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A wheal is an edematous papule that may enlarge to form a pink, sharply circumscribed, elevated plaque. The typical lesions of urticaria, pictured here, have a suggestion of central clearing. By definition, the individual lesions of urticaria evolve quickly and resolve within 24–48 hours. They are usually accompanied by severe pruritus. The most common etiologies of urticaria are medications, foods, and viral (Fig. 13-2) and bacterial infections. Autoimmune disease and malignancy are extremely rare causes. In the child with chronic urticaria, it is often difficult or impossible to identify a single cause.

This type of urticaria is causally related to elevation in body temperature and sweating. The lesions usually develop rapidly after the beginning of exercise and resolve rapidly and spontaneously with cooling. The urticaria that develops in this condition has a typical appearance, illustrated here. The lesions consist of red or flesh-colored papules 2–3 mm in diameter that are surrounded by an ill-defined erythematous macule. The use of antihistamines will decrease the frequency and severity of attacks. Doxepin may also be effective.

There are urticarias in which stroking, pressure, cold, heat, or sun exposure are causative. Figure 13-4 is whealing in linear form produced by stroking the skin with a degree of force that would ordinarily cause nothing more than transient erythema. The phenomenon, called dermographism, is present in a small percentage of normal individuals. Figure 13-5 is a huge wheal produced by resting an ice cube on the forearm. Cold-induced urticaria may be acquired or inherited. In the most common, acquired form, patients develop lesions shortly after ingesting cold foods or liquids or shortly after exposure to a drop in environmental temperature. Patients with this form of sensitivity are at risk for laryngeal edema or circulatory collapse as a result of significant cold exposure. Antihistamines or doxepin is of some help in preventing attacks. In a very rare syndrome, contact of the skin with water, without respect to its temperature, produces wheals (aquagenic urticaria).

This disorder is termed multiforme because the morphology of its lesions is so variable. Figure 13-6 is an overview of a severe case. The primary lesion is most often an erythematous macule that evolves into a papule. Early in the course, these lesions may easily be mistaken for urticaria. As the lesions enlarge, they form round or irregularly shaped plaques. The central area may blister or become dusky in color; this change represents the necrosis of keratinocytes in areas of active involvement (Fig. 13-7...

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