lichen planus pigmentosus ( lpp ) , an uncommon variant of lp , is characterized by mottled or reticulated hyperpigmented , dark brown macules in sun - exposed areas and flexural folds .
the histopathologic findings of lpp consist of hyperkeratosis , atrophic epidermis with vacuolar alteration of the basal cell layer , and scarce lymphohistiocyte or lichenoid infiltrates in the dermis with pigmentary incontinence and the presence of melanophages ( 1 ) .
although there have been a few reports of linear lichen planus , there has been no report of lpp with a linear pattern ( 2 , 3 ) .
we describe two patients with lpp who had skin lesions in a linear distribution related to blaschko 's lines on their extremities .
a 23-yr - old korean woman presented with asymptomatic dark brown macules on the left leg for 2 yr . one or two dark brown macules first appeared and spreaded gradually without any preceding erythematous or scaly skin eruption .
there was no history of prolonged sun exposure or trauma on the lesion site . on examination , she had linear streaks of dark brown macules from the left ankle , across the calf and to the thigh , consistent with the pattern of blaschko 's lines ( fig .
skin biopsy showed atrophic epidermis , basal hydropic degeneration with sparse perivascular lymphohistiocytic infiltrates , and numerous melanophages ( fig .
these findings were consistent with lpp . a 16-yr - old korean woman presented with linear dark brown pigmentation on the left arm for one year .
she stated that recurrent asymptomatic erythematous papules first appeared and then the lesion had developed into dark brown macules during 1 - 2 weeks .
examination showed two linear streaks of dark brown macules from the left dorsum of hand to the upper arm probably related to blaschko 's lines ( fig .
skin biopsy showed orthokeratosis , focal basal liquefaction , a sparse perivascular inflammatory infiltrate , and pigmentary incontinence , confirming the diagnosis of lpp ( fig .
a 23-yr - old korean woman presented with asymptomatic dark brown macules on the left leg for 2 yr . one or two dark brown macules first appeared and spreaded gradually without any preceding erythematous or scaly skin eruption .
there was no history of prolonged sun exposure or trauma on the lesion site . on examination , she had linear streaks of dark brown macules from the left ankle , across the calf and to the thigh , consistent with the pattern of blaschko 's lines ( fig .
skin biopsy showed atrophic epidermis , basal hydropic degeneration with sparse perivascular lymphohistiocytic infiltrates , and numerous melanophages ( fig .
a 16-yr - old korean woman presented with linear dark brown pigmentation on the left arm for one year .
she stated that recurrent asymptomatic erythematous papules first appeared and then the lesion had developed into dark brown macules during 1 - 2 weeks .
examination showed two linear streaks of dark brown macules from the left dorsum of hand to the upper arm probably related to blaschko 's lines ( fig .
skin biopsy showed orthokeratosis , focal basal liquefaction , a sparse perivascular inflammatory infiltrate , and pigmentary incontinence , confirming the diagnosis of lpp ( fig .
lpp has been described as a condition of unknown etiology which clinically differs from the classical lichen planus by exhibiting dark brown macules and/or papules and a longer clinical course without pruritus or scalp , nail , or mucosal involvement .
the lpp is most common on sun - exposed areas such as the face , neck , and flexural folds including the axilla , inguinal , and submammary regions ( 1 ) .
some authors observed a striking predominance of lesions in an intertriginous location , with most of them in the axillae , thus they proposed the designation lpp - inversus ( 4 ) .
less common presentations include zosteriform pattern on the trunk ( 5 ) and involvement of non sun - exposed areas such as thigh ( 6 ) .
the linearity of the lesions is probably related to blaschko 's lines , which suggests that the predisposition to develop lpp might be determined during embryogenesis .
the differential diagnoses of our cases include lichen striatus ( 7 ) , linear and whorled nevoid melanosis and incontinentia pigmenti .
in particular , because lichen striatus may have post - inflammatory hyperpigmentation ( 8) and lacks a well - defined histopathological picture ( 9 ) , it should be differentiated in our patients .
lichen striatus almost always have preceding inflammatory papules or scaly eruption , which last for 4 months to 4 yr ( 8) . there was no previous papule in patient 1 and the short duration of preceding papules ( 1 - 2 weeks ) favor the lpp in patient 2 .
there were no epidermal changes consisting of spongiosis and exocytosis , and a deeper dermal inflammatory infiltrate around adnexal structures which are features frequently seen in lichen striatus ( 9 ) . in summary
, lpp can present with a linear pattern and thus should be considered in the differential diagnosis of linear hyperpigmented skin lesions .
 we report two cases of lichen planus pigmentosus ( lpp ) that developed in a unilateral linear pattern . the patients presented with unilateral linear brown macules on the extremities . 
 skin biopsy showed orthokeratosis , basal hydropic degeneration with scarce lymphohistiocytic infiltrates , and numerous melanophages in both patients . 
 these patients , to the best of our knowledge , are the first cases of lpp presenting with a linear pattern . 
 lpp should be considered in the differential diagnosis of linear hyperpigmented skin lesions . 
INTRODUCTION
CASE REPORT
Patient 1
Patient 2
DISCUSSION