonproliferative DR and the remaining 5-10% to proliferative DR. Here is the need for a holistic approach to the patient aimed at prevention and proper medical management.
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