Multiple irregular hyperpigmented plaques present in a linear distribution.

A 14-years-old girl born to non-consanguineous parents presented to the outpatient department with multiple dark lesions in the left upper limb in a linear pattern, since the last 7 months, with slow enlargement of the old lesions and new lesions appearing along the same line. There was no history of similar lesions in the family. On examination, there were multiple well-demarcated, irregular, hyperpigmented plaques present in a linear distribution along the length of the left upper limb extending from the middle of the arm to just proximal to the wrist [Figure 1]. The plaques showed a fine raised border. The systemic examination was within normal limits. The routine hematological and biochemical investigations were also within normal limits. A skin biopsy done from the edge of one of the lesions showed a hyperkeratotic stratum corneum, a column of poorly staining parakeratotic stratum corneum cells, and absence of granular layer just below it [Figure 2].

Figure 1

Irregular well-defined plaques surrounded by a keratotic raised wall

Figure 2

Epidermal invagination with column of poorly staining parakeratotic stratum corneum cells

Question

What is your diagnosis?

Answer

Linear porokeratosis

Porokeratosis is an autosomal dominant disorder characterized by sharply demarcated scaly lesions, histologically showing a column of parakeratotic keratinocytes (cornoid lamella). Immunosuppression, ultraviolet exposure, and radiation therapy may promote the development of skin malignancies in porokeratosis lesions. Among the different types of porokeratosis, linear porokeratosis usually appears in childhood and follows the lines of Blaschko. It probably results due to an abnormal clone of epidermal precursors. The individual lesions are irregular well-defined plaques surrounded by a keratotic wall or collar. This wall is grayish or brownish and frequently surmounted by a linear ridge running along its summit. Two distinct varieties of linear porokeratosis have been described. The commoner variety consists of unilateral lesions confined to an extremity, whereas the rare generalized form shows multiple lesions affecting several extremities and may involve the trunk.[1] The diagnosis is confirmed by the characteristic histopathology seen at the edge of the lesion. It shows a column of parakeratotic stratum corneum cells, known as cornoid lamella, absence of granular layer just below it. The underlying keratinocytes are edematous with spongiosis, shrunken nuclei, and a moderate dermal lymphocytic infiltrate under the lamella. Topical 5-fluorouracil is an effective treatment, but the benefits must be weighed against the risk of treatment.[1] Other less effective treatments, which have been tried, include topical vitamin D3 analogs, topical retinoids, topical imiquimod, and oral retinoids. Destructive modes of therapy include cryotherapy, electrodessication and curettage, CO2 laser, pulsed dye laser, Nd-YAG laser, dermabrasion, and Grenz ray.[23] Linear porokeratosis has the highest risk of malignant transformation (19%), hence strict follow-up is necessary.[4] This is a rare case of linear porokeratosis, the late onset being an unusual feature.