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Vol.18, No. 7

July 1996

V

Continuing Education Article

Otitis Externa
FOCAL POINT # Otitis externa occurs secondary
to predisposing factors or other diseases and is also a primary condition caused by microorganisms and parasites. University of Minnesota

Patrick J. McKeever, DVM, MS

O

KEY FACTS
I Atopic dermatitis is the most common disease predisposing to otitis externa. ICytologic evaluation of otitic exudate should be performed in every case to determine the types of microorganisms present. I Otitic ears should be thoroughly cleaned of exudate, wax, and debris so that appropriate medication can reach diseased tissue. I Detailed client education concerning the correct methods of cleaning and applying medication is necessary for successful management of otitisexterna.

titis externa is inflammation of the epithelium that lines the external auditory canal. Diagnosis of the condition can be frustrating and challenging, as a number of causative agents can be responsible for its development. Failure to diagnose and treat the primary cause (whether attributable to disease or environment) may lead to less than satisfactory results. The incidence of otitisexterna in the canine population has been reported to be from 4% to 16% of hospital admissions and up to 20% of the general population.1 In cats, the incidence has been reported to be from 2% to 6.6% of hospital admissions.2 Such factors as time of year when the surveys were taken, breed popularity in a given area, and varying diagnostic criteria may be responsible for the wide range of reportedincidence.

ANATOMY OF THE EXTERNAL EAR The funnel-shaped pinna, which is formed from distal flaring of the auricular cartilage, receives air vibrations and transmits them by way of the ear canal to the tympanic membrane. The pinna is covered on both sides with skin that is tightly attached to the periochondrium. The skin that lines the inner surface of the pinna generally contains fewer hairs. Eachpinna functions independently of the other and is very mobile.3 The shape of the pinna is breed specific. The diameter of the ear canal or external acoustic meatus formed from the auricular and annular cartilages ranges from 5 to 10 mm, depending on the age, breed, and size of the dog. The canal is approximately 2 cm in length and ends proximally with the tympanic membrane or eardrum.4 Thetympanic membrane is thin, semitransparent, and elliptical (Figure 1). It serves as a wall between the tympanic cavity and the external acoustic meatus. The membrane is thinnest in the center and becomes progressively thicker toward its periphery. At the periphery, the membrane is attached to a fibrocartilaginous circular pad that is fastened to a collar of bone in the external acoustic meatus.4 Themanubrium of the malleus is a white fingerlike projection that extends into the tympanic membrane. The epithelial lining is thinnest on the deep, or proximal, portion of the ear canal. The lining becomes thicker as it progresses to the distal or superficial part of the canal and reaches its greatest thickness on the distal third of the concave surface of the pinna.5 Close to the tympanic membrane,the epithelial lining is aglandular. In short-haired breeds, no hair follicles may be found. In long-haired dogs, however, very fine hairs are present.5 In the dermis of the ear canal and especially the more distal or superficial areas of the canal, sebaceous

Small Animal

The Compendium July 1996

glands form the superficial glandular bed. Apocrine glands, which form below the sebaceousglands in the deeper dermis, open directly to the skin surface either immediately adjacent to the opening of an associated hair follicle or at some distance from it.6 The number of sebaceous and apocrine glands varies. In long-haired and fine-haired breeds (e.g., Irish setters or spaniels), the glands are more developed and numerous than in short-haired breeds.5

primary condition from parasites....