Author: Dr Amanda Oakley, Dermatologist, Hamilton, New Zealand, 2001.
A fixed drug eruption is an allergic reaction to a medicine that characteristically recurs in the same site or sites each time a particular drug is taken.
Exactly how a fixed drug eruption arises and why just certain areas of skin are affected, is unclear.
It is thought that an antigen from the drug activates cytotoxic T cells in the epidermis. These release cytokines (inflammatory mediators), such as interferon-γ, granzyme B, and perforin. The cytokines, with helper T cells and neutrophils, destroy the local skin cells (keratinocytes and melanocytes). The cytotoxic T cells then remain in the epidermis and release more cytokines when again exposed to the causative drug.
Fixed drug eruption presents as well defined, round or oval patches of redness and swelling of the skin, sometimes surmounted by a blister. This then fades to a purplish or brown colour and the blister shrinks and peels off. In mucosal sites (lips, vulva, penis), extensive ulceration can occur.
The hands and feet, lips, eyelids, genitalia and perianal areas are common sites.
The lesions usually develop within 30 minutes to 8 hours of taking the drug. They are sometimes solitary at first, but with repeated attacks, new lesions may appear, and existing ones may increase in size.
As healing occurs, crusting and scaling are followed by a persistent dusky brown colour at the site. This may fade but often persists between attacks. Pigmentation tends to be more extensive and pronounced in people with brown skin. Pigmentation from fixed drug eruptions fades when the causative drug is avoided.
Non-pigmenting fixed eruptions have been reported due to pseudoephedrine and piroxicam.
Local or general symptoms accompanying a fixed drug eruption are mild or absent.
Sometimes a patient with fixed drug eruption may present with multiple sites simultaneously in which there are bullae and erosions. These are often intertriginous sites. When extensive, the principal differential diagnosis is Stevens-Johnson syndrome / toxic epidermal necrolysis.
The number of drugs capable of causing fixed eruptions is large. Most reactions are due to the following medicines.
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