Porokeratosis

Porokeratosis appears as a papule or plaque with an elevated border that expands peripherally. The term porokeratosis is a misnomer, as the lesion was originally thought to overlie the pore of a sweat gland. It is commonly thought to result from a disorder of keratinization, but the cause is not certain. Several variants of porokeratosis are recognized, including:

Porokeratosis of Mibelli. This is the originally reported porokeratosis (classic porokeratosis), presenting during infancy or childhood as an asymptomatic small, brown or skin-colored papule anywhere on the skin or mucous membranes, but favoring the extremities. Multiple lesions may be present, usually regionally localized. The plaque is circumscribed by raised keratotic border. The lesion gradually expands, and can reach several centimeters in diameter. It is rare, and thought to be inherited in an autosomal dominant inheritance pattern.

Disseminated superficial porokeratosis (DSP) and disseminated superficial actinic porokeratosis (DSAP). These variants are the most commonly observed, typically in the third or fourth decades of life. The lesions are small (2-5 mm), often pink papules that may be pruritic. The lesions expand centrifugally, leaving an atrophic center with a well-demarcated hyperkeratotic rim. DSP occurs in a more widespread pattern than porokeratosis of Mibelli, usually involving the extremities bilaterally but sparing the mucous membranes, palms and soles. DSAP is usually seen in Caucasians with a history of significant sun exposure and affects only sun-exposed areas, most often the legs. Both of these variants are thought to be inherited as an autosomal dominant trait.

Porokeratosis palmaris et plantaris disseminata (PPPD). This uncommon variant occurs during adolescence and is also inherited in an autosomal dominant fashion. This variant appears similar to classic porokeratosis, but each lesion is smaller and the border is less raised. The palms and soles are affected, but any area of the body or mucous membranes can exhibit lesions as well.

Linear porokeratosis. This rare form has its onset during infancy or childhood; its pattern of inheritance is uncertain. The lesions are similar to classic porokeratosis but distributed along the lines of Blaschko.

Punctate porokeratosis. This form begins during adolescence or young adulthood, and may occur with other types of porokeratosis. It appears as small punctate papules with a raised rim on the palms and soles.

With what can porokeratosis be confused?

Classic porokeratosis is usually obvious clinically. On histopathology, porokeratosis can be confused with other conditions with focal parakeratosis or columns of parakeratosis, such as verruca vulgaris, actinic keratosis, or guttate posriasis. Actinic keratosis may have a cornoid lamella, but cellular atypia is present. Verruca vulgaris also may have focal keratosis that appears as a cornoid lamella, but koilocytosis will usually be apparent. Linear porokeratosis can clinically resemble stage II of incontinentia pigmenti, linear lichen planus, and epidermal verrucous epidermal nevus, but none exhibit a cornoid lamella on histopathology. Punctate porokeratosis can clinically resemble Darier's disease, Cowden's disease, keratoderma, and arsenical keratoses, but biopsy can usually distinguish among these.

How is porokeratosis diagnosed?

Clinical. Clinically, the diagnosis of classic porokeratosis of Mibelli is usually apparent. DSAP usually affects the legs and not the face, whereas actinic keratosis will usually involve both areas.

Histopathology. Porokeratosis has distinctive histopathology. The cornoid lamella is a column of parakeratosis, with pyknotic nuclei, with underlying hypogranulosis and dyskeratosis. It extends from an invagination in the epidermis, often extending above the surrounding stratum corneum. It corresponds to the annular keratotic ridge observed clinically. Sine the lesions is circular, a section will often include a cornoid lamella on either edge of the specimen. The superficial portion often leans toward the center of the lesion. The balance of the epidermis can be atrophic, normal or hyperplastic, with effaced rete ridges. A perivascular and lichenoid lymphocytic infiltrate may be present, often localizing beneath the cornoid lamella.

How is porokeratosis treated?

Porokeratosis are usually treated with various destructive modalities. They are typically more resistant to treatment than actinic keratoses or seborrheic keratoses. Cryotherapy, topical 5-FU, shave excision, CO2 laser, curretage, excision, have all been used. Oral acitretin may be beneficial in reducing the size of lesions, but will not eradicate them.

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What is the prognosis for porokeratosis?

Development of squamous cell carcinoma has been reported to occur in all variants of porokeratosis with the exception of punctate porokeratosis. Squamous cell carcinoma, when it arises, usually appears on the border of the lesion, giving rise to an asymmetric appearance. Despites its association with actinic damage, DSAP has the lowest risk of malignant degeneration.

Image links

DSAP from American OSteopathic College of Dermatology
DSAP Fact Sheet and Photos from DermNet NZ
Porokeratosis Hi stopathology from Dr. William Weems
Case report with photo and histopathology from Medical Observer