Physical urticarias.

Chronic urticaria can be produced by a number of stimuli that cause mast cell mediator release. Patients with urticaria caused by physical agents account for roughly one-fifth of all cases of chronic urticaria. There are about 20 different types of physical urticaria. Two forms, dermographism and cholinergic urticaria, are quite common and represent more than two-thirds of all cases of physically caused urticaria. More than one agent may precipitate urticaria in a given individual. Urticarial response can be easily reproduced in the sensitive patient and, generally, lasts less than one hour. Systemic features such as flushing, dizziness, headaches, and even hypotension, may occur during severe episodes. Identification of the causative physical agent is necessary for effective therapy.