Parapsoriasis is a term encompassing several disorders of the skin that possess a similar clinical presentation to psoriasis. They are considered to be lymphoproliferative in origin, as a number of patients progress to cutaneous T-cell lymphoma or mycosis fungoides. Pityriasis lichenoides, large plaque parapsoriasis (> than 5 cm in diameter) and small plaque parapsoriasis (< 5 cm) are the three main subtypes. The diagnosis is made on clinical grounds and a biopsy with a subsequent histopathological examination.
The term parapsoriasis was initially coined more than 100 years ago and included pityriasis lichenoides, large plaque parapsoriasis, and small plaque parapsoriasis, which exhibit a similar clinical appearance but different etiology from psoriasis  . The conditions that fit the clinical and histological description of parapsoriasis seem to represent a spectrum of a lymphoproliferative disorder because many studies have established that patients with either small plaque or large plaque psoriasis eventually progress to a T-cell cutaneous lymphoma (known as mycosis fungoides)   . Furthermore, infectious pathogens, such as human herpesvirus type 8 (HHV-8), were identified in a substantial number of lesions , implying that microorganisms might play an important role in the development of this clinical entity. As mentioned previously, three main subtypes of parapsoriasis are currently identified    :
A properly conducted physical examination and a meticulously obtained patient history are essential steps in making an initial diagnosis of parapsoriasis . For this reason, physicians must obtain data regarding the progression of symptoms, their pattern of appearance (relapses, recurrences, etc.), and the location where they appear. A full inspection of the skin reveals crucial information about the lesions (their size, type, and stage), which is why the physical examination is perhaps the most important component of the workup. But because the diagnosis cannot be made solely on clinical grounds, and because progression to mycosis fungoides may occur in unrecognized patients (carrying a significantly poorer prognosis), a prompt sampling of the lesion and a histopathological evaluation is necessary. Small plaque parapsoriasis is distinguished by nonspecific epithelial hyperplasia, acanthosis, spongiosis, focal hyperkeratosis, and exocytosis  , whereas infiltration of lymphocytes, lichenoid findings, variable epithelial appearance (hyperplastic, atrophic, or even normal) and necrosis of keratinocytes are known hallmarks of large plaque parapsoriasis  . However, lymphocytic infiltrates (although much milder) in the dermis have been identified in small plaque parapsoriasis as well , illustrating the frequent histological overlapping of the parapsoriasis subtypes.