CHAPTER 68 ■ RASH: DRUG ERUPTIONS MELINDA V JEN DRUG ERUPTIONS The spectrum of cutaneous drug eruptions ranges from the relatively benign, where the medication can be continued if essential, to the severe, where there can be significant morbidity and mortality Thus, prompt and accurate diagnosis is critical and can be lifesaving The primary morphology of the eruption helps guide the clinician to a diagnosis Herein, we summarize the most salient features of the most common drug reactions, with a particular focus upon their primary morphologies URTICARIA Urticaria (hives, wheals) consists of erythematous, edematous papules and plaques that can coalesce into larger polycyclic, arcuate, and annular plaques ( Figs 68.1 and 68.2 ) A key diagnostic feature is that individual lesions are transient, resolving within 24 hours, but with new lesions appearing elsewhere As they resolve, purpuric macules secondary to capillary leak and hyperpigmentation may remain Pruritus and angioedema, particularly of the eyelids, hands, and feet, are common Urticaria results from IgE degranulation of mast cells Although the most common cause of urticaria is infection, medications can sometimes trigger urticaria Urticaria typically appears within the first weeks of starting the culprit medication Cephalosporins, β-lactam antibiotics, sulfonamides, and anticonvulsants are common causes of drug-induced urticaria Some medications, such as nonsteroidal anti-inflammatory drugs (NSAIDs), may cause urticaria through both immunologic and nonimmunologic pathways (via increased leukotriene synthesis) Urticaria is often confused for erythema multiforme (EM) The key features differentiating urticaria from EM are morphology, individual lesion duration, symptomology, and distribution Urticaria can be annular, polycyclic, and arcuate, but does not have the classic target appearance of EM Additionally, urticaria does not vesiculate, while the central areas of EM lesions may be bullous As noted above, individual lesions of urticaria last less than 24 hours, while lesions of EM are fixed and take several days to resolve If symptomatic, urticaria is pruritic while EM may itch or burn Urticaria can occur anywhere on the body,