Lichen planus

Overview

A chronic T-cell-mediated inflammatory disorder of skin and mucous membranes. Remember the six Ps: Purple, Pruritic, Polygonal, Planar (flat-topped) Papules and Plaques. Wickham striae and flexor-wrist distribution are the giveaways.

Recognise

• Violaceous, flat-topped, polygonal papules with fine white lacy lines (Wickham striae), on the flexor wrists/forearms, ankles and lower back
• Koebner phenomenon; marked itch; post-inflammatory hyperpigmentation as lesions resolve
• Mucosal (white lacy reticular pattern on buccal mucosa; can be erosive), nail (longitudinal ridging, pterygium) and scalp (lichen planopilaris → scarring alopecia) variants

Differentials & how to tell them apart

Clinical image

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Investigations

Usually clinical. Biopsy if uncertain (saw-tooth rete ridges, a band-like lymphocytic infiltrate). Check the drug history and consider hepatitis C (an association).

Management

Potent topical corticosteroid + reassurance that cutaneous disease is usually self-limiting

1. 1Potent topical corticosteroid + emollient/antihistamine for itch; reassure (cutaneous lichen planus usually clears within 1–2 years, leaving hyperpigmentation).Gate: Review the drug history first — a lichenoid DRUG eruption (thiazides, antimalarials, ACE inhibitors, beta-blockers, gold) is the mimic and resolves when the culprit drug is stopped
2. 2Extensive or unresponsive cutaneous disease → dermatology (phototherapy, oral corticosteroids, acitretin). Oral/erosive disease → topical steroid or calcineurin inhibitor + good oral hygiene, with monitoring for malignant change.

Monitor & prognosis

Resolution of cutaneous disease; long-term review of erosive mucosal disease.

Cutaneous disease self-limits over months to 2 years; mucosal disease is more chronic.