Lichen planus

Lichen planus (LP) is a chronic inflammatory and autoimmune disease that affects the skin, nails, hair, and mucous membranes.^[1][2] It is not an actual lichen, but is named for its appearance.^[3] It is characterized by polygonal, flat-topped, violaceous papules and plaques with overlying, reticulated, fine white scale (Wickham's striae), commonly affecting dorsal hands, flexural wrists and forearms, trunk, anterior lower legs and oral mucosa.^[4] The hue may be gray-brown in people with darker skin.^[5] Although there is a broad clinical range of LP manifestations, the skin and oral cavity remain as the major sites of involvement.^[6] The cause is unknown, but it is thought to be the result of an autoimmune process with an unknown initial trigger. There is no cure, but many different medications and procedures have been used in efforts to control the symptoms.

The term lichenoid reaction (lichenoid eruption or lichenoid lesion) refers to a lesion of similar or identical histopathologic and clinical appearance to lichen planus (i.e., an area which resembles lichen planus, both to the naked eye and under a microscope).^[7][8] Sometimes dental materials or certain medications can cause lichenoid reactions.^[7] They can also occur in association with graft versus host disease.^{[7][9]: 258}