Porokeratosis is a disorder of keratinization characterized by hyperkeratotic papules or plaques surrounded by a distinct peripheral, thin, ridge-like scale that expands centrifugally. Several variants of porokeratosis have been described, and a wide overlap between variants exists, suggesting that they are close in relation. Clinical variants, which are most commonly seen in the pediatric population, include linear porokeratosis, porokeratosis of Mibelli, and porokeratosis palmaris et plantaris disseminata (also known as plantar porokeratosis). Porokeratosis ptychotropica is a rare variant that presents in adults in the intergluteal cleft and extending onto the buttocks. Eruptive pruritic papular porokeratosis is another rare variant that is seen in adults. All variants share a distinctive histological feature known as the cornoid lamella, which corresponds to the hyperkeratotic ridge surrounding these lesions.

Linear porokeratosis: An uncommon variant that presents in infancy or early childhood as unilateral lesions contained within the lines of Blaschko. A less-common generalized form affects multiple extremities as well as the trunk. Recently, mutations in the mevalonate kinase pathway have been identified in linear porokeratosis and are thought to be the pathogenetic basis of this disorder. The linear variant of porokeratosis has the highest potential of malignant transformation of all porokeratoses, with cutaneous squamous cell carcinoma (SCC) being the most frequently associated malignancy.

Porokeratosis of Mibelli: The classic porokeratosis of Mibelli presents in infancy to early adulthood as an asymptomatic to slightly pruritic, brown to skin-colored, annular plaque with a peripheral hyperkeratotic, ridge-like scale. Lesions can range in size from 1 to 2 mm up to several centimeters in diameter. Larger lesions tend to form on the lower leg and foot, although giant facial and genital lesions have been described. The condition may be inherited as an autosomal dominant trait. Antecedent trauma or immunosuppression may be present. Lesions often persist indefinitely.

Porokeratosis palmaris et plantaris disseminata (plantar porokeratosis): An autosomal dominant inherited genodermatosis presenting in adolescence or young adulthood. Initial lesions first appear on the palms and soles as small, uniform hyperkeratotic lesions with characteristic leading ridge with a longitudinal furrow. Subsequent lesions spread to involve other areas of the body, including non-sun-exposed sites as well as the mucous membranes. Males are affected twice as often as females.

Porokeratosis ptychotropica: A rare variant that has been reported in adults. Most reported cases have been in males. Giant plaques with a verrucous surface may be seen on the buttocks. It has been reported to coexist with porokeratosis of Mibelli.

Eruptive pruritic papular porokeratosis: Typically very pruritic and is seen in older men predominantly. Annular papules appear abruptly on the trunk and extremities. Some cases have been seen in association with hematologic or gastrointestinal malignancies.