Patients suffering from chronic external otitis had developed contact allergies to one or more compounds of topical preparations in 40% of 142 tested patients. Neomycin and framycetin caused most of the allergic reactions (16.2%) followed by chinoform (7.0%), chloramphenicol and polymyxin (4.2%). Preservatives of the topical otic preparations such as benzethonium chloride (8.5%), benzalkonium chloride (6.3%) and thimerosal (merthiolate) (5.6%) were also common causes of allergic reactions. An epicutaneous test (patch test) using compounds in topical preparations should be done in cases of prolonged, treatment resistant external otitis.
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Fräki et al. (1985) studied this question.