ACUTE URTICARIA IN INFANCY

Urticaria, or hives, is a common disorder in the young child. The characteristic clinical finding in urticaria is a well-circumscribed, raised, erythematous plaque, which is typically evanescent and frequently shows central pallor. The lesions are usually very pruritic. The mast cell is the key cell in urticaria. Its activation causes the release of vasoactive mediators, predominantly histamine, leading to oedema in the superficial dermis. There are many causes of urticaria. However, in many cases the cause remains unknown. The causes of urticaria can be classified as IgE-mediated hypersensitivity reactions, chemically induced mast-cell degranulation, physical urticaria, arachidonic acid metabolism and complement-mediated reactions (Table I). It is important to differentiate urticaria from erythema multiforme, which shows epidermal necrosis, with the characteristic target-like appearance. While urticaria and angio-oedema (in which the oedema is largely in the subcutaneous tissue) are clinically distinct entities, they may be seen in the same patient, either concurrently or at different times during the illness. Urticaria is defined as ‘acute urticaria’ if it lasts for less than 6 weeks and ‘chronic urticaria’ if it lasts for more than 6 weeks.