Presentations of Cutaneous Disease in Various Skin Pigmentations: Porokeratosis

Abstract

Description

Porokeratosis was first described in 1893. It is a relatively rare disorder with over 9 subtypes. Lesions are clinically characterized as well-demarcated, erythematous papules (raised, <1 cm) or plaques (raised, >1 cm), with an atrophic center, and raised scaly border. Porokeratosis is an important diagnosis to identify because it may undergo malignant transformation and mimics many commonly encountered diagnoses. These commonly mimicked diagnoses include squamous cell carcinoma, tinea corporis, nummular dermatitis, and psoriasis vulgaris, to name a few. The clinical images in this review focus on identifying porokeratosis along the full spectrum of skin tones.

Introduction

Porokeratosis is a heterogeneous group of skin disease with many clinical variants caused by disordered keratinization.¹ Initially described in 1893, there are now at least 9 subtypes identified. ² All variants of porokeratosis are characterized histologically by the presence of an invaginated column of parakeratotic cells within the epidermis, known as a cornoid lamella.³ The typical clinical presentation is an irregularly shaped annular papule or plaque consisting of an atrophic center bordered by a peripheral hyperkeratotic ridge.⁴ The lesions themselves may be variably asymptomatic or pruritic.⁴ Persons with porokeratosis have an increased risk of developing skin cancer.⁵ The pathogenesis of porokeratosis is complex and incompletely elucidated.⁶ A recently discovered autosomal dominant enzymatic defect has led to the development of novel treatments.⁷

Porokeratosis is a relatively common disorder with a prevalence of 24.2 per 100 000 reported in the Swedish population.⁵ It is an important diagnosis as it mimics other commonly encountered lesions. This differential includes actinic keratosis, squamous cell carcinoma (SCC), nummular dermatitis, tinea corporis, psoriasis, and seborrheic keratosis. Porokeratosis may also rarely exhibit malignant transformation.⁸ This series of clinical images focuses on identifying porokeratosis along the full spectrum of skin tones, as commonly described by the Fitzpatrick scale (Figure 1).⁹ Additional background information on the Fitzpatrick scale and a description of the classification of skin types are discussed further elsewhere.⁹

Figure 1.

The Fitzpatrick scale provides a classification system for an individual’s skin type based on the ability to burn and/or tan when exposed to ultraviolet light. It is used to approximate the degree of skin pigmentation.

Clinical Images

Figure 2 shows a White man with Fitzpatrick type I (always burns, never tans) skin. Figure 2A shows an image of the skin on the right lower leg demonstrating the typical presentation of classical porokeratosis of Mibelli with a sharply demarcated, annular, papule (raised, <1 cm) or plaque (raised, >1 cm), an atrophic center, and raised scaly border. The image in Figure 2B outlines distinct histologic features of porokeratosis. A column of parakeratotic cells within an invagination of the epidermis represents the cornoid lamella. Parakeratosis refers to retained nuclei in the stratum corneum (outermost layer of skin), representing proliferating skin cells that have not completely matured. This correlates clinically to the scaly raised rim bordering porokeratosis lesions. There is unilateral thinning of the stratum corneum on histopathology. This correlates to the central atrophy that can be appreciated visually on palpation. Central erythema is also readily apparent in this light-skin type.

Figure 2.

Fitzpatrick I (always burns, never tans): (A) Skin on the right lower leg demonstrates the classical presentation of porokeratosis of Mibelli. This variant is characterized by a sharply demarcated annular (ring-shaped) plaque (raised, >1 cm) with a peripheral hyperkeratotic ridge that surrounds an atrophic center. There is a clear boundary along the periphery of the raised edge, making it sharply demarcated. The scales themselves are limited to the edge. The center of the lesion is atrophic, evidenced by a lighter, pale pink color, thin quality, and central depression. Erythema is easily appreciated as bright pink. (B) The classic histopathologic presentation of the cornoid lamella is shown, which consists of a column of parakeratosis overlying an invagination of the epidermis (arrows). Epidermal atrophy is highlighted with stars. This area of thinned stratum corneum correlates to the atrophic center of porokeratosis.

Figure 3 illustrates the clinical presentation of cornoid lamella on the lower leg of a White man with Fitzpatrick type II (mostly burns, rarely tans) skin. A faintly erythematous papule with a white atrophic center is surrounded by a thin, raised, erythematous border. Figure 3B shows the results of an ink test that can be performed to delineate a cornoid lamella when it is difficult to appreciate clinically. The skin is colored with gentian violet from a surgical marking pen. This is left on for several seconds to allow for complete penetration. Excess ink is then removed with an alcohol wipe. Retained ink accentuates the keratotic rim within the cornoid lamella, distinguishing porokeratosis from similarly-appearing entities. A dermatoscopic examination (Figure 3C) of this lesion reveals a clearly defined erythematous hyperkeratotic border. The thin central skin results in easily visualized blood vessels underlying the atrophic surface.

Figure 3.

Fitzpatrick II (mostly burns, rarely tans): (A) Skin on the lower leg demonstrates another classical porokeratosis of Mibelli. This lesion has a less appreciable scale on its thread-like border. The central atrophy is again apparent. The erythema may vary with the Fitzpatrick type. This lesion is a dull red compared to the bright pink seen in Figure 2. Notice the mottled red and brown pigmentation interspersed with the skin surrounding the porokeratosis, representing sun-damaged skin. (B) The cornoid lamella is accentuated by surgical pen ink (gentian violet). The pen was used to mark the lesion, and the ink persisted on the raised rim after wiping with 70% isopropyl alcohol (ink test). (C) A dermoscopic view shows the sharply demarcated borders of the cornoid lamella.

Figure 4 shows a Hispanic woman with Fitzpatrick type IV (rarely burns, mostly tans) skin with porokeratosis of Mibelli on her thigh. Compared to lighter skin types, the erythema seen at the center begins as red-brown to violaceous, with hues of gray at the periphery. Dark brown erythema outlines the hyperkeratotic ridge. This variety of hues contributes to the notable hyperpigmented quality of the lesion.

Figure 4.

Fitzpatrick IV (rarely burns, mostly tans): (A) Skin of the thigh demonstrates a patient with classical porokeratosis of Mibelli. There is an annular plaque (raised, >1 cm) with an atrophic center surrounded by a thin hyperkeratotic, ridge-like border (cornoid lamella). These common features are seen in the previous figures but are distinctly unique in darker skin. The lesion itself is substantially darker with hues of brown, red, violet, and gray. Erythema in skin of color may present as violaceous-brown rather than pink-red. (B) A dermoscopic view demonstrates a heavily accentuated white scaly ridge corresponding to cornoid lamella. A light brown atrophic center can be appreciated in both figures.

Figure 5 is an Indian man with Fitzpatrick type V (almost never burns, always tans) skin with porokeratosis of Mibelli. This photo demonstrates a striking contrast between the peripheral and central aspects of the lesion. The raised hyperkeratotic ridge is heavily accentuated by the dark brown hyperpigmentation at the peripheral edge. The atrophic center is readily apparent by its hypopigmented, lighter pink coloration and depressed quality.

Figure 5.

Fitzpatrick V (almost never burns, always tans) skin demonstrating classic porokeratosis of Mibelli variant. The hyperkeratotic ridge-like border of this plaque (raised, >1 cm) is accentuated by dark brown hyperpigmented skin at the periphery and an atrophic, light pink interior. The center of the lesion is depressed. (Image reproduced with permission.¹⁰)

Figure 6 is a Brazilian woman with Fitzpatrick type IV (rarely burns, mostly tans) skin with disseminated superficial actinic porokeratosis (DSAP) on the face. The dark brown raised borders and atrophic centers of these papules (raised, <1 cm) and plaques (raised, >1 cm) are easily appreciated. The hyperkeratotic ridges are thin and are reddish-brown. Consistent with Figure 4, which also is Fitzpatrick type IV skin, erythema in a darker skin type is dark brown to violaceous in color.

Figure 6.

(A) Fitzpatrick IV (rarely burns, mostly tans) skin demonstrating disseminated superficial actinic porokeratosis (DSAP) on the face. It is characterized by multiple small porokeratosis lesions affecting a sun-exposed distribution. (B) There are multiple hyperpigmented papules (raised, <1 cm) and plaques (raised, >1 cm). The sharp demarcation is attributed to the dark brown borders encircling each lesion. Erythema is appreciated as faint pink to violet hues centrally. (Images reproduced with permission.¹¹)

Figure 7 is a White woman with Fitzpatrick type II (mostly burns, rarely tans) skin demonstrating a linear porokeratosis variant. There are dark reddish-brown linear plaques with pink erythema. The clinical diagnosis of porokeratosis is made by identifying the individual lesions. Individual papules that combine (coalesce) into linear plaques have the characteristic appearance of porokeratosis of Mibelli (sharply demarcated, annular papules or plaques with a peripheral hyperkeratotic ridge surrounding an atrophic center). These linear plaques are unilateral and appear to be swirled or wave-like (Figure 7B and 7C), known as a Blaschkoid distribution.

Figure 7.

Fitzpatrick II (mostly burns, rarely tans) skin demonstrating a variant called linear porokeratosis. In this patient, papules (raised, <1 cm) have coalesced into linearly oriented plaques (raised, >1 cm). (A) The plaques are distributed on the left chest, axilla, arm, and lower extremity. (B and C) The plaques follow a unilateral distribution with swirl or wave-like shapes that do not cross the midline. (D) Individual papules have the typical appearance of porokeratosis with sharply-demarcated raised borders (cornoid lamella) and atrophic centers. Identifying the individual peripheral papules allows the diagnosis to be made clinically.

Figure 8 is an Indian patient with Fitzpatrick type V (almost never burns, always tans) skin demonstrating a linear porokeratosis variant on the foot. There is less redness, and the overall color is gray to dark blue, which correlates to less inflammation compared to prior images. Between the plaques are areas of hypopigmentation, where the skin is a lighter shade than that which surrounds it. This likely represents post-inflammatory pigmentary changes associated with porokeratosis.

Figure 8.

Fitzpatrick V (almost never burns, mostly tans) skin demonstrating linear porokeratosis on the foot. It is easily identified as porokeratosis as individual papules (raised, <1 cm) have ridge-like borders with atrophic centers. The lesions feature more blue-grey pigmentation compared to the previous example in a lighter skin type. There is a faint violaceous erythema present between the coalesced papules, masked by the natural skin tone. (Image reproduced with permission.¹⁰)

Discussion

Common clinically encountered variants of porokeratosis include porokeratosis of Mibelli, DSAP, and linear porokeratosis. These entities are categorized based on variations in morphology, distribution, and clinical course.⁴ Other rare porokeratosis variants that will not be discussed are porokeratosis palmaris et plantaris disseminata, porokeratosis ptychotropica, punctate porokeratosis, eruptive bullous porokeratosis, follicular porokeratosis, and lichen planus-like porokeratosis. The diagnosis of porokeratosis can be readily identified by the distinct appearance of a well-demarcated, irregularly-shaped annular lesion, consisting of an atrophic center and a distinct, hyperkeratotic, ridge-like border (Figures 2A, 3A, and 4A).¹² This often mimics other more commonly seen annular lesions. The differential diagnosis of porokeratosis includes nummular dermatitis, tinea corporis, seborrheic keratosis, psoriasis, SCC, and actinic keratosis (Figures 9–12).

Figure 9.

Fitzpatrick I (always burns, never tans) skin demonstrating acute (A) and chronic (B) nummular dermatitis. There are multiple round plaques (raised, >1 cm) distributed symmetrically on the extremities. Acute lesions may have bright red erosions. (C) These “coin-shaped” lesions are diffusely scaly and erythematous. They are distributed linearly due to the pattern of scratching. Lesions that have been present longer have a light pink hue and are violaceous from postinflammatory hyperpigmentation.

Figure 10.

Fitzpatrick IV (rarely burns, mostly tans) skin demonstrating tinea corporis, a common differential diagnosis for porokeratosis. (A) Notice the heavily accentuated, clearly defined erythematous border that uniformly surrounds the annular plaques (raised, >1 cm). There is hypopigmentation and scale uniformly distributed throughout the center. (B) On the left thigh, there are more annular plaques beginning to form a polycyclic arrangement. In contrast to porokeratosis, tinea infections may feature a sharp red margin with interspersed papules or pustules.

Figure 11.

Fitzpatrick I (always burns, never tans) skin demonstrating multiple seborrheic keratoses on the right breast. They are light brown papules (raised <1 cm) with a waxy quality. This contributes to their classic “stuck-on” appearance. The erythema of the central lesion, which demonstrates inflammation, is easily perceived from its bright pink color. Compared to porokeratosis, the pigment and scale are overall uniform throughout every lesion. There are no central atrophic areas.

Figure 12.

Fitzpatrick IV (rarely burns, mostly tans) skin demonstrating psoriasis on the neck. There is a well-defined papule (raised <1 cm) with a thick silver scale. It features a characteristic guttate (teardrop) shape. The erythema is uniformly distributed throughout the lesion. In contrast to porokeratosis, the scale of psoriasis has a thick white (micaceous) quality.

In 1889, Vittorio Mibelli described a man with lesions of varying size and form on the arms and hands present since the age of 2.¹³ The lesions were whitish-red and bordered by an elevated, uninterrupted ridge.¹³ The patient’s father, brother, and sister were similarly affected. ¹³ He referred to this condition as “porokeratosis,” and it subsequently became known as porokeratosis of Mibelli.¹

Lesions of porokeratosis of Mibelli appear as one or more asymptomatic papules that slowly enlarge in a centrifugal fashion.¹⁴ The classic clinical presentation is a sharply demarcated, irregularly shaped annular lesion with a peripheral hyperkeratotic ridge and a longitudinal furrow surrounding an atrophic center.¹⁵ The central portion becomes hairless, atrophic, and hypopigmented as the lesion grows.¹⁴ It may occur anywhere on the body but is usually confined to the extremities.¹⁴ Porokeratosis of Mibelli may be acquired or inherited in an autosomal dominant pattern.¹⁴ Onset is typically during childhood, especially in hereditary cases.¹⁴ Non-hereditary cases occur later in life.¹⁴ Men are more often affected at an incidence of 2.17 to 1.4 The etiology is multifactorial, with triggering factors including genetics, drugs, trauma, and immunosuppression.^16–18 Malignant transformation of histologically confirmed porokeratosis of Mibelli has been reported in 6.4% of patients.⁵

Disseminated superficial actinic porokeratosis is the most common porokeratosis variant.^19,20 First described in 1967 by Chernosky, DSAP presents as multiple small, scaly papules with raised borders in sun-exposed areas during the third to fourth decades of life.²¹ The number of lesions varies from 3 to several hundred.²² DSAP is usually asymptomatic but may also be pruritic.²¹ It is an autosomal dominant disorder with reduced penetrance at young ages.²³ Patients may experience exacerbations during the summer.²⁰ This observation led to the hypothesis of ultraviolet induction as a pathogenic factor.²⁰ It has also been associated with other triggers such as immunosuppression and UVB irradiation.^24–26 Facial involvement in DSAP is uncommon, occurring in less than 15% of patients. ²⁷ This may vary by race as studies in the Chinese population have shown DSAP involvement of the face in up to 82.4% of patients.²⁸

In 1974, Rahbari described linear porokeratosis as a distinct entity from porokeratosis of Mibelli. ²⁹ Recent genetic evidence suggests linear porokeratosis arising as a result of postzygotic germline mutations.³⁰ Linear porokeratosis is most commonly diagnosed during childhood, though adult onset is possible.³¹ Linear porokeratosis shares identical morphology to classical porokeratosis of Mibelli; however, the lesions are grouped in a linear distribution.¹⁵ Lesions are distributed unilaterally, but bilateral involvement has been reported.³² Linear porokeratosis most often involves a distal extremity and may engulf an entire side of the body.¹⁵ Clinical identification of linear porokeratosis may be made through inspecting individual lesions (Figures 7 and 8). Compared to other variants of porokeratosis, linear porokeratosis carries the highest rate of malignant transformation. This has been estimated as high as 19%.³³

The differential diagnosis of porokeratosis includes scaly lesions that are sharply defined and ovoid-shaped. Common skin diseases that fit this pattern include nummular dermatitis (Figure 9A, B), tinea corporis (Figure 10), seborrheic keratosis (Figure 11), psoriasis vulgaris (Figure 12), SCC and actinic keratosis (Figure 13). A summary of their differences is presented in Table 1. Cutaneous SCC typically presents as a rough scaly papule on an erythematous base in sun-exposed areas.³⁴ Compared to porokeratosis, SCC usually has poorly defined borders with an adherent scale.³⁴ The presentation of an advanced actinic keratosis is very similar to SCC; therefore, it is difficult to differentiate the two by clinical inspection.³⁵ SCC tends to present as a larger papule, plaque, or nodule, that may be crateriform or ulcerated in appearance.³⁵ Nummular dermatitis is characterized by pruritic, sharply defined, coin-shaped erythematous papules or plaques. The diagnosis is made on a clinical basis in a patient with symmetric, linearly distributed lesions on the trunk and/or extremities with a background of xerotic skin.³⁶ Tinea corporis is classically a scaly annular plaque with central clearing.³¹ The border of tinea corporis is defined but not as sharply as porokeratosis. The border seen in tinea is more erythematous than porokeratosis and features scale that may expand outside of the lesion. Pustules and vesicles may also be seen.³⁷ The clinical presentation for seborrheic keratosis may vary with the type. They are generally characterized as a waxy, “stuck-on” appearing papule or plaque varying in colors from light gray to dark brown.³⁸ Seborrheic keratoses are erythematous if inflamed.³⁹ Seborrheic keratoses also feature scale throughout the lesion. Psoriasis vulgaris presents as sharply defined erythematous annular papules or plaques with a thick silver scale.⁴⁰ Lesions are often symmetrically distributed on the extensor surfaces of the body. Compared to porokeratosis, the scale of psoriasis is coarse, and the erythema is uniformly bright throughout.

Figure 13.

Fitzpatrick I (always burns, never tans): Skin demonstrates a cutaneous squamous cell carcinoma (SCC) overlying sun-damaged skin. This is a common differential diagnosis for porokeratosis. This is classically a firm, hyperkeratotic (thickened stratum corneum, the outer layer of skin), scaly papule (raised, <1 cm) with a rim of erythema. In lighter skin types, the erythema is easily visualized as bright pink-red. The background skin is sun damaged, visualized as brown mottled pigmentation with telangiectasias. In contrast to porokeratosis, neither a peripheral rim of scale (cornoid lamella) nor an atrophic center is observed in SCC. Actinic keratosis exists along a spectrum with SCC. This makes advanced actinic keratosis difficult to distinguish from early SCC. Late-stage SCC may present as large nodules (raised, solid, >1 cm) with ulceration and hemorrhagic crust.

Table 1.

Porokeratosis Differential Diagnosis

Condition	Border	Scale	Erythema
Porokeratosis	Raised hyperkeratotic ridge with atrophic center	Located at the periphery	Varies
Nummular dermatitis	Discrete round coin-shaped appearance	Seen throughout the lesion on background lichenification	Early lesions are pink-red; later lesions are hyperpigmented
Tinea corporis	Well-demarcated with central clearing	Variable with accentuation at the periphery of the lesion	Variable with accentuation at the border
Seborrheic keratosis	Uniformly raised “stuck-on” appearance	If present is throughout the lesion	Present if inflamed
Psoriasis vulgaris	Well-demarcated	Thick, silvery white “micaceous”	Uniform throughout
Squamous cell carcinoma	Well-demarcated but may have crusting and ulceration	Adherent and thick	Bright pink-red
Actinic keratosis	Well-demarcated but may merge with surrounding skin	Adherent and fine	Bright pink-red

Funding Statement

This research was supported (in whole or in part) by HCA Healthcare and/or an HCA Healthcare-affiliated entity.