Histochemical properties of upper airway muscles: comparison of dilator and nondilator muscles.

The upper airway dilator muscles (UADMs) represent a subgroup of muscles in the pharyngeal area which, in addition to their roles in mastication, vocalization, etc., also have an important respiratory function. Failure of these muscles to maintain upper airway patency during sleep is important in the development of the obstructive sleep apnoea syndrome. In the present study, we evaluated the histochemical properties of the UADMs and compared them to those of neighbouring muscles without respiratory functions, and to the diaphragm, to determine whether the UADMs are specifically adapted to their respiratory role. Our results, both in dogs and rats, indicate that the dilator and nondilator upper airway muscles are similar and differ from the diaphragm. In rats, there were significantly less type I fibres (<12% as compared to 42% for the diaphragm) and more type IIb fibres (39-67% as compared to 27% for the diaphragm). A similar pattern was seen in dogs: type I fibres <38% as compared to 46% for the diaphragm, and type IIb fibres, 29-35% as compared to 10% for the diaphragm. These findings suggest that the upper airway dilator muscles are not specifically designed for their respiratory role. They may fail in the presence of increased loads, often encountered in patients with obstructive sleep apnoea, unless appropriate adaptive structural changes take place.