A self-limiting rash that resolves after 10 weeks characterised by a herald patch and subsequent fir-tree pattern eruption

Aetiology

May be triggered by:

• Viruses: such as HHV6/7, COVID-19, and Flu
• Drugs: such as gold, ACEi, penicillamine, and biologics.
• Many vaccines: BCG, hepatitis B, and pneumococca

Clinical features

• Usually, a few days after a viral infection e.g. an URTI, the patient presents with a 'herald patch'
• This is a red-pink oval/discoid plaque with a peripheral colarette of scale on the trunk or upper arm/thigh
  • May be misdiagnosed as ringworm
• 1-20 days later, a similar widespread eruption (but the individual lesions may be smaller) occurs on the body that follows a fir tree (Christmas tree) pattern appears, with the long axis of the lesions sweeping from back to front as if following Langer's lines
• In most cases, there are virtually no symptoms except for the rash

Management

• Reassurance - it is self-limiting and will resolve on its own
• 25% patients may have intense pruritus - antihistamines, emollients, and topical steroids may be of use in these patients