Obstructive Sleep Apnea (Apnoea)

Obstructive sleep apnea (apnoea) is a common disorder characterized by repetitive episodes of pharyngeal (upper) airway collapse or narrowing during sleep (Figure 1). In people with OSA, pharyngeal muscles relax during sleep and gradually allow the pharynx to collapse. The level of pharyngeal collapse varies between patients, but most often occurs at the velopharyngeal level and/or retroglossal level. Collapse of the pharyngeal airway can block airflow or significantly restrict airflow (hypopnea), both of which can cause oxyhemoglobin desaturation. An episode of apnea or hypopnea is terminated by a brief arousal or a lighter stage of sleep, accompanied by activation of the upper airway dilator muscles and restoration of airway patency. This cycle occurs repeatedly throughout the night, commonly resulting in daytime hypersomnolence, excessive fatigue, and long-term co-morbidities.



In general, people with OSA have an anatomically narrow upper airway (e.g., due to obesity or anatomical variation), and a narrow airway is more prone to collapse. During wakefulness, people with OSA reflexively compensate for their more collapsible airway with increased airway dilator muscle activity. During sleep, however, their airway dilator muscle activity is decreased and airway collapse can result from negative airway pressure at the beginning of inspiration and/or the absence of substantial positive airway pressure at the end of expiration.

The largest upper airway dilator muscle is the genioglossus muscle, which is one of several different muscles in the tongue that facilitate multiple purposes including speech, swallowing and breathing. The genioglossus muscle causes tongue protrusion and stiffening of the anterior pharyngeal wall.

In people with OSA, the neuromuscular activity of the genioglossus muscle falls markedly at sleep onset, contributing to airway collapsibility. In people without OSA, the genioglossus muscle shows tonic neuromuscular activity during expiration, plus bursts of neuromuscular activity during inspiration to prevent airway collapse and allow air to flow freely (Figure 2).