Medical Treatment of Otitis Externa

Stephen White United States

Medical treatment of the ear with otitis externa may be divided into three stages: ear cleaning with management of predisposing causes, treatment of perpetuating causes, and identification and treatment of primary causes.

Ear cleaning and predisposing causes

Cleaning the ear is important for a variety of reasons. Wax, oil, and other debris may be irritating, prevent medicaments from contact with the ear canal wall, produce a favorable envi�ronment for microorganisms to proliferate, and may inactivate certain antibiotics. Hair should be removed from the ear canal. This is best done by grasping the hair with a forceps and twisting the hairs out by twirling the forceps (rather than plucking, which is more painful). Excessive hair at the base of the medial pinna should also be clipped away.

Arguably, the best results of ear cleaning are obtained with the patient under general anesthesia. Such a patient must be intubated to prevent aspiration of flushing fluids that can drain through the eustachian tube. Instilling the ear with a ceruminolytic agent greatly helps the cleaning process. The author prefers a product containing dioctyl sodium sulfosuccinate, urea peroxide, and lidocaine hydrochloride (Clearx^® Ear Cleansing Solution, DVM) although I limit its use to anesthetized patients because occasionally irritation occurs. The ear is subsequently flushed using a rubber ear bulb syringe and either a mild disinfectant solution (dilute chlorhexidine works well) or just saline solution. This solution should then be removed via suction, with either a suction pump or a 12-ml syringe and propylene tomcat catheter. After the cleaning procedure has been performed, an otic examination is indicated to ensure that all remaining wax and debris are removed. The use of an ear curette is particular�ly helpful in removing wax.

In cases where either the owner declines anesthesia, the animal is not a good candidate for anesthesia due to concurrent health problems, or where the degree of canal occlusion and exudate are not severe, a reasonable cleaning program may be obtained on an outpatient basis utilizing products which have both cleansing and drying properties. Most of these products contain various types of acids (lactic, malic, benzoic, salicyl�ic, acetic, etc.) often with a cerumenolytic agent or alcohol added. A number of these substances have antimicrobial properties as well. The author prefers a product containing salicylic acid, lactic acid, and propylene glycol (Epi-otic^®, Allerderm/Virbac). Cleansing/drying solutions are usually instilled into the ear once or twice daily, preceding any treat�ment solution also prescribed.

Some predisposing causes of otitis externa are relatively easy to determine and remedy. Ear canal maceration caused by excessive moisture is best treated with one of the cleans�ing/drying solutions mentioned above. Treatment errors such as ineffective treatments or a hypersensitivity-type reaction are easily monitored by periodic rechecks of the patient once a therapy is selected. Obstructive ear disease and conformation abnormalities are best dealt, when practical, with surgical removal or correction, respectively. The exact roles of pyrexia or anal sac disease in the predisposition for otitis externa have always remained obscure to the author.

Perpetuating causes

The veterinarian is usually able to deal with perpetuating causes of otitis externa such as bacteria and yeast with the use of topical medications alone, instilled in the ear twice daily. In severe cases, or when the owner is unable to instill medica�tion into the ear (an aggressive patient; a swollen or occluded ear canal) systemic medications may be invaluable. For most cases of cocci seen on microscopic examination of ear exudates, the author finds the following effective:

 Tresaderm^® (neomycin, thiabendazole and dexamethasone) MSD AGVET Merck.

 �Otomax^® (gentamycin, betamethasone, and clotrimazole) Schering.

The author can also recommend any of the following in cases of severe Pseudomonas infection:

 Gentocin ^® (gentamycin, betamethasone]; Schering.

 Otomax ^® [gentamycin, betamethasone, clotrimazole) Schering.

 TrizEDTA^® [Tris-EDTA]

 Dermafet Ear/Skin Cleanser ^® [boric acid, acetic acid] DermaPet.

 Baytril otic^® (enrofloxacin, silver sulfadiazine) Bayer.

 Cortisporin^® solution (polymixin B, neomycin, hydrocortisone) Glaxo Wellcome.

 Silvadene Cream^® (1% silver sulfadiazine), Marion Laboratories (dissolved in water at a 1:10 ratio.)

 Timentin^® (Ticarcillin-Clavulanate) SmithKline Beecham.

Dilute according to manufacturer�s directions, then draw into 2 ml aliquots and freeze. Thaw and use each aliquot as 0.5 ml in each ear, twice daily.

When a systemic antibiotic is needed, the author prefers enrofloxacin (or other quinolones), 5�15 mg/kg q12h (pending culture and sensitivity results). In cases of yeast infection, most medications with thiaben�dazole or one of the newer imidazole compounds (miconazole, clotrimazole) are advised. Otic preparations containing chlorhexadine or acetic and boric acids also work well. When a systemic anti-yeast medication is needed, the author has had excellent results with ketoconazole, 5 mg/kg q24h (in dogs) or itraconazole 5 mg/kg q24h (in cats).

It is important to monitor patients treated for bacteria and/or yeast, preferably two to three weeks after initiating therapy. At that time, otic examination and microscopic examination of the ear exudate are repeated. If there has been no improvement and compliance has been good, a change in medications is indicated. The therapy should be continued until both otic and microscopic examination approach that of a normal dog. In the author's experience, this will take at least one month. An important point to remember is that frequently, although not always, bacteria and yeast are perpetuating causes of otitis externa and if the ear examinations never become normal, or if the infections relapse upon discontinuation of therapy, the presence of otitis media or primary causes should be inves�tigated (see below). Another important point is that the ear must be kept relatively clean, for the treatment to work. This often means continuance of a cleans�ing/drying solution as part of the treatment protocol.

Corticosteroids are important to use in the treatment of otitis externa to relieve the inflam�mation present (along with its concurrent discomfort), especially if treating obstructive ear disease is due to progressive pathologic change. Such change is initially due to tissue swelling and progresses to fibrosis and fibroproliferative pathology. Calcification of the cartilage of the external ear canal may result. Corticosteroids are helpful in controlling or even reversing these changes, in the early stages. Usually, topical steroids alone will suffice. The author will start with a high-potency cortico�ste�roid (usually included as one of several types of medicaments in a topical preparation) such as fluocinolone, betamethasone, or dexamethasone. These are instilled into the ear twice daily until the swelling and inflammation are under control (1-4 weeks). The frequency of treatment is then reduced to every other day, and/or, a topical corticosteroid of lower potency (hydrocortisone) may be used. Use of corticosteroids in the ear may lead to increases in the levels of serum enzymes such as alkaline phos�phatase, to a hypoadrenal (Addisonian) response on adrenal function tests, and rarely, to visible Cushinoid signs (alopecia, abdominal distension). In cases where systemic corticosteroids are indicated (compliance problems or an external canal opening that is swollen and occulded), prednisone may be given orally, ideally at the lowest possible dose and frequency to be effective.

A frequent concern of veterinarians is what effect a rup�tured tympanic membrane has on any of the recommendations for therapy. One should use solutions rather than ointments or creams, and avoid the use of topical aminoglycosides, if possi�ble. However, the actual incidence of ototoxicity due to the aminoglycosides (or any other medications) in dogs and cats is unknown (and probably uncommon). There�fore, the veterinarian should not avoid using aminoglycosides in a patient with a ruptured tympanic membrane if they are the only option for the animal.

Otitis media should be considered as a perpetuating cause of recurrent otitis externa. Radiographs or CT scans and cultures should be obtained.

Primary causes

Parasites

The most common parasitic cause of otitis externa is the ear mite Otodedes cynotis. Typically, it presents with a brown-black crumbly otic exudate. Pets may be infested with these mites but show no clinical signs; in addition, the mites may live on the body outside the ears. For these reasons the author treats with a system�ic acari�cide (including all in-contact pets). Selamectin (Revolution^®, Pfizer) works well as a cutaneously absorbed ectoendoparasiticide applied to the skin interscapular. Ivermectin works well at a dosage of 0.3 mg/kg given once weekly, either orally or by subcutaneous injec�tion, for one month. The author uses a bovine product (Ivomec^®, Merck, Sharp and Dohme, Rahway, New Jersey). However, this preparation, and the aforementioned dosage, is not approved for use in dogs and cats. Ivermectin at this dose is contraindicated in collies, old English Sheepdogs, Australian herding breeds, and their crosses. In those breeds, selamectin or milbemycin (Interceptor^®, Novartis at 2 mg/kg once weekly for three weeks) may be used. Topically, the author prefers the previously mentioned Tresaderm, which contains thiabendazole to kill the mites (and their eggs) and dexamethasone to relieve accompanying inflamma�tion. The solution is instilled in the ear twice daily for a minimum of two weeks.

Demodex sp in both dogs and cats have been noted as otic parasites, and in cats the condition may resemble otitis due to O. cynotis. The ears may be the only place on the body affected by Demodex in cats, but in dogs, demodectic otitis is usually a manifesta�tion of generalized demodicosis. Therapy for the ears, there�fore, must be part of a generalized treatment plan. The author has found a mixture of one ounce of mineral oil to 1 ml of a 19.9% amitraz solution (Mitaban^®, Upjohn, Kalamazoo, Michigan, USA) instilled into the ears once daily to be effective.

Foreign bodies

Alliga�tor forceps are extremely useful for removal. It should be remembered that wax and other debris may act as a foreign body in its ability to elicit an inflammatory response; this is one of the reasons why keeping the ear clean is so important. Even if a foreign body is successfully removed from one ear, the other ear should always be examined; the first ear may simply have been the one that was the most uncomfortable to the patient.

Hypersensitivity diseases

In the opinion of the many veterinarians, including the author, chronic otitis externa in the dog should be considered secondary to hypersensitivity until proven otherwise. This includes atopic dermatitis and food allergy, and less commonly contact and drug allergies. In most cases the ears are not affected alone; frequently the feet (especially interdigitally) and sometimes the face and axilla are involved. Thus, it becomes evident how important a thorough history and physical examination is. Occasionally atopic dogs will have otitis externa as the only manifestation of their disease, initially with a seasonal presentation. Rarely, otitis externa may be the sole sign of a food allergy.

Keratinization/Cornification Disorders

Idiopathic seborrhea is the most common keratinization disorder causing otitis externa in the dog. The yellowish exudate fails to take up stain on microscopic examination. The otitis almost always occurs with seborrhea elsewhere on the body. Endocrinopathies, especially hypothyroidism, may mimic such a presentation, and should always be investigated. In otitis externa due to idiopathic seborrhea the veterinarian will often need to use long-term topical corticosteroid therapy. The author's preference is a fluocinolone-DMSO (dimethyl sulfoxide) preparation (Synotic^®, Diamond Laboratories), provided no infection is present; if there is, a topical antimicrobial preparation must be used first. Fluocino�lone and DMSO are very effective in suppressing excess sebum and cerumen production, as well as inflammation; they are, however, also very effective in suppressing the local immune response. Thus, if well-controlled ears should become inflamed with this medication, infection must be suspected.

Rarely, a dog with sebaceous adenitis (idiopathic granuloma�tous inflammation targeting the sebaceous glands) may have an otitis externa as the initial sign of the disease. Ideally, such a diagnosis is made via a biopsy of the ear canal. More common�ly, a dry scaling of the medial pinna in association with an otitis externa is seen concurrently with other skin signs of the disease (severe seborrhea, alopecia, etc.).

References

1.  Cole LK, Kwochka KW, Kowalski JJ, Hillier A. Microbial flora and antimicrobial susceptibility patterns of isolated pathogens from the horizontal ear canal and middle ear in dogs with otitis media. J Am Vet Med Assoc. 1998 212: 534-8.

2.  Colombini S, Merchant S R and Hosgood G. Microbial flora and antimicrobial susceptibility patterns from dogs with otitis media. Veterinary Dermatology 2000, 11:235-40.

3.  White SD. Otitis externa. Waltham International Focus, 2:2-9, 1992