Urticaria refers to a group of disorders affecting adults and children, in which red patches and weals occur in the skin. A weal is a swelling of the surface skin. It may be spelled 'wheal'.

The skin swelling seen in urticaria is due to the release of chemicals such as histamine from mast cells and basophils in the skin, which causes small blood vessels to leak. The weals can be a few millimetres or several centimetres in diameter, coloured white or red, often surrounded by a red flare, and frequently itchy. Each weal may last a few minutes or several hours, and may change shape. Weals may be round, or form rings, a map-like pattern or giant patches.

About 40 percent of people with chronic hives also have angioedema. Signs and symptoms of angioedema include large welts or swelling of the skin that may occur around the eyes and lips, hands, feet, genitalia, and inside the throat. Swelling in the throat can obstruct breathing and requires emergency treatment. Angioedema may itch less than hives do, but can cause pain or burning.

Symptoms may not occur all the time. They may come and go with no apparent trigger. For some people, certain conditions — such as heat, exertion or stress — can make symptoms worse.

Acute urticaria is sometimes due to allergy. Allergy depends on previous exposure to the material, and the development of an immune reaction to it. An immunoglobulin called IgE is involved, which attaches itself to a receptor on the mast cell and causes it to release its chemical mediators.

The cause of an allergic urticaria may be:

• Drug eruption: most often an antibiotic, but many other drugs have been reported as well as vaccines.
• Food allergy: Fish, eggs, nuts, vegetables, fruits, Food preservatives etc can cause urticaria

Non-allergic causes of acute urticaria include:

• Infection, including sinusitis, Helicobacter (a cause of stomach ulcers), Viral hepatitis, Mycoplasma penumonia, Infectious mononuclosis and Candida (thrush).

Chronic urticaria is often due to autoimmune disease (allergy to one's self), and may be associated with other autoimmune conditions such as thyroid disease and coeliac disease. Circulating ‘anti-idiotypic’ antibodies can be detected in 10% of patients with chronic urticaria. These activate IgE bound on mast cells to cause excessive release of chemicals.

Recurrent angioedema without urticaria may be due to C1 esterase deficiency (the protein C1 INH is missing or abnormal); there is often a family history of similar problems. It may also be caused by angiotensin converting enzyme (ACE) inhibitors such as captopril, quinapril, enalapril and others, which are used to treat heart failure and hypertension. These drugs inhibit kinin breakdown. Angioedema may also be idiopathic (of unknown cause).

Investigations in urticaria

Full blood count to identify eosinophilia caused by allergy or parasitic infestation, and low white blood count from systemic lupus erythematosus.
Thyroid antibodies and function in chronic urticaria if autoimmune origin is considered likely.
Skin prick testing and blood tests for specific allergy (RAST, or radiollergosorbent tests, or CAP fluoroimmunoassay).
Autologous serum skin prick test in chronic urticaria.

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