Stenotic Ear Canals

A common finding in dogs with otitis externa is a narrowed ear canal. Within the tube of skin that makes up the external ear canal, any swelling translates to decreased lumen diameter. When the lumen of the ear canal becomes narrowed or occluded, stenosis results (Figure 7-2). The stenosis magnifies the severity of the ear disease, making examination and treatment of the otitis externa more difficult.

Figure 7-2 Stenotic ear canal as the result of trauma. Swelling from inflammation or increased tissue mass from fibrosis in the ear canal closes off the lumen.

A brief review of the anatomy of the external ear canal is helpful in delineating the mechanisms involved in creating the stenotic ear canal. The ear canal of the dog is lined by keratinizing stratified squamous epithelium.

Vertical Ear Canal

The skin of the vertical canal is approximately 1 mm thick and contains a well-developed dermis and subcutaneous layer. Numerous long, coarse hairs are present along the vertical canal. Surrounding the hair follicles are numerous sebaceous and ceruminous glands (modified apocrine glands). No eccrine sweat glands are located in the external ear canal. Hairs are most numerous toward the opening of the ear canal; they decrease along the ear canal toward the eardrum. Conversely, the ceruminous glands increase in density in the vertical canal distally.

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.

The ear canal of the Shar-Pei has an abundant mucinous dermis under the epithelial layer that increases the thickness and folding of the dermal-epidermal layer. Owing to the anatomically normal thick lining, the lumen diameter in this breed is decreased. The Shar-Pei’s ear canal is thus predisposed to higher humidity and greater glandular secretions, promoting bacterial and yeast colonization.

Anatomically, the vertical canal is more prone to becoming stenotic because of the vascularity and glandular structures found there. The inflammation and edema lead to narrowing of the ear canal. The stenosis prevents drainage of exudates out of the ear canal and complicates therapy by preventing topical medications from achieving therapeutic levels beyond the stenotic portion. Increases in fluid and air pressure beyond the stenosis can cause excessive pressure on the eardrum, predisposing it to rupture.

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 7-4 Histopathologic section of an ear canal with extensive fibrosis below a hyperplastic epithelium. Numerous epithelial folds are present and there is an exudate in the stenotic ear canal.

Figure 7-5 Sebaceous hyperplasia usually results in a smooth, stenotic canal. In this Shetland Sheepdog’s ear, the epithelium is also hyperplastic.

Figure 7-6 Ceruminous gland hyperplasia causes a stenosis in the vertical ear canal of this Cocker Spaniel.

Figure 7-7 Radiograph of a stenotic, calcified ear canal in an American Cocker Spaniel. This radiographic sign of pathology indicates a nonreversible, end-stage ear that will require surgical removal.

Figure 7-8 Fibrosis and granulation tissue can result from chronic otitis externa.

Figure 7-9 Cerumen gland adenocarcinomas in an American Cocker Spaniel. This ear was also secondarily infected with Staphylococcus intermedius, which remained until surgery was performed. Laser ablation of these tumors opened up the ear canal.

Horizontal Ear Canal

Fortunately, the skin of the horizontal canal is often spared the devastating effects of inflammation associated with otitis externa. The thin epidermis lining the horizontal canal firmly attaches to the underlying auricular cartilage in the lateral aspect of the horizontal canal (cartilaginous portion) and is approximately 0.2 mm thick. The ventral portion of the horizontal canal has an extension of the pertrous temporal bone underlying the thin epidermis (bony portion). The thin epithelium of the bony horizontal canal is continuous with the epithelium on the lateral aspect of the tympanic membrane, which is one or two cells thick. Epidermal rete ridges, skin adnexal structures, and a subcutaneous layer are absent in the skin of the horizontal canal. Because the skin is adherent to the underlying cartilage and to the periosteum of the bony portion of the horizontal canal, pathologic changes of the horizontal canal are usually limited to hyperplasia. The exception to this anatomic feature is the American Cocker Spaniel. This breed supports much more glandular tissue in the horizontal canal than other breeds. This has been verified by an analysis of histopathologic specimens from the horizontal canal of several breeds.

Overtreatment

Long-standing overtreatment of ears with ear cleaners may macerate the epithelium, causing swelling and greater folding of the epithelial surface. Ear cleaners usually contain a combination of several ingredients, including alcohols, acids, detergents, propylene glycol, and water. Prolonged contact of these substances with the ear canal causes the cells to swell as the ear canal skin loses its waterproof properties. Drugs such as neomycin and silver sulfadiazine are known to trigger a contact dermatitis within the ear canal, leading to erythema and swelling in the ear canal. If any of these causes is suspected, suspending all topical treatment for 1 or 2 weeks may decrease the tissue reaction and relieve the swelling.

Stenosis

Stenosis presents a special problem in examination of the ear canal. Instruments cannot be inserted through the narrowed canal, and the integrity of the eardrum cannot be determined otoscopically. Occasionally, a short course of potent corticosteroids, applied topically, injected directly into the stenotic tissue, or administered parenterally, decreases the inflammatory infiltrates and reduces edema (Figure 7-10). In addition, corticosteroids decrease the amounts of glandular secretions, making them less viscous so that they are easier to flush out of the ear canal; they also reduce the secretion and dilation of ceruminous glands. If corticosteroids successfully increase the lumen diameter, adequate visualization of the ear canal and tympanic membrane becomes possible.

Figure 7-10 Potent corticosteroids can be infused through a small catheter through a stenotic portion of the ear canal to reduce inflammation.

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

In some of the haired breeds, such as the Poodle and many of the terriers, long hairs normally grow from the skin of the ear canal. The number of hair follicles in predisposed breeds gradually decreases along the length of the external ear canal. The highest density of hair follicles occurs at the entrance to the ear canals at the pinna and along the proximal portion of the vertical canal. Occasionally hairs may be found along the deeper parts of the canal.

Excessive hair or knots of ear hair may occlude the ear canals and interfere with adequate drying of the canal. The excessive moisture created by hair plugs predisposes these breeds to otitis externa. Certain bacteria such as Pseudomonas and Proteus thrive in a humid ear canal. When long hairs become matted and tangled in the ear canal, cerumen, exudates, and other secretions mold to the hair mass and form an occlusion.

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.