Otitis in cats: what is different from dogs (Proceedings)


Otitis in cats: what is different from dogs (Proceedings)

Nov 01, 2010

Anatomic Differences

The cat has some important anatomic differences from the dog. The obvious is that there is less breed variation in pinna shape and conformation as well as relatively short and straighter ear canal. The manubrium is also less curved than in the dog. The most clinically relevant differences related to treatment of otitis are in the middle ear where the cat has a much different ventral tympanic bulla. The cat's ventral bulla is divided by an incomplete septum the divides it into two communicating compartments. Dorsally the compartment is more lateral and ventrally more medial. A branch of the sympathetic nerve runs in this septum and when damaged results in Horner's syndrome. This septum is more readily damaged when flushing, cleaning or using instruments in the middle ear, which explains why Horner's syndrome is a much greater risk of complication in cats with otitis. In addition the septum it is somewhat concaved saucer shaped with the convex surface facing ventrally. Any fluid reaching the ventral compartment will have a much more difficult time being removed by positional changes or through the auditory canal. It is important to realize once medication reaches the middle ear it is more likely to stay and if irritating more likely to cause damage. As such it is wise to be very cautious when cleaning cat middle ears and avoiding harsh cleaning ingredients such as dioctyl sulfo succinate and ototoxic drugs.

Etiologic Differences

The incidence of ear disease in the cat is reported as 2-6%, much lower than in the dog. Though no studies have compared chronic otitis I would speculate that is even much lower compared to the incidence in dogs. The relative importance of Otodectes is also greater in cats than dogs which likely contribute to this observation. In a study of the ten most common feline skin disease at a University Otodectes was the fourth most common with only flea related disease and dermatophytosis more prevalent.In addition non Otodectes otitis externa was roughly one fourth the frequency. The low incidence in cats likely reflects that cats are not affected with many of the chronic skin diseases, such as hypothyroidism and hereditary based keratinization/epithelialization disorders, which often affect dogs. Even allergy is much less common in cats than dogs and when cats do have allergy the ear is not affected in the majority of cases. Why this difference between species is seen is unknown.

What is even more unusual in its difference from dogs is the role of secondary infections and perpetuating factors. Secondary bacterial otitis plays a relatively minor role. In one study the most common bacterial pathogen isolated in 31 of 75 ears was coagulase neg Staphylococcus while Pseudomonas was not a problem. Referrals for Pseudomonas otitis in cats are exceedingly rare in my practice and we actually see more otitis media from Malassezia than bacteria alone in my practice. Several other studies have shown Malassezia to be a significant problem in feline otitis. FADDIN EN.CITE One study cultured Malassezia from 63 of 99 cases and 33 had only Malassezia cultured. When antibiotics are needed then clavamox and clindamycin generally are preferred in cats as compared to dogs.

Demodex mites may be found in cats with otitis as well in cases with just mild excessive cerumen build up. When no otitis is present and it is found in just mild ceruminous build up then no treatment is necessary. In some cases with otitis the otitis will not respond until the ears have been treated with lime sulfur. Inflammatory PolypsInflammatory polyps are non neoplastic masses of epithelial tissue, fibroblasts, macrophages, lymphocytes, plasma cells and neutrophils. The variety of cells makes it easy to differentiate from neoplastic tissue with cytologic evaluation of a fine needle aspirate. It is believed they most often arise from the mucosa of the middle ear or auditory tube. The exact cause is unknown though in some cases chronic inflammation from chronic otitis or upper airway disease is believed to be a factor. Cases often occur that had no prior otitis or upper respiratory symptoms. Though one study found no evidence of a viral etiology a role for virus as a cause or triggering factor has not been ruled out. Cats are more commonly affected by inflammatory polyps than dogs. They tend to arise from a small pedicle but enlarge and then can extend into the external ear canal or pharynx. They occur more in young cats, though may be found from a few weeks to 15 years of age.

Clinical finding may relate to otitis externa/media or upper respiratory disease. Chronic or recurrent otitis with head shaking, aural pruritus, discharge, nystagmus, Horners syndrome, head tilt may be seen. Though unilateral disease is more common some cases may have bilateral ear disease and bilateral polyps. Some cats may have difficulty swallowing and one cat had secondary pulmonary hypertension. In many cases the masses are visible in the ear canal or behind or under the soft palate. One recent study revealed 38% of cats are deaf in the affected ear and this appears to be sensory deafness not conductive hearing loss.

Presently, there are 2 main treatment options for removal of inflammatory (nasopharyngeal) polyps: traction avulsion and surgery usually by a ventral bulla osteotomy. Traction avulsion is believed to have a higher recurrence rate. Though studies have not been done the recurrence rate may be affected by how the avulsion occurs and post avulsion treatment with systemic glucocorticoids. When the ear canal contains the mass it is important to grasp the mass far down the stalk and try not to tear in into pieces but keep the mass intact. Twisting the base while applying slow even traction is helpful to remove the mass intact. After removal oral triamcinolone 0.1 to 0.2 mg/kg for four days then taper to eod is prescribed as well as topical ear drops with glucocorticoids, usually dexamethasone 0.1% and antimicrobials selected based on cytologic findings.