Head Posture and Upper Cervical Spine Morphology in Patients with Obstructive Sleep Apnea

The main aim of this chapter is to describe the role of upper cervical spine morphology and head and neck posture in the etiology, diagnosis, and treatment in patients with obstructive sleep apnea (OSA). Previously it has been documented that the posture of the head and neck was related to the morphology of the facial profile, dysfunction of the jaws, and obstruction of the upper airway. It has been shown that head posture in relation to the upper cervical spine was extended in OSA patients. New findings have been added concerning the occurrence and pattern of deviations of the upper cervical spine morphology in OSA. Furthermore, associations between upper cervical spine morphology and the morphology of the facial profile, including the cranial base in OSA patients have been reported. In addition, the occurrence of upper cervical spine morphological deviations in OSA patients seems to affect the outcome of the treatment with a mandibular advancement devise (MAD). Accordingly, it is suggested that upper cervical spine morphology and posture of the head and neck are important factors in the etiology, diagnosis, and treatment considerations in OSA patients.


Introduction
Obstructive sleep apnea (OSA) is by far the most common sleep-related breathing disorder, affecting 2-4% of the adult population, particularly males aged 60 years and older where the prevalence is 30-60% [1,2]. OSA is defined as cessation of airflow with persistent respiratory effort, due to repeated anatomical obstruction or partial collapse of the oropharyngeal region, involving the soft palate, dorsum of the tongue, and the posterior pharyngeal wall [1,3]. The majority of the patients have symptoms such as loud snoring and excessive daytime sleepiness [4,5]. Nightly choking or gasping, morning headache, memory loss, decreased concentration, increased irritability, and nocturia are also reported [4,6]. Thus, OSA has consequences for the quality of life, working ability, and traffic safety as well as comorbidities as hypertension [4][5][6][7]. OSA is multifactorial with age, gender, and body mass index (BMI) as predisposing factors [1,3]. The authors agree that there are craniofacial morphological and postural characteristics in OSA patients such as reduced posterior airway space, abnormally long soft palate, low position of the hyoid bone, and an extended head posture [8,9]. The primary treatments of OSA are based on physical effects and consist of continuous positive airway pressure (CPAP), mandibular advancement device (MAD), and upper airway surgery [6,7].
This chapter focuses on the role of head posture and the morphology of the upper cervical spine in the etiology, diagnosis, and treatment in patients with OSA.

Head posture in relation to OSA
Associations between head posture and pharyngeal airway dimensions have been documented on lateral cephalograms [9][10][11][12]. It was found that an extension of the head in relation to the upper cervical spine resulted in an increase of the anterior-posterior dimension of the pharynx. Furthermore, studies have shown the influence of airway obstruction on head posture [9,13,14] where airway obstruction resulted in an extension of the head in relation to the upper cervical spine. Due to the head posture's close associations with the pharyngeal airway, it seems relevant to focus on the relationship between the head posture and OSA.

Definition of head posture
Natural head position is a standardized and reproducible position of the head in an upright position determined by the subjects' own postural control system [14][15][16]. Accordingly, the posture of the head and neck can be defined in two ways: with or without external reference. The "self-balance position" is without external reference (the subjects' proprioceptive system) and the "mirror position", with external reference (the subjects' proprioceptive and visual system) [13,14]. In this chapter, the head posture refers to the OSA patients' "self-balance position" or the "mirror position" evaluated on lateral cephalograms and defined as the following angels [17][18][19] (Figure 1).

Head posture in OSA patients
In OSA patients, an extended posture of the head in relation to the upper cervical spine in the upright awake position was found to be associated with larger pharyngeal airway dimensions [9,[20][21][22][23][24][25]. It was especially the lower part of the pharyngeal airway that was increased in relation to an extended head posture. Furthermore, an extended posture of the head has also been demonstrated in men with OSA compared to healthy controls [9,24] (Figure 2). The severity of OSA was also associated with head posture. The more severe OSA, the more extended and forward head posture was observed [20][21][22][23]. The extended head posture in OSA patients may be a compensatory physiological postural mechanism that serves to maintain airway adequacy in OSA patients [9,[20][21][22][23][24][25]. It is suggested that airway obstruction via neuromuscular control triggers an increase in the cranio-cervical angle in order to relieve the obstruction by facilitating oral breathing due to enlargement of the naso-and oropharyngeal airway space [9,13,24,26]. The hypothesis is supported by a study in OSA patients showing that the airway resistance significantly influences the head posture [21]. A decreased airway resistance (less obstructive) was seen in OSA patients with an extended head posture. Thus, an extended head posture in the upright awake position was found in OSA patients. The results were considered to reflect a compensatory physiological postural mechanism that serves to maintain airway adequacy in OSA patients in the awake upright posture.

Definition of upper spine morphology
The upper cervical spine morphology can be obtained from conventional two-dimensional (2D) lateral cephalograms or from three-dimensional (3D) cone beam computed tomography (CBCT). One method to describe the upper cervical spine morphology on either lateral cephalograms or on CBCT is by visual assessment of the first five cervical vertebral units as referred to in this chapter. The morphological deviations are divided into two categories "Posterior arch deficiency" and "fusion anomalies" [14,40,45] (Figure 3):

1.
Posterior arch deficiency consisted of partial cleft: failure of the posterior part of the neural arch to fuse and dehiscence: failure of part of a vertebral unit to develop (Figure 3).

2.
Fusion anomalies consisted of fusion: fusion of one unit with another at the articulation facets, neural arch or transverse processes, block fusion: fusion of more than two units at the vertebral bodies, articulation facets, neural arch or transverse processes and occipitalization: assimilation either partial or complete of the atlas (C1) with the occipital bone (Figure 3).

Upper spine morphology in OSA patients
Previous studies have shown that morphological deviations in the upper cervical spine evaluated on 2D lateral cephalograms and 3D CBCT occurred significantly more often in OSA patients compared to healthy controls [53,54]. The morphological deviations occurred in 32-46% as fusion anomalies: fusions either between the second and third vertebrae, between the third and fourth vertebrae, or between the fourth and fifth cervical vertebrae; block fusions: fusions either between the second, third, or fourth vertebrae, between the second, third, fourth, and fifth vertebrae, or between the third, fourth, and fifth vertebrae; occipitali-  zation in combination with fusions, block fusions or as a single deviation. Posterior arch deficiency: partial cleft of the first cervical vertebra or dehiscence of the third cervical vertebra and the fourth cervical vertebra [53,54] (Figure 4). The pattern of morphological deviations in the upper spine seen in OSA patients is more severe and occurred more caudally than seen in healthy subjects and in orthodontic patients with severe malocclusion [45][46][47][48][49]. Occipitalization, block fusion, and dehiscence were the phenotypes, which were characteristic of sleep apnea (Figure 4).
It is presumed that the pattern and location of upper cervical spine morphological deviations is connected to different locations of neural crest cell migration along the body axis [55]. Accordingly, it is hypothesized that the level of pharyngeal obstruction in sleep apnea is associated with the caudally/cranially positioned cervical spine deviation. Furthermore, the craniofacial profile of OSA patients with upper spine morphological deviations was significantly different from the craniofacial profile in OSA patients without deviations in the upper spine [56] (Figure 5). A long and retrognathic facial profile together with an extended head posture was characteristic of OSA patients with upper spine morphological deviations. The background for the interrelationship between the cervical spine and the craniofacial profile can be traced back to early embryological development of these structures [57]. It has been documented that the development of the body axis is regulated by the notochord [58,59]. It is also well known that the notochord runs in its full extent from the sacral region to the sella turcica in the posterior part of the cranial base to which the jaws are attached [55]. Different genes act in different segments along the path [60]. A deviation in the development of the notochord may influence the surrounding bone tissue in the spine as well as in the posterior part of the cranial base. On lateral cephalograms and CBCTs, it can be observed that the bone tissues formed around the notochord are the vertebral bodies and the basilar part of the occipital bone (Figure 6). The shared origin of the spine and posterior part of the cranial base is the basis for the hypothesis of associations between the spine and the cranial base to which the jaws are attached [60,61]. The findings indicated that morphological deviations of the upper cervical spine may play a role in the phenotypical subdivision and diagnosis of OSA. In addition, OSA patients with morphological deviations in the upper spine may respond poorer to MAD treatment compared to OSA patients without morphological deviations in the upper spine [62]. This finding further supports the role of upper spine morphological deviations in OSA patients. So far, the complex aetiology of OSA is not fully understood and the explanation for the association between upper spine morphological deviations and OSA is still unknown. Thus, the findings indicated that the aetiology in OSA patients with morphological deviations in the upper spine is characterized by other factors or combinations of different factors than in OSA patients without upper spine morphological deviations and that upper spine morphological deviations therefore may influence the MAD treatment outcome in OSA patient [53,54,56,62].

Conclusion
When head position and upper cervical spine morphological deviations are evaluated on 2D lateral cephalograms and 3D CBCTs taken in the standardized upright position of the head determined by the subjects' own postural control system, the following is concluded: on average, an extended posture of the head and a significantly larger occurrence of upper spine morphological deviations were seen in patients with OSA. The craniofacial profile of OSA patients with upper cervical spine morphological deviations differed significantly from the craniofacial profile of other OSA patients without morphological deviations in the upper spine. OSA patients with morphological deviations in the upper spine may respond poorer to MAD treatment compared to OSA patients without morphological deviations in the upper spine. The findings indicated that head posture and morphological deviations of the upper cervical spine play a role in the phenotypical subdivision and diagnosis of OSA and thereby for the treatment outcome.