Jon D. Plant, DVM, DACVDParaplegia. 1983 Apr;21(2):86-93.
The frequent occurrence of occult and/or recurrent otitis externa in Pets can give owners the false impression that this disorder is easily managed or cured. Given the numerous possible predisposing and primary causes, it is clear that resolving a case of otitis externa is often more challenging than many Pet owners appreciate. The previous article outlined a diagnostic approach to otitis externa, emphasizing the need to uncover and control the primary cause. This article will focus on the medical management of otitis externa, particularly the steps you can take to eliminate and prevent the bacterial and yeast infections that are often the result of a diseased ear canal. Managing otitis externa usually begins, but does not end, with treating for these organisms.
Understanding the normal versus diseased ear canal The normal ear canal produces cerumen that serves multiple purposes: lubrication of the tympanic membrane (TM) and epithelium, protection against transepidermal water loss and collection of debris and organisms.1 Cerumen is a mixture of secretions from the sebaceous glands and ceruminous glands (modified apocrine sweat glands). Breed differences exist; American Cocker Spaniels, for example, have more prominent ceruminous glands than other breeds, explaining, in part, their predisposition to otitis externa. Ceruminous glands are more abundant close to the tympanum, while sebaceous glands predominate near the ear canal opening and are associated with hair follicles. The normal secretion of ceruminous glands is a thinner consistency than that of sebaceous glands. Inflammation of the ear canal results in increased cerumen production, which shields bacteria and yeast from topical therapy and increases the humidity of the ear canal, favoring bacterial and yeast growth.3 In dogs with otitis externa, the lipid content of cerumen is significantly lower, and the mean pH is different (lower in acute, higher in chronic otitis externa) than in normal ears.4 A healthy ear canal cleans itself. The mechanism by which cerumen, desquamated keratinocytes, and trapped debris and bacteria are slowly cleared from the normal ear canal involves the migration of the epithelium toward the external opening as it matures from deeper to superficial layers, a process that begins with germinal cells in the tympanic membrane. Most of the predisposing and primary causes of otitis externa (See Pursuing the causes of otitis externa, Table 1, page 20) may result in the failure of epithelial migration, causing a buildup of cerumen and keratinocytes in the proximal ear canal. Soft wax plugs or inspissated concretions (ceruminoliths) may form next to the eardrum (Figure 1, page 34), causing inflammation and compromising the tympanic membrane. It is not unusual to discover a perforated or missing tympanic membrane once ceruminoliths are removed. Pet owners should be explicitly advised before an ear flush procedure that the status of the eardrum is unknown if you haven’t been able to visualize it yet.