Immune-mediated condition characterised by typical or atypical target lesions and potential blistering

Aetiology

• Erythema multiforme is most frequently caused by infections, notably the herpes simplex virus (HSV), and less commonly by mycoplasma
• Other potential infectious triggers include Epstein-Barr virus (EBV), cytomegalovirus (CMV), hepatitis, human immunodeficiency virus (HIV), varicella-zoster virus (VZV), group A streptococci, campylobacter, and salmonella
• Drug-induced causes (rare) - antibiotics, anti-inflammatories, anti-epileptics, and allopurinol

Pathophysiology

• Erythema multiforme results from a delayed T-cell-mediated skin reaction, meaning the reaction may not appear until several weeks after the initial insult, which may have already resolved
• The disease is classified into two main categories:
  • Erythema multiforme minor, which does not involve mucous membranes and is usually triggered by an infection
  • Erythema multiforme major, which includes mucous membrane involvement and is more commonly drug-induced

Clinical features

• The sudden appearance of target lesions characterises erythema multiforme
• These lesions can be typical (three colours) or atypical (two colours), and often have a central blister
• In erythema multiforme major, mucous membranes may be involved
• There is usually no prodrome, and the skin lesions can be burning or painful
• Oral lesions can be extremely painful
• The rash typically starts in the palms and soles before spreading to the rest of the body

Investigations

• Primarily clinical diagnosis
• Skin biopsy: may be required in uncertain cases
• Further investigations to identify cause may be required e.g. bloods, cultures, viral swabs