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Lichen planus (Greek leichen, “tree moss”; Latin planus, “flat”) is a unique, common inflammatory disorder that affects the skin, mucous membranes, nails, and hair. The appearance of lichen planus-like lichenoid dermatoses has been likened to the scurfy, finely furrowed, dry excrescences of the symbiotic vegetation known as lichen. Although this morphologic comparison may be antiquated, lichen planus is a distinctive entity with prototypic “lichenoid” papules that show distinctive color and morphology, develop in typical locations, and manifest characteristic patterns of evolution. Microscopic features are also distinctive, although the microscopic pattern of inflammation and skin response is shared by several dermatoses. The term lichenoid reaction1 is the histologic description used to capsulize the pathologic characteristics of skin diseases resembling lichen planus.
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The four Ps—(1) purple, (2) polygonal, (3) pruritic, and (4) papule—is the mnemonic device often used to recall the constellation of symptoms and skin findings that characterize lichen planus.
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The exact incidence and prevalence of lichen planus are unknown, but the overall prevalence is believed to be somewhat around 1% of the general population. Estimates between 0.14% and 1.27% have been reported worldwide and approximately 0.44% in the United States. No racial predilection has been observed.2,3
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At least two-thirds of cases occur between the ages of 30 and 60 years of age. No sexual predilection is evident. Females are usually affected in their 50s and 60s, whereas males develop lichen planus at a somewhat earlier age. The disease is less common in the very young and the elderly. The development of lichen planus may be affected by seasonal or environmental factors.2
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Fewer than 100 cases of familial lichen planus have been reported. The familial form tends to be more protracted and severe and presents in erosive, linear, or ulcerative patterns or with atypical features affecting young adults and children.4 Some believe that the familial form represents a separate, unique dermatosis. Different HLA haplotypes were reported in familial lichen planus, including HLA-B7, -Aw19, -B18, and -Cw8. In nonfamilial lichen planus, HLA-A3, -A5, -A28, -B8, -B16, and -Bw35 are more common.5 HLA-B8 is more common in patients with oral lichen planus as a sole manifestation, and HLA-Bw35 is more strongly associated with cutaneous lichen planus.
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It is evident that specific immunologic mechanisms control the development of lichen planus. T-cell mediated pathologic alterations involving proinflammatory and counterregulatory mechanisms function in the pathogenesis of lichen planus.6 No consistent alterations in immunoglobulins (Igs) have been shown in lichen planus, and humoral immunity most likely is ...