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 environment 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 particularly 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, salicylic, 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 treatment 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 cleansing/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 medication 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 thiabendazole 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 investigated (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 cleansing/drying solution as part of the treatment
protocol.
Corticosteroids are important to use in
the treatment of otitis externa to relieve the inflammation 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 corticosteroid (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
phosphatase, 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 ruptured 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 possible. However,
the actual incidence of ototoxicity due to the aminoglycosides (or any
other medications) in dogs and cats is unknown (and probably uncommon).
Therefore, 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 systemic acaricide
(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 injection, 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
inflammation. 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 manifestation
of generalized demodicosis. Therapy for the ears, therefore, 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
Alligator 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. Fluocinolone 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 granulomatous 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 commonly, 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