Identifying erythema multiforme

A skin eruption may be the potentially serious condition erythema multiforme, writes Dr Nigel Stollery.

Erythema multiforme (EM) is an acute self-limiting condition characterised by distinctive 'target' lesions. The minor form is more common and presents with round lesions on the forearms, hands, feet and knees.

Erythema multiforme

The lesions may be a uniform pink but more usually appear as a ring - so-called target lesions - in the centre of which there may be fluid-filled blisters. Occasionally blisters form on the lips.

The major forms are more unusual and, as well as the skin, also affect mucous membranes. Patients are usually systemically unwell, and eating and drinking may be difficult.

Immune response
The pathophysiology of EM is still not really understood, but herpes-associated EM appears to be a cell-mediated immune response to the herpes simplex antigen, affecting keratinocytes in the skin. Men are affected slightly more than women; the highest incidence occurs in those aged 20-40 years.

Target lesions are the hallmark of EM. Lesions start as red macules or urticarial plaques that enlarge over 24-48 hours to a maximum 2cm diameter. In the centre, a small vesicle forms that flattens and clears; the periphery then changes to a cyanotic or violaceous colour, completing the typical target appearance.

The lesions are generally not itchy, but patients often describe a burning sensation within them. They typically appear on extensor surfaces, palms, neck and face. Oral lesions present in 70 per cent of cases and are usually mild in EM minor. Genital mucosa may also be affected, with more severe lesions.

[The Nikolsky sign can be a useful way to distinguish EM from pemphigus (see box for other differential diagnoses). The sign is typically negative in EM; it is positive when apparently normal skin can easily be dislodged under shearing stress.

Differential diagnoses
  • Behcet's disease
  • Urticarial vasculitis
  • Toxic epidermal necrolysis
  • Drug eruptions
  • Pemphigus
  • Kawasaki disease

Infection
There are many causes of EM, the most common being infection, especially with herpes simplex. This virus is thought to be responsible for almost all cases of EM minor and to contribute to 55 per cent of more severe cases.

There are many other causative agents, such as mycoplasma, fungal infections, chlamydia, drugs, food and chemical contacts (for example, poison ivy), and mechanical factors, such as tattooing, radiotherapy, cold and sunlight.

A biopsy may be required to confirm diagnosis, but once made, it is very important to determine cause. Investigations looking for infection may be useful and a careful drug history should be taken. If a drug is thought to be the cause, it should be stopped immediately.

Any infection should be treated appropriately. If mucosal involvement is severe and eating and drinking are difficult, IV support may be needed. Systemic corticosteroids remain controversial. Other drugs include ciclosporin, thalidomide, dapsone and cyclophosphamide.

It is important not to miss ocular involvement, which presents as a bilateral purulent conjunctivitis and increased lacrimation. Refer to ophthalmology if unsure. In some cases, attacks may be recurrent. Where these occur more than five times a year, prophylaxis may be helpful in the form of daily oral aciclovir, which may be required for six to 12 months or longer.

Alternative prophylactic treatments include dapsone, antimalarials, azathioprine, cimetidine and thalidomide.

  • Dr Stollery is a GP in Kibworth, Leicestershire, and a clinical assistant in dermatology at Leicester Royal Infirmary

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