A fixed drug eruption is an exclusively drug-induced cutaneous reaction. The lesions develop usually less than two days after the drug intake. Clinically, they are characterized by a solitary or few, round, sharply demarcated erythematous and oedematous plaques, sometimes with a central blister (Figure 34.7, between pp. 426 and 427). The eruption can be located on every site of the body and may involve mucous membranes, principally the lips and genitalia. The eruption progressively fades in a few days, to leave a post-inflammatory brown pigmentation. With rechallenge with the causative drug, the lesions recur at exactly the same sites. After several relapses the eruption may involve large areas of the body. This Generalized Fixed Drug Eruption may be difficult to distinguish from TEN.
Histopathology reveals a superficial and deep dermal and perivascular infiltrate (composed of lymphocytes, eosinophils, and sometimes neutro-phils) associated with necrotic keratinocytes. Dermal macrophages pigmented by melanin
(melanophages) when present are considered an important clue to the diagnosis.
The drugs most frequently associated with fixed drug eruption are phenazone derivates, barbiturates, tetracycline, sulphonamides, and carbama-zepine (Kauppinen and Stubb, 1984).
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