Feline middle ear disease

Abstract

ANATOMY AND PHYSIOLOGY OF THE MIDDLE EAR The ear is composed of structures at three levels the outer, middle and inner ear. The middle ear comprises the tympanic membranes, tympanic cavity, ossicles and the auditory tube, and transmits sound to the inner ear. The major difference between the cat and dog lies in the structure of the middle ear. Whereas the mesotympanic cavity and epitympanic recess of the middle ear of the dog are partially separated by a bony shelf from the larger ventromedial hypotympanic cavity, the cat’s tympanic chamber is separated into two almost distinct cavities communicating through a small slit-like opening in the bony separation, which widens into a distinct foramen caudally (Fig. 1).This separation gives rise to the characteristic ‘double shell’ profile of the tympanic structures visible on anteroposterior radiographic views (Fig. 2). The lining of the cat’s tympanic chambers is reported to contain more abundant ciliated and secretory cells than the dog. A number of nerves pass through the tympanic chamber and their distribution within the middle ear is similar to that of the dog; the facial nerve and the vestibulocochlear nerve reach the internal auditory meatus in close association. The facial nerve lies exposed in the dorsal aspect of the tympanic cavity. On entering the middle ear, the tympanic plexus (facial nerve mixed with vagal branches) distributes widely across the bony promontory. Sympathetic postganglionic fibres (from the first cervical nerve) pass through the bulla. In the cat, the tympanic plexus is reported to be more exposed, or possibly more sensitive to, iatrogenic trauma.