The porokeratoses are a group of benign epidermal proliferations. The most common and best-described clinical variants include disseminated superficial actinic porokeratosis (DSAP), porokeratosis of Mibelli, porokeratosis palmaris et plantaris disseminata, and punctuate porokeratosis. The underlying disease state is the same for all variants, as are the characteristic and diagnostic histopathological findings. There are many other clinical variants that are infrequently seen.
Clinical Findings: Porokeratoses are typically inherited in an autosomal dominant fashion. They manifest beginning in the third to fourth decades of life and are more common in sun-exposed areas. The lesions can be minute to a few centimeters in diameter. They usually are 1- to 2-cm, thin, flesh-colored to slightly pink or hyperpigmented patches with a characteristic hyperkeratotic surrounding rim. This rim encompasses the entire lesion and is almost pathognomonic for porokeratosis.
The DSAP form is the most common and most easily recognized clinical variant. Patients present with a family history of similar skin growths. The lesions are almost entirely located in sun-exposed regions of the body. Patients who have had more ultraviolet light exposure over their lifetime are more likely to have multiple and more noticeable lesions. Most porokeratoses are asymptomatic, and patients typically present because of the appearance of the lesions and the fact that they continue to develop more lesions over time. Most lesions are flesh colored to slightly pink or red. Some can be frankly inflamed, with redness and crusting. Transformation into squamous cell carcinoma has been reported. and patients should be counseled to be reevaluated if they develop growths or ulcerations within the porokeratosis. The lesions of DSAP are much more likely to affect the skin on the extremities than the facial skin.
The porokeratosis of Mibelli is a solitary lesion, or a group of lesions with a linear array that have an identical morphology of a thin patch with a thin hyperkeratotic rim. They may occur anywhere on the body.
Porokeratosis palmaris et plantaris disseminata is a unique variant that affects the skin of the palms and soles initially and then can disseminate into a generalized pattern. The lesions of the palms and soles can be tender. This variant is also inherited in an autosomal dominant manner. The lesions begin on the palms and soles during the third to fourth decades of life and slowly spread to other areas of the skin in a generalized pattern.
Punctate porokeratosis of the palms and soles is a rare clinical variant that is localized to the palms and soles. The lesions tend to be 0.5 to 1 cm in diameter and have a well-defined rim of hyperkeratosis. Occasionally, they can be mistaken for plantar warts.
Pathogenesis: The pathogenesis of porokeratosis, no matter which variant, is believed to be an abnormality of keratinocyte proliferation. A clonal expansion of the abnormal keratinocytes leads to development of the expanding rim of hyperkeratotic tissue. This rim of hyperkeratosis is recognized histopathologically as the cornoid lamella. No genetic defect has been identified. Porokeratosis is more commonly found in patients taking chronic immunosuppressive medications (e.g., after solid organ transplantation) and in those infected with the human immunodeficiency virus. This is indirect evidence that chronic immunosuppression may lead to a lack of tumor surveillance and the development of porokeratosis.
Histology: On biopsy, the hallmark of porokeratosis is recognition of the cornoid lamella. The cornoid lamella is the pathological representation of the hyper- keratotic peripheral rim of tissue seen on clinical examination. The cornoid lamella is positioned at an angle away from the center of the lesion. The granular cell layer underneath the cornoid lamella is often absent or severely thinned. The appearance of the center of the lesion is dependent on the clinical variant seen. The area can be atrophic or acanthotic. It is not uncommon to see an inflammatory infiltrate underneath the lesion, composed predominantly of lymphocytes.
Treatment: Treatment is difficult and often unsuccessful for widespread areas such as those involved in DSAP. Sun protection and sunscreen use are recommended. Solitary lesions can be removed surgically. Multiple disseminated lesions can be ablated with carbon dioxide laser ablation, 5-fluorouracil, or dermabrasion. These therapies are not always effective and may be associated with scarring. It is imperative to continue to monitor these patients with routine skin examinations, because porokeratoses have a potential for malignant degeneration.