Paul B. Bloom, DVM, DACVD, DABVP (Canine and Feline Specialty)
Allergy, Skin and Ear Clinic for Pets, Livonia, MI; Department of Dermatology, Michigan State University, East Lansing, MI, USA
It is important to understand that ear disease is only a symptom (no more specific than 'pruritus') It is appropriate therefore to approach the diagnosis of ear disease just as you would for any other skin disease.
Reviewing the signalment (age, breed and sex) may give you some insight as to the underlying disease. Of course a detailed history is essential. You need to know when the problem started, what other symptoms the dog has (gastrointestinal?), what medication, if any, has been used and was it successful, any current medication, any perceived seasonality and what is the current and past diet(s). Ask if any other pets are in the household and if so, are they affected? Also inquire about any humans in the household - do they have any new skin disease?
Be sure to do a complete dermatologic examination since ear disease is usually a component of more wide spread skin disease. Next, perform an otoscopic examination. Be sure to note any alopecia, erythema, ulceration, crusting, scaling or swelling of the pinna. Then, palpate the canals for pain, calcification or thickening. This is followed by an otoscopic examination of the ear canals. The presence, degree and location of inflammation, ulceration & proliferative changes should be noted (i.e., cobblestone hyperplasia). Describing the size of both the vertical and horizontal canals along with the type, location and quantity of debris or exudate should also be included in the medical record. Next, it should be documented whether the tympanic membrane (TM) is visualized. If it is not, then note why the membrane is not seen - is it due to swelling in the ear canal, the presence of a ceruminolith or is there just debris in the proximal horizontal canal obstructing the view? If the animal is too painful to allow a thorough examination, sedation may be required. If you can visualize the TM you need to describe if it is normal in appearance or not. Evaluation for concurrent middle or inner ear disease should be included in the examination. This is because dogs with chronic recurrent otitis externa (OE) may have concurrent otitis media (OM). This step may require heavy sedation or general anesthesia. Evidence of middle ear involvement includes a ruptured TM or an abnormal appearing TM.
Horner's syndrome (sympathetic nerve), keratoconjunctivitis sicca (parasympathetic nerve) and facial nerve paralysis may be present in cases of OM due to the close association of the respective nerves to the middle ear. Deafness may also be present with OM.
Now diagnostics and treatment need to be pursued. The first step is to identify and treat the primary (underlying) cause(s) of the ear disease. There are many causes, but the most common include parasites (Demodex, Otodectes, Sarcoptes), foreign bodies, allergies (may be unilateral disease!) and autoimmune disease (e.g., vasculitis, pemphigus foliaceus).
Secondary factors must be addressed if possible. Secondary factors don't cause ear disease but increase the risk of developing ear disease and may make successful treatment more difficult. Secondary factors include anatomical factors (e.g., long pendulous ears in the Basset Hound or stenotic ear canals in Shar Peis), excessive moisture in ears (swimming) and iatrogenic trauma (plucking hairs from the ear canals, cleaning ear canals with cotton-tip applicators).
Lastly, perpetuating factors must be identified and treated. These factors don't initiate the problem, but will cause the disease to continue, even with the elimination of the primary factor, once the disease has been established, until these perpetuating factors have also been addressed. The most common perpetuating factors are infectious, structural (pathological changes) and otitis media
Laboratory tests are a necessary component to the proper workup of a case of canine ear disease. CBC, serum chemistry profile, urinalysis, skin scrapings, fungal culture, endocrine testing and skin biopsies may be necessary depending on what the differential diagnoses are for that patient.
Cytologic examination of a roll swab sample should be performed on any exudate, being sure to quantitate numbers & type of bacteria, yeast and inflammatory cells.
In my opinion, bacterial culture and susceptibility (c/s) is rarely, if ever, performed in cases of OE and, when performed, it is done in conjunction with cytology. Remember that cultures interpret susceptibility based on antibiotic levels, measured in µg/ml, that are achieved systemically, not topically. When applied topically, mg/ml is the concentration being used. This level is 1,000 times (at least) the measured MIC.
With the information gathered above, the treatment is directed toward the primary cause(s) (e.g., parasiticidal treatment, food trial, intradermal testing, allergen-specific immunotherapy) and perpetuating factors. Ear cleaning is performed in the clinic before dispensing medication for the owner. If the ear is very swollen, ear cleaning may be postponed for 10–14 days during which topical glucocorticoids (GC) +/- systemic GC are administered.
Once the swelling has decreased, it will be much easier to examine the ear canals and visualize the TM.
Cleaning agents contain substances that soften and emulsify wax and lipids. This initial cleaning is necessary in order to remove debris that may interfere with the effectiveness of topical agents and reduce inflammatory debris (bacterial toxins). There are many reasons for this approach, but the main one is that the base in the otic ointments/suspensions (mineral oil, liquid paraffin) acts as a ceruminolytic agent, dissolving the cerumen as you treat the ear. The author doesn't usually have the owner do cleaning after the initial exam since it seems that many owners have trouble with just medicating the ear, let alone cleaning too.
After ear cleaning, topical agents are dispensed. The author prefers ointments over drops, because of the impression that ointments get the medication to the region of the tympanic membrane better than drops do (this may be a volume issue more than the formulation - it has been reported that it takes 1.0 cc of medication to get down to the TM in a medium-sized (40 pound) dog - personal communication).
Most topical products contain a combination of glucocorticoids, antibacterial and antifungal agents. The list of products and ingredients is quite extensive, but one of the key points when selecting a product is to reserve fluoroquinolones for culture-resistant infections - don't use when it is for owner compliance problems!
Systemic antibiotics or antifungal agents are used only if otitis media with bacteria, other than Pseudomonas (see below about Pseudomonas) or Malassezia is present on cytology and there are severe proliferative changes in the ear canals that have failed to respond to the author's topical treatment (very rare occurrence).
Systemic glucocorticoids are used if the ear canals are edematous, ulcerated and/or stenotic. Even proliferative changes may decrease with steroid administration since secondary edema may be present. Prednisone at 0.50–1.0 mg/kg BID for 7–14 days is dispensed. Reassessment is made in 7–14 days. At that time, if the canals are completely open and the ulcers are healed, the prednisone can be discontinued. If the ears are better but not normal, then make a clinical decision whether to maintain that dose or decrease the dose for another 7–14 days, recheck again in 7–14 days. If the ear canals are not opened by this second recheck, a total ear canal ablation with a bullae osteotomy would most likely need to be performed.
Pseudomonas infections are especially challenging because of Pseudomonas intrinsic multi-drug resistance (MDR). Many of the clinically relevant resistance mechanisms in P. aeruginosa are attributed to synergy between its outer membrane, which has a very low permeability to drugs, and the presence of an active drug efflux pump (MEX). Because of the intrinsic MDR with Pseudomonas infections, successful treatment must be aggressive before other resistance develops.
References are available upon request.