eMedicine Specialties > Dermatology > Benign Neoplasms

Porokeratosis

Author: Linda V Spencer, MD, Consulting Staff, Department of Dermatology, St Clare Medical Center
Contributor Information and Disclosures

Updated: Aug 3, 2009

Introduction

Background

Porokeratosis is a clonal disorder of keratinization characterized by 1 or more atrophic patches surrounded by a clinically and histologically distinctive hyperkeratotic ridgelike border called the cornoid lamella. Five clinical variants of porokeratosis are recognized: classic porokeratosis of Mibelli (PM), disseminated superficial actinic porokeratosis (DSAP), porokeratosis palmaris et plantaris disseminata (PPPD), linear porokeratosis, and punctate porokeratosis. Several other variants have been described, including hyperkeratosis types, a pruritic papular variant, and an unusual verrucous variant that is localized to the buttocks and mimics psoriasis.¹ Occasionally, a patient may develop more than one type of porokeratosis.

Pathophysiology

Clonal hyperproliferation of atypical keratinocytes leads to the formation of the cornoid lamella, which expands peripherally and forms the raised boundary between abnormal and normal keratinocytes. Local or systemic changes in immune function may allow the development of atypical clones of keratinocytes. Loss of heterozygosity has been proposed as a mechanism for linear porokeratosis associated with a personal or family history of DSAP.

Several risk factors for the development of porokeratosis have been identified; these factors include genetic inheritance, ultraviolet radiation, and immunosuppression. Immunosuppression associated with porokeratosis may be secondary to a disease process such as HIV infection or lymphoma or an iatrogenic suppression such as with immune-modulating drugs used to prevent organ transplant rejection or to treat autoimmune diseases.^2,3 An autosomal dominant mode of inheritance has been established for familial cases of all forms of porokeratosis. Sun exposure is thought to cause DSAP, although the typical sparing of facial skin is unexplained. Excessive natural or artificial ultraviolet radiation, electron beam therapy, and extensive radiation therapy are well-established trigger factors. Immunosuppression may induce new lesions or cause preexisting lesions to flare.

The formation of squamous or basal cell carcinomas has been reported in all forms of porokeratosis.^4,5,6 Chromosomal instability and reduced immune surveillance with overexpression of p53 are hypothesized to play a role in the development of cutaneous malignancies within porokeratosis.^7,8,9,10,11

Frequency

United States

DSAP is relatively common. The other forms of porokeratosis are rare.

Mortality/Morbidity

• Most lesions are asymptomatic. Ulcerative lesions have been described. Giant PM in a facial or acral location may cause destruction of underlying soft tissue or pseudoainhum with amputation.¹²
• Malignant degeneration has been reported in all forms of porokeratosis, with risks of 7.5% and 11% determined from 2 published reviews of the literature. Large lesions, lesions of long-standing duration, and the linear type of porokeratosis were found to be at greatest risk. A squamous cell carcinoma developing within a large PM lesion, associated with extensive metastases and hypercalcemia, has been reported.¹³

Race

• Porokeratosis most commonly occurs in fair-skinned individuals. It is rare in darker-skinned populations.

Sex

• PM and PPPD affect men twice as often as women.
• DSAP is 3 times as likely to develop in women compared with men.
• Linear porokeratosis is seen with equal incidence in men and women.

Age

• PPPD and linear porokeratosis may be seen at any age, from birth to adulthood.
• PM usually develops in childhood.
• Disseminated superficial porokeratosis (DSP) generally develops in the third or fourth decade of life.

Clinical

History

• Classic porokeratosis (Mibelli)
  • During childhood, a small, asymptomatic or slightly pruritic lesion develops, expanding over a period of years.
  • Less commonly, lesions may develop during adulthood and enlarge rapidly, usually in the clinical setting of immunosuppression.
  • Occasionally, patients have a history of an antecedent trauma, such as a burn wound.
• Disseminated superficial (actinic) porokeratosis
  • Multiple, brown, annular, keratotic lesions that develop predominantly on the extensor surfaces of the legs and the arms characterize DSAP. They are usually asymptomatic, but they may itch slightly. Sunless tanning lotions containing dihydroxyacetone cause the cornoid lamellae to darken and become more clinically prominent. Dermoscopy reveals a "white track" structure with brown pigmentation on the inside of the track that can be seen at the edge of an individual lesion, corresponding to the cornoid lamella. Centrally, a white area with red dots, globules, and lines is present that corresponds to capillary vessels; it is more easily observed through the atrophic epithelium.
  • Facial lesions are seen in approximately 15% of patients, but the face may be the only area of involvement.
  • Patients are typically women in their third or fourth decade of life, with a history of excessive ultraviolet exposure.
  • Patients may have a history of phototherapy for psoriasis.
  • Nonactinic forms may be seen following electron beam total skin irradiation, organ transplantation, hepatitis C virus–related hepatocellular carcinoma,¹⁴ HIV infection, renal failure, or in association with other causes of immunosuppression.
• Linear porokeratosis: Linear porokeratosis lesions typically develop during infancy or early childhood. They arise as reddish-brown patches or linear keratotic papules and plaques, typically distributed along Blaschko lines, suggesting cutaneous mosaicism. Linear porokeratosis can be localized, zosteriform, generalized, or systematized and can have a higher risk of malignant degeneration, possibly because of allelic loss.
• Porokeratosis palmaris et plantaris disseminata
  • Small, relatively uniform lesions are first seen on the palms and the soles. They may then develop in a generalized distribution, including the mucosal membranes.
  • The lesions may itch or sting, but they are usually asymptomatic.
  • The onset is typically during adolescence or early adulthood, and males are affected twice as often as females.
• Punctate porokeratosis: Multiple, asymptomatic, tiny, hyperkeratotic papules with thin, raised margins develop on the palms and the soles during adulthood.

Physical

• Classic porokeratosis (Mibelli)
  • The lesion develops as a small, light brown, keratotic papule that slowly expands to form an irregularly shaped, annular plaque with a raised, ridgelike border. This border may be hypertrophic or verrucous and is usually greater than 1 mm in height. A thin furrow is typically seen in the center of the ridge, causing a Great Wall of China effect. The lesion is slightly hypopigmented or hyperpigmented, minimally scaly, slightly atrophic, hairless, and anhidrotic. The size may vary from a few millimeters to several centimeters.
  • Lesions may be found anywhere, including the mucous membranes, although they most commonly occur on the extremities. Generally, few lesions are observed. A verrucous variant that is localized to the buttocks and resembles psoriasis has been reported in several patients.

• Disseminated superficial (actinic) porokeratosis
  • Dozens of small, indistinct, light brown patches with a threadlike border are seen on the extensor surfaces of the arms and the legs.
  • Dermoscopy shows a "white track" structure at the periphery of the lesion, with a brownish pigmentation on the inner side and with a double white track in some parts of the lesion. This corresponds to the cornoid lamella. The center may show a white homogeneous area, corresponding to acanthotic epithelium, or red dots, globules, and lines corresponding to enlarged capillary vessels that can be seen because the epithelium is atrophic.¹⁵
  • Facial lesions are seen in approximately 15% of patients.
  • Nonactinic DSP may have a generalized distribution, sparing the palms and the soles.
  • Bullous and pruritic variants have been described.

• Linear porokeratosis
  • Grouped, linearly arranged, annular papules and plaques with the characteristic raised peripheral ridge are seen unilaterally on an extremity, the trunk, and/or the head and neck area. The lesions commonly follow a dermatomal distribution, and they may arise initially as reddish-brown patches.
  • Multiple linear groups may be seen in one patient, typically on the same side. They may be seen in association with other forms of porokeratosis.
  • Individual lesions within the linear grouping have a well-developed border, often with a central furrow similar to that seen in classic PM.
  • Clinical changes consistent with the development of a basal or squamous cell carcinoma are more common in linear porokeratosis than in other forms of porokeratosis.
• Porokeratosis palmaris et plantaris disseminata
  • The lesions are small, superficial, relatively uniform, with a slightly hyperpigmented, atrophic center and a minimally raised peripheral ridge. Mucosal lesions are small, annular or serpiginous, and pale.
  • Squamous cell carcinoma has been reported to develop within lesions of PPPD.
• Punctate porokeratosis
  • Dozens of discrete or grouped seedlike hyperkeratotic lesions with characteristic thin, raised ridgelike margins develop on the palms and the soles. Patients usually have other forms of porokeratosis as well, most commonly the linear or Mibelli types.
  • Punctate porokeratosis may be clinically and histologically indistinguishable from punctate porokeratotic keratoderma, which is considered to be a cutaneous sign of an internal malignancy.

Causes

Risk factors for porokeratosis include genetic inheritance, ultraviolet light exposure, and immunosuppression. One study found that approximately 10% of patients who had undergone renal transplantation developed porokeratosis.¹⁶

• Classic porokeratosis (Mibelli)
  • Autosomal dominant inheritance and immunosuppression are the usual causes.
  • PM has been seen following radiation therapy, at burn wounds,¹⁷ and at hemodialysis sites.
• Disseminated superficial (actinic) porokeratosis
  • Sun exposure and/or artificial ultraviolet radiation exposure in a patient who is genetically predisposed causes DSAP. Exacerbations have been reported following prolonged sun exposure, repeated tanning bed exposure, electron beam radiation therapy, and therapeutic phototherapy or photochemotherapy for psoriasis. Drug-induced photosensitivity may play a role. Protection from ultraviolet radiation may lead to spontaneous resolution.
  • Immunosuppression predisposes patients to both DSAP and nonactinic DSP. Because of this, a viral etiology has been hypothesized.
• Linear porokeratosis
  • No definite inheritance pattern has been established.
  • Loss of heterozygosity has been proposed as a genetic mechanism and may explain the higher risk of malignant degeneration seen in linear porokeratosis in comparison to other forms of porokeratosis.
• Porokeratosis palmaris et plantaris disseminata
  • Familial PPPD is transmitted in an autosomal dominant mode with variable penetrance.
  • Acquired PPPD may be caused by immunosuppression, or it may be a cutaneous marker of internal malignancy.
• Punctate porokeratosis: This condition has no unique inheritance pattern and is usually associated with other forms of porokeratosis.

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