Version May 2024
INTRODUCTION — Pityriasis rubra pilaris (PRP) is a rare, cutaneous, inflammatory, papulosquamous disorder classically characterized by follicular, hyperkeratotic papules; waxy, yellow palmoplantar keratoderma; and erythroderma with islands of sparing (picture 1A-K). PRP may be acquired (most common) or familial, may have adult or childhood onset, and may have widespread or focal involvement. Other synonyms for PRP include Devergie's disease, lichen ruber pilaris, lichen acuminatus, and lichen ruber acuminatus.
The pathogenesis, clinical manifestations, and diagnosis of PRP will be reviewed here. The prognosis and management of PRP is discussed separately.
●(See "Pityriasis rubra pilaris: Prognosis and management".)
EPIDEMIOLOGY — Clinical findings consistent with PRP were first recognized in the 1800s [1-3]. The incidence and prevalence of PRP are uncertain. A private PRP social media group associated with the nonprofit patient advocacy organization PRP Alliance has approximately 2000 active members worldwide:
●Age – PRP may occur at any age. The peak incidence has a bimodal distribution, with the first peak at age six to seven years [4-6]. The second peak occurs around the fifth or sixth decade of life [7-10]. Of note, the classical childhood-onset subtype of PRP usually develops in the late teenage years but may also be seen in the first few years of life [5,8]. (See 'Clinical subtypes' below.)
●Sex – PRP affects equal numbers of males and females in most case series [4,6-12]. However, other case series suggest a male predominance [5,13-15].
●Geographic distribution – PRP occurs worldwide. The largest published case series describe patients from the United States [7,9,10,16], Denmark [14], and Thailand [6].
Estimates of incidence range widely, with studies suggesting incidences of 1 in 500 new dermatology patients in Kenya [17], 1 in 5000 new dermatology patients in Great Britain [8], and 1 in 50,000 new dermatology patients in India [18]. It is unclear whether observed differences in incidence relate to differences in risk for PRP in different populations, local differences in the diagnostic approach to PRP, or other factors.
●Family history – Most cases of PRP are sporadic. There is an autosomal dominant inherited form that likely represents <5 percent of patients [6,12,16,19]. (See 'Pathogenesis' below.)
PATHOGENESIS — The pathogenesis of PRP is not fully understood. The interleukin (IL) 23/Th17 axis may play an important role; this concept is supported by the response of PRP to therapies targeting IL-17 and IL-23. (See "Pityriasis rubra pilaris: Prognosis and management", section on 'IL-17 inhibitors' and "Pityriasis rubra pilaris: Prognosis and management", section on 'IL-23 inhibitors'.)
Familial PRP has been linked to autosomal dominant mutations in the CARD14 gene. (See 'CARD14, CCL20, and the IL-23/IL-17 axis' below.)
CARD14, CCL20, and the IL-23/IL-17 axis — Insight into the cutaneous, inflammatory pathways involved in PRP has been gained through the study of genetic variants and transcriptomic analysis:
●Role of CARD14 – CARD14 has been identified as a pathogenetic gene variation in families with autosomal dominant PRP [20] as well as in some individuals with sporadic PRP [21]. CARD14 is part of the PSORS2 gene locus and is also implicated in familial pustular psoriasis [22]. (See "Pustular psoriasis: Pathogenesis, clinical manifestations, and diagnosis", section on 'Genetics'.)
Two mouse models recapitulating human CARD14 genetic variants have helped elucidate the disease mechanism [23,24]. The mouse models exhibited cutaneous scale and erythema. The genetic variations led to constitutive activation of the CARD14 gene product, CARMA2, with a downstream increase in the transcription factor nuclear factor kappa B (NF-kB) and resulting increase in the chemokine, CCL20, a potent activator of the Th17 pathway [23]. Injection of an anti-IL 23p19 antibody resulted in improvement in the skin disease [24].
Sporadic PRP is less understood, although there is emerging evidence that it may involve similar immune pathways to inherited PRP. An increase in CARD14, NF-kB, and CCL20 was found in cutaneous biopsies of patients with PRP who lacked germline CARD14 mutations [25-27].
●Other insights – Other studies suggest that heterogeneity in the pathogenesis of PRP may contribute to differences in responses to treatment. In a transcriptomic analysis study of 11 adults with PRP, normalization in expression of CCL20, IL17C, and IL23A occurred in patients who responded well to treatment with an IL-17A inhibitor, whereas patients who responded poorly had higher baseline and post-treatment levels of CCL20 and IL17C expression [27]. Activation of innate signaling pathways has also been detected in patients with poor responses to IL-17A inhibition [28]. (See "Pityriasis rubra pilaris: Prognosis and management", section on 'IL-17 inhibitors'.)
Overlap in the pathogenic mechanisms of psoriasis and PRP is also proposed. In a study that compared gene expression in PRP and psoriasis, all dysregulated genes found in PRP skin were also found in psoriasis skin, and no genes were unique to PRP [29].
Drug-induced pityriasis rubra pilaris — Drug exposure has been linked to the development of PRP (table 1). Tyrosine kinase inhibitors (TKIs) and phosphoinositide 3-kinase (PI3K) inhibitors are the most often reported associated drugs (table 1).
Based upon a systematic review that included reports of 25 patients with drug-associated PRP, the time period between drug initiation and PRP onset may range from three days to five months [30]. Proposed hypotheses for the mechanism of drug-induced PRP include drug-induced changes in regulatory immune surveillance, epidermal growth, and proinflammatory cytokine production, as well as effects related to drug exposure in the setting of underlying immunologic disorders [30].
Other theories — PRP was initially thought to be a disorder of vitamin A deficiency, but this association was subsequently dismissed. Early reports also linked PRP with neuromuscular disorders. This presentation was later recharacterized as Wong-type dermatomyositis, which presents with both PRP-like and dermatomyositis-like, cutaneous eruptions with or without myositis [31]. (See 'Differential diagnosis' below.)
No human leukocyte antigen (HLA) association has been found in PRP [19]. Infectious or environmental triggers of PRP have been proposed but have not been definitively linked to PRP.
CLINICAL SUBTYPES — Multiple classification schemata for PRP have been proposed (table 2):
●Griffiths' classification – The most commonly cited classification schema for PRP is the Griffiths' classification, which consisted of five subtypes of PRP prior to the later addition of HIV-associated PRP [8,32]. However, it is important to note that these categories have not been validated to predict disease outcomes or treatment response:
•Type I – Classical adult
•Type II – Atypical adult
•Type III – Classical juvenile
•Type IV – Circumscribed
•Type V – Atypical juvenile
•Type VI – HIV associated
Classical adult PRP is considered the most common of these subtypes [8,33].
●Other classification schemata – Additional classification schemata include classification based upon PRP that is familial or acquired, childhood or adult onset, and typical or atypical in presentation [16]; classification by the body surface areas involved [6]; and classification of childhood-onset PRP by the duration of disease (table 2) [12].
CLINICAL FEATURES — PRP has a range of clinical presentations.
Primary features — Two primary features (follicular, hyperkeratotic papules and palmoplantar keratoderma) link the various clinical presentations of PRP.
Follicular, hyperkeratotic papules — Follicular, hyperkeratotic papules manifest as multiple small, approximately 1 to 2 mm diameter papules with a central, smooth or sharp, keratotic plug (picture 1C-D, 1L-M). A surrounding yellow-orange ring may be visible in individuals with lightly pigmented skin, or the color may be red-brown in individuals with more deeply pigmented skin [34]. Although follicular, hyperkeratotic papules on the dorsal fingers has been proposed to be pathognomonic for PRP, this finding is often absent (picture 1M).
The appearance of follicular, hyperkeratotic papules has been likened to the round, sharp, grid array on the surface of a nutmeg or fine cheese grater. The term "nutmeg-grater sign" has been used to refer to this finding.
Palmoplantar keratoderma — Palmoplantar hyperkeratosis often appears early in the course of PRP. The palms and soles become diffusely hyperkeratotic and waxy with a yellow to orange hue (picture 1E, 1I). Severe cases exhibit dramatic hyperkeratosis and multiple painful fissures, which may have a profound, negative effect on daily activities, such as ambulation (picture 1J) [35].
Specific subtypes — Classical, circumscribed, atypical, and HIV-associated presentations of PRP have additional characteristic features.
Classical pityriasis rubra pilaris — In addition to follicular hyperkeratosis and palmoplantar keratoderma, classical PRP in adults presents with inflamed plaques, islands of sparing, pityriasiform scale, and erythroderma [36]. Skin involvement often progresses in a cephalocaudal direction. Ectropion, hair loss, and nail abnormalities may also occur.
Classical PRP in childhood (classic juvenile PRP) is similar to the classical adult presentation, but fewer children develop complete erythroderma and ectropion (picture 2):
●Cutaneous findings:
•Erythematous plaques and lichenification – Follicular papules coalesce into larger, often deeply erythematous plaques, resulting in a lichenified appearance with exaggerated furrowing of the skin (picture 1H) [12]. In patients with highly pigmented skin, erythema may be subtle, and plaques may become hyperpigmented. (See 'Skin of color' below.)
•Islands of sparing – The term "islands of sparing" refers to areas of normal-looking skin in the midst of affected areas. Islands of sparing usually appear or expand during the resolution phase of the disease (picture 1A-B, 1G). The presence of islands of sparing strongly suggests PRP, but the finding may also occur in erythroderma from other causes [36].
•Pityriasiform scale – Scale in classical PRP is fine, powdery, and nonadherent and has been likened to the bran of wheat [36]. Scaling is most prominent on the face and scalp (picture 3) [36]. Because of the nonadherent nature, patients may report clouds of dust falling from their skin despite having minimal scale visible on the surface of the skin.
•Erythroderma – Erythroderma is generally defined by the presence of erythema involving more than 90 percent of the body surface area. Erythema may be subtle in patients with highly pigmented skin. Erythroderma typically develops rapidly over approximately one month [37]. (See "Erythroderma in adults", section on 'Cutaneous signs and symptoms'.)
Erythroderma is generally accompanied by heat and cold intolerance, edema of the legs (and sometimes the arms), and tachycardia. The skin is typically warm to touch. In addition, many patients with PRP report anhidrosis (loss of the ability to sweat) [38,39].
•Cephalocaudal progression – Cephalocaudal progression of skin involvement is common in classical PRP (picture 1F, 1K). Erythema, follicular papules, and powdery, nonadherent scale often first appear on the scalp, followed by the face, chest, and upper back, and may progress in a cephalocaudal distribution until the patient becomes erythrodermic.
•Pruritus – Pruritus can be severe and is often disruptive to sleep [35]. Patients also frequently describe a stinging or burning sensation in the skin. Cutaneous dysesthesia often results in difficulty wearing clothing [39].
•Photosensitivity – PRP exacerbation by ultraviolet light is common [40-47], although a smaller subset of patients improve with phototherapy.
●Other findings:
•Ectropion – Prolonged facial involvement can result in ectropion and resulting eye pain, blurry vision, eye dryness, and epiphora [36]. Rarely, ectropion is severe and may lead to keratitis [48,49].
•Nail abnormalities – Nail abnormalities can include terminal and subungual hyperkeratosis, splinter hemorrhages, and longitudinal ridging (picture 4A-B). Psoriasiform pits in the nail plate are an occasional finding, but the onycholysis and salmon patches commonly present in psoriasis are absent [36]. Nails may become thick, powdery, and may shed completely (onychomadesis) [50].
•Hair loss – Diffuse, nonscarring hair loss is common in classical PRP. However, complete alopecia does not occur, and hair loss is generally reversible after disease treatment or resolution [38].
•Arthralgias – Generalized, noninflammatory arthralgias are commonly reported by patients with PRP, although true inflammatory arthritis is rare [38,39].
•Ear canal blockage – Hyperkeratosis in the external auditory canals may lead to muffled hearing and require manual removal by a medical provider [39].
Circumscribed pityriasis rubra pilaris — Circumscribed PRP is the most common presentation of PRP in prepubertal children [4,5]. This subtype may also occur in adults.
Characteristic features of circumscribed PRP include:
●Circumscribed plaques – Well-demarcated plaques studded with follicular plugs develop over the extensor knees and elbows (picture 5). Other sites of involvement of follicular papules and plaques may include the dorsal hands, dorsal feet, and buttocks. Patients may also develop subtle, perioral and periorbital papules and plaques.
●Palmoplantar keratoderma – Most patients also have palmoplantar keratoderma. Plantar involvement often extends over the Achilles tendon. (See 'Palmoplantar keratoderma' above.)
In addition, the scalp may contain large, adherent scales, and nails are also occasionally involved. Rarely, circumscribed PRP progresses to more classical, widespread PRP. (See 'Classical pityriasis rubra pilaris' above.)
Atypical adult-onset pityriasis rubra pilaris — Atypical adult-onset PRP is rare.
Characteristic clinical features include [8]:
●Coarse, lamellated scale on the palms and soles (picture 6)
●Ichthyosis-like (larger, plate-like) scale on the legs
●Eczematous, ill-defined, erythematous patches
●Sparseness of scalp hair
In addition, some areas of follicular hyperkeratosis should be present.
The disease course is prolonged, typically lasting two decades or more [8].
Atypical childhood-onset pityriasis rubra pilaris — Atypical childhood-onset PRP (atypical juvenile PRP) is rare, usually develops in early childhood, and is often familial.
Typical clinical features may include:
●Prominent facial involvement with well-demarcated, erythematous plaques on the cheeks and chin, sparing the infralabial region
●Follicular, hyperkeratotic papules with minimal, but persistent, erythema
●Erythematous, thin plaques with ichthyosiform or psoriasiform scale on the trunk and extremities
Occasionally, atypical childhood-onset of PRP is accompanied by scleroderma-like changes of the hands [8]. The course is typically chronic.
Of note, most patients considered to have atypical childhood-onset PRP would likely be better characterized as having either CARD14-associated papulosquamous eruption (CAPE) or a type of congenital ichthyosis. Genetic testing can be useful in this setting. (See 'CARD14-associated papulosquamous eruption' below and "Overview and classification of the inherited ichthyoses", section on 'Diagnostic approach'.)
HIV-associated pityriasis rubra pilaris — HIV-associated PRP is a rare subtype of PRP that can occur in the setting of either low or normal CD4 T cell counts. An association between HIV and PRP was first proposed in 1991 [51].
Patients with HIV-associated PRP present with cutaneous findings of classical PRP. There are reports of a tetrad of HIV-associated, follicular syndrome that includes PRP, acne conglobata, hidradenitis suppurativa, and lichen spinulosus [52,53].
CARD14-ASSOCIATED PAPULOSQUAMOUS ERUPTION — CARD14 gene variations have been implicated in familial cases of PRP [20]. CARD14-associated papulosquamous eruption (CAPE) is a proposed term to describe the presence of PRP-like or psoriasis-like features in patients with CARD14 variations.
A study that identified 15 kindreds with CARD14 gene variations among patients referred for participation in a genetic study of inherited disorders of keratinization led to the proposal of CAPE [54]. Early disease onset was common, with 13 of the 15 index patients with onset of disease at or before the age of one year. Physical findings included cutaneous findings that were primarily PRP like, primarily psoriasis like, or exhibited features of both conditions. Classical PRP findings detected included palmoplantar keratoderma (12 out of 15 patients), typical follicular papules (6 out of 15 patients), and islands of sparing (5 out of 15 patients). Fourteen out of 15 patients had prominent facial involvement, and two patients had scleroderma-like changes of the hands. Three of the 15 patients had arthritis.
CAPE shows similar histopathologic features as classical PRP [54], although acantholysis has not been described [55]. (See 'Histopathology' below.)
PSYCHOSOCIAL IMPACT — PRP is associated with a severe, negative impact on quality of life. A survey study that compared Skindex-29 quality-of-life scores from 85 patients in a PRP support group with existing data from other skin conditions found patients with PRP had worse quality of life than patients with all other dermatologic conditions included in the analysis except for epidermolysis bullosa [47]. Palmoplantar keratoderma was the largest factor that interfered with daily activities due to difficulty using the hands for fine motor tasks and feet for ambulating [39,47].
Moreover, reports of PRP-related depression, anxiety, and suicidal ideation were common in patient surveys [9,35,39]. Focus groups and surveys of participants with PRP also identified feelings of embarrassment, loss of self-worth, loneliness from having a rare disease, loneliness from not participating in social activities, demoralization by ineffective treatments, and uncertainty about diagnosis and prognosis [39]. The physical and mental impacts of PRP led to many patients feeling housebound and being unable to perform their usual work or home duties. (See "Pityriasis rubra pilaris: Prognosis and management", section on 'Patient support'.)
COMPLICATIONS — Secondary bacterial infections are uncommon in PRP; however, secondary herpetic infections (Kaposi's varicelliform eruption) have been reported [56-58].
HISTOPATHOLOGY — There is no single histopathologic feature or known immunocytochemical marker that is diagnostic of PRP. A constellation of features supports the diagnosis and helps differentiate PRP from similar cutaneous diseases, such as psoriasis:
●Common features – Histopathologic features commonly found in established lesions of PRP include:
•Alternating orthokeratosis and parakeratosis – Stratum corneum with orthokeratosis and parakeratosis alternating in both vertical and horizontal directions, comparable to a checkerboard [59]
•Keratotic, follicular plugging – Keratotic, follicular plugging with perifollicular parakeratosis [19]
•Psoriasiform hyperplasia – Thickening of the epidermis with broad, short, irregular elongation of the rete ridges, known as psoriasiform hyperplasia, with normal or thickened suprapapillary plates (in contrast to psoriasis, which is associated with thinning of the suprapapillary plates) [60,61]
•Normal or thickened granular layer – Normal or thickened granular cell layer under areas of parakeratosis (in contrast to other inflammatory diseases that have a diminished or absent granular cell layer under areas of parakeratosis) [61]
•Mitotic figures – Mitotic figures above the basilar layer of the epidermis, although fewer than may be found in psoriasis [60,61]
•Acantholysis – Focal areas of acantholysis present in around one-quarter of PRP biopsies [62], with occasional widespread acantholysis mimicking pemphigus vulgaris [63-65]
•Lymphocytic, inflammatory infiltrate – Dermis with sparse, perivascular, inflammatory infiltrate composed primarily of lymphocytes [60,61]
●Occasional features – Occasional histologic features include spongiosis; a brisk, lichenoid infiltrate with eosinophils [10]; scattered plasma cells; and dilated, superficial blood vessels (although not tortuous, as would be seen in psoriasis).
●Features suggestive of other disorders – Interface dermatitis should be absent; the presence of interface dermatitis would be suggestive of Wong-type dermatomyositis. Intraepidermal lymphocytes should be minimal; the presence of intraepidermal lymphocytes would raise concern for cutaneous T cell lymphoma. (See 'Differential diagnosis' below.)
●Impact of disease stage – Early, evolving lesions of PRP are more likely to show a normal, basket-weave stratum corneum [60]. Late or partially resolved biopsies of PRP show a normal, basket-weave or compact, orthokeratotic stratum corneum with less psoriasiform hyperplasia [60].
ASSOCIATED DISORDERS — There are multiple case reports of PRP occurring in patients with concurrent medical conditions and malignancies, although whether these are stochastic events or related by disease mechanism is uncertain.
Inflammatory arthritis has also been associated with PRP in case reports [66-74]. Rarely, PRP has been temporally associated with eruptive seborrheic keratosis [75-77], immunologic abnormalities [78-80], autoimmune disorders [5,81-85], endocrine disorders [12,86-89], and Down syndrome [90-92].
PRP is not generally thought to be a paraneoplastic condition. Multiple malignancies have been suggested to be associated with PRP, but each is supported by isolated case reports. These include adenocarcinoma [93-95], bronchogenic carcinoma [96], cholangiocarcinoma [97], hepatocellular carcinoma [98], leukemia [99], meningioma [100], monoclonal gammopathy [93], melanoma [101,102], myelodysplastic syndrome [103], papillary renal cell carcinoma [104], and squamous cell carcinoma [105,106].
DIAGNOSIS — The diagnosis of PRP is based on the recognition of consistent clinical and histopathologic features (see 'Clinical features' above and 'Histopathology' above). There are no validated diagnostic criteria for PRP.
Caution is indicated when diagnosing atypical adult-onset or atypical childhood-onset PRP. In the author's experience, adults with idiopathic erythroderma are sometimes labeled with atypical PRP despite lack of supporting clinical evidence of PRP. In addition, many patients considered to have childhood-onset PRP may be better classified as having CARD14-associated papulosquamous eruption (CAPE) or a form of congenital ichthyosis. (See 'Differential diagnosis' below and 'Atypical childhood-onset pityriasis rubra pilaris' above and 'Genetic testing' below.)
Patient history — The patient history should include discussion of the age of onset, progression of skin involvement, and symptoms. A medication history should be obtained to explore the possibility of drug-induced PRP. Obtaining a family history can be helpful for identifying patients who may have CAPE or inherited PRP. (See 'Genetic testing' below and 'CARD14-associated papulosquamous eruption' above.)
A description of the progression of skin involvement can be particularly helpful for patients with classical PRP; once PRP has become erythrodermic, it can be difficult to discern PRP from other types of erythroderma. Patients with classical PRP typically first describe the initial development of scale and erythema of the scalp, with spread to the face, chest, and upper back, followed by involvement of the remainder of the body. Waxy keratoderma of the palms and soles starts early in the disease course and may become thicker with fissuring as progression occurs. (See 'Classical pityriasis rubra pilaris' above.)
Physical examination — A complete skin examination should be performed to detect common signs of PRP (eg, follicular, hyperkeratotic papules; palmoplantar keratoderma; plaques; pityriasiform scale; islands of sparing; or erythroderma). (See 'Clinical features' above.)
Dermoscopic examination may be useful in differentiating PRP from similar inflammatory disorders, although this has not been well validated. Examples of reported dermoscopic findings include white scale; dotted vessels with orange, structureless areas; and reticular vessels in the islands of spared skin [107-109]. This contrasts with the more regular pattern of dotted and glomerular vessels observed in erythrodermic psoriasis [108,109]. Areas of follicular hyperkeratosis, as well as the scalp, may show a central hair with a follicular plug surrounded by an orange/yellow halo [110-114].
Skin of color — In patients with highly pigmented skin, follicular papules and erythema have a red-brown color, rather than a pink-orange hue [34]. Postinflammatory hyperpigmentation may also occur, causing papules or plaques to appear darker than surrounding skin. Careful attention to other features of PRP, such as the morphology and distribution of lesions and the progression of disease (eg, cephalocaudal progression in classical PRP), and histopathologic examination can aid with diagnosis.
Skin biopsy — Skin biopsies to identify consistent histopathologic findings are an important diagnostic tool for PRP. (See 'Histopathology' above.)
The skin biopsy specimen should be taken from an involved area of the skin, ideally in a location where hyperkeratotic, follicular, scaly papules are observed. Typically, two 4 mm punch biopsies are preferred, given the sometimes nonspecific histopathologic changes that may be found. A shave biopsy through the mid-dermis may also be adequate.
ADDITIONAL TESTS
HIV testing — Because of the possibility of HIV-associated PRP, we screen adolescent and adult patients with PRP for HIV infection. (See "Screening and diagnostic testing for HIV infection".)
Genetic testing — We suggest genetic testing for CARD14 gene variations for patients with PRP onset in the first year of life, children who have PRP symptoms that persist for longer than three years, or children with family members with known or suspected PRP or psoriasis. Of note, individuals with CARD14-associated papulosquamous eruption (CAPE) may have either classical or atypical features (see 'CARD14-associated papulosquamous eruption' above). The Foundation for Ichthyosis and Related Skin Disorders is a resource for genetic testing.
DIFFERENTIAL DIAGNOSIS — Examples of disorders in the differential diagnosis of PRP include seborrheic dermatitis, erythrodermic psoriasis and other forms of erythroderma, chronic plaque psoriasis, Wong-type dermatomyositis, and erythema gyratum repens:
●Seborrheic dermatitis – Early classical PRP may be mistaken for seborrheic dermatitis because of the cephalocaudal progression of PRP. Seborrheic dermatitis commonly presents with erythematous, scaly patches on the scalp and may also involve the eyebrows, nasolabial folds, and, less commonly, the central chest and intertriginous areas (picture 7A-B). The characteristic greasy scale, absence of follicular hyperkeratosis and palmoplantar keratoderma, and relatively stable distribution of seborrheic dermatitis distinguish this disorder from PRP.
●Erythrodermic psoriasis and other forms of erythroderma – Erythroderma in PRP can closely resemble erythroderma secondary to other causes. Psoriasis is one of the most common causes of erythroderma (picture 8). Examples of other causes include atopic dermatitis, drug reactions, and cutaneous T cell lymphoma (Sézary syndrome). Erythroderma may also be idiopathic. Palmoplantar keratoderma, pruritus, and lymphadenopathy are common in various forms of erythroderma. (See "Erythroderma in adults".)
The patient history, physical examination, and skin biopsy can be helpful for identifying erythrodermic psoriasis. Patients with erythrodermic psoriasis often have a history of psoriasis. Psoriasis also tends to have a more adherent scale than PRP, but this can be subtle. On histopathologic examination, the presence of keratotic, follicular plugging; a normal or increased granular cell layer; acantholysis; and the notable absence of neutrophils in the stratum corneum support a diagnosis of PRP [61]. Dermoscopy has also been proposed to help differentiate between erythrodermic PRP and psoriasis, with the presence of regular, dotted, and glomerular vessels observed in psoriasis versus less regular, dotted vessels within orange, structureless areas in PRP [107-109]. (See "Erythrodermic psoriasis in adults" and 'Physical examination' above.)
As with other forms of erythroderma in the differential diagnosis, cutaneous T cell lymphoma should lack the follicular papules of PRP. However, the erythrodermic stage can be difficult to discern clinically from PRP, and there is a case report of Sézary syndrome initially presenting with follicular papules suspected to be PRP [115]. Blood flow cytometry or T cell clonality studies are important when cutaneous T cell lymphoma is suspected. (See "Clinical presentation, pathologic features, and diagnosis of Sézary syndrome".)
In our experience, idiopathic erythroderma is sometimes inappropriately diagnosed as atypical PRP despite the lack of histopathologic features suggestive of PRP. (See 'Histopathology' above.)
●Chronic plaque psoriasis – Chronic plaque psoriasis is characterized by well-demarcated, inflamed plaques with micaceous scale. Similar to circumscribed PRP, common locations for psoriasis are the extensor elbows, knees, scalp, palms, and soles (picture 9A-B). However, psoriatic plaques lack the follicular nature of PRP. A skin biopsy can be helpful for distinguishing plaque psoriasis and PRP [61]. (See 'Histopathology' above and "Psoriasis: Epidemiology, clinical manifestations, and diagnosis".)
●Wong-type dermatomyositis – Wong-type dermatomyositis is a rare disorder that presents with clinical and histopathologic features of both PRP and dermatomyositis (eg, follicular, keratotic papules on the trunk and waxy, orange keratoderma of the soles similar to PRP and a heliotrope eruption on the eyelids with Gottron papules over the dorsal hands similar to dermatomyositis). Patients with Wong-type dermatomyositis may exhibit myositis or may be amyopathic. An additional skin biopsy taken from a cutaneous eruption suggestive of dermatomyositis, as well as a muscle examination and muscle enzymes, can be helpful for distinguishing Wong-type dermatomyositis from PRP.
●Erythema gyratum repens – Erythema gyratum repens is a rare, paraneoplastic, cutaneous eruption with unique and characteristic migratory, annular, and arcuate plaques resembling wood grain (picture 10). It may be accompanied by palmoplantar keratoderma. Erythema gyratum repens is most often associated with lung carcinoma. Resolving PRP can mimic erythema gyratum repens but lacks the migratory nature of erythema gyratum repens [116-122]. (See "Cutaneous manifestations of internal malignancy", section on 'Erythema gyratum repens'.)
Other mimickers of PRP may include hypereosinophilic syndrome, severe tinea versicolor [123,124], and PRP-like graft-versus-host disease [125,126].
SUMMARY AND RECOMMENDATIONS
●Overview – Pityriasis rubra pilaris (PRP) is a rare, cutaneous, inflammatory, papulosquamous disorder that occurs in both adults and children. PRP can have a profound, negative impact on quality of life. (See 'Epidemiology' above and 'Psychosocial impact' above.)
●Pathogenesis – The pathogenesis of sporadic PRP is not fully understood. The interleukin (IL) 23/Th17 axis may play an important role.
Most cases of PRP are sporadic, with familial PRP accounting for a small subset of cases. Familial PRP has been linked to mutations in the CARD14 gene. Drug exposure may contribute to some cases of PRP (table 1). (See 'Epidemiology' above and 'Pathogenesis' above.)
●Clinical subtypes – The most common classification scheme divides PRP into six subtypes (table 2):
•Type I – Classical adult
•Type II – Atypical adult
•Type III – Classical juvenile
•Type IV – Circumscribed
•Type V – Atypical juvenile
•Type VI – HIV associated
Classical adult PRP is the most common of these subtypes. (See 'Clinical subtypes' above.)
●Clinical features – The clinical presentation of PRP varies (picture 1A-K). Follicular, hyperkeratotic papules and palmoplantar hyperkeratosis are characteristic features.
Patients with classical PRP may also exhibit plaques, pityriasiform scale, islands of sparing, cephalocaudal progression of skin involvement, and erythroderma. Other subtypes exhibit different features. Limited, focal skin involvement with plaques studded with follicular plugs is characteristic of circumscribed PRP, which is more common in children. Coarser scale may be present on the palms, soles, and legs in atypical PRP. (See 'Clinical features' above.)
●Diagnosis – A diagnosis of PRP can be made based upon the presence of consistent clinical and histopathologic findings. The patient history, physical examination, and skin biopsy are important for the evaluation and may be particularly helpful for distinguishing psoriasis and PRP. (See 'Diagnosis' above.)
●Role of genetic testing – Genetic testing is appropriate for children with PRP onset before one year of age, children with PRP that does not remit within three years, or children with a family history of PRP or psoriasis. (See 'Genetic testing' above.)
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