Linear lichen planus.

A 50-year-old man presented with pruritic, hyperpigmented papules and plaques on the right lower extremity following Blaschko lines. A skin biopsy specimen was consistent with lichen planus (LP). Linear LP accounts for less than 0.2 percent of all patients with LP, and the segmental formation is thought to be due to a postzygotic mutation that affects one of the genes predisposing its development. This loss of heterozygosity may occur from a mutation, deletion, or DNA recombination and leads to the formation of a keratinocyte clone that is more susceptible to development of the skin disease. Histopathologically, linear LP is identical to LP, with the presence of hyperkeratosis, focal hypergranulosis, irregular acanthosis with a sawtooth appearance, vacuolar change of the basal-cell layer, and a dense band-like lymphocytic infiltrate at the dermal-epidermal junction. It is important to differentiate linear LP from lichen striatus, inflammatory linear verrucous epidermal nevus, linear psoriasis, and linear Darier-White disease, which have different presentations clinically and histopathologically.