Porokeratosis
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.
