linear psoriasis is a rare form of psoriasis , characterized by a linear distribution of the psoriatic lesions along blaschko 's lines . the pathogenesis of linear psoriasis is not clear , but it could be explained by the well - established concept of genetic mosaicism . a 56-year - old previously healthy woman presented with a 3-month history of multiple confluent erythematous papules and plaques covered by scales . the papules and plaques had well - defined and sharply demarcated boundaries , ranged from 3 to 10 mm in diameter , were limited to the right side of her body and were arranged along blaschko 's lines with a marked mid - line cutoff . the lesions were first noticed in the right scapular region and progressively extended within a few weeks to the lateral aspect of the right superior limb , the right side of the trunk , the abdomen and the anterior aspect of the inferior limb , especially pronounced in the proximal third of the thigh and the knee ( fig . the patient denied any episodes of infection preceding the dermatosis . no psoriasis - form nail or scalp lesions were observed , and there was no personal or family history of psoriasis . the histopathologic analysis of a skin biopsy revealed orthokeratosis , parakeratosis and spongiosis - form munro 's microabscesses , in association with a regular elongation of rete ridges , suprapapillary thinning , and an absence of granular cell layers ( fig . a slight improvement was noticed after 1 month of combined treatment with betamethasone valerate and calcipotriol ointment . after complete laboratory exams including blood cell count , renal , hepatic and lipid panels , hbv and hcv serologies , and a chest radiograph , the patient was started on methotrexate 10 mg weekly with excellent response . in the 26th week of treatment , as only residual lesions persisted , the methotrexate dose was reduced to 7.5 mg weekly and maintained until the present time ( cumulative dose of 465 mg ) with no clinical relapse . there are few cases reported in the literature [ 1 , 2 , 3 ] , and therefore there is no estimated prevalence . also because of its clinical and histological similarity to ilven , the main differential diagnosis is ilven , and the distinction between the two entities has been discussed in the literature [ 4 , 5 , 6 ] . a late onset of asymptomatic to slightly itchy lesions with a possible involvement of scalp and nails and a favorable response to antipsoriatic treatment favors the diagnosis of linear psoriasis [ 8 , 9 ] . in contrast , ilven lesions usually occur in the first months of life , are slowly progressive , very pruritic and highly refractory to antipsoriatic therapy . as for the histological features , ilven classically demonstrates hypergranulosis and parakeratosis alternating with hypogranulosis and orthokeratosis . on the other hand , the classic features of psoriasis are hyperkeratosis , parakeratosis , the absence of granular cell layers , an elongation of rete ridges , suprapapillary thinning and munro 's microabscesses , which can also be seen in ilven . immunohistopathologic studies may be helpful in distinguishing the two conditions , as done by vissers et al . : in ilven patients , the number of ki-67-positive nuclei tended to be reduced , whereas the number of keratin-10-positive cells was increased compared with psoriasis patients . involucrin expression , another marker , is absent in ilven patients but detectable in psoriasis . the pathogenesis of linear psoriasis is not clear , but it could be explained by the well - established concept of genetic mosaicism .
a 56-year - old woman presented with a 3-month history of erythematous plaques covered by scales and limited to the right side of her body . 
 the plaques were arranged along blaschko 's lines with a marked mid - line cutoff . 
 the histopathologic analysis of a skin biopsy in conjunction with the anamnesis allowed the diagnosis of linear psoriasis . 
 our patient showed a good clinical response to antipsoriatic treatment .