Pityriasis rubra pilaris some reflections

Abstract

Some reflections. I have presented a working classification of five types of PRP which gives a guide to prognosis and helps to consider like-with-like for research purposes. To focus unduly on taxonomy only obscures the more important aspect of aetiology. After many years of looking at PRP there remain many outstanding problems. It is clear that classical adult PRP has little to do with the other types aetiologically. The clinical features were reviewed and are not repeated here as they can be found in the standard texts. I have focused here on problems that may provide clues for others to advance the study of PRP. Psoriasis or PRP? There is an enigmatic relationship with psoriasis which is difficult to penetrate: • relative resistance to drugs usually effective in psoriasis, such as topical and systemic steroids, methotrexate, cyclosporin; • normal numbers and distribution of peripheral T-cell markers; • absence of prominent even parakeratosis despite an enhanced epidermal turnover; • sparse inflammatory infiltrate lacking intra-epidermal neutrophil migration; • absence of a PRI? arthropathy; • personal family history of psoriasis in PRI? patients lying half-way between that of the general population and of psoriasis patients; • a patient with clinically and histologically unequivocal PRI? cleared in 3 years except for occipital pityriasiform scaling. Ten years later plaques of clinically and histologically unequivocal psoriasis developed -coincidence of two diseases or PRI? becoming psoriasis? Further observations. A few patients have repeated attacks of PRP separated by intervals of several years. The morphology in each episode is that of classical PRP, not PRP on one occasion and psoriasis on another. The erythema in PRP shows with a striking orange hue not limited to the palmoplantar keratoderma, so not simply reflecting the thickness of the callus of the keratoderma (red + yellow = orange). Carotenoids and retinoids are yellow or red but their levels in PRP are normal. Could there be some significance that hypovitaminosis-A induces localized follicular hyperkeratosis with histopathology similar to that of PRP? Pathogenesis. What is the mechanism of production of the rash? The initial lesion is an erythernatous macule soon followed by areas of perifollicular erythema, which only later produce a hyperkeratinized follicular plug. The erythema triggers the localized burst of epidermopoiesis showing first as a follicular plug as the follicle is a column of epithelium. At this stage the inter-follicular epithelium shows hyperkeratosis only visible histologically as it does not possess the vertical ‘epidermal escalator’ of the follicle. In time the inter-follicular epidermis ‘catches up’ with the hyperkeratosis seen at the follicular orifices. This is exactly what is observed clinically as follicular papules coalesce into two's, three's then many lesions, producing confluent erythema. This process typically spreads in a cephalocaudal direction. Epidermal turnover is some six- to 10-fold more rapid on the head and neck than the legs. An increase of 10% in the stimulus to epidermopoiesis (as found experimentally) if sustained, will in time produce the observed evolution of PRP − early confluent erythema on the head and neck, palms and soles (fast turnover), follicular papules on the trunk becoming confluent, involvement of the legs last. If this supposition is correct the problem can be stated thus: What is the abnormal stimulus to vasodilation showing first at the follicles and from which the clinical features develop? A number of patients have repeated attacks of PRP separated by intervals of several years. In each case the morphology is that of classical PRP, not PRP on one occasion and psoriasis on another. PRP shows an erythema with a striking orange hue visible not only on the palmoplantar keratoderma − so not simply a function of the thick callous of the keratoderma − but all over the body (red + yellow = orange), carotenoids and retinoids are yellow or red but their levels in PRP are normal. Is there some significance that hypovitaminosis-A induces localized follicular hyperkeratosis with histopathology similar to that of PRP? The relative rarity of PRP make it a difficult disease to study but it is hoped that these reflections based on personal experience of many cases may provide some clues to a challenging condition. Further reading 1 Griffiths WA. Pityriasis rubra pilaris- an historical approach. Trans St John's Hospital Dermatol Soc 1975; 61: 58–69. 2 Griffiths WA. Pityriasis rubra pilarls- an historical approach 2. clinical features. Clin Exp Dermatol 1976; 1: 37–50. 3 Griffiths WA. & Ozluer Pityriasis rubra pilaire. Ann Dermatol Venereol 2001; 128: 931–4. 4 Griffiths WA. Pityriasis rubra pilaris. In: Champion RH, Burton JL, Burns DA, Breathnae SM, eds. Textbook of Dermatology, 6th edn. Oxford: Blackwell Science, 1998: 1539–45. 5 Griffiths WA. Pityriasis rubra pilaris. In: Harper J, Oranje A, Prose N. eds Textbook of Pediatric Dermatology, 1st edn. Oxford: Blackwell Science, 2000: 668.