Urticaria is a pruritic rash involving the epidermis and upper portions of the dermis which results from localised capillary vasodilatation, followed by transudation of protein-rich fluid in the surrounding tissues.
Pathologically, there is an IgE-mediated hypersensitivity response with histamine release and, in some cases, complement-mediated reactions.
Urticaria affects people of all ages. About 20 per cent of the population will have at least one episode of urticaria during their lifetime. It occurs more commonly in atopic individuals.
Acute urticaria is more common in children and adolescents and accounts for 80 per cent of cases. The condition is reclassified as chronic urticaria after six weeks.
Chronic urticaria is more common in the elderly, and affects about 0.1 per cent of the UK population.
Urticaria classically presents as nettle rash: elevated red or white, non-pitting small plaques that change shape and size over time and which last for hours or days.
Less commonly, the rash is confluent with annular configurations and central pallor. Wheals usually last less than 24 hours and can be circular, linear or arcuate.
Urticaria can be migratory. Dermographism and angio-oedema may occur separately or together. Patients are systemically well with no fever.
Blisters do not occur.
There is a variety of reasons that could trigger urticaria (see box below).
The diagnosis is usually clear from the history and examination and absence of blisters. If there is purpura or wheals lasting over 24 hours, consider a skin biopsy to exclude vasculitis. Blisters suggest pemphigoid.
Acute contact eczema may appear similar to urticaria, but urticaria does not weep, blister or scale. Acute herpes and drug reactions should also be considered.
Routine investigations are usually not helpful. A full blood count may show eosinophilia secondary to parasite infection or atopy.
Consider an ESR, ANA, thyroid function tests and serum complement if vasculitis or urticaria associated with systemic disease is considered. Allergy testing is not helpful.
You should reassure the patient that urticaria is harmless.
Identify and remove the cause if possible, and avoid salicylate medications.
Oral antihistamines are the treatment of choice and they are safe for long-term use.
Chlorpheniramine is fast acting but has a sedative effect and lasts only a few hours; newer-generation once-daily non-sedating antihistamines are usually preferred.
Oral corticosteroids may be needed for a few days in severe cases (1-2mg/kg in children and 30-40mg/day in adults). Topical steroids are not effective and should not be used.
Tricyclic antidepressants used at night may help with sleep and break the cycle of itching and scratching.
Other therapies, for example H1-blockers such as ranitidine, may be useful adjuncts in severe cases.
Most acute urticaria clears within a few days. If urticaria becomes chronic, only 25 per cent of patients are symptom-free after five years.
However, the disorder usually disappears as mysteriously as it first appeared.
- Dr Crockett is a GP and a hospital practitioner in respiratory medicine in Swindon, Wiltshire
TRIGGERS OF URTICARIA
- Food: nuts, eggs, shellfish and strawberries.
- Food additives: salicylates, benzoates and sulphites.
- Drugs: penicillin and aspirin.
- Systemic diseases: SLE and polycythaemia rubra vera.
- Infections: any viral, bacterial or fungal infection.
- Physical factors: heat, cold, pressure and exercise.
- Inhalants: spores, dander, pollens and moulds.
- Contact (non-immunological): hairy caterpillars and plants.
- Other: stress, cosmetics and pregnancy.