Disseminated superficial actinic porokeratosis (ORPHA:79152)
A rare skin disease that is the most common form of porokeratosis characterized by the presence of several small annular plaques with a distinctive keratotic rim found most commonly on sun-exposed areas of the skin, particularly the extremities.
Epidemiology
Disseminated superficial actinic porokeratosis (DSAP) prevalence is not precisely known, although DSAP is the most common form of porokeratosis. It is more frequently seen in women, probably because they more readily seek advice for cosmetic concerns.
Clinical description
The disease usually starts during the third to fourth decade of life (only rarely during childhood). DSAP is characterized by several small (0.5-1 cm), round, pink-brownish plaques, surrounded by a distinctive keratotic rim, corresponding microscopically to the cornoid lamella. They are painless, but pruritus is reported in one third of patients. The lesions appear on skin that is exposed to sunlight (usually the extremities) but never on the palms or soles. They usually appear in summer and may improve or disappear during winter. DSAP is usually a benign disease, although squamous cell carcinoma can very rarely develop within the lesions.
Etiology
Mutations in the mevalonate kinase (MVK) gene, located to chromosome 12q24, have been found in up to one third of DSAP cases. MVK encodes an enzyme in the mevalonate pathway, which is thought to be crucial for the biosynthesis of cholesterol and isoprenoid as well as the regulation of calcium-induced keratinocyte differentiation. More recently, pathogenic mutations in the SLC17A9 gene (20q13.33) were also found in DSAP patients. Risk factors for DSAP include exposure to ultraviolet light and immunosuppression.
Diagnostic methods
Histopathological examination of a cutaneous biopsy confirms the clinical diagnosis of DSAP. The characteristic feature is the presence of a cornoid lamella, i.e. a vertical stack of parakeratotic corneocytes within the horny layer, seated on a shallow depression of the underlying epidermis that is devoid of a granular layer. Molecular genetic testing for a mutation in the MVK gene can also confirm diagnosis.
Differential diagnosis
Differential diagnoses include (pre)neoplastic or hyperplastic keratotic skin lesions as well as other forms of porokeratosis, such as porokeratosis of Mibelli or superficial disseminated porokeratosis (similar to DSAP but not triggered by sunlight).
Genetic counseling
DSAP often follows an autosomal dominant pattern of inheritance, but sporadic cases have also been reported.
Management and treatment
There is no standard treatment for DSAP. Topical imiquimod 5% cream, topical 5-fluorouracil (5-FU) and topical vitamin D-analogues (tacalcitol, calcipotriol) have shown to be beneficial in treating the lesions of DSAP in some patients. Cryotherapy, electrodessication, laser ablation and photodynamic therapy have been tested with varying results. DSAP patients should limit their exposure to sun.
Prognosis
DSAP has a good prognosis as it very rarely progresses to carcinoma. However the disease may cause cosmetic concern and thereby exerts a negative effect on a patient's quality of life.

                            Last update: 
                            August 2020


                            - Expert reviewer(s): 
                            Pr Jean KANITAKIS