Non-allergic causes of acute urticaria include:
- Infection, including sinusitis, helicobacter (a cause of stomach ulcers), dental abscess, viral hepatitis, infectious mononuclosis and candida (thrush).
- Serum sickness, due to blood transfusion, viral infection or medicines (e.g. Ceclor™); urticaria is accompanied by fever, swollen lymph glands, painful joints and nausea. It is thought to result from immune complexes of the allergen and antibodies lodging in small blood vessels.
- Non-allergic release of mast-cell granules by medicines, especially morphine, codeine, other opiates, and radiocontrast agents. Urticaria provoked by aspirin and other non-steroidal anti-inflammatory drugs involves leukotriene formation.
- Non-allergic food reactions, from salicylates in fruit, azo dye food colouring agents, benzoate preservatives and other food additives, or from histamine due to bacterial decomposition e.g. scombroid fish poisoning.
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 activate IgE bound on mast cells to cause excessive release of chemicals. More commonly there is no evidence for autoantibodies, and the patient is said to have chronic idiopathic urticaria.
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).
Urticaria should be distinguished from urticarial vasculitis, in which weals persist for longer than 24 hours and vasculitis is found on skin biopsy. It results from immune complex deposition.
Physical urticaria refers to urticaria induced by external physical influences. The weals take about 5 minutes to develop, and last 15 to 30 minutes. Some people suffer from a mixture of different types of physical urticaria and generalized urticaria. The cause is unknown.
Dermographism means ‘skin writing’. Stroking the skin causes it to weal in the line of the stroke. This is very itchy, but scratching causes more wealing. Dermographism usually starts quite suddenly. Weals come up where clothes or furniture touch, especially when the affected person is hot or upset. A warm shower followed by rubbing with a towel can result in itchy weals all over.
Cholinergic urticaria results from sweating. In severe cases, hundreds of tiny red itchy spots develop after running, when warm, or when concentrating.
Cold urticaria affects skin warming up after a reduction in temperature, especially in winter. Weals can be widespread and may cause fainting attacks. Affected individuals should not expose large areas of the skin to the cold or wind. They should be advised never to swim alone.
Contact urticaria results from absorption of an elicting substance through the skin or through a mucous membrane. It may be allergic or non-allergic in origin. It may result in wealing confined to the site of contact or spreading more widely. IgE antibodies on mast cells react to chemicals in white flour, cosmetics, and textiles, or to proteins in latex rubber, saliva, meat, fish and vegetables may cause contact urticaria. Non-allergic examples include the stinging reaction of certain plants (e.g. nettles), animals (hairy caterpillar) and medicines.
Localised heat urticaria, aquagenic urticaria (water contact), solar urticaria (sunlight), vibratory angioedema and delayed pressure urticaria are less common.
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