Contact Dermatitis

Contact dermatitis is a common cause of skin lesions on the concave surface of the pinna, probably because it is hairless.⁹ Topical ear treatments are a common cause.

Figure 12-5

Erythema and Iichenification on the inner pinnae of a female Bichon Frise with food allergy.

Typically, the animal is treated topically for otitis externa; 1 to 3 days after the start of treatment, the condition worsens. Almost any treatment can cause an allergic reac­tion. In humans and animals, neomycin has been most commonly involved.^9,10 Propylene glycol is a very common vehicle of topical auricular medications. It is considered by Griffin to be the most common cause of contact dermatitis of the pinna.¹¹

Some ointments can be applied on the skin for transdermal absorption such as vasodilators for cardiac disease (e.g., nitroglycerine). The concave pinna is the usual site because it is glabrous and vascular and the animal cannot lick the product easily. The drug and its vehicle are potential causes of contact dermatitis. Erythema is often observed after repeated application of medications, especially if applied on the same pinna.¹² Recently, ear pinna transdermal methimazole application for hyper­thyroidism has been evaluated in cats.¹³ Contact dermatitis is also possible, so it is recommended that pinnal application be alternated.

In contact dermatitis associated with topical ear ointments, the concave surface of the pinna is erythematous and swollen. Erosion or even ulceration can be present, especially if an irritant product (diethyl ether, toluene, essential oil) has been applied. Pruritus is usually more severe, and ear scratching and rubbing cause alopecia and excoriation. In some cases the ear is painful. A definite diagnosis can rarely be achieved because drug challenge is usually not performed due to a possible worsening of the symptoms.

When symptomatic treatment is opted for, two points should be considered:

1. Steroids can be the cause of the adverse reaction.

2. The drug vehicle can also be implicated. Because it is identical in most ointments, change of drug does not always resolve the symptoms.

Ear flushing and pinna cleaning with saline (with or without systemic antiinflam­matory treatment) is a possible option.