Lichen planus (LP) is an inflammatory disease of the skin that may involve the hair follicles, nail unit, cutaneous surface, and mucous membranes. (See oral lichen planus for oral mucosal presentation and lichen planopilaris for scalp presentation.) The etiology is unknown, but viruses, medications, and contact allergens have all been implicated. In adults, LP occurs most frequently in the fifth and sixth decades. The skin changes of LP are manifested by small, flat-topped, reddish to purple papules that are most commonly seen on the volar wrists and flexural surfaces.

Nail involvement occurs in less than 25% of patients with skin or mucosal disease. Isolated nail involvement can also be the first or only manifestation of the disease, but isolated nail LP is relatively uncommon. Clinical features depend on the location, duration, and severity of the disease in the nail apparatus. Involvement of the entire nail matrix is the most frequent presentation and results in thinning of the nail plate, onychorrhexis, nail atrophy with dorsal pterygium formation, and trachyonychia. Focal nail matrix involvement produces longitudinal splitting, longitudinal ridging, and pitting. Involvement of the nail bed is rare and induces distal subungual hyperkeratosis with or without associated onycholysis. If untreated, permanent scarring of the nail matrix may occur, resulting in dorsal pterygium formation.

LP is a cause of twenty-nail dystrophy, which is more commonly seen in children than adults. LP has been reported as a cause of longitudinal melanonychia.

Typically the diagnosis of nail LP is delayed, and many patients will have changes for more than 3 years before the diagnosis is made.

LP is more common in adults, and prevalence in children is 2%-3%.