Current terminology of parapsoriasis refers to 2 disease processes that are caused by T-cell–predominant infiltrates in the skin. These disease processes are large plaque parapsoriasis and small plaque parapsoriasis.
As the nomenclature and description of the disease spectrum under the descriptive term parapsoriasis evolved, the primary focus has been on the distinction of whether the disorder progresses to mycosis fungoides (MF) or cutaneous T-cell lymphoma (CTCL). Small plaque parapsoriasis is a benign disorder that rarely if ever progresses. Large plaque parapsoriasis is more ominous in that approximately 10% of patients progress to CTCL. Controversy exists in the classification of large plaque parapsoriasis because some think it is equivalent to the earliest stage CTCL, the patch stage.
The duration of parapsoriasis can be variable. Small plaque disease lasts several months to years and can spontaneously resolve. Large plaque disease is chronic, and treatment is recommended because it may prevent progression to CTCL.
Pathophysiology: The initiating cause of parapsoriasis is unknown, but the diseases likely represent different stages in a continuum of lymphoproliferative disorders from chronic dermatitis to frank malignancy of CTCL.
Small plaque parapsoriasis likely is a reactive process of predominantly CD4+ T cells. Genotypic pattern observed in small plaque parapsoriasis is similar to that observed in chronic dermatitis, and the pattern of clonality of T cells is consistent with the response of a specific subset of T cells that have been stimulated by an antigen. Multiple dominant clones can be detected by polymerase chain reaction (PCR) of T-cell receptor gene usage, which supports a reactive process. Lymphocytes do not show histologic atypia to suggest malignant transformation. Southern blot analysis of T-cell receptor genes from parapsoriasis does not identify a dominant clone of T cells.
Some physicians believe that small plaque parapsoriasis is an abortive T-cell lymphoma; however, no clear distinguishing evidence, such as genetic changes observed in other malignancies, exists to support this contention. Nevertheless, a hint to the verity of this hypothesis is the recent identification of increased telomerase activity in T cells from CTCL at low-grade stages, high-grade lymphoma, and in parapsoriasis, which is activity not exhibited in normal T cells. A better understanding is likely to develop from further molecular characterization.
Large plaque parapsoriasis is a chronic inflammatory disorder, and the pathophysiology has been speculated to be long-term antigen stimulation. This disorder is associated with a dominant T-cell clone, one that may represent up to 50% of the T-cell infiltrate. If the histologic appearance is benign, without atypical lymphocytes, classification of large plaque parapsoriasis is made. If atypical lymphocytes are present, many would classify such patients as having patch stage CTCL.
