Urticarial rashes

Contributed by Dr Jean Watkins, a sessional GP in Hampshire

Acute ordinary urticaria

Weals appear spontaneously anywhere on the body as the result of a stimulus that causes degranulation of mast cells and the release of histamine. The rash may persist for a few minutes or some hours.

There may or may not be associated areas of angioedema.

Urticarial rashes tend to occur more frequently in atopic patients. Aggravating factors such as sun, heat, cold, scratching or certain drugs may be identified.

Treatment with antihistamines and topical corticosteroids will usually be sufficient. Oral corticosteroids should be reserved for more severe cases.

Pruritic urticaria of pregnancy

It develops in the third trimester and is thought to be an allergic response to stretch marks. It is more common in a first pregnancy, with large babies or multiples when the stretching of the skin is greater.

It starts around the umbilicus and may spread to the thighs and buttocks. The rash of small, pink papules is very itchy. The papules coalesce to form larger urticarial plaques. It persists until after delivery when it gradually fades.

Treatment is symptomatic only with emollients, topical steroids and an oral antihistamine if necessary.


The rash appears as very itchy, smooth blotches, blisters or weals that vary in size and shape. Sometimes they may be pale or red at the edges with central pallor. Investigations are not normally necessary for acute ordinary urticaria.

In more severe cases that fail to respond to treatment, FBC, ESR, thyroid auto-antibodies and TFTs should help to identify an autoimmune aetiology, an eosinophilia in a helminth infection of the bowel or systemic lupus erythematosis that could be underlying the problem. However, often a cause is not found (idiopathic urticaria).

Allergic contact urticaria

In acute urticaria, investigations are usually unnecessary unless a particular allergy is suspected. The reaction is usually limited to the area of contact and tends to develop some hours later. It will subside within a few days, providing the contact is not continued.

This patient had a reaction to latex gloves but possible allergens include white flour, cosmetics, meat, fish or vegetables. The reaction may remain localised or may progress to anaphylaxis in highly sensitised patients. If necessary, skin prick tests will confirm the allergen or related substances to be avoided.

Child: Urticaria pigmentosa

Urticaria pigmentosa is uncommon. As the result of a gene mutation there is an abnormal proliferation of histamine-releasing mast cells. It often starts in the first few months of life.

The child presents with widespread reddish-brown macules that swell and become red when rubbed, especially after a hot bath and vigorous drying (Darier sign). Often the lesions are mistaken for insect bites. The child may become irritable. By the teens, most are completely clear of the rash.

Antihistamines or corticosteroids may be offered if the condition is troublesome.

Adult: Urticaria pigmentosa

In some patients urticaria pigmentosa will present for the first time in adult life. The macules urticate on rubbing and the patient may find them itchy and unsightly.

Unlike in children, the condition is likely to persist indefinitely. Lesions may also accumulate in other organs such as the liver, spleen, lymph nodes, bone or GI tract. It is then known as systemic mastocytosis. Patients may be unwell with fever, weight loss or diarrhoea.

Treatment is the same as for children with the addition of PUVA. Interferon or imatinib may help in severe cases.

Papular urticaria

Papular urticaria is a common reaction to insect bites. The initial bite may pass unnoticed, until the patient develops crops of itchy, red spots that blister.

The legs are commonly affected, as in this patient who had been out for a walk in the country a few days before. Not everyone will have the same reaction.

A topical steroid cream should ease the irritation, together with an oral antihistamine if the itching is severe. An antiseptic cream may help if secondary infection is a problem and a topical or oral antibiotic if necessary.

Erythema annulare centrifugum

Erythema annulare centrifugum is a rare condition and may occur at any age. It mostly appears on the legs but may affect the arms, face and trunk.

The rash is a non-scaling, annular or arcuate, raised, erythematous rash with central clearing. The lesions may be up to 8cm in size. It is sometimes associated with infections, systemic illness, malignancy or certain drugs.

Spontaneous resolution can be expected. Topical corticosteroids may reduce the inflammation and ease the itching.

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