Clinical features
The attack often starts without warning with the skin becoming hot and flushed with marked local itching. The weals develop and spread with a surrounding flare and sometimes blanching of the skin around margins of the rash.
The weals can vary in size from a few millimetres to several centimetres in diameter and may merge into continuous areas covering a large proportion of a limb, the trunk or the abdomen. They are often circular but may be irregular in shape. Often there is blanching of the skin in the centre area of the weal. The weals eventually flatten and the rash fades over 12 to 24 hours but may persist for a few days. The eruption may recur in other areas. Recurring episodes would not be expected to last for more than six weeks in acute urticaria.
Angio-oedema is a more severe effect associated with urticaria where swelling occurs in the deeper tissues as well as the surface of the skin. In more severe reactions, often as seen in nut allergies, other food allergies or bee stings, the swelling can occur in the area of the face, mouth or throat and can cause obstruction of the air passages with resulting difficulty in breathing. At worst it can lead to anaphylactic shock and circulatory collapse.
