Pityriasis Rubra Pilaris

Pityriasis rubra pilaris (PRP) is a rare chronic inflammatory papulosquamous disorder with follicular hyperkeratosis, orange red scaly plaques, and palmoplantar keratoderma.

• Incidence is about 1 in 400,000; bimodal onset (first and fifth decade). Most adult cases are acquired; juvenile familial form (type V) is linked to CARD14 mutation.
• Griffiths classification: type I classic adult (most common; 50% resolve in 3 years), type II atypical adult (ichthyosiform, chronic), types III and IV juvenile, type V atypical juvenile (familial), type VI HIV-associated (poor response to retinoids).
• Associated with infection, autoimmune disease, and rarely malignancy.

• Perifollicular keratotic papules ("nutmeg grater"), progressive salmon or orange red scaly plaques that may become confluent erythroderma with distinct islands of sparing.
• Waxy, yellow orange palmoplantar keratoderma ("sandal" pattern), nail changes (subungual debris, distal yellow brown discolouration, thickening), and ectropion.
• Scalp scale, facial erythema; mucosa spared.
• Complications: erythroderma with fluid imbalance, temperature dysregulation, secondary infection.

• Clinical, supported by biopsy: alternating orthokeratosis and parakeratosis in vertical and horizontal directions ("checkerboard"), follicular plugging, thick granular layer, acanthosis, and a sparse perivascular infiltrate.
• Exclude psoriasis (larger plaques, nail pits), seborrhoeic dermatitis, erythrodermic atopic dermatitis, CTCL, drug eruptions, ichthyosis, and Darier disease.
• HIV serology; look for associated malignancy in the elderly if features are atypical.
• Test for CARD14 in familial cases.

• Topical: emollients and keratolytics (urea 10 to 20%, salicylic acid 3 to 5%), mid- to high potency steroids, calcipotriol, and tazarotene for localised disease.
• First line systemic:
  • Acitretin 0.5 to 1 mg/kg/day (25 to 75 mg), for 3 to 6 months; effective in about 60 to 70% (requires strict contraception for women for 3 years after stopping).
  • Methotrexate 7.5 to 25 mg weekly with folate 5 mg.
• Biologics: adalimumab 80 mg then 40 mg every 2 weeks, infliximab 5 mg/kg at weeks 0, 2, 6 then every 8 weeks, ustekinumab 45 mg (90 mg if over 100 kg) at weeks 0, 4 then every 12 weeks, secukinumab 300 mg, ixekizumab, guselkumab, risankizumab (IL-17 and IL-23 inhibitors show high response rates).
• Alternatives: ciclosporin 3 to 5 mg/kg, narrow band UVB or PUVA with retinoid (RePUVA), isotretinoin 0.5 to 1 mg/kg.
• Erythrodermic care: warm environment, fluid and protein replacement, emollient wraps, antibiotics if infected, and monitoring of lipids, liver tests, and bone health during retinoids.