What is acute urticaria?
Urticaria (sometimes called hives) is an itchy rash caused by tiny amounts of fluid that leak from blood vessels just under the skin surface. Urticaria is classed as:
- Acute urticaria - if it develops suddenly and lasts less than six weeks. Most cases last 24-48 hours. About 1 in 6 people will have at least one bout of urticaria in their life. It can affect anyone at any age. Some people have recurring bouts of acute urticaria.
- Chronic urticaria - if it persists longer than six weeks. (Chronic means persistent or ongoing.) This is uncommon.
This rest of this leaflet deals only with acute urticaria. There is a separate leaflet for information about chronic urticaria.
What does the rash of acute urticaria look like?
Are there any other symptoms?
- Most people with acute urticaria do not feel 'ill', but the appearance of the rash and the itch can be troublesome.
- In some cases a condition called angio-oedema develops at the same time as urticaria. In this condition some fluid also leaks into deeper tissues under the skin which causes the tissues to swell.
- The swelling of angio-oedema can occur anywhere in the body but most commonly affects the eyelids, lips and genitals.
- Sometimes the tongue and throat are affected and become swollen. The swelling sometimes becomes bad enough to cause difficulty breathing.
- Symptoms of angio-oedema tend to last longer than urticarial weals. It may take up to three days for the swollen areas to subside and go.
- A variation called 'vasculitic urticaria' occurs in a small number of cases. In this condition the weals last more than 24 hours, they are often painful, may become dark red, and may leave a red pigmented mark on the skin when the weal goes.
What causes acute urticaria?
A 'trigger' causes cells in the skin to release chemicals such as histamine. The chemicals cause fluid to leak from tiny blood vessels under the skin surface. The fluid pools to form weals. The chemicals also cause the blood vessels to open wide (dilate) which causes the flare around the weals. The trigger is not identified in about half of cases. Some known triggers include:
- Allergies - for example:
- Food allergies such as allergies to nuts, strawberries, citrus fruit, egg, food additives, spices, chocolate, or shellfish. Sometimes you can develop an allergy to a food even if you have eaten it without any problem many times before.
- Allergies to insect bites and stings.
- Allergies to medicines such as penicillin, aspirin, anti-inflammatory painkillers, etc.
- A viral infection such as a cold or 'flu can trigger an urticarial rash in some people. (You 'react' to the virus.) A mild viral infection which causes few other symptoms is probably a common trigger of an urticarial rash that develops without an apparent cause.
- Skin contact with 'sensitizers' causes a local area of urticaria in some people. For example, chemicals, latex, cosmetics, plants, ointments, nettle stings, etc.
- Physical urticaria. This is when a localised rash appears when the the skin is physically stimulated. The most common is called dermographism when a rash develops over areas of skin which are firmly stroked. Sometimes an urticarial rash is caused by heat, cold, emotion, exercise, or strong sunlight. See separate leaflet called 'Physical Urticarias'.
Is acute urticaria serious?
- Food allergy. If a food allergy is the cause then the rash is likely to return each time you eat the particular food. This is more often a 'nuisance' than serious.
- Severe allergies. People who have a severe allergic reaction to peanuts, insect stings, etc, often have an urticarial rash as one of the symptoms. This is in addition to other symptoms such as severe angio-oedema, breathing difficulties, collapse, etc. A severe allergic reaction is called anaphylaxis. But note: most people with acute urticaria feel well and do not have anaphylaxis.
- Chronic urticaria. This means the rash keeps coming and going on most days for six weeks or longer. This is uncommon. See separate leaflet called 'Chronic Urticaria'.
What is the treatment for acute urticaria?
- Often no treatment is necessary as the rash commonly goes within 24-48 hours.
- A cool bath or shower may ease the itch.
- Antihistamine tablets can ease symptoms. Antihistamines block the action of histamine which is involved in causing urticaria. You can get antihistamines on prescription. You can also buy them from pharmacies. There are several brands. The pharmacist will advise.
- Modern brands usually work well and are unlikely to cause side-effects. They sometimes cause drowsiness - particularly if you drink alcohol.
- Older brands will often make you drowsy - but this effect may be welcome at bedtime if itch is troublesome.
- If you can identify a 'trigger' such as a food, then it would be sensible to avoid it in the future.
- A short course of steroid tablets is sometimes prescribed in severe cases to help reduce swelling in the skin.