Parapsoriasis—A Diagnosis with an Identity Crisis: A Narrative Review

Parapsoriasis is an uncommon inflammatory skin disease characterized by chronic patches that may be resistant to therapy. It was primarily introduced and classified 120 years ago, and the original classification incorporated parapsoriasis and pityriasis lichenoides under the umbrella term parapsoriasis. After a major change in classification, parapsoriasis now exclusively refers to small plaque parapsoriasis (SPP) and large plaque parapsoriasis (LPP). However, debates still frequently occur regarding various nomenclatures and classifications used by different authors. Moreover, parapsoriasis may progress to overt cutaneous lymphoma, most commonly mycosis fungoides (MF), and it is very difficult to distinguish these two conditions despite modern histologic and molecular testing techniques. As parapsoriasis is a rare disease, there is a lack of studies and clinical guidelines to assist physicians in clinical practice. In our comprehensive review, we review several aspects of parapsoriasis, from the history of nomenclature and classification, clinical characteristics, immunohistopathology, and advanced molecular techniques for the diagnosis of this condition, to the most current treatments. We also propose a scheme for distinguishing parapsoriasis from early-stage MF in this review.