Mediator release in local heat urticaria: protection with combined H1 and H2 antagonists.

Two patients with local heat urticaria were examined for evidence of histamine release and for changes in mast cell morphology after local heat challenge. Both patients demonstrated significant release of histamine into the draining venous blood in the challenged arm. Biopsy specimens revealed mucosal edema after 30 min, followed by a mononuclear, perivascular infiltrate by 6 hr, and increasing in intensity by 24 hr. Degranulated mast cells were observed by electron microscopy. Heat challenge was repeated after treatment with hydroxyzine, cimetidine, or a combination of both drugs. Neither hydroxyzine nor cimetidine alone affected the clinical response to challenge, but both drugs together completely abolished the clinical response. In addition, the combination of both drugs taken regularly completely prevented symptoms. This observation suggests that histamine is one of the major mediators of this disorder and that both H1 and H2 receptors are operative.