[Hypersensitivity reaction to allopurinol]

Major hypersensitivity reactions to allopurinol are rare. They are characterized by systemic vasculitis associated with a grave clinical picture. 20 days after beginning treatment with allopurinol, a 70-year-old patient presented with a maculo-papular, erythemato-squamous eruption which developed into erythroderma with fever, edema, polyadenopathy, marked eosinophilia, cholostatic jaudice, and aggravation of preexisting renal insufficiency. Skin biopsy showed vasculitis with fibrinoid necrosis and a chiefly lymphocytic infiltrate suggestive of a malignant lymphoma-type process. As soon as allopurinol was discontinued, and without steroid treatment, the patient spontaneously recovered. A lymphocyte transformation test was positive for this drug. The poorly know mechanism is immunological, with formation of immune complex deposits on the endothelial cells and at the dermo-epidermal junction, fibrinoid necrosis of small vessels and cellular reaction which is lymphocytic. It is not a toxic reaction related to the dose administered, though most authors have emphasized that preexisting renal insufficiency could favor hypersensitivity to allopurinol.