Author: Assoc Prof Patrick Emanuel, Dermatopathologist, Auckland, New Zealand, 2014.
Pityriasis rubra pilaris or PRP presents clinically as a papulosquamous eruption with follicular plugging and perifollicularerythema. The various clinical forms share the same histopathologic features.
Histology of pityriasis rubra pilaris
In PRP, the epidermis shows regular acanthosis and psoriasiformhyperplasia (figures 1-3). The horn is thickened with parakeratotic foci between orthokeratosis both vertically and horizontally (not seen clearly in the presented images). The hyperkeratosis tracks down the openings of follicular structures forming follicular plugs (figures 1-3). The dermis may contain a mixed sparse infiltrate which may be lichenoid. Rarely, foci of acantholysis may be seen (figure 3) and some authors have made associations with Darier disease.