Hives are itchy red or white bumps on the skin. This itchy rash is also known as urticaria, or as nettle rash. In some cases this itchy rash is triggered by a physical stimulus. If this is the case, the condition is called inducible urticaria or physical urticaria. Examples of physical factors which can trigger hives include pressure, friction, sweating, cold, heat, sunlight and water. Treatments include avoiding the trigger (where possible), and antihistamines.
What are hives?
When you have hives (urticaria), you have an itchy rash caused by tiny amounts of fluid that leak from blood vessels just under the skin surface. Hives can be classed as follows:
- Acute - if it develops suddenly and lasts less than six weeks. Most cases last less than 48 hours. It can affect anyone at any age. Some people have repeated episodes of acute hives. See separate leaflet called Hives (Acute Urticaria) for more details.
- Chronic - if the rash persists longer than six weeks. (Chronic means persistent or ongoing.) This is uncommon. In most cases the cause is not known. See separate leaflet called Hives (Chronic Urticaria) for more details.
The inducible type of hives (described below) is triggered by a physical stimulus and can be either acute or chronic. Physical causes can result in repeated episodes of acute hives, or a persisting long-term condition.
What does the rash of hives look like?
The rash usually appears suddenly and can affect any area of skin. Small raised areas called weals (or wheals) develop on the skin. The weals look like mild blisters and they are itchy. Each weal is white or red and is usually surrounded by a small red area of skin which is called a flare. The weal and flare rash of hives looks similar to the rash caused by a nettle sting.
The weals are commonly 1-2 cm across but can vary in size. For example, in cholinergic hives (described below) they are much smaller. There may be just a few weals but sometimes many develop on the skin. Sometimes weals that are next to each other join together to form larger ones. The weals can be any shape but are often round. As a weal fades, the surrounding flare remains for a while. This makes the affected area of skin look blotchy and red. The blotches then fade gradually and the skin returns to normal. Each weal usually lasts less than 24 hours. Others may then appear elsewhere.
What causes hives?
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. It is not known what the trigger is in about half of cases.
Some known triggers include:
- A physical stimulus. Where this is the case the condition is called inducible urticaria. In this type of hives, a rash appears when the skin is physically stimulated (for example, by heat, cold or pressure).
- 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 some substances causes a local area of contact hives in some people. For example, chemicals, latex, cosmetics, plants, ointments, nettle stings, etc.
Pine tree contact:
The rest of this leaflet deals only with inducible urticaria.
Different types of inducible urticaria
Hives can be triggered by a number of different physical causes. The reason why a rash appears in affected people is not clear. The physical stimulus somehow causes a release of histamine and other chemicals, which causes the rash.
The main types of inducible urticaria include the following:
Dermatographism (also known as dermographism and dermatographia) means skin writing. People with this condition develop the rash on areas of skin that have been firmly stroked. (Because of this, you could 'write' on someone's skin by applying firm pressure with a finger or other object. The rash follows the line of the writing on the skin.) The affected area of skin is usually very itchy but in some mild cases it does not itch much. Although any part of the skin can be affected, the palms, soles of the feet, genital skin and scalp are less commonly affected. Dermatographism is more prone to occur when you are hot. For example, it may develop more easily when you rub yourself firmly with a towel after a hot shower. The rash tends to last less than an hour. In many cases the pressure needed to be applied to cause the rash is quite firm. However, some people develop dermatographism with just light pressure.
Up to 1 in 20 people will develop dermatographism at some stage of life. It most commonly first develops in early adult life. In most cases, the condition tends to improve gradually over a few years and it goes or becomes less severe. However, in some cases the condition remains troublesome for many years.
Cholinergic hives are quite common. The condition is caused by sweating and is sometimes called heat bumps. The hives rash is quite distinct as the weals are very small (2-3 mm) with a red flare around each one. The rash appears within a few minutes of sweating and tends to be worst on the chest, back and arms. The rash lasts from 30 minutes to an hour or more before fading away. Some people become slightly wheezy and short of breath for the duration of the rash.
The sweating that triggers the rash may be due to exercise, heat, high temperature (fever), emotion or eating spicy food. It can be a real nuisance when trying to exercise. In severe cases, hundreds of tiny weals develop when you run or do other types of exercise. Sometimes the tiny weals join together to form larger ones. Cholinergic hives most commonly first develop in early adult life. In many cases the condition tends to improve after a few years and it goes or becomes less severe. However, in some cases the condition remains troublesome for many years.
Cold hives are quite an uncommon condition. An urticarial rash develops after being exposed to cold, including rain, cold winds and cold water. It may be the cold that triggers the rash, or the re-warming of the skin after coming in from the cold. The rash affects the chilled parts of the skin. If a large area of skin has been chilled, the rash can be very extensive. For example, swimming in cold water may cause a widespread and severe rash over most of the body that can make you dizzy and faint. (For this reason, if you are known to develop cold hives, you should never go swimming alone.)
Delayed pressure hives
Delayed pressure hives are uncommon. The condition can develop alone but it commonly affects people who also have chronic hives. In this type of hives the rash develops 4-6 hours after the affected area of skin has had deep prolonged pressure applied. For example, after wearing a tight seatbelt, or wearing a tight watch strap, or after gripping a tool such as a screwdriver for a reasonable period of time. The rash can be painful and tends to last several hours, or even a day or so.
This is a rare condition in which an urticarial rash develops on skin exposed to sunlight.
Water contact (aquagenic) hives
In this rare condition, an urticarial rash develops on skin exposed to water of any temperature.
In this rare condition, the rash comes up after using vibrating tools.
What is the treatment for inducible urticaria?
Once you know what causes the rash, it may be possible to avoid situations that trigger it. For example, dermatographism can often be prevented by avoiding firm pressure against the skin as much as possible. In mild cases, no additional treatment may be needed.
Many types of hives are helped by taking antihistamine medicines. (However, these medicines may not help some cases of delayed pressure hives.) Antihistamines block the action of histamine which is involved in causing hives. The most commonly used antihistamines for hives are cetirizine, fexofenadine and loratadine. These do not usually cause drowsiness. Your doctor may advise doses which are higher than the usual recommended dose in order to control the rash. If the itch is making it difficult to sleep, sometimes an antihistamine which makes you sleepy can also be taken at night. Examples of antihistamines used in this way include chlorphenamine or hydroxyzine.
Antihistamines can be bought over-the-counter without a prescription. However, if you need to take them for more than a few days, or on a regular basis, it is best to see your doctor for further advice. Do not take more than the recommended dose without discussing it with your doctor. Antihistamines are not usually advised in pregnancy.
Some people take antihistamines now and then when a rash flares up. If you take an antihistamine as soon as the rash appears, the rash tends to fade more quickly than it would do normally. If the rash recurs frequently then your doctor may advise a regular dose of an antihistamine to prevent the rash from occurring. If you have cholinergic urticaria and know that you get a rash on exercise, a dose of an antihistamine just before you do exercise may help to prevent or limit a flare-up of the rash.
Unlike most other types of hives, those caused by a physical stimulus are not usually helped by steroids. Occasionally for severe flare-ups of hives, a course of prednisolone tablets taken for a week may be helpful.
If hives are not controlled by avoiding known triggers and/or antihistamines you may be referred to a skin specialist (dermatologist). Occasionally other treatments may be used, such as a tablet called montelukast, which is usually used for asthma. This may be particularly useful in delayed pressure urticaria, where antihistamines often do not help. Omalizumab is a newer medication which acts against autoantibodies produced by the body's own immune system. It has to be given by injection once a month. This has been shown to be effective for some types of inducible urticaria in some studies. In the UK the National Institute for Health and Care Excellence (NICE) so far has not recommended its use for this particular type of hives.
Treatment with ultraviolet light (phototherapy) can cause an improvement in symptoms but, unfortunately, this may only last for a few months.
What is the outlook (prognosis)?
Inducible urticaria most commonly occurs in healthy young adults. You can have two or more different types of hives at the same time. As a rule, hives triggered by physical stimuli tend to improve and become less troublesome with time (often going, or being much less troublesome, after 2-5 years).
However, it is not possible to predict for an individual how long the problem will last and, in some cases, the condition lasts many years. In many cases, by avoiding the trigger and/or by taking antihistamines, the condition can be controlled.
Further help & information
Dr Tim Kenny
Dr Mary Harding
Dr Helen Huins