Hives, urticaria, nettle rash or welts
Hives, also known as urticaria, welts or nettle rash, is an itchy, raised rash on the skin.
The hives usually appear suddenly, sometimes because of an allergic reaction or being stung by a nettle.
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Hives usually cause itching, but may also cause burning or stinging. They can appear anywhere on the body, including the face, lips, tongue, throat or ears. Hives vary in size (from a pencil rubber to a dinner plate), and may join together to form larger areas known as plaques. They usually last for less than 24 hours, but in some cases they can last for several days or months before fading.
What causes hives?
Hives form in response to histamine causing tissues to swell. Histamine and other chemicals cause blood vessels to dilate and become leaky allowing fluid from the blood to leak out of the blood vessels causing itchiness and swelling.
Allergic reactions, chemicals in foods, insect stings, sunlight exposure or medicines can all cause histamine release. Sometimes it's impossible to find out exactly why hives have formed.
There are several different types of hives including:
- Acute urticaria: Hives or swelling lasting less than six weeks. The most common causes are foods, medicines, latex or infections. Insect bites and internal disease may also be responsible. The most common foods that cause hives are nuts, chocolate, fish, tomatoes, eggs, fresh berries, soya, wheat and milk. Fresh foods cause hives more often than cooked foods. Certain food additives and preservatives may also be to blame. Medicines that can cause hives include aspirin and other non-steroidal anti-inflammatory medications such as ibuprofen, some high blood pressure medications and some painkillers such as codeine.
- Chronic urticaria: Hives or swelling lasting more than six weeks. The cause of this type of hives is usually more difficult to identify than those causing acute urticaria. The causes can be similar to those of acute urticaria but can also include autoimmunity, chronic infections, hormonal disorders and malignancy.
- Physical urticaria: Hives caused by direct physical stimulation of the skin - for example cold, heat, sun exposure, vibration, pressure, sweating and exercise. The hives usually occur directly where the skin was stimulated and rarely appear anywhere else. Most of the hives appear within one hour after exposure.
- Dermatographism: Hives that form after firmly stroking or scratching the skin. These hives can also occur along with other forms of urticaria.
How are hives diagnosed?
Your GP will need to ask you various questions in an attempt to find the possible cause of hives. Since there are no specific tests for hives, testing will depend on your medical history and a thorough examination by your GP, allergy specialist, immunologist or dermatologist.
Skin tests may be performed to determine the substance that you are allergic to. Routine blood tests are done to determine if a system-wide illness is present.