Pityriasis Rubra Pilaris
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Direct answer
Six Griffiths types, one histology: pityriasis rubra pilaris (PRP) is a papulosquamous disorder of follicular plugging that spreads cephalocaudally into salmon-orange erythroderma with palmoplantar keratoderma, unmistakably punctuated by islands of sparing — normal skin islands within the red sea. Classical adult type 1 begins in the scalp and descends over weeks, may cause ectropion, and clears in one to three years in the majority; type 6 rides alongside HIV with acne conglobata and lichen spinulosus and responds poorly. Histology shows the quotable checkerboard sign — alternating horizontal and vertical orthokeratosis and parakeratosis — with thick suprapapillary plates and a sparse infiltrate. Acitretin is first-line therapy, with methotrexate and off-label biologics behind it.
What you must remember
- Type map (Griffiths): 1 classical adult (over half of cases, scalp-first descent, best prognosis); 2 atypical adult (alopecia, long course); 3 classical juvenile (onset in the first decade, behaves like type 1); 4 circumscribed juvenile (prepubertal, elbows and knees); 5 atypical juvenile, often familial; 6 HIV-associated (acne conglobata, lichen spinulosus, nodulocystic lesions, refractory).
- Three clinical signatures: salmon-orange generalised erythema with follicular plugging, waxy orange palmoplantar keratoderma, and islands of sparing — the triad the photographic question is built on.
- Cephalocaudal spread: scalp and face first, trunk then extremities, ending in exfoliative erythroderma with ectropion and alopecia in severe type 1.
- Histology quote: alternating orthokeratosis and parakeratosis in horizontal and vertical planes ("checkerboard"), thickened suprapapillary plates, sparse dermal infiltrate — a favourite slide description.
- Therapy ladder: acitretin 0.5-1 mg/kg (practical start 25-50 mg daily) is first line; methotrexate next; TNF, IL-12/23 and IL-17 inhibitors reported in series — the IL-17 logic mirrors psoriasis.
- Type 1 natural history: most patients clear within 1-3 years and stay clear — the prognostic sentence exams reward; types 2, 4 and 5 run chronic courses.
- Supportive care: emollients by the kilogram, keratolytics for keratoderma, ocular lubrication for ectropion, and monitor erythroderma complications — thermoregulatory loss, high-output cardiac strain, sepsis risk.
- Indian practice note: the psoriasis mimic is strong; biopsy before labelling, because PRP lacks the Auspitz sign and responds poorly to phototherapy.
Telling it apart from erythrodermic psoriasis: a reasoned comparison
Both diseases can put a red, scaly patient in the bed; the discrimination is clinical first and histological second. Walk the bedside: PRP shows follicular, nutmeg-grater papules, orange-hued palmoplantar waxy keratoderma, and — decisively — sharp islands of sparing scattered across trunk and limbs. Erythrodermic psoriasis tends to carry larger flaky sheets, nail pitting or oil drops, a personal history of plaques and arthritis, and Auspitz positivity on scraping. When the clinical picture sits on the fence, the biopsy arbitrates: PRP's checkerboard parakeratosis with thick suprapapillary plates contrasts with psoriasis's regular acanthosis, parakeratosis and clubbed rete ridges with neutrophils in the stratum corneum. The stakes are practical — phototherapy, a psoriasis staple, can flare PRP, and biologics chosen on the wrong label behave unpredictably — so the biopsy before immunosuppression is not academic caution but the hinge of management.
How the exam frames it
Expect a photograph with normal-skin islands inside orange erythroderma, or a histology slide with alternating ortho- and parakeratosis; either way the answer spells PRP. The one-liner comparisons are predictable: versus psoriasis (islands of sparing, negative Auspitz, checkerboard histology), versus ichthyosis (acquired pattern without follicular plugging), and the HIV vignette — young man with generalized erythroderma plus acne conglobata and lichen spinulosus, where type 6 is the expected answer and antiretroviral therapy is part of it. Dose questions like acitretin's weight-based range appear in viva form; the defensible answer quotes 0.5-1 mg/kg with monitoring of lipids and liver enzymes, and a firm statement that no therapy is curative — most type 1 disease clears itself on a clock of one to three years.
Frequently asked questions
Which feature most reliably distinguishes PRP from erythrodermic psoriasis?
Sharp islands of uninvolved skin within confluent salmon-orange erythroderma, supported by the checkerboard histology of alternating ortho- and parakeratosis.
What characterises Griffiths type 6 PRP?
HIV-associated disease with acne conglobata, lichen spinulosus and nodulocystic lesions, resistant to conventional therapy.
What is the first-line systemic treatment?
Acitretin at 0.5-1 mg/kg per day, with methotrexate and off-label biologics as alternatives in refractory disease.
How does classical adult PRP typically evolve?
Cephalocaudal spread from the scalp to erythroderma over weeks, then spontaneous clearance within one to three years in the majority.
Why is phototherapy used cautiously in PRP?
PRP can flare with phototherapy, unlike psoriasis, so retinoids are preferred before light-based treatment is attempted.