Chapter 59 Otitis Externa
The external ear canal consists of skin overlying cartilage, which provides the structural support to keep the canal open. The auricular cartilage is the framework for the pinnae and vertical aspect of the ear canal, and the annular cartilage supports the horizontal component of the external ear canal. The cartilage is covered by skin, which contains sebaceous sweat glands, apocrine (ceruminous) glands, and hair follicles. Otitis externa is defined as inflammation of the skin and adnexal structures of the ear canal. This condition is one of the most common and frustrating problems encountered in small animal practice.
ETIOLOGY AND PATHOGENESIS
Primary Factors
• Primary factors are those conditions or disorders that initiate the inflammatory process within the ear canal.
• Examples include parasites (Otodectes cynotis); allergies (food, atopic dermatitis, contact); foreign bodies (grass awns, foxtails); keratinization disorders (seborrhea); and, less frequently, trauma, autoimmune disease (pemphigus), sebaceous adenitis, zinc-responsive dermatoses, and endocrinopathies (hypothyroidism)
• Primary factors may initially induce disease outside the external ear canal. Otitis externa may be an extension of a pinnal disorder (see Chapter 58), otitis media, or otitis interna (see Chapter 61).
Predisposing Factors
• Predisposing factors facilitate the inflammation by promoting an environment conducive to survival of perpetuating factors.
• Examples include conformation of the ear canal (long canal with a deep vertical component), moisture in the canal (dogs that swim, residual cleaning agents in the ear canal), hair in the ears (e.g., in poodles and terriers), breed predisposition (e.g., Chinese Shar-Pei, stenotic canals), immuno-deficiency syndromes, endocrine imbalances, iatrogenic ear trauma (e.g., unnecessary hair removal and cleaning with cotton-tipped applicators), and obstructive disease (e.g., cancer, polyps, and hyperplasia).
Perpetuating Factors
• Mechanisms include occlusion of the canal, which prevents drying or proper application of medication; secretion of irritating factors; alterations in pH of the canal; and formation of a focus of infection (otitis media).
• Examples include bacterial infections (Staphylococcus intermedius, Proteus mirabilis, Pseudomonas aeruginosa, and Escherichia coli), and yeast infection (Malassezia pachydermatis). Otitis media serves as a source of infectious agents, and chronic hyperplastic changes of the ear canal may obstruct the canal.
CLINICAL SIGNS
Signs Directly Related to Ear Involvement
• Behavioral changes: pets may become irritable and aggressive toward family members as a result of ear pain.
Signs Reflecting an Underlying (Predisposing) Dermatologic Disorder
• Face rubbing, sneezing, foot licking, anal scooting, and generalized scratching suggest underlying allergic disease.
• Severe pruritus may incriminate parasitic (scabies, Notoedres mange) or allergic (flea allergy dermatitis) causes.
• Scaling and crusting may indicate seborrheic disease, sebaceous adenitis, or pemphigus foliaceus as a primary disease.
• Recurrent bacterial dermatitis may suggest an allergy, an endocrine imbalance, or immunologic insufficiency.
Signs of Associated Otitis Media and Otitis Interna
Typically, the most common clinical sign of otitis media is chronic and recurrent otitis externa. Neurologic signs are more specific for otitis media (sympathetic and facial neuropathy) and otitis interna (peripheral vestibular syndrome) but are not always present in dogs with middle or inner ear disease. Refer to Chapter 61.
DIAGNOSIS
History
• Use the history to detect evidence of allergies (seasonality), parasites (possible exposure), and environmental factors of concern.
Physical Examination
• Physical evaluation includes palpation of the external ear canals for pain or evidence of calcification of the external canals (firm or hard on palpation), smelling of the ears, and careful examination of the skin over the entire body for evidence of systemic disease.
Otoscopic Examination
• For equipment, a standard otoscope with a diagnostic or operating head is generally adequate; however, a video otoscope is strongly recommended for enhanced imaging and its specialty applications.
• Evaluate for the size of the ear canals; the presence of parasites, exudate, hair, or foreign material; the color of the epithelium; the presence of ulcers or masses; and the appearance and integrity of the tympanic membrane (see Chapter 1).
• Sedation of the animal may be necessary.
• Topical anesthesia with 1% to 2% lidocaine HCl, 0.5% proparacaine, or other similar agents may be sufficient.
• Avoid trauma to the ear canal by advancing the otoscope cone only while directly visualizing the canal.
Otoscopic Abnormalities
• Exudation: dark, dry, granular exudate is generally found with ear mite infection; moist, yellow, odoriferous exudate is generally a sign of bacterial infection; brown, waxy exudate is generally consistent with yeast infection and overgrowth; and yellow, waxy-to-dry scale may be found with keratinization disorders. Cytology of otic exudates is always indicated to confirm these clinical clues.
• Hyperplasia (lichenification, hyperpigmentation) is a sign associated with chronicity. Surgical management may be necessary in patients with severe hyperplasia (occlusion) that is unresponsive to therapy.
Cytology
• Cytology often provides an indication for the best initial treatment plan and is essential in the assessment of secondary bacterial and yeast infections.
• Use a cotton-tipped applicator to swab the external canal, if possible at the outer end of the horizontal ear canal. Remove and gently roll the applicator onto a clean glass slide.
• Heat fix and stain the slide with a modified Wright-Giemsa preparation (Diff-Quik; American Scientific Products). Examine a separate slide after adding mineral oil (and without heat fixing or staining) to look for external parasites.
• Examine for parasites, cellular components, and infectious agents (bacteria, yeast, fungi). Notice whether infectious agents are present within inflammatory cells or free in the exudate and whether the infectious agents are present as a single population or part of a mixed population.
Culture and Susceptibility Testing
• Bacterial culture of the external ear canal is recommended when:
• Cytologic examination of exudate from the external ear canal shows a uniform population of gram-negative bacteria (or rod-shaped bacteria if Diff Quick is the stain used).
• Ulcers are present in the external ear canal and bacteria are present on cytologic examination of exudate.
• Otitis media should also be suspected in animals with chronic (recurrent or persistent) otitis externa even when the tympanic membrane is intact. Bacterial culture of the middle ear (by myringotomy if the tympanic membrane is intact) may be indicated if the tympanic membrane appears at all abnormal (see Chapter 61).
Biopsy
• Biopsy instruments designed for endoscopic procedures are useful to collect a small tissue sample from the ear canal.
Diagnostic Imaging
• Radiography is occasionally indicated (especially in severe or chronic otitis) to evaluate the patency of the ear canal, to aid in detecting the presence of otitis media and otitis interna, and to determine the extent of involvement of surrounding structures.
• Computerized tomography (CT scans) and magnetic resonance imaging (MRI) are more sensitive diagnostic tests to evaluate the patency of the ear canal (and involvement of the middle ear).
Other Tests
• Miscellaneous diagnostic tests are helpful to identify predisposing and primary factors. See respective chapters for details on these tests. Tests frequently recommended include hematology, serum biochemistry profiles, urinalysis, thyroid function tests, adrenal function tests, intradermal allergy tests, serum (in vitro) allergy tests, skin scrapings, fungal cultures, and dietary trials.
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