Histologic criteria for the diagnosis of pityriasis rubra pilaris.

Even though the clinical appearance of pityriasis rubra pilaris is often distinctive enough to allow specific diagnosis of it, criteria for its histologic diagnosis have not yet been well defined. On the basis of a study of 55 cases of pityriasis rubra pilaris, we found the following histologic features helpful for diagnosis throughout most of the course of the disease: alternating orthokeratosis and parakeratosis in both vertical and horizontal directions; focal or confluent hypergranulosis; thick suprapapillary plates; broad rete ridges; narrow dermal papillae; and sparse superficial perivascular infiltration, mostly of lymphocytes. Plugging of follicular infundibula by cornified cells was seen only in biopsy specimens that came from lesions that were clinically indubitably follicular. Psoriasis has many histologic features in common with pityriasis rubra pilaris, but evolving and fully developed lesions of psoriasis have neutrophils in mounds of parakeratosis, thin rete ridges, thin suprapapillary plates, broad dermal papillae, and mixed inflammatory cell infiltrates of variable density.