Aetiology
Genetic and environmental factors are thought to have roles in OSA. Aggregation of OSA has been demonstrated in studies of families that included obese and non-obese adults and children.[14][15] The estimated heritability for OSA, as defined by the Apnoea-Hypopnoea Index, is 0.30 to 0.40.[16]
In OSA, an episode of apnoea is caused by dynamic narrowing of the upper airway during sleep. Airway narrowing may be triggered by neuromuscular mechanisms within an anatomically small upper airway. Anatomical narrowing of the pharynx may be mediated by maxillomandibular anomalies or adenotonsillar hypertrophy.[17] Increases in lateral pharyngeal, soft palatal, and tongue tissue mass commonly seen with obesity may also reduce the pharyngeal cross-sectional area.[18] The involvement of neuromuscular mechanisms is suggested by histological demonstration of pharyngeal muscular and neurological lesions, by the augmented tone of pharyngeal dilators when people with OSA are awake (loss of tone occurs with sleep onset), and by response to OSA treatment with hypoglossal nerve stimulation.[19][20] Neuromuscular dysfunction may prevent maintenance of pharyngeal dilator tone during sleep in people with OSA who have a narrowed pharynx.
Pathophysiology
Upper pharyngeal dilator muscle activity decreases with sleep onset, and both tonic and phasic dilator activity is further reduced during rapid eye movement sleep. During sleep, the pharynx is most vulnerable to collapse at end expiration secondary to the loss of the neural tone of the pharyngeal dilators, and especially at end expiration due to the loss of the positive intraluminal pressure.[21] People with OSA have a narrow pharyngeal cross-sectional area, and so have an increased risk of an episode of apnoea during sleep.[21][22] When awake, genioglossus activity is increased in OSA patients to compensate for reduced pharyngeal area and to maintain pharyngeal patency. However, this tone is decreased during sleep and the pharynx obstructs.[23] Hypoxaemia and hypercapnia may result from airway obstruction, their magnitude also depending on the presence of pulmonary disease and reserve. Episodes of apnoea and hypopnoea terminate with cortical or subcortical arousal. Autonomic sympathetic activation occurs, which may result in cardiac dysrhythmias and vasoconstriction. If sleep is resumed after the arousal, pharyngeal obstruction may recur and the cycle is repeated.
Hypoxaemia, sympathetic activation, and arousals may be related to the higher prevalence of cardiovascular, metabolic, and neurocognitive dysfunction in OSA patients.[24] Rates of hypertension, stroke, myocardial infarction, heart failure, cardiac dysrhythmias, cognitive dysfunction, depression, metabolic syndrome, oxidative stress, and motor vehicle accidents are all higher in patients with OSA.[12][24][25][26][27]
Classification
International classification of sleep disorders (ICSD-3-TR) - third edition, text revision[1]
Defines OSA as a PSG- or HSAT-determined obstructive respiratory event ≥5 events/hour associated with one or more of the typical symptoms of OSA (e.g., unrefreshing sleep, daytime sleepiness, fatigue or insomnia, awakening with a gasping or choking sensation, loud snoring, or witnessed apnoeas), or an obstructive respiratory event ≥15 events/hour (even in the absence of symptoms).
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