Lichen planus is an inflammatory mucocutaneous condition characterized by violet colored lesions. These lesions are best described by the six 'P's: pruritic, polygonal, planar, purple, papules, and plaques. The lesions commonly appear on the flexor surface of the extremities, notably the wrists. They also tend to occur in areas exposed to trauma, such as lacerations, a phenomenon known as isomorphic response. Apart from cutaneous surfaces, lichen planus lesions can also appear, sometimes exclusively so, on the oral mucosa, the genital mucosa, the nails, and the scalp. The WHO, meanwhile, has classified lichen planus as a potentially malignant disorder.
The prevalence of lichen planus is unknown, but it is estimated to occur in less than 1 percent of the population, with no racial predilection. It most commonly affects middle-aged adults of both sexes, with a slight predominance in women. Diagnosis of the condition is based on the clinical appearance and a punch biopsy of the lesions. Typically, the tissue under the microscope shows a band-like infiltrate of lymphocytes at the epidermal-dermal junction and damage to the basal cell layer.
Topical steroids are the mainstay of treatment in patients with early or localized disease. For patients with generalized disease, oral corticosteroids may be recommended. In some cases where the lesions are resistant to steroids, other treatment strategies, such as topical and systemic retinoids, and oral PUVA may be used.
The exact pathogenesis of lichen planus is unknown, although many studies support an immunologic pathogenesis. Lymphocytes, particularly T-cells, play a major role. While most cases of lichen planus are idiopathic, some are linked to medication use, infection with hepatitis C virus, or hepatitis B vaccination. Studies of familial cases of lichen planus have indicated that polymorphisms in certain genes, such as TNFR2 and P53 might be associated with providing susceptibility to the disease. However, this association is not clear.