Parapsoriasis and premycotic eruption
Parapsoriasis is a contentious term, which many would like to drop. We still find it useful clinically for lesions that look a little like psoriasis but which scale subtly rather than grossly, and which persist despite anti-psoriasis treatment.
It is worth trying to distinguish a benign type of parapsoriasis from a premycotic type, which is a forerunner of mycosis fungoides, a cutaneous T-cell lymphoma (Fig. 6.9)aalthough they can look alike early in their development. However, even the term ‘premycotic’ is disputed, as some think that these lesions are mycosis fungoides right from the start, preferring the term ‘patch stage cutaneous T-cell lymphoma’.
The cause is otherwise unknown.
Pink scaly well-marginated plaques appear, typically on the buttocks, breasts, abdomen or flexural skin. The distinguishing features of the small-plaque (benign) and large-plaque (premycotic/prelymphomatous) types are given in Table 6.3.
Perhaps the most important point to look for is the presence of poikiloderma (atrophy, telangiectasia and reticulate pigmentation) in the latter type. Both conditions are stubborn in their response to topical treatment, although often responding temporarily to PUVA. Itching is variable.
Patients with suspected premycotic/prelymphomatous eruptions should be followed up carefully, even though the development of cutaneous T-cell lymphoma may not occur for years. If poikiloderma or induration develops, the diagnosis of a cutaneous T-cell lym-phoma becomes likely
This includes psoriasis, tinea and nummular (discoid) eczema. In contrast to psoriasis and pityriasis rosea, the lesions of parapsoriasis, characteristically, are asymmetrical. Topical steroids can cause atrophy and confusion.
Several biopsies should be taken if a premycotic erup-tion is suspected, if possible from thick or atrophic untreated areas. These may suggest an early cutaneous T-cell lymphoma, with bizarre mononuclear cells both in the dermis and in microscopic abscesses within the epidermis. Electron microscopy may show abnormal lymphocytes with convoluted nuclei in the dermis or epidermis, although the finding of these cells, especially in the dermis, is non-specific. DNA probes can determine monoclonality of the T cells within the lymphoid infiltrate of mycosis fungoides based on rearrangements of the T-cell receptor genes. The use of these probes and of immunophenotyping
Treatment is controversial. Less aggressive treatments are used for the benign type of parapsoriasis. Usually, moderately potent steroids or ultraviolet radiation bring some resolution, but lesions tend to recur when these are stopped. For premycotic/prelymphomatous eruptions, treatment with PUVA with topi-cal nitrogen mustard paints, is advocated by some, although it is not clear that this slows down or pre-vents the development of a subsequent cutaneous T-cell lymphoma.