To the Editor: Food, medication or insect stings are the major causes of systemic allergic reactions.1 That topical agents can mimic such reactions is not commonly appreciated. I report here a systemic allergic reaction to a topical medication (initially attributed to food).

A 7-year-old boy experienced generalised urticaria and facial swelling within an hour of eating a peanut-containing slice. His father recalled applying a topical antiseptic (Medi Creme [Pharmacare]) to a graze over the boy’s right elbow at about the same time. There were no respiratory or cardiovascular symptoms, and the urticaria settled within 2 hours of taking oral promethazine.

Six months later, the same cream applied to a graze over the boy’s right chest resulted in a localised 15 cm urticarial welt. Intercurrent problems included atopic dermatitis but no known food or drug hypersensitivity.

The active ingredients of Medi Creme are hexamidine isethionate, chlorhexidine acetate, cetrimide and lignocaine hydrochloride. With the assistance of the manufacturer, skin prick tests using a 10% weight/volume suspension of Medi Creme or a 10% suspension of hexamidine isethionate in normal saline produced 5 mm itchy weals at 15 minutes in the patient (but not controls). By contrast, skin prick tests to the other active ingredients, inert vehicles and relevant foods (including peanut, almond, brazil nut, cashew, hazelnut, pecan, walnut, sunflower seed and sesame seed) were negative. Avoidance of hexamidine was advised. The child has eaten peanut products before and since without any adverse reaction.

Hexamidine is an aromatic diamidine antiseptic (other members of the group include pentamidine and dibrompropamidine). These drugs have broad antibacterial and antifungal properties and are also used topically to treat corneal infections and some skin infections.2 In Australia, hexamidine is an ingredient of one topical local anaesthetic/antiseptic cream (Medi Creme) and one nappy rash cream, as well as some tinea treatment creams, medicated shampoos, sunscreens and cosmetic facial wipes in other countries. Adverse reactions (such as contact allergic dermatitis and photodermatitis3) are rare — only four reports of localised dermatitis have been reported to Australia’s Adverse Drug Reactions Advisory Committee (ADRAC) in the past 6 years (Dr K Mackay, Acting Director, ADRAC, Adverse Drug Reactions Unit, Therapeutic Goods Administration, personal communication). There have been more reports of systemic allergic reactions (including anaphylaxis) triggered by chlorhexidine or cetrimide,4 with one description of anaphylaxis to hexamidine after patch testing, but none with clinical use.3 Underlying dermatitis is a risk factor for sensitisation to topical agents.5

This case emphasises the importance of documenting exposure to potential allergenic triggers in the setting of a short-lived episode of urticaria (where the search for an avoidable trigger is more likely to be productive) or anaphylaxis. Exposure to stinging insects is usually obvious, whereas exposure to particular foods or medications is often poorly recalled. That topical allergens can also trigger systemic reactions should be considered.