Cholinergic urticaria is a very common form of hives. It manifests as multiple, small, 2 to 3 mm red hives on the upper trunk and arms, although it can occur from the neck to the thighs. These hives appear in response to a rise in body temperature from exercise, overheating, or stress.
Usually the hives develop within 2 to 20 minutes after the body temperature rises, although they can take up to an hour to occur. They cause itching, tingling, burning, and warmth of the skin. The hives last for minutes to hours with an average of 30 minutes. It is believed that histamine is released in response to stimulation by the parasympathetic nervous system.
Cholinergic urticaria is diagnosed by history and reproducing the hives under certain conditions. Many times the patient is asked to exercise by jogging in place or riding a stationary bike and the time it takes for hives to develop is noted.
Cholinergic urticaria can be treated by limiting strenuous exercise. It responds well to a medication called hydroxyzine, which is an antihistamine. However, the main side effect of sleepiness is often not tolerated well. Showering with hot water may cause a release of histamine throughout the body depleting histamine stores and causing a 24-hour refractory period.