Histopathology of Linear Lichen Planus

To the Editor: We read with interest the paper by Rousseau et al1 on the histopathology of linear lichen planus (LLP), which concluded that it was identical to the conventional nonlinear or generalized lichen planus (LP). Our findings were different. As part of a prospective case series of acquired inflammatory Blaschko-linear disorders, we recruited 47 consecutive cases of LLP.2 LLP presented as discrete classical LP lesions in a narrow blaschkoid distribution in young adults in the third or fourth decade of life, with progression to reach its complete extent in months. We found 3 prominent histopathological patterns in the cases of LLP (Table 1). Interestingly, there were 2 distinct findings, which differed from findings described for classical LP. TABLE 1. - Histopathologic Findings of Linear Lichen Planus (n = 47) Serial No. Pattern Number Percentage 1 Basal cell damage and lichenoid infiltrate limited to upper dermis—either confluent band-like (5), unifocal (4), or multifocal (2) infiltrate in the upper dermis 11 23.4 2 Deeper infiltrates in mid or lower dermis—they had either confluent band-like lichenoid, unifocal (4), or multifocal (7) inflammatory infiltrate in the upper dermis, or just pigmentary incontinence, no active inflammation in the upper dermis 23 48.9 3 Only upper dermal pigmentary incontinence, no active inflammation 12 25.6 4 Nonspecific perivascular inflammation without current or previous interface dermatitis 1 2.1 One, the upper dermal infiltrate was not band-like, but instead was patchy and multifocal in 17/47 (36.2%) cases. Interestingly, this phenomenon of skip lesions has been described in various epidermal nevi, where it is proposed to indicate mosaicism at a microscopic level.3 Similarly, multifocal infiltrates in an acquired inflammatory disease such as LLP might indicate the same. Two, there were focal, deep dermal infiltrates in almost half (23/47, 48.9%) the cases. The deeper infiltrates were noted around vessels in 13, hair follicles in 4, and eccrine glands in 18 cases. In contrast, only 2 out of 50 controls (4.0%) with ordinary lichen planus showed deep perifollicular infiltrates (P < 0.001). Both of these cases had hypertrophic LP on the legs. In conclusion, some cases of LLP show histopathological findings that differ from those seen in classical lichen planus. The significance of these findings and their relation to the segmental/linear clinical presentation requires further study.