Past exam of the medicine course of the University of Oxford / 1865 / Second examination / Pathology / 4 / Solution

The historical name pityriasis grouped finely scaling eruptions; its varieties were not all one fungal infection. A useful period list is simple or scalp pityriasis, pityriasis rubra, pityriasis versicolor and pityriasis nigra. The names describe distribution or colour, and classifications differed. In particular, “nigra” was sometimes a very dark form of “versicolor,” sometimes a pigmentation disorder with little true scaling. It cannot safely be translated into one present species of fungus.^[ref]

Simple pityriasis, often called pityriasis capitis on the scalp, gives loose, whitish, bran-like scales with little or variable redness. Microscopy of a scale principally shows flattened keratinised epithelial cells and debris. An oily or inflamed scalp can also contain organisms normally living there; this does not make every case identical to pityriasis versicolor. Present descriptions of dandruff and seborrhoeic dermatitis explain many such appearances, but the historical label cannot retrospectively specify the cause of each case.^[ref]

Pityriasis rubra denoted a conspicuously red, scaling eruption, sometimes extensive. Detached scales are again principally epidermal material. A modern skin biopsy may show an inflammatory pattern, but there is no single microscopical organism that defines the historical category. It must not automatically be renamed pityriasis rubra pilaris, a distinct follicular disorder. An extensive red and peeling skin is a pattern requiring causal diagnosis, rather than proof of one infection.^[ref]

Pityriasis versicolor produces irregular pale, yellowish-brown or brown patches, commonly on the upper anatomical trunk, with a fine surface scale. Here the microscopy is especially discriminating: a superficial scraping, cleared with potassium hydroxide, shows clusters of round yeast forms and short curved hyphae within the stratum corneum. Modern classification places the organisms in Malassezia. The older “Microsporon furfur” terminology should not be read as identification of a modern Microsporum dermatophyte. On a histological section the organisms occupy the horny surface layer and are made conspicuous by suitable fungal stains.^[ref]

Pityriasis nigra calls first for verification of what is actually being described. If it is dark pityriasis versicolor, the same superficial yeast and hyphae may be found; if the lesion is pigmentation without a fungal eruption, this finding cannot be assumed. A contemporary discussion explicitly recorded the overlap of nigra and versicolor and disputes over the significance of the supposed parasite.^[ref] Thus epidermal scales are common to the description, but demonstrable fungal structures particularly distinguish pityriasis versicolor. Examine actual scrapings with a optical microscope, compare several fields and use appropriate controls before attributing a different scaling disorder to a fungus.