Lichen planus is a chronic, inflammatory disorder of the skin characterized by violaceous, flat-topped papules and plaques. The surface scale, known as Wickham's striae, is fine, white and reticulated. Histologically, lichen planus shows orthohyperkeratosis, wedge-shaped hypergranulosis, irregular acanthosis with 'saw-toothing' of the rete ridges, vacuolization of the basal layer and a dense, band-like lymphocytic infiltrate abutting the epidermis with pigmentary incontinence. While the aetiology is unknown, typically the eruption will spontaneously subside after months to years. Ultraviolet light, oral and topical steroids may speed resolution, although many alternative treatments are documented. Several specific variants of lichen planus are described: actinic lichen planus, lichen planopilaris, lichen planus pigmentosus and lichen planus pemphigoides. Lichenoid drug eruptions can mimic the clinical picture of lichen planus. In rare cases, the clinical and histological presentations of lichen planus and lupus erythematosus can overlap and should be recognized. Lichen nitidus is another chronic, inflammatory disorder of the skin. Minute, 1-2 mm shiny papules evenly spaced and grouped define the presentation of lichen nitidus. Histological examination reveals a infiltrate pattern resembling a 'claw clutching a ball', consisting of a focal mixed collection of lymphocytes and histiocytes grasped by descending rete ridges. Treatments are variably effective but may alleviate associated pruritus.
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