- At least five or more clinical variants of porokeratosis are recognized
- The prototype, porokeratosis of Mibelli, is a plaque that appears during infancy or childhood
- Disseminated superficial actinic porokeratosis (DSAP) is the most common type, with multiple thin papules appearing most commonly on the legs of adult women
- Linear porokeratosis develops during infancy or childhood and follows the lines of Blaschko
- Punctate porokeratosis appears during or after adolescence as 1-2mm papules of the palms and soles
- Porokeratosis palmaris et plantaris disseminate (PPPD) is a variation of punctuate porokeratosis with lesions also present on other areas of the body
- Some forms of porokeratosis are inherited in autosomal dominant fashion, but many patients have no family history
- A thread-like raised hyperkeratotic border is characteristic
- Histologically, a thin column of parakeratosis, the cornoid Iamella, is seen in all variants and corresponds to the raised hyperkeratotic border seen clinically
- Development of squamous cell carcinoma is possible
Porokeratosis presents as a hyperkeratotic papule or plaque with an annular appearance due to its thread like elevated border that expands centrifugally.
Disseminated superficial porokeratosis and disseminated superficial actinic porokeratosis (DSAP) are the most common variants. Clinical manifestations occur at a later age than in other forms of porokeratosis, i.e., the condition usually presents in the third or fourth decade of life. There is a female predilection. DSAP is found most commonly in Caucasians with a history of significant sun exposure. It is rarely seen in blacks.
Porokeratosis palmaris et plantaris disseminate (PPPD) is uncommon. It is inherited in an autosomal dominant manner and the lesions first appear during adolescence or early adulthood. Men are affected twice as often as women.
Linear porokeratosis is also uncommon, with onset during infancy or childhood. It has been reported in monozygotic twins, and it has been seen in families with other types of porokeratosis, but a specific heritable pattern thus has not been identified.
Punctate porokeratosis appears during adolescence or adulthood and may be seen concomitantly with other types of porokeratosis.
It is thought to be a disorder of keratinization but the definitive pathogenesis remains unclear.
Classic porokeratosis of Mibelli begins during infancy or childhood as asymptomatic small brown to skin colored keratotic papules that gradually enlarge over the years to form plaques that may measure several centimeters in diameter. Plaques of classic porokeratosis are surrounded by a raised sharply demarcated keratotic border with a longitudinal furrow. The center of the lesion may be hyperpigmented, hypopigmented, depressed, atrophic or anhidrotic. Lesions may occur anywhere on the body including mucous membranes, but the extremities are most frequently involved. Multiple lesions may arise, but they are almost always regionally localized and unilateral.
Lesions of disseminated superficial porokeratosis and DSAP initially appear in the 3rd to 4th decade of life as a small asymptomatic or mildly pruritic keratotic papules ranging from 2 to 5 mm in diameter; they are often pink in color. As the lesions progress, they expand radially, the older, central area becomes atrophic and anhidrotic while the well demarcated border evolves as a thin, elevated, furrowed keratotic rim. Lesions occur in a more widespread pattern than other types of porokeratosis.
Disseminated superficial porokeratosis usually involves the extremities bilaterally and symmetrically, sparing the palms, soles and mucous membranes, while the actinic form occurs exclusively in sun exposed areas most commonly the legs and forearms. Majority of patients with DSAP report that the lesions become more prominent or erythematous during the summer. DSAP patients have involvement of the legs without involvement on the other areas of sun damage, as in idiopathic guttate hypomelanosis.
Linear porokeratosis arises in infancy or childhood, and it consists of one or more plaques that are similar in appearance to classic porokeratosis; however, the plaques follow the lines of Blaschko, most commonly on the extremities.
Punctate porokeratosis is the most difficult type to recognize clinically because of the small size of the lesions. It appears during adolescence or adulthood as small seed-like keratotic papules with a peripheral raised rim on the palms and/or soles.
Individual lesions of PPPD also resemble those of porokeratosis of Mibelli except they are smaller and have less pronounced keratotic border. They may be asymptomatic or pruritic, and generally arise during childhood or adolescence. The name suggests that the palms and soles are initially affected but any surface, including mucous membranes may be involved.
Development of invasive SCC in lesions of porokeratosis has been reported in all variants except the punctuate variety. Lesions in older patients, those of long standing duration and linear lesions all have higher rates of malignant degeneration. DSAP has the lowest risk of malignant change. Squamous cell carcinoma generally appears as a papule or plaque arising in contiguity with the individual lesion of porokeratosis, leading to asymmetry.