Sleep apnea has a plurietiological character, combining anatomical, functional, neurological, muscular and neuroventilatory factors which converge sparking the development of the disease.
Anatomical and functional alterations of the upper airway are the principal factors, but the real cause of this syndromes still remains a mystery. With the current knowledge we can only assume there are a wide range of conditioning factors. It is still unknown why some subjects with the same conditioning factors develop OSAHS whilst others don't. Although the etiopathology of OSAHS is unknown there are a variety of hypotheses e.g. balance of pressures and balance of forces theory which takes into account the potential significance of the neuromuscular and structural conditioners. All authors agree that the development of OSAHS is linked with anatomical, neuromuscular and neuroventilatory factors.
The relaxation or inactivity of the dilator muscles during sleep produces insufficient tone in the pharyngeal muscles and predisposes to airway obstruction by causing a disruption of the forces which oppose the action of the diaphragm. In healthy subject the dilator muscles, tensor veli palatine, geniohyoid, sternohyoid and genioglossus, maintain the patency of the upper airway during sleep; their actions sufficient to counteract the negative pressure exerted by the diaphragm during inspiration. Studies suggest that sufferers of OSAHS present a dysfunction of the sensory receptors causing a delayed response of the dilator muscles which counteract the pressure changes of the pharynx.
The development of OSAHS is also associated with anatomical factors. Morphological abnormalities which involve the reduction of the diameter of the upper airway predispose the airway to collapse. Tonsillar hypertrophy, retrognathia, macroglossia and subjects with a Class II skeletal pattern are all anatomical factors which cause a narrowing of the airways and studies of OSAHS patients have shown that even whilst awake these patients have smaller airway diameters than healthy subjects. This indicates a hypertrophy of the walls of the pharynx in OSAHS sufferers likely to favour the development of the disease but its cause is still unknown. As shown by Carrera M. et al the pharyngeal space is comparative in healthy subjects when compared to OSAHS patients however it is very different in terms of shape and spatial arrangement. The pharynx is healthy subjects has a circular shape but in OSAHS subjects, the anteroposterior diameter is less than the transverse diameter, acquiring an oval or elliptical form. This has pathophysiological implications because the pharyngeal dilator muscles (genioglossus, geniohyoid, sternohyoid and tensor veli palatine) are inserted into the anterior wall of the pharynx and when tensed act in a predominantly anteroposterior fashion to increase the diameter of the upper airway. It follows that these muscles are not as effective at dilating the pharynx as much in OSAHS sufferers as in healthy subjects. Any anatomical obstruction serves to reduce the size of the pharynx, promoting collapse. Many of these alterations are common in OSAHS subjects and are easily detectable upon physical examination of the patient.
Motor dysfunction in the pharyngeal muscles, as mentioned above, explains the reduction in the calibre of the upper airway, predisposing to its collapse in OSAHS patients. However, morphology alone does not explain why this collapse happens only during sleep and not also whilst awake. To that extent the pathophysiology of OSAHS is intrinsically linked to the neurological mechanisms that regulate sleep and adjust the tone of the respiratory muscles in order to maintain a patent upper airway. Research suggests that OSAHS sufferers have some neurological disturbances. The activity of the respiratory system depends upon the extrinsic regulation of the Central Nervous System (CNS) which is stimulated by central and peripheral chemoreceptors located in the brainstem, responsible for the involuntary control of ventilation during sleep. In the absence of voluntary breathing, which is present during wakefulness, breathing becomes more vulnerable, relying solely on the involuntary control system. It follows that any alteration of the CNS control system, which regulates breathing and dilator muscle tone, would have a direct affect on the pathogenesis of sleep apnea.