Chapter 7 Factors that Predispose the Ear to Otitis Externa
Careful examination of a clean, dry ear canal in a dog or cat with otitis externa may reveal many conditions that affect the ear canal. Because the ear canal lining is actually modified skin, the same basic types of lesions found on the skin of the trunk may be found in the ear canal. Certain conditions, called predisposing factors, are responsible for altering the anatomy and physiology of the ear canal and increasing the likelihood of otitis externa. These factors increase the susceptibility of the ear canal to support bacteria and yeast growth.
Vertical Ear Canal
The external acoustic meatus and the skin on the pinna contain numerous adnexal structures and have a significant subcutaneous layer, which can respond to disease. Frequently, the stenotic portion of the ear is limited only to the external acoustic meatus (Figure 7-3). In that situation, the otoscope tip may be passed through the stenosis, revealing a normal vertical canal beyond it.

Figure 7-3 Hyperplastic epithelium on the concave pinna closes off the opening to the vertical ear canal.
Inflammation and edema increase the thickness of the subcutaneous layer of the ear canal, leading to stenosis. Chronic otitis externa leads to progressive pathologic changes of the lining epithelium such as hyperkeratosis and hyperplasia (Figure 7-4). The marked thickening of the epithelial layer may significantly reduce ear canal diameter. Increases in the number and size of sebaceous glands (Figure 7-5) and dilated apocrine glands also reduce lumen diameter (Figure 7-6). In addition, pathologic changes that lead to calcification (Figure 7-7) and thickening of the auricular cartilage (especially in American Cocker Spaniels) or to fibrosis and formation of excessive granulation tissue resulting from chronic infection also lead to narrowing of the ear canal lumen (Figure 7-8). Tumors such as ceruminous adenocarcinoma may also occlude the ear canal lumen (Figure 7-9).

Figure 7-6 Ceruminous gland hyperplasia causes a stenosis in the vertical ear canal of this Cocker Spaniel.
Stenosis
When medical therapy for stenosis is ineffective because of severe pathologic changes to the ear canal, surgical ablation of the vertical canal, horizontal canal, or both is required. Surgical treatment of stenotic ears relieves the pain associated with chronic otitis and allows the remaining ear canal to be ventilated.
Hair in Ear Canals
Routine plucking of hairs by groomers may not be necessary in a dog whose ears are normal, and it can sometimes be detrimental. Plucking the hairs with curved hemostats can create an inflammatory reaction that can predispose the ear to infection. In a dog that has recurrent ear infections and excessive hair growth in the ear canal, however, the hair should be routinely removed to prevent a mass of tangled hairs from blocking the ear canal lumen. Plucking the excessive hairs from the ears in patients predisposed to otitis externa is recommended for the prevention and management of otitis externa.
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