Pityriasis rubra pilaris
Also called PRP
A rare psoriasis mimic that reads orange: follicular keratotic papules coalescing into red-orange plaques with sharply spared islands of normal skin, plus a waxy orange palmoplantar keratoderma. Some adult forms clear on their own within years.
How it looks
Red-orange follicular papules coalescing into scaly plaques with discrete islands of sparing, and a waxy orange keratoderma of the palms and soles.
Rose is the lesion, grey the pattern the lesions make, green where on the body it lives.
Across skin tones
The orange hue that names the disease is muted in deeply pigmented skin, where plaques look violaceous or grey-brown; the islands of sparing and the waxy keratoderma then do the diagnostic work.
Symptoms & course
- Itch is variable, often milder than the surface area suggests
- Tight, fissuring palms and soles
- Can progress to erythroderma
Diagnostic approach
- Biopsy, which shows alternating ortho- and parakeratosis in a checkerboard
- Screen adult-onset disease for HIV, which drives one subtype
Differential, and how to separate them
- Psoriasis
Favours it: Scaly red plaques and keratoderma with nail change
Against it: PRP's follicular papules, orange tone, and islands of sparing are not psoriatic
- Seborrheic dermatitis
Favours it: Early PRP often starts on the scalp and face with greasy scale
Against it: Seborrheic dermatitis stays in its zones and never builds keratoderma
Management principles
- Emollients and keratolytics for the keratoderma
- Systemic retinoids first-line for classic disease
- Methotrexate or biologics for the refractory
Clinical pearl: Islands of completely normal skin inside a sea of red-orange is PRP until proven otherwise; psoriasis does not leave islands.
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