OTITIS EXTERNA

• Otitis externa is inflammation of the epithelium that lines the external auditory canal
• Not necessarily a diagnosis for one distinct problem, but often a presentation of clinical signs with multifactorial causes
• Otitis externa was recently number one reason for dog pet claims (VPI pet insurance, 2012)

The causes of otitis externa have been classified into causes (primary and secondary) and factors (predisposing and perpetuating).

1. Primary causes: create disease in a normal ear. Once a primary cause alters the aural environment, secondary infections often develop.
   Examples: allergy, foreign bodies, parasites, endocrine disease, immune-mediated disease, epithelialization disorders and others
2. Secondary causes: create disease in an abnormal ear.
   Examples: bacteria, yeast, fungal, medication reaction, physical trauma (e.g. Q-tip)

Factors are elements of the disease or pet that contributes to or promote otitis externa, generally by altering the structure, function, or physiology of the ear canal. Factors are classified as:

• Often all categories are involved, but each category must be identified and addressed separately.
• In this way a more accurate prognosis can be provided, a specific and safe therapeutic plan formulated, and the best possible outcome from treatment assured.

• Pruritus (e.g., head shaking, ear scratching) and malodor = most apparent clinical signs
• Aural redness, discharge, and pain are also common

• History (otic, dermatologic)
• Physical Examination (otic, dermatologic)
• Ancillary tests such as cytology

Otic history: unilateral or bilateral disease, age of onset, pruritus, seasonality, inflammation vs infection, response to therapy

Dermatologic history: concurrent skin disease, age of onset, pruritus, seasonality, family history, response to therapy

Otic exam:

1. gross (unilateral or bilateral, primary or secondary, acute vs chronic, characterization of discharge)
2. microscopic/cytologic evaluation of exudate or cerumen obtained from the horizontal ear canal is imperative and can provide immediate diagnostic information.
   Diagnostic technique: Exudate obtained with a cotton-tipped applicator is rolled onto a glass slide and stained with a 3-step quick stain or modified Wright’s stain, and examined microscopically. Smears should be examined first under low-power magnification and then under high-power (using immersion oil) for numbers and morphology of bacteria, yeasts, and white blood cells; evidence of phagocytosis of microorganisms; fungal hyphae; and acantholytic or neoplastic cells.
   In addition to stained cytology, otic exudate should be examined for eggs, larvae, or adults of the ear mite Otodectes cynotis and other ectoparasites. Smears are made by combining cerumen and otic discharge with a small quantity of mineral oil on a glass slide. A coverglass should be used, and the smear examined under low-power magnification. Rarely, refractory ceruminous otitis externa may be associated with localized proliferation of Demodex sp in the external ear canals of dogs and cats and may be the only area on the body affected.
3. otoscopic (i.e. changes in ear canal diameter, pathologic changes in the epithelium, quantity and type of exudate, parasites, foreign bodies, neoplasms, and changes in the tympanic membrane)

Dermatologic exam: concurrent skin lesions and history of other skin disease often helps lead to proper primary diagnosis of otitis externa. In one study, concurrent skin lesions were present in 76% of animals with chronic otitis externa.

Ancillary tests: 
Culture and susceptibility testing are indicated if otitis media is present or when systemic therapy will be prescribed for severe otitis externa (e.g. cytology reveals large numbers of rods or empirical therapy has been ineffective). Samples for culture should be taken with a sterile culturette from the horizontal canal (the region where most infections arise) or from the middle ear in cases of tympanic rupture and/or otitis media.

Histopathologic changes associated with chronic otitis externa are often nonspecific. Biopsies from chronic, obstructive, unilateral otitis externa may reveal whether neoplastic changes are present. Otherwise, biopsy for histopathology is generally not recommended from the ear canal, however biopsies of pinnal lesions may be helpful in diagnosis of immune-mediated diseases and/or vasculitis.

Imaging (Computed tomography or Magnetic Resonance Imaging ) should be performed for cases of severe, chronic otitis or when proliferative tissues prevent adequate visualization of the tympanic membrane or when otitis media is suspected, and when neurologic signs accompany otitis externa.

• Treatment of otitis externa depends on identifying and controlling, to the extent possible, all the causes and factors involved in the disease
• Client education is critical to achieve good compliance
• Follow-up otoscopic exams and cytology evaluations important to achieving successful outcomes
• Major categories of treatments for otitis externa are listed in the table below.

• The best treatment approach is to develop a plan that considers each cause or factor and how the response to treatment will be monitored
• Ear cleaning is an essential component of effective management of otitis externa
• Client education on the importance of compliance and the need for a diagnostic work-up in recurrent or non-responsive cases is crucial
• Treatment plans must be feasible for the pet owner in order to achieve compliance and successful treatment outcomes
• Unresolved cases can erode pet owner confidence in their veterinarian's abilities
• Timely referral to a local dermatologist for resistant or recurrent cases is strongly recommended
• A variety of surgical procedures may be appropriate to recommend to owners of fractious animals, when severe proliferative changes and calcification of the ear cartilage is present, and when neoplasia is diagnosed.

References

1. Miller W, Griffin C, Campbell K. Muller and Kirk's Small Animal Dermatology, ed 7, Philadelphia: Elsevier, 2013, pp 741-767.
2. The Merck Veterinary Manual. Otitis Externa. Available at: http://www.merckvetmanual.com/mvm/index.jsp?cfile=htm/bc/30900.htm&word=otitis%2cexterna. Accessed February 1, 2013.
3. Morris DO: Medical therapy of otitis externa and otitis media. Vet Clin North Am Small Anim Pract 34(2):541-555,vii-viii, 2004.
4. Nuttall T, Carr MN: Topical and systemic antimicrobial therapy for ear infections. In Affolter VK, Hill PB, editors: Advances in Veterinary Dermatology, Ames, 2010, Wiley-Blackwell, pp 402-407.