221 (22)

Diagnostic approach to otitis extema

Otitis extema is an acute or chronię inflammation of the extemal auditory canal and tympanic membranę ' It is the reason for consultation in 25% of cats presented for a dermatological problem in first opinion practice and in 2% of cats referred to a dermatologist^{3 6}. Long thought of as a local phenomenon, otitis externa should be reclassified in a broader dermatological context, as it is usually just one local manifestation of an underlying dermatosis⁷. Its pathogenesis involves predisposing factors, primary factors, microbial amplifying factors, and aggravating factors which prevent healing (Table 22 : 1) ‘'³'⁷'⁹.

Table 22 : 1: Aetiology of otitis extema

When dealing with otitis externa in the cat, it is essential that all causes of inflammation of the extemal auditory canal are correctly identified and treated⁷. To achieve this, a thorough diagnostic approach is necessary, not forgetting however, the importance of parasites as a cause of feline otitis extema.

History

The history directs the clinician towards certain differential diagnoses and is a critical part of the diagnostic process^10,1Important points to consider are :

Age of onset ofclinical signs: otoacariasis, caused by Otodectes cynotis, is seen particularly in young cats aged less than a year (Fig. 22 : 1). On the other hand, an erythemato-ceruminous otitis extema in an adult cat is morę suggestive of an allergic dermatitis (Fig. 22 : 2). A purulent otitis in an old cat strongly suggests an auto-immune dermatosis (Fig. 22 : 3) or a tumour of the extemal auditory canal (Fig. 22:4).

22.1