Urticaria is characterised by a red, raised, itchy rash that appears on the skin. It is also known as hives, “nettle rash” or welts. The short-lived swellings of urticaria are known as wheals. The rash usually disappears within a few days, but sometimes it can last longer.
Urticaria is described as “acute” if it lasts less than 6 weeks and “chronic” if it persists beyond this. Urticaria is common. The acute form affects about 20% of people at some point in their lives. Chronic urticaria is much rarer. Urticaria happens when a trigger provokes a substance called histamine to be released in the skin. This substance is normally carried by cells in the skin called mast cells.
Acute continuous urticaria can last up to 6 weeks, beyond which it becomes chronic urticaria. If the urticaria becomes continuous (occurring at least 3 times a week) it is not likely to be caused by an allergy. Sudden development of acute continuous urticaria that lasts several days or weeks is commonly caused by a viral or bacterial infection. Acute episodic urticaria (single episode) is often brought by allergic triggers (e.g. foods, drugs, irritants).
It can be difficult to identify external causes for the common, “ordinary” form of urticaria. In some patients with ordinary chronic urticaria, the release of histamine from skin mast cells is triggered by factors circulating in the blood, such as antibodies directed against their own mast cells – a process known as autoimmunity.
Commonly, no obvious cause or trigger is found and the condition is known as idiopathic. Sometimes urticaria can be brought by physical findings. For example, if hives develop after cold exposure, this is called cold urticaria, sunlight (solar urticaria), deep pressure (delayed pressure urticaria), water (aquagenic urticaria), stress or heat (cholinergic urticaria), skin scratching (dermatographism) or chemicals such as latex (contact urticaria). These are all forms of physical urticaria.
The main symptom of urticaria is itchiness. The wheals are raised and may be flesh-coloured, pink or red. They can be of different shapes and sizes, but usually look like nettle stings.
Most episodes of urticaria peak between 8-12 hours and then disappear after 24 hours. New wheals may then appear in other areas. In ordinary urticaria, the wheals can occur anywhere on the body, at any time.
Approximately 50% of people with chronic urticaria and 25% of individuals with acute urticaria present with angioedema. Angioedema is a much more severe form of urticaria, causing large raised bumps under the surface of the skin as well as puffiness typically around the eyes, lips and sometimes the mouth