Allergic cutaneous reactions to drugs

Because the pathophysiology of many drug eruptions is unknown, the presumption that a drug eruption is due to immune mechanisms is often based on clinical features. The drug exanthem, urticaria, and contact dermatitis are the most common adverse cutaneous reactions to medications. Drug exanthems occur in 2 to 3 per cent of medical inpatients and are most commonly caused by antibiotics and blood products. The incidence of drug exanthems is much higher in certain patient populations (for example, patients with AIDS treated with trimethoprim-sulfamethoxazole). Urticaria is the second most common allergic cutaneous reaction to drugs. Individual urticarial lesions last for less than 24 hours and do not leave hyperpigmentation or scarring. Urticaria not accompanied by systemic symptoms should not be treated with systemic corticosteroids or parenteral epinephrine. Allergic contact dermatitis is commonly caused by neomycin, benzocaine, ethylenediamine, diphenhydramine, and transdermal patches. The clinical spectrum of other, less common drug eruptions is wide. Toxic epidermal necrolysis, erythema multiforme, and fixed drug eruptions share similar pathologic features, are caused by many of the same drugs, and may have a similar pathogenesis. Photoallergic drug reactions require the interaction of drugs, UV irradiation, and the immune system. Drugs implicated in causing photoallergy include thiazide diuretics, sulfonamides, and phenothiazines. Eruptions seen in serum sickness include the drug exanthem, urticaria, vasculitis, urticarial vasculitis, and erythema multiforme. Identifying and discontinuing the responsible drug is usually essential for successful therapy of drug eruptions.