Sleep-related breathing disorder.2. Pathophysiology of obstructive sleep apnoea.

Abstract

Obstructive sleep apnoea is a disorder char-acterised by recurrent sleep-induced collapse of the pharyngeal airway.' The principal sequel, hypersomnolence (or sleepiness during the day), results from the repeated arousals re-quired to reopen the airway and resume breath-ing and thus poor sleep quality during the night in these patients.2 The recurrent hypoxaemia and hypercapnia which characterise sleep apnoea may also lead to both pulmonary' and systemic hypertension,4 cardiac arrhythmias,5 and possibly decreased survival.6 As a result, this disorder can impact on the health and wellbeing ofthe afflicted individual. This review will address the pathophysiology of this dis-order with the principal focus being the mech-anisms that lead to pharyngeal collapse during sleep. No attempt will be made to address exhaustively every possible hypothesis re-garding this sleep-induced airway event. How-ever, prominent theories will be discussed. In addition, a somewhat briefer discussion on the mechanisms of apnoea termination will be in-cluded. Awake Epiglottis-Figure 1 Diagrammatic representation ofthe pharyngeal airway ofa patient with apnoea. During wakefulness airway patency can be maintained. During sleep collapse occurs as shown between the choanae and the epiglottis. Printed with permission from White.'2 Pharyngeal collapse The principal problem in the patient with ob-structive apnoea is the collapse of the pharyn-geal airway (fig 1) which characteristically occurs at the onset ofsleep in these individuals.' Although respiratory effort generally continues, the obstructed (or largely obstructed) pharyn-geal airway prevents effective ventilation pro-ducing either apnoea (total respiratory ces-sation) or hypopnoea (a substantial reduction in ventilation). In either case, hypoxia and hypercapnia quickly develop necessitating arousal to re-establish airway patency and nor-mal ventilation.7 The location of this ob-struction is virtually always between the choanae and the epiglottis, generally either behind the uvula and soft palate (the velo-pharynx), or behind the tongue (the or-opharynx), or some combination of the two.8 Rare reports of collapse of the airway at the level of the epiglottis have emerged,9 but this is probably unusual. PHARYNGEAL ANATOMY