Managing otitis externa (Proceedings)

Canine otitis externa is certainly one of the most common problems veterinarians are dealing with in small animal practice. However, recurrent ear diseases can be very challenging to manage and be quite frustrating for the practitioner as well as for the pet owner. To be successful in the treatment of ear infections, especially in recurrent cases, it is important to understand the different aspects of primary, predisposing and secondary factors which may contribute to the disease. Causes such as ectoparasites (e.g. ear mites, ticks), auto-immune diseases (e.g. Pemphigus foliaceus), foreign body (e.g. grass awn), keratinization defects (e.g. idiopathic seborrhea, hypothyroidism), neoplasia, and most importantly allergies, are responsible for directly inducing an otitis. Ear conformation and breed (e.g. Cocker Spaniel, Shar Pei), environmental factors (e.g. increased humidity), or excessive ear cleaning are considered predisposing factors. Secondary or perpetuating factors such as irreversible anatomical changes, otitis media and especially infections with bacteria and Malassezia will lead to chronicity.

Inflammation within the ear canal, caused for example by trauma or hypersensitivity, is probably the most important process in the pathomechanism of otitis. This is characterized by increased humidity, temperature, pH and edema, as well as apocrine hyperplasia, fibrosis, and hyperkeratosis, resulting in stenosis, decreased ventilation and perfect environment for secondary infections.

Common reasons for failure to successfully managing otitis externa are multifaceted, and may include: too much focus on symptomatic therapy without indentifying underlying problems, insufficient ear cleaning, resistant bacteria and poor antibiotic selection, side effects, treatment duration and treatment discontinuation without follow-up exams.

Otitis work-up

Before treatment can be initiated it is important to obtain a thorough history and to perform an in-depth ear examination. However, it is not unusual that the tympanic membrane cannot be assessed at the initial visit due to the debris and pathological changes (e.g. pain, stenosis) within the ear canal. Before affected ears are flushed samples for ear cytology and eventually culture should be obtained to determine what organisms are involved and to pick the right treatment. It is important to remember that cytology is much more reliable for the quantitative assessment of the microorganisms involved than bacterial cultures. However bacterial cultures are valuable in cases of otitis which will be treated with oral antibiotics.

Ear flushing

The main purpose of a thorough ear flush is the removal of excessive cerumen, foreign body material, microorganism and their toxins, and inflammatory mediators. The goal should be to obtain a clean ear canal so that topical ear medication is able to directly interact with the epithelium. Treating ear infections without removing the excessive debris will often result in treatment failures. Initial ear cleaning should be performed by the veterinarian, under heavy sedation or even general anesthesia, because affected ears are commonly painful, irritated and the debris may sit deep in the horizontal ear canal. Severely inflamed, swollen and painful ear canals are difficult to assess and even under full anesthesia a proper cleaning and evaluation may not be possible. I such cases it is recommended to postpone the procedure and to treat the patient with oral steroids (e.g. 1mg/kg every 12-24 hours for 3-7 days). This will allow the ear canal to open up, to reduce cerumen formation and inflammation and to evaluate the condition of the tympanic membrane. The owner should be advised before the procedure that a fragile intact ear drum may rupture and that temporary side effects, such as head tilt, nystagmus, and deafness may occur.

Because many topical ear products contain potentially ototoxic compounds, the use of them without knowing whether the ear drum is intact or not is discussed controversially among veterinarians. However, using an effective ear cleaner will help with loosing up debris resulting in shorter ear flushing time and so irritation. To minimize potential ototoxic effect the ear canals should be flushed thoroughly with saline after using an ear cleaner.

Many ear cleaners are available, but their efficacy, especially regarding their cerumenolytic activity, varies. The following cerumenolytic components can be found in ear cleaners.

     - Organic oils

     - Solvents: propylene glycol (Cerulytic; Virbac), lanolin, glycerin, squalene (e.g. Cerumene; Evsco), butylated hydroxytoluene, cocamidopropyl betaine (e.g. OtiFoam; DVM Pharm or Tricide; Molecular Therapeutics), isopropanol (Nolvasan Otic; Fort Dodge), mineral oil

     - Surfactants: dioctyl sodium, sulfosuccinate, calcium sulfosuccinate

For a thorough deep ear flush immobilize the patient and instill 1-3ml of the ear cleaner into the affected ear. The solution should stay in contact for 2-5 minutes with the ear epithelium and the ear canal should be gently massaged. To flush the ear canal a new red rubber feeding tube or TomCat catheter attached to a syringe can be used while visualizing the ear canal with an otoscope. The tube should be trimmed to a length of 4-6 inches. Saline should be used to flush the ear canal until the ear cleaner solution and debris is removed. Once removed the tympanic membrane area should be assessed. In situations where the ear drum is still not visible due to marked stenosis or complete rupture, indirect assessment can be made by hitting the bony structure of the middle ear bulla with the tip of the tube or catheter, observing the tube tip disappearing from view, and the patient swallowing after instilling large amount of fluid into the ear canal.

The use of ear cleaner at home depends on the severity of cerumen production and consistency, amount of purulent discharge, chronicity, and the presence of bacteria and yeast. If infection and inflammation is present a topical ear medication should be prescribed and the ear cleaner used as an adjunct treatment. Various ear cleaner for the at home use are available and should be selected carefully and not used excessively in sensitive ears. For maintenance ear cleaning solution should be only used every 7-14 days.

Topical ear therapy

Most of the time otitis externa can be managed successfully with topical therapy. Many commercially available topical ear drugs contain a combination of the following active ingredients:

     - Anti-inflammatory properties: hydrocortisone, betamethasone, dexamethasone, mometasone, triamcinolone

     - Antibacterial properties: gentamicin, neomycin, chloramphenicol, tobramycin, enrofloxacin, acetic acid, polymyxin B sulfate, silver sulfadiazine, TrisEDTA, chlorhexidine, amikacin

     - Antifungal properties: clotrimazole, miconazole, nystatin, zinc undecylenate, thiabendazole, acetic acid

     - Antiparasitic properties: pyrethrin, rotenone, thiabendazole, fipronil (Frontline), amitraz (Promeris), ivermectin, moxidectin (Advantage Multi)

     - Potentiating properties: Tris-EDTA (chelating agent).

Commercially available topical ear drugs

Topical application of ear medication has the following advantage:

     - achieves a much higher local drug concentration than systemic therapy

     - systemic side effects are minimal. Hydrocortisone for example is a very potent steroid within the skin, but is rapidly metabolized into less potent compounds before reaching the blood stream

     - faster response

Besides the amount of debris and pus, which can inactive or reduce the effect of ear medication, the volume of the topically applied drug determines the efficacy of the treatment. It has been suggested that a volume of 0.5-1ml may be necessary to cover the entire epithelium. Considering that 1 drop is ~0.025ml, it is possible that some dogs may not be treated with an appropriate amount of topical ear drugs.

Ototoxicity may be a concern when topicals are used in the management of otitis externa with a ruptured ear drum. Damage to the vestibular system has been reported and depending on the severity mild signs of such problems may go undetected. However, ototoxicity is observed rarely despite frequent local therapy and damaged tympanic membranes. The most important compounds which have been associated with ototoxicity are: aminoglycosides (except gentamicin), propylene glycol, and chlorhexidine.

After initiating therapy the patient must be re-evaluated every 2-3 weeks to repeat ear cytology and assess the ear canal and tympanic membrane, until the infection is cleared and the lesions healed. If underlying problems such as allergies are present therapy may be necessary for an extended period of time until the underlying causing disease has been identified and corrected.