Porokeratosis is a clonal disorder of keratinization characterized by 1 or more atrophic patches surrounded by a clinically and histologically distinctive ridgelike border called the cornoid lamella. Five clinical variants of porokeratosis are recognized: classic porokeratosis of Mibelli (PM), disseminated superficial actinic porokeratosis (DSAP), porokeratosis palmaris et plantaris disseminata (PPPD), linear porokeratosis, and punctate porokeratosis. Several other variants have been described, including hyperkeratosis types, a pruritic papular variant, and an unusual verrucous variant that is localized to the buttocks and mimics psoriasis. Occasionally, a patient may develop more than one type of porokeratosis
* Classic porokeratosis (Mibelli) The lesion develops as a small, light brown, keratotic papule that slowly expands to form an irregularly shaped, annular plaque with a raised, ridgelike border. This border may be hypertrophic or verrucous and is usually greater than 1 mm in height. A thin furrow is typically seen in the center of the ridge, causing a Great Wall of China effect. The lesion is slightly hypopigmented or hyperpigmented, minimally scaly, slightly atrophic, hairless, and anhidrotic. The size may vary from a few millimeters to several centimeters. Lesions may be found anywhere, including the mucous membranes, although they most commonly occur on the extremities. Generally few lesions are observed. A verrucous variant that is localized to the buttocks and resembles psoriasis has been reported in several patients. * Disseminated superficial (actinic) porokeratosis Dozens of small, indistinct, light brown patches with a threadlike border are seen on the extensor surfaces of the arms and the legs. Dermoscopy shows a "white track" structure at the periphery of the lesion, with a brownish pigmentation on the inner side and with a double white track in some parts of the lesion. This corresponds to the cornoid lamella. The center may show a white homogeneous area, corresponding to acanthotic epithelium, or red dots, globules, and lines corresponding to enlarged capillary vessels that can be seen because the epithelium is atrophic. Facial lesions are seen in approximately 15% of patients. Nonactinic DSP may have a generalized distribution, sparing the palms and the soles. Bullous and pruritic variants have been described.
* Facial lesions are seen in approximately 15% of patients, but the face may be the only area of involvement. * Patients are typically women in their third or fourth decade of life, with a history of excessive ultraviolet exposure. * Patients may have a history of phototherapy for psoriasis. * Nonactinic forms may be seen following electron beam total skin irradiation, organ transplantation, hepatitis C virus–related hepatocellular carcinoma, HIV infection, renal failure, or in association with other causes of immunosuppression.
* Classic porokeratosis (Mibelli) During childhood, a small, asymptomatic or slightly pruritic lesion develops, expanding over a period of years. Less commonly, lesions may develop during adulthood and enlarge rapidly, usually in the clinical setting of immunosuppression. Occasionally, patients have a history of an antecedent trauma, such as a burn wound. * Disseminated superficial (actinic) porokeratosis Multiple, brown, annular, keratotic lesions that develop predominantly on the extensor surfaces of the legs and the arms characterize DSAP. They are usually asymptomatic, but they may itch slightly (see Media File 2). Sunless tanning lotions containing dihydroxyacetone cause the cornoid lamellae to darken and become more clinically prominent. Dermoscopy reveals a "white track" structure with brown pigmentation on the inside of the track that can be seen at the edge of an individual lesion, corresponding to the cornoid lamella. Centrally, a white area with red dots, globules, and lines is present that corresponds to capillary vessels; it is more easily observed through the atrophic epithelium.