Porokeratosis is a clonal disorder of keratinization characterized by one or more atrophic patches surrounded by a clinically and histologically distinctive hyperkeratotic ridgelike border called the cornoid lamella. Historically, porokeratosis is a misnomer that was erroneously coined on the assumptions that cornoid lamellae emerged from pores of the sweat glands.  Multiple clinical variants of porokeratosis are recognized. Malignancies, typically squamous cell carcinomas, may develop within lesions of porokeratosis.
The most common forms of porokeratosis are as follows:
Classic porokeratosis of Mibelli (PM) 
Disseminated superficial actinic porokeratosis (DSAP) and its nonactinic variant disseminated superficial porokeratosis (DSP)
Porokeratosis palmaris et plantaris disseminata (PPPD)
Punctate porokeratosis, which might represent a variant of PPPD 
A patient may develop more than one type of porokeratosis simultaneously or consecutively. 
Less commonly reported clinical entities that share the histopathologic characteristic of cornoid lamellation include the following:
Porokeratosis ptychotropica, a verrucous variant localized to the buttocks 
Porokeratoma, also referred to as porokeratotic acanthoma, an entity arising as a solitary keratotic or verrucous nodule with prominent multiple and confluent cornoid lamellae 
Porokeratotic adnexal ostial nevus (PAON), a rare congenital disorder of keratinization with eccrine and hair follicle involvement: This name was proposed to incorporate porokeratotic eccrine ostial and dermal duct nevus (PEODDN) and porokeratotic eccrine and hair follicle nevus (PEHFN). 
Pigmented porokeratosis: A case published in 2016 showed a new possible variant of porokeratosis. Biopsies showed prominent melanocytic hyperplasia in 8 of 31 patients studied. The case represents a distinct variant of porokeratosis; biopsies were performed to clinically exclude melanoma. An erroneous diagnosis of melanoma in situ can be avoided with increased recognition of this entity. 
Punctate follicular porokeratosis has been described in two separate reports, which supports the contention that this is a unique form rather than just a variation in histology. In both cases, the cornoid lamellae originated exclusively from hair follicles with no associated annular plaque. Diagnosis relies heavily on proper histopathologic sampling, and the rarity of this variant could be explained by the low prevalence of this condition in the population or perhaps low biopsy rate of lesions that appear to represent a localized form of keratosis pilaris to physicians. 
Clonal proliferation of atypical keratinocytes showing abnormal terminal keratinocyte differentiation leads to the formation of the cornoid lamella. Cornoid lamellation is typically seen in porokeratosis, where it corresponds to the threadlike scale present at the lesional border.  This expands peripherally and forms the raised boundary between abnormal and normal keratinocytes. The atypical keratinocytes show abnormal differentiation but do not show an increased rate of proliferation. [11, 12]
It is not known what triggers this process, and more than one causative factor may be involved. Several risk factors for the development of porokeratosis have been identified, including genetic inheritance, ultraviolet radiation, and immunosuppression.
Inherited or sporadic genetic defects, possibly creating a change in immune function and/or keratinocyte function, are thought to be responsible for several forms of porokeratosis. Familial cases of all forms of porokeratosis have been reported and appear to have an autosomal dominant inheritance pattern with incomplete penetrance. Several chromosomal loci have been identified for DSAP, DSP, and PPPD. [13, 14, 15, 16, 17, 18, 19] The focal variants of porokeratosis (PM and linear porokeratosis) may occur through mosaicism, in which somatic mutations cause focal loss of heterozygosity.  Genetic mutations in the SART3 and MVK genes have been found in DSAP pedigrees.
A somatic mutation in the GJB2 mutation associated with keratosis-ichthyosis-deafness syndrome was found to cause porokeratotic eccrine ostial and dermal duct nevus. 
Natural or artificial ultraviolet radiation, electron beam therapy, and extensive radiation therapy are well-established trigger factors for DSAP and PM. Sun exposure in genetically susceptible individuals is thought to cause DSAP, although the sparing of facial skin in most patients is unexplained.
Immunosuppression associated with porokeratosis may be secondary to a disease process or medications. Diseases reported in association with porokeratosis include HIV infection,  diabetes mellitus,  liver disease,  and hematologic or solid organ malignancy. [25, 26] Immunomodulating drugs used to treat autoimmune diseases or to prevent organ transplant rejection may also trigger porokeratosis. [27, 28, 29, 30, 31, 32, 33, 34, 35] Localized cutaneous immunosuppression due to long-term application of a potent topical steroid was reported to induce PM.  The incidence of porokeratosis in organ transplant recipients has been reported to be anywhere from 1-11%. [28, 31]
Immunosuppression may induce new lesions or cause preexisting lesions to flare. New lesions of porokeratosis have occurred as quickly as 4 months after initiation of immunosuppressive therapy, or as long as 14 years [31, 33] and may resolve following cessation of immunosuppressive therapy. [34, 35]
Trauma, such as a burn, may also trigger porokeratosis. 
DSAP and PM are the most commonly seen variants. The other forms of porokeratosis are rare.
Porokeratosis most commonly occurs in fair-skinned individuals. Porokeratosis is rare in darker-skinned populations.
PM and PPPD affect men twice as often as women.
DSAP is twice as likely to develop in women compared with men. 
Linear porokeratosis is seen with equal incidence in men and women.
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.
The prognosis is generally excellent. This is especially true for disseminated superficial porokeratosis (DSP).
Large lesions of porokeratosis, linear porokeratosis, or porokeratosis in an immunocompromised patients should be monitored carefully for the development of cutaneous malignancies within the porokeratosis lesions. Malignant transformation can occur in about 7.5% of patients. [38, 39, 40] Lesions that are large, long-standing, or linear have the greatest risk of developing an associated malignancy.  Chromosomal instability and reduced immune surveillance with overexpression of p53 are hypothesized to play a role in the development of cutaneous malignancies within porokeratosis. [42, 43, 44, 45, 46]
PM circumferentially involving the digits may induce pseudoainhum. 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. 
Patients must practice strict sun precautions. These measures include wearing protective clothing; applying sunblock; avoiding exposure to midday sunlight; and discontinuing exposure to artificial ultraviolet light, such as tanning beds and therapeutic phototherapy.
Consideration for vitamin D3 supplementation should be made given the need for careful sun avoidance and protection.
Patients must periodically examine their skin for lesions suggestive of malignancy. A qualified physician should promptly evaluate any change in a porokeratosis lesion.
Family members should be examined for porokeratosis if familial porokeratosis is suspected.
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