Parapsoriasis

There are three entities described as parapsoriasis: small-plaque parapsoriasis, large-plaque parapsoriasis, and parapsoriasis variegata.

Large-plaque parapsoriasis and parapsoriasis variegata are best considered as early stages of cutaneous T-cell lymphoma/mycosis fungoides.

The small-plaque parapsoriasis is also known as xanthoerythrodermia perstans of Crocker  and as digitate dermatosis . Pink-to-yellow, slightly scaly, oval or elongated, often fingerprint-like patches 1 to 5 cm in diameter are symmetrically distributed over the trunk and the proximal portions of the extremities following the tension lines of the skin . The eruption is usually asymptomatic, has a chronic course, and tends to persist. In some instances, cases diagnosed originally as small-plaque type later showed reticulate pigmentation and atrophy and were reclassified as the large-plaque type .

Histopathology.

 The small-plaque parapsoriasis shows focal epidermal involvement consisting of slight spongiosis, exocytosis of lymphocytes, mild acanthosis, and parakeratosis . Elongated mounds of parakeratosis with collections of plasma above a basket-weave cornified layer is a characteristic finding . In the papillary dermis, there is a mild superficial perivascular lymphocytic infiltrate that in some instances is more pronounced and resembles that seen in the large plaque type; such cases require inclusion in the large­plaque category . It must be conceded that, in some instances, a clinical or histologic distinction of small-plaque parapsoriasis from large-plaque parapsoriasis is difficult, so that only the subsequent course decides the issue .

Pathogenesis. The inflammatory infiltrate in small-plaque parapsoriasis is dominated by CD4+ (helper-inducer) T lymphocytes with a small proportion of the CD8+ (cytotoxic-suppressor)

T lymphocytes subset. Langerhans cells are increased in the epidermis and dermis .

Relationship to Lymphoma.

 Although it is generally accepted that small-plaque parapsoriasis, or digitate dermatosis, is a benign disorder without the potential for transformation into mycosis fungoides , it has been shown that in some cases there is a dominant clonal rearrangement of the infiltrating T lymphocytes . Accordingly, some authors hypothesize that small-plaque parapsoriasis could be placed into the category of "abortive lymphomas," which includes conditions in which clonality can be demonstrated

but conversion into a systemic lymphoma does not occur . However, some believe that small-plaque parapsoriasis is a form of mycosis fungoides , and in a retrospective cohort study of more than 100 patients in a 26-year follow-up period, 10% of those diagnosed with small-plaque para-psoriasis and 35% of those diagnosed with the large-plaque variant developed histologically confirmed mycosis fungoides . Another study detected a clonal T-cell proliferation in the peripheral blood but not in the skin of patients with small-plaque parapsoriasis, suggesting that this disorder is not strongly related to mycosis fungoides, where there is clonal T­cell proliferation in both peripheral blood and skin .