Parapsoriasis

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 [1] [2]. 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) [2] [3] [4]. Furthermore, infectious pathogens, such as human herpesvirus type 8 (HHV-8), were identified in a substantial number of lesions [5], 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 [1] [4] [6] [7]:

• Pityriasis lichenoides - It is described as a rare disorder of the skin with several clinical forms - pityriasis lichenoides et varioliformis acuta (PLEVA), pityriasis lichenoides chronica (PLC) and febrile ulceronecrotic Mucha-Habermann disease (FUMHD) [7]. Main findings include an acute eruption of small red papules on multiple sites that may leave residual hypo/hyperpigmentation and scarring after resolution (PLEVA), a slower onset of flat macules and papules that are reddish-to-brown in color (PLC), or severe generalized eruption of necrotic plaques, ulcers, and purpuric lesions that has a 25% mortality rate (FUMHD) [7].
• Small plaque parapsoriasis - The appearance of small (< 5 cm in diameter), pruritic macules, plaques, and patches on the extremities and the trunk in a relapsing and remitting fashion is the hallmark of small plaque parapsoriasis [1] [6]. They are more frequently encountered in middle-aged adults and a predominance toward male gender is observed [1] [6]. Digitate dermatosis, a separate type of small plaque parapsoriasis, is described when symmetrically distributed finger-like patches develop on the flanks [6].
• Large plaque parapsoriasis - In addition to a larger diameter (> 5 cm, but many patients present with lesions of > 10 cm), atrophy of the plaques is a typical sign of large-plaque parapsoriasis. The lower extremities (thighs and buttocks), as well as the flexures of all extremities, are the main location where skin lesions appear [6]. Retiform parapsoriasis is a term used to describe a web-like disseminated pattern of lesions [1].

A properly conducted physical examination and a meticulously obtained patient history are essential steps in making an initial diagnosis of parapsoriasis [6]. 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 [1] [6], 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 [1] [6]. However, lymphocytic infiltrates (although much milder) in the dermis have been identified in small plaque parapsoriasis as well [1], illustrating the frequent histological overlapping of the parapsoriasis subtypes.

1. Baderca F, Chiticariu E, Baudis M, Solovan C. Biopsying parapsoriasis: quo vadis? Are morphological stains enough or are ancillary tests needed? Rom J Morphol Embryol. 2014;55(3 Suppl):1085-1092.
2. Sarveswari KN, Yesudian P. The conundrum of parapsoriasis versus patch stage of mycosis fungoides. Indian J Dermatol Venereol Leprol. 2009;75(3):229-235.
3. Väkevä L, Sarna S, Vaalasti A, Pukkala E, Kariniemi AL, Ranki A. A retrospective study of the probability of the evolution of parapsoriasis en plaques into mycosis fungoides. Acta Derm Venereol. 2005;85(4):318-323.
4. Salava A, Pereira P, Aho V, et al. Skin Microbiome in Small- and Large-plaque Parapsoriasis. Acta Derm Venereol. 2017 Feb 8.
5. Kreuter A, Bischoff S, Skrygan M, et al. High association of human herpesvirus 8 in large-plaque parapsoriasis and mycosis fungoides. Arch Dermatol. 2008;144(8):1011–1016.
6. Lewin J, Latkowski JA. Digitate dermatosis (small-plaque parapsoriasis). Dermatol Online J. 2012;18(12):3.
7. Khachemoune A, Blyumin ML. Pityriasis lichenoides: pathophysiology, classification, and treatment. Am J Clin Dermatol. 2007;8(1):29-36.