Lichen planus Lichen planus is an uncommon skin complaint. It is thought to be due to an abnormal immune reaction provoked by a viral infection (such as hepatitis C ) or a drug. Inflammatory cells seem to mistake the skin cells as foreign and attack them. Lichen planus may cause a small number of skin lesions or less often affect a wide area of the skin and mucous membranes. In 85% of cases it clears from skin surfaces within 18 months but it may persist longer especially when affecting the mouth or genitals. Clinical features Lichen planus takes several forms. Classical lichen planus Classical lichen planus is characterised by shiny, flat-topped, firm papules (bumps) varying from pin point size ('guttate') to larger than a centimetre. They are a purple colour and often are crossed by fine white lines (called 'Wickham's striae'). They may be close together or widespread, or grouped in lines (linear lichen planus) or rings (annular lichen planus). Linear lichen planus can be the result of scratching or injuring the skin. Although sometimes there are no symptoms, it is often very itchy. Lichen planus may affect any area, but is most often seen on the front of the wrists, lower back, and ankles. On the palms and soles the papules are firm and yellow. Very thick scaly patches are particularly itchy and are most likely to arise around the ankles (hypertrophic lichen planus). New lesions may appear while others are clearing. As the lichen planus papules clear they