Physiologic Factors

Neuromuscular Reflexes

Upper airway neuromuscular reflexes are a primary protective response to upper airway obstruction. Stretch mechanoreceptors in the pharynx and lung and chemo-receptors (responsive to O2 and CO2) are recognized sensors for these reflexes to protect the patency of the upper airway by activating pharyngeal dilator muscles, such as the tensor palatini and genioglossus muscles.

In patients with OSA, upper airway dilator muscles exhibit higher activation during wake (32,33). The decrease of pharyngeal muscle activation that occurs with sleep onset in these patients results in upper airway obstruction (34). Studies examining gender differences in upper airway neuromuscular reflexes with inspiratory flow limitation (IFL) are limited, but genioglossal and tensor palatini activity response to upper airway resistance loading do not appear to differ by gender (25). Similarly, genioglossus activity and diaphragmatic response to hypoxia are not different by gender (35). While these studies were performed in healthy men and women, it is unclear whether gender differences in neuromuscular reflexes exist in patients with OSA.

Local pharyngeal trauma due to repetitive upper airway collapse, resulting in diminished neuromuscular reflexes, is another proposed pathophysiology for development of apneas and hypopneas (36). Neuromuscular reflexes are physiologic compensatory mechanisms to protect the patency of the upper airway in the setting of upper airway collapse. While current studies have not explored possible gender differences in the susceptibility to local pharyngeal trauma, if one were to exist, this may explain the gender difference in the clinical manifestation of upper airway obstruction, since UARS occurs equally in men and women, whereas OSA is two to three times more common in men than women (36). The difference between these two entities may be due to gender differences in predisposition to lose neuromuscular reflexes with repetitive upper airway trauma or the severity of upper airway collapse.

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