Linear psoriasis, a rare form of psoriasis, is characterized by the linear distribution of psoriatic lesions along Blaschko's lines1,2. Histopathologically, the classic features of psoriasis may be observed1,2. Herein, we report a case of linear psoriasis on the right side of the trunk.

A 14-year-old girl presented with a 1-year history of occasionally pruritic, well-defined, “S”-shaped, erythematous, and scaly patches from the right side of the pubic area to the back (Fig. 1). There were no nail or scalp lesions and no personal or family history of psoriasis. A skin biopsy from the back exhibited parakeratosis, elongation of rete ridges, Munro's microabscess, dilated tortuous blood vessels, and perivascular lymphocytic infiltration (Fig. 2). With the unilateral distribution in a linear pattern, the lesion was diagnosed as linear psoriasis and was treated with a topical betamethasone dipropionate/calcipotriol ointment. After 4 weeks, the lesion improved considerably and only a trace remained.

Linear psoriasis is an unusual subtype of psoriasis first described in 19513. The pathogenesis is still unclear but could be explained by the concept of genetic mosaicism4. Happle proposed that loss of heterozygosity in somatic cells during early embryogenesis results in somatic recombination, i.e., homozygosity to one of the genes that predispose to psoriasis4. In this instance, one of the daughter cells may become homozygous for a psoriasis gene, becoming the stem cell of a clone proliferating in a linear pattern during the skin's embryonic development4.

Because of clinical and histologic similarity, the main differential diagnosis is inflammatory linear verrucous epidermal nevus (ILVEN)1,2. Late onset but rapid progression of asymptomatic or slightly pruritic lesions with a possible involvement of the scalp and nails, and a favorable response to antipsoriatic treatment would indicate linear psoriasis1,2. In contrast, ILVEN lesions that usually appear in the first months of life, are slowly progressive, very pruritic, and highly refractory to antipsoriatic treatment1,2. Histologically, ILVEN classically presents with hypergranulosis and orthokeratosis alternating with hypogranulosis and parakeratosis, while Munro's microabscess is considered as characteristic of psoriasis1,2. Immunohistochemical studies could be helpful in distinguishing the two diseases1,2. In psoriasis patients, the number of Ki-67-positive nuclei is usually higher than that in ILVEN patients1,2. There is little expression of keratin 10 in psoriasis while these levels remain normal in ILVEN1,2. Furthermore, involucrin would be detectable in psoriasis but absent in ILVEN1,2. In our case, we easily excluded ILVEN because of characteristics such as late onset, mild pruritus, positive response to antipsoriatic treatment, and histological features.

We report a rare, but a typical and an interesting case of linear psoriasis. Dermatologists should be aware of this entity of linear psoriasis and keep this condition in mind in clinical settings.