Porokeratosis of Mibelli (PM) is a form of porokeratosis (see this term) that is characterized by the presence of brown single or multiple annular plaques of varying size, that are sometimes confluent, with a distinctive sharply-defined keratotic border.
The prevalence is unknown. It is more prevalent in males than females with a ratio of 2-3:1.
Disease onset usually occurs in children, adolescents or adults (sometimes immunosuppressed). It presents with brownish dry hyperkeratotic plaques of varying size that may coalesce. The lesions have a sharply-defined keratotic border and are usually asymptomatic, but rarely pruritic. They most commonly occur on the limbs (hands and feet) but other areas such as the shoulders and genitalia can also be affected. Facial and mucosal lesions are rare. In around 7 % of cases, PM can undergo malignant transformation, mostly toward squamous cell carcinoma, or less commonly, basal cell carcinoma.
The exact etiology is unknown but the lesions are thought to originate from the localized expansion of a clone of abnormal keratinocytes. Contributing factors include immunosuppression, exposure to ultraviolet radiation and drugs, as well as genetic factors (chromosome 3p14-p12 instability).
Diagnosis is based on physical examination and a cutaneous biopsy showing the distinctive cornoid lamella, i.e. a narrow, vertical stack of parakeratotic corneocytes within the horny layer seated on a depression of the underlying epidermis.
The main differential diagnoses are psoriasis, actinic keratosis, elastosis perforans serpiginosa, annular lichen planus, circumscribed palmoplantar hypokeratosis, focal palmoplantar keratoderma (see these terms) and Bowen's disease.
Autosomal dominant inheritance has been reported although PM is often sporadic.
In certain patients, lesions have improved with topical 5-fluorouracil (5-FU), imiquimod, cryotherapy, dermabrasion, shave excision, carbon dioxide laser ablation or photodynamic therapy. However the lesions often recur after treatment. Regular follow-up is recommended to monitor the development of any possible malignancies, and sun exposure should be limited in order to decrease their risk.
PM is a chronically progressive disease and can have a negative impact on a patient's quality of life due to the presence of lesions. Rarely (in less than 10% of cases), long-standing lesions may transform into squamous cell carcinoma, which has exceptionally proven fatal.
Last update: October 2015