HISTOCHEMICAL PROPERTIES OF UPPER AIRWAY MUSCLES - COMPARISON OF DILATOR AND NONDILATOR MUSCLES

The upper airway dilator muscles (UADMs) represent a subgroup of muscl es in the pharyngeal area which, in addition to their roles in mastica tion, vocalization, etc., also have an important respiratory function. Failure of these muscles to maintain upper airway patency during slee p is important in the development of the obstructive sleep apnoea synd rome. In the present study, we evaluated the histochemical properties of the UADMs and compared them to those of neighbouring muscles withou t respiratory functions, and to the diaphragm, to determine whether th e UADMs are specifically adapted to their respiratory role. Our result s, both in dogs and rats, indicate that the dilator and nondilator upp er airway muscles are similar and differ from the diaphragm, In rats, there were significantly less type I fibres (<12% as compared to 42% f or 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 r espiratory role. They may fail in the presence of increased loads, oft en encountered in patients with obstructive sleep apnoea, unless appro priate adaptive structural changes take place.