Erythema Multiforme

Definition/diagnostic criteria Erythema multiforme (EM) is an acute, self-limiting, and often recurring skin condition, characterised by the distinctive target lesion – a central dark red area surrounded by a paler ring and a dark outer ring. EM is typically classified into two forms: EM minor, which is usually triggered by infections (most commonly herpes simplex virus and mycoplasma), and EM major, often drug-induced and associated with mucosal involvement. Erythema multiforme is now seen as a separate condition from the more severe illnesses Stevens-Johnson Syndrome (SJS) and Toxic Epidermal Necrolysis (TEN).

Epidemiology EM is relatively uncommon, with an estimated incidence of 1.2 cases per 100,000 people annually in the UK. It is more prevalent in the younger population, particularly affecting individuals aged 20-40 years. Men are slightly more affected than women.

Diagnosis
Clinical features: Diagnosis is primarily clinical. EM presents as symmetrical, red, papular or vesicular lesions, evolving into the classic target lesions. EM minor typically involves the limbs and is less severe, while EM major includes mucosal involvement, often oral.

Investigations: Investigations are not routinely required but may be considered to determine the underlying cause. Skin biopsy is rarely needed but can be helpful in uncertain cases. Typical histological findings include epidermal necrosis and lymphocytic infiltration. Laboratory tests (e.g complete blood count, liver function tests and viral serologies) may be indicated to exclude other causes.

Treatment Treatment is primarily supportive. EM minor often resolves without intervention. For symptomatic relief, topical steroids and oral antihistamines can be used. In cases associated with herpes simplex, oral antiviral therapy, such as aciclovir, may prevent recurrence. EM major requires more intensive management, potentially including systemic corticosteroids, though their use is controversial.

Mucosal involvement necessitates analgesic mouthwashes and systemic analgesia. Severe cases may require hospital admission for supportive care and monitoring.

Prognosis EM minor has an excellent prognosis, with lesions usually resolving within 2-3 weeks without scarring. Recurrences are common, particularly in herpes-associated cases. EM major, while more severe, also usually resolves completely, but the risk of recurrence varies depending on the causative agent. Long-term follow-up is advised to manage and prevent recurrences.