What causes Urticaria?
Acute Urticaria
The skin reaction in acute urticaria is probably due to the release of a mediator substance called histamine produced by mast cells in the body in response to the action of an agent that triggers an allergic reaction. This reaction can result from allergy to foods, medicines, certain infections, irritants, physical agents and insect bites or stings. However in around 50% of cases the trigger cannot be determined. The duration of the reaction to histamine injected into the skin is very short-lived and other chemical mediators in addition to histamine appear to be involved in maintaining the reaction in urticaria.
Common triggers are:
- Medicines - antibiotics and anti-inflammatory drugs such as aspirin and non-steroidal anti-inflammatory drugs (NSAIDS)
- Foods - such as nuts, eggs, chocolate, citrus fruit
- Infections - colds and flu, glandular fever, hepatitis and intestinal parasites
- Physical triggers - scratching or stroking the skin (dermographia), irritants such as latex, plants, chemicals and insect bites; exposure to cold, exercise, emotion and sunlight
Chronic Urticaria
Chronic urticaria is rarely activated by trigger factors and seems to be due to an abnormal immune response within the body where normal antibodies inappropriately react to cause release of histamine and other mediators. The underlying cause of this can rarely be found.
There are several other manifestations of urticaria:
Cold urticaria occurs with sudden drop in temperature such as on exposure to cold wind, jumping into cold water, or handling ice or snow. It occurs in two forms: -
- Familial cold urticaria is an inherited condition that usually presents in infancy with the rash appearing a short time after exposure to cold. It is usually accompanied by fever and joint pains.
- Acquired cold urticaria develops spontaneously for no apparent reason. The rash presents within minutes of exposure to sudden cold and can be a cause of sudden death for example when plunging into ice-cold water.
Heat urticaria is a rare type when the typical rash appears some hours after exposure to a hot environment.
Cholinergic urticaria tends to occur, like blushing, in adolescents and young adults and may be associated with emotion, exercise or a hot bath. The cause is not known and it is usually short-lived, lasting only a few minutes.
Pressure urticaria tends to occur in susceptible individuals at sites of pressure on the skin and may appear under a shoulder strap of a heavy bag or under the line of a tight belt. It can also appear on the soles of the feet.
Solar urticaria is a rare form where the typical rash appears within a few minutes of exposure of the sun and precedes any erythema due to sunburn. Tolerance to the effect may develop in areas continually exposed to light such as the face and hands but in sensitive areas the effect can occur even on cloudy days or when sunscreens have been used. A similar effect can occur due to photosensitisation of the skin caused by some medication and certain diseases.
Papular urticaria is a local reaction of one or more small weals, to an insect bite such as that of a flea, a midge or a mosquito.
