Facial contact dermatitis due to metronidazole

Allergic contact dermatitis to topical antibiotics: Epidemiology, responsible allergens, and management

Topical antibiotics are widely used to treat cutaneous, ocular, and otic infections. Allergic contact dermatitis to topical antibiotics is a rare but well-documented side effect, especially in at-risk populations. The purpose of this article is to review the epidemiology, responsible allergens, and management of allergic contact dermatitis to topical antibiotics.

LEARNING OBJECTIVE

After completing this learning activity, participants should be able to describe the epidemiology of allergic contact dermatitis related to topical antibiotics; show knowledge of the most common allergenic topical antibiotics; and understand the allergenic cross-reactivity pattern amongst topical antibiotics.

Rosacea and contact allergy to cosmetics and topical medicaments - retrospective analysis of multicentre surveillance data 1995-2002

The role of contact allergy in rosacea has rarely been investigated. In this retrospective study, 361 out of 76,697 patients tested and documented by the Information Network of Departments of Dermatology between 1995 and 2002 had rosacea. Patch tests included standard series and constituents of cosmetics and topical medicaments. 118/361 had additionally been patch tested with their own cosmetics/medicaments. Positive reactions occurred to nickel (II) sulfate in 9.3%, fragrance mix in 8.8%, thimerosal in 6.9%, Myroxylon pereirae resin in 5.9%, potassium dichromate in 4.6% and propolis in 2.8%. Whereas rosacea patients had a significantly higher risk of contact allergy to propolis compared to the remaining patients, in an age- and sex-adjusted analysis, contact allergy to nickel was significantly less frequent in this group. For Lyral, the risk was elevated, albeit not significantly. Only 2/329 patients were positive to neomycin sulfate and 1/100 to gentamicin sulfate, among the panel of (topical) antibiotics tested. Among 118 patients tested with their own products, 3 were tested to metronidazole, 1 reacting positively. Irritant or doubtful patch test reactions were provoked by various substances (vehicles, oxidants and preservatives of various creams), which might also be clinically important, considering the heightened sensitivity of rosaceous skin.

Co-existing sensitivity to metronidazole and isothiazolinone

A 68-year-old woman developed allergic contact dermatitis to topical metronidazole gel as proven by positive patch tests to the gel and to metronidazole. She was also allergic to methylchloroisothiazolinone and methylisothiazolinone (MC/MI). The similarity between the two molecules and the fact that the patient reacted to the gel after the very short incubation period of 1 day (i.e. not long enough for acquiring an active sensitization) makes the possibility of a cross-reaction between these substances very plausible. As the isothiazolinones are widely used and comprise an important and relatively frequent cause of allergic contact dermatitis, a cross-reactivity with metronidazole means that perhaps there should have been more cases of metronidazole allergy is more common than the current literature suggests.