It often is difficult to make a diagnosis with specificity of pityriasis rubra pilaris, even in the context of features clinical diagnostic of that disease. The reason is that many of the changes associated with pityriasis rubra pilaris also may be seen in conditions like psoriasis and chronic contact dermatitis or chronic nummular dermatitis. As examples of the similarities, psoriasis and pityriasis rubra pilaris have in common findings histopathologic of parakeratosis, psoriasiform acanthosis, an increased number of mitotic figures in basal keratocytes and, at times, suprabasal ones, dilated tortuous capillaries in dermal papillae, sparse infiltrates of lymphocytes, and erythrocytes extravasated in the papillary dermis and epidermis.
Chronic allergic contact dermatitis and chronic nummular dermatitis share several changes histopathologic with pityriasis rubra pilaris, namely, orthokeratosis and parakeratosis, a prominent granular zone, uneven psoriasiform acanthosis, thick suprapapillary plates, and a superficial perivascular infiltrate of lymphocytes.
One of the clues most helpful for differentiation of pityriasis rubra pilaris from all other psoriasiform dermatitides is alternation of orthokeratosis and parakeratosis in both vertical and horizontal directions in a thickened cornified layer. That checkerboard sign, when present, is virtually diagnostic of pityriasis rubra pilaris. In solar keratoses, orthokeratosis and parakeratotic cells also alternate vertically, but not horizontally. No condition other than pityriasis rubra pilaris has orthokeratotic and parakeratotic cells that alternate in both vertical and horizontal directions.
Unfortunately, not every biopsy specimen of pityriasis rubra pilaris provides sections of tissue that show signs of alternation, in both horizontal and vertical direction, of orthokeratotic and parakeratotic cells in the cornified layer. In sections from some specimens of pityriasis rubra pilaris, the parakeratotic nuclei stain so faintly that no judgment firm about alternation of parakeratosis and orthokeratosis can be made. When such sections are stained more vibrantly by H&E, the subtle changes in the cornified layer of pityriasis rubra pilaris sometimes may become evident. It should be noted that many specimens of unquestionable pityriasis rubra pilaris are devoid of infundibula plugged by cornified cells, especially those extracted from regions erythrodermic in which no discrete keratotic papules are apparent clinically. Because that sign, touted by textbooks to be crucial to diagnosis of pityriasis rubra pilaris, often is absent, it should not be surprising that no plugs of corneocytes are discernable in infundibula of sections from some specimens of pityriasis rubra pilaris.
As a rule in inflammatory diseases of the skin, prominent parakeratosis is associated with decrease in the thickness normal of the granular zone. Examples of that phenomenon are encountered in psoriasis, Mucha-Habermann disease, and the sextet of pellagra, acrodermatitis enteropathica, necrolytic migratory erythema, kwashiorkor, Hartnup’s disease, and maple syrup disease. In the case of pityriasis rubra pilaris, however, notable parakeratosis is present above a granular zone that is prominent. That seeming paradox is yet another clue to diagnosis of pityriasis rubra pilaris and can be explained by the intervals between acceleration and deceleration of the process pathologic as it affects the epidermis being much longer than those of psoriasis. When in pityriasis rubra pilaris acceleration of epidermopoiesis occurs, parakeratosis results with subsequent diminution of the granular zone; deceleration follows and the granular zone then forms anew.