Lichen planus pigmentosus in a blaschkoid distribution.

Lichen planus pigmentosus is a pigmentary disorder of unknown etiology, with diffuse hyperpigmentation of sun-exposed areas, more commonly seen in some ethnic and racial groups. We report an unusual case of lichen planus pigmentosus in a 40-year-old man with Fizpatrick type III skin that was present in a blaschkoid distribution on the trunk, a distribution that has been rarely reported. This unique presentation of lichen planus pigmentosus may contribute to better understanding of the etiology, as the blaschkoid distribution may reflect underlying cutaneous mosaicism that renders those cells more susceptible to an insult that results in lichen planus pigmentosus. This disorder should be considered in the differential diagnosis of macular hyperpigmentation, especially in those from more commonly affected ethnic and racial groups, even when the distribution is atypical and in the absence of history of sun exposure.