“Pruritic, purple, polygonal, planar papules, and plaques” are the tongue-twisting Ps that describe this disorder of skin and squamous mucosa. The lesions may result from a CD8+ T cell–mediated cytotoxic response against antigens in the basal cell layer and the dermoepidermal junction that are produced by unknown mechanisms, perhaps as a consequence of a viral infection or drug exposure.
MORPHOLOGYCutaneouslesionsoflichenplanusconsistofpruritic, violaceous, flat-topped papules thatmaycoalescefocallytoformplaques(
Fig.24.4A ).ThesepapulesarehighlightedbywhitedotsorlinestermedWickham striae. Hyperpigmentationmayresultfrommelaninlossintothedermisfromdamagedkeratinocytes.Microscopically,lichenplanusisaprototypicalinterface dermatitis, socalledbecausetheinflammationanddamageareconcentratedattheinterfaceofthesquamousepitheliumandpapillarydermis.Thereisadense,continuousinfiltrateoflymphocytesalongthedermoepidermaljunction(see
Fig.24.4B ).Thelymphocytesareintimatelyassociatedwithbasalkeratinocytes,whichoftenatrophyorbecomenecrotic.Perhapsasaresponsetodamage,thebasalcellstakeontheappearanceofthemorematurecellsofthestratumspinosum(squamatization).Thispatternofinflammationcausesthedermoepidermalinterfacetoassumeanangulated,zigzagcontour(sawtoothing). Anucleate,necroticbasalcellsareseenintheinflamedpapillarydermisandarereferredtoascolloidbodiesorCivatte bodies. Althoughthesechangesbearsomesimilaritiestothoseinerythemamultiforme(anothertypeofinterfacedermatitisdiscussedearlier),lichenplanusshowswell-developedchangesofchronicity,includingepidermalhyperplasia,hypergranulosis,andhyperkeratosis.
Fig. 24.4 Lichenplanus.(A)Flat-toppedpink-purplepolygonalpapulewithwhitelacelikemarkingsreferredtoasWickham striae. (B)Microscopicexaminationshowsabandlikeinfiltrateoflymphocytesalongthedermoepi-dermaljunction,hyperkeratosis,hypergranulosis,andpointedreteridges(“sawtoothing”),whichresultsfromchronicinjuryofthebasalcelllayer.
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Lichen planus is an uncommon disorder that usually presents in middle-aged adults. The cutaneous lesions are multiple and are usually symmetrically distributed, particularly on the extremities, and often occur around the wrists and elbows and on the vulva and glans penis. Approximately 70% of cases also involve the oral mucosa, where the lesions manifest as white papules with a reticulate or netlike appearance. The cutaneous lesions of lichen planus usually resolve spontaneously within 1 to 2 years, but the oral lesions may persist and be of sufficient severity to interfere with food intake.
Lichen simplex chronicus manifests as roughening of the skin, which takes on an appearance reminiscent of lichen on a tree. It is a response to local repetitive trauma, usually from rubbing or scratching. Nodular forms exist that are referred to as prurigo nodularis. The pathogenesis of lichen simplex chronicus is not understood, but the trauma probably induces epithelial hyperplasia and eventual dermal scarring.
MORPHOLOGYLichensimplexchronicusischaracterizedbyacanthosis, hyperkeratosis, andhypergranulosis. Alsoseenareelongationofthereteridges,fibrosisofthepapillarydermis,andadermalchronicinflammatoryinfiltrate(