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.
For example, excessive skin folds at the base of the tail may predispose the English Bulldog to a tail fold pyoderma. In the ear of this breed, excessive skin folds also predispose to otitis. The presence of a predisposing factor in a patient makes the ears more susceptible to otitis externa even when the patient is not exhibiting symptoms of otitis externa. By analogy, not all English Bulldogs with excessive tail folds suffer from tail fold pyoderma. Because such dogs have the predisposing factor (the excessive folds), tail fold pyoderma is more likely to occur in this breed than in the Beagle, which has no tail folds.
The floppy ear carriage common to many dog breeds may predispose them to otitis because of inadequate ventilation of the ear canal, which ultimately leads to higher humidity there. The dark, warm, moist area increases the chance that bacteria and yeasts will have a more favorable environment for growth and reproduction. It is not surprising that the incidence and severity of otitis externa are greater in dog breeds with floppy ears. However, there are many floppy-eared breeds that do not have increased incidence of ear disease. The decrease in cosmetic ear trimming in recent years has not resulted in increased otitis in the Miniature Schnauzer or the Doberman Pinscher. Some of the predisposed floppy-eared breeds such as the Cocker Spaniel, the Labrador Retriever, and the Springer Spaniel have increased densities of glandular tissue, also contributing to the increased humidity. Many of the floppy-eared breeds have the predisposing factor of excessive hair growth in their ears. Excessive hair can matt or knot and seal the ear canal. Hairs can also accumulate wax, creating a plug. Cats and many dog breeds have erect ears that are well ventilated, and these pets have a lower incidence of otitis externa.
In developing a therapeutic plan for otitis externa, the veterinarian must give attention to the treatment of predisposing factors that effectively reduce the likelihood that the patient’s ear disease will become chronic or recurrent. Some treatments to remove predisposing factors are very simple, such as plucking the excessive hairs found in a Poodle’s ears to reduce the humidity within the ear canal. Some breeders and veterinarians advocate taping or fixation of the ears of a Cocker Spaniel over the top of its head to allow ear canal ventilation. Lateral canal ear resection in a nondiseased ear opens it up and provides ventilation as well as prevents accumulation of exudates that may form. Treatment of other predisposing factors is not such a simple matter. Diffuse cerumen gland adenocarcinoma, for example, requires total ear canal ablation to resolve.
These anatomic and physiologic alterations create favorable climates for the proliferation of infectious organisms. High temperature and humidity in the ear canal, greater amounts of substrates for growth of bacteria and yeasts, and damaged epithelium result in the failure of the ear’s normal immune mechanisms. Commensal bacteria and yeasts may be induced to reproduce in the favorable climate created. Otitis externa is the eventual result.
How does the presence of a predisposing factor in a patient’s ear canal affect the course of otitis externa? Some pathophysiologic mechanisms have been explained, but other mechanisms remain a challenge. An understanding of the mechanisms involved in how these predisposing factors affect the normal physiology of the ear canal helps clinicians formulate more complete therapeutic plans for the treatment of otitis externa.
The normally smooth epithelial surface of the ear canal has a mechanism for clearing surface debris out of the ear canal; the process has been termed epithelial migration. Surface keratinocytes slowly slide along the epidermal layer of the ear canal, carrying cerumen and microorganisms out of the ear canal. When the anatomy of the epithelium of the ear canal is altered by the presence of abnormal tissue, the movement of kerotinocytes is also altered. The smooth surface of the normal ear canal becomes roughened or obstructed, and the debris accumulates at the point where the epithelial movement stops (Figure 7-1). Inflammation and colonization of microorganisms occur at these points.
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.
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.
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.
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 7-7 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.
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.
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 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.
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.
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.