History
Classic porokeratosis (Mibelli)
Porokeratosis of Mibelli (PM) is the second most common form of porokeratosis, accounting for about a third of reported cases. [54] It is seen in men more than twice as often as women and may first appear in childhood or young adulthood. PM initially appears as a small, asymptomatic, or slightly pruritic lesion develops that slowly expands 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.
Most PM lesions reach several centimeters in diameter, although “giant porokeratosis” lesions might grow to 10 cm or 20 cm. [55] Classically, PM lesions are located on an extremities, although they may occur anywhere, including the palms, soles, genitalia, or mucous membranes. [54, 56, 57] The center of the lesion may be slightly hypopigmented or hyperpigmented, minimally scaly, slightly atrophic, and hairless. Occasionally lesions may be confluent thick hyperkeratotic or verrucous plaques. [58, 59]
Disseminated superficial (actinic) porokeratosis
Disseminated superficial (actinic) porokeratosis (DSAP) is the most common form of porokeratosis, and may account for almost half of all cases. [54] Patients develop a few to several dozen tan, annular macules with raised peripheral ridges, developing predominantly on the distal extensor surfaces of the legs and the arms. Palms and soles are spared, and facial lesions may be seen in less than 15% of patients. Hyperkeratotic variants have been described. [60] The lesions are usually asymptomatic, but they may itch or sting slightly. [60] Extensive exposure to natural or artificial ultraviolet radiation may trigger or worsen DSAP.
The cornoid lamellae may be stained and accentuated by sunless tanning lotions containing dihydroxyacetone.
Patients are typically women in their third or fourth decade of life, with a history of ultraviolet light exposure. Patients may have a history of phototherapy for psoriasis. There is frequently a family history of DSAP, especially in other females in the family.
Lesions of disseminated superficial porokeratosis (non-actinic) appear very similar except in a generalized distribution. Patients with DSP may be more likely to be immunosuppressed and to be less likely to have worsening with sun exposure than patients with DSAP. [61]
Linear porokeratosis
Linear porokeratosis lesions typically develop during infancy or early childhood and may represent a segmental form of disseminated superficial porokeratosis (DSP). [20, 62] Females are slightly more likely to be affected than males. [34] Lesions arise usually as unilateral reddish brown patches or linear keratotic papules and plaques that are typically distributed along the lines of Blaschko, suggesting cutaneous mosaicism. Less commonly, patients may have bilateral involvement with a generalized or systematized forms.
All cases of linear porokeratosis have a higher risk of malignant degeneration than other forms of porokeratosis, possibly because of the allelic loss hypothesized to be at fault for the formation of the lesions.
Porokeratosis palmaris et plantaris disseminata
Small, relatively uniform lesions first develop in adolescence or early adulthood on the palms and the soles. They may then spread in a more generalized distribution, occasionally along lines of Blaschko. The lesions are usually asymptomatic, but they may itch or be tender to palpation and plantar lesions may cause discomfort with walking. Disseminated areas of involvement may include the mucosal membranes, where they occur as multiple small, depressed, opalescent rings with hyperemic borders. [63] Males are affected twice as often as females.
Punctate porokeratosis
Punctate porokeratosis presents as multiple, asymptomatic, tiny, hyperkeratotic papules with thin, raised margins developing on the palms and the soles during adulthood. Some authors consider this to be a forme fruste of porokeratosis palmaris et plantaris disseminata (PPPD), rather than a separate entity. [64]
Other variants
Porokeratosis ptychotropica presents with inflammatory keratotic plaques with histopathologic foci of cornoid lamellae on the buttocks or genitals, and may be mistaken for psoriasis. [65, 66]
Follicular porokeratosis is described as an eruptive papular porokeratosis in the setting of DSP. Porokeratosis may occur syndromically with craniosynostosis and anal anomalies (CAP syndrome). [67]
Porokeratotic adnexal osteal nevus is the name proposed to incorporate porokeratotic eccrine ostial and dermal duct nevus (PEODDN) and porokeratotic eccrine and hair follicle nevus (PEHFN). It is considered a congenital lesion, usually developing during infancy or childhood. Lesions consist of hyperkeratotic papules and plaques, and occasionally punctate pits filled with a comedolike keratin plug. It develops typically on the distal extremities, although proximal extremities, trunk, and face may be involved. During the neonatal period, the lesions may initially appear as erosions. Lesions are often aligned along the lines of Blaschko and may extend into nailbeds. See the image below.
A young boy with a linear lesion of porokeratotic eccrine ostial and dermal duct nevus extending onto the nailbed, causing pterygium formation.
Physical Examination
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 may be 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. Porokeratosis ptychotropica is a verrucous variant that is localized to the buttocks and clinically resembles psoriasis. Note the image below.
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.
Facial lesions are seen in approximately 15% of patients. Nonactinic disseminated superficial porokeratosis (DSP) has a generalized distribution, sparing the palms and the soles. Bullous and pruritic variants have been described. Note the images below.
Disseminated superficial actinic porokeratosis on the lower legs of a female patient.
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 or blaschkoid distribution, and they may arise initially as reddish-brown patches.
Multiple linear groups may be seen in one patient. 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 porokeratosis of Mibelli (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 porokeratosis palmaris et plantaris disseminata (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.
Punctate follicular porokeratosis
Follicular involvement is contiguous with an annular cornoid lamella. It can have the appearance of multiple static perifollicular punctate keratotic lesions that occur in a patch distant from the usual location of keratosis pilaris on the lateral upper arms or anterior lateral thighs. [10]
Other variants
Porokeratosis ptychotropica presents with multiple plaques involving the medial buttocks and gluteal cleft, clinically resembling psoriasis.
Patients with porokeratotic adnexal ostial nevus develop hyperkeratotic papules and plaques, and occasionally punctate pits filled with a comedolike keratin plug. The lesions are typically on the extremities and occasionally the trunk and/or face. If present at birth, they may present as erosions that evolve into darker brown, well-marginated linear plaques. Pterygium formation can occur if lesions extend into the nail bed. Lesions are often distributed along the lines of Blaschko.
Rare case reports of porokeratotic adnexal ostial nevus (PAON) describe other findings, including psoriasis, focal anhidrosis, developmental delay, seizures, scoliosis, hemiparesis, polyneuropathy, hyperthyroidism, hearing loss, and breast hypoplasia. Squamous cell carcinoma has been reported to develop within well-established lesions. Additionally, late-onset lesions developing during adulthood may be clinically indistinguishable from PPPD. [68]
Complications
The most important complication to watch for is the development of cutaneous malignancy. Functional impairment due to involvement of critical anatomical locations may develop. Prophylactic excision could be considered in appropriate situations.
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Porokeratosis of Mibelli on the lower leg in a renal transplant recipient.
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Disseminated superficial actinic porokeratosis on the lower legs of a female patient.
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A 42-year-old woman with multiple lesions on the pretibial aspects of the legs.
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A young boy with a linear lesion of porokeratotic eccrine ostial and dermal duct nevus extending onto the nailbed, causing pterygium formation.
