[Obstructive sleep apnea syndrome and the upper airway muscles]

Pharyngeal muscles are prone to distorsion during inspiratory negative pressure. Some pharyngeal muscles, called pharyngeal dilatators, exhibit tonic and/or phasic inspiratory activity. At the velar level, tensor palatini, glossopharyngeus, palatopharyngeus and musculus uvula drive the airflow towards nasal or buccal breathing. Genioglossus and geniohyoid muscles exert a forward propulsion to the tongue. The contraction of the dilatators muscles precedes the diaphram contraction. These muscles show a poor endurance. In obstructive sleep apnea syndrome, they contract during unfavorable metabolic (hypoxia and hypercapnia) as well as mechanical (excentrical contraction) conditions. Histological changes occur in upper airway dilator muscles: muscle volume and proportion of type Ila fibers are increased. These changes, considered as compensatory mechanisms, are variable within the different studied pharyngeal muscles. The initial pharyngeal obstructive site, and its further extension, could be determined by the different strength and endurance properties of these muscles.