The adolescent presented with a 5 year history of an itchy, linear rash involving her left arm and hand. Over the last 2 months extension of the lesion had been noted. Punch biopsy from the left hand showed marked hyperkeratosis and parakeratosis with the presence of a cornoid lamella. Porokeratosis refers to a set of genetically determined disorders of keratinization. It is characterized by circular or serpiginous lesions showing a disctinctive peripheral, raised keratotic ridge. Microscopically, this corresponds to a cornoid lamella, which is column of parakeratotic cells overlying an absent granular layer. Dyskeratotic cells are often seen in the basal layer. There are five distinct clinical variants: plaque-type porokeratosis of Mibelli, disseminated superficial actinic porokeratosis (DSAP), porokeratosis punctata, porokeratosis palmaris et plantaris, and linear porokeratosis. Each type is thought to represent a different phenotypic expression of a common genetic disorder. Linkage anaylsis has identified two genetic loci on chromosomes 12q24 and 15q25 that are associated with DSAP. The lesions have malignant potential, and are exacerbated by immunosuppression, ultraviolet light and radiation.
Linear porokeratosis is rare clinical variant that arises in childhood as a linear array of annular, keratotic papules most commonly seen on the extremities. Lesions may coalesce into larger plaques with central atrophy and a raised peripheral border which follow Blaschko’s lines. Nail dystrophy and bony abnormalities have also been described. Linear porokeratosis appears to be a nonhereditary trait, suggesting that somatic recombination resulting from somatic crossing-over during early embryogenesis may result in a clone of epithelial cells that proliferate in a linear fashion. Loss of homozygosity could explain the observation that the lesions of linear porokeratosis are far more pronounced than those of DSAP. Linear porokeratosis has the highest risk of malignant transformation into squamous cell carcinoma or basal cell carcinoma.
Treatment of porokeratosis with topical agents such as keratolytics, 5-fluorouracil, corticosteroids, retinoids, and calcipotriene have been variably successful. Oral retinoid therapy has been effective, but recurrence upon discontinuation is common. Surgical modalities such as curettage, excision, cryotherapy, electodessication, and carbon dioxide laser have been used for localized lesions, but may be impractical for more widespread disease.
linear configuration of papules coalescent into plaques, some with elevated edges, without apparent nail changes.