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Parapsoriasis at a Glance
  • Also known as parapsoriasis en plaques.
  • Parapsoriasis occurs worldwide and affects mainly adults.
  • Large-plaque parapsoriasis (LPP) and small-plaque parapsoriasis (SPP) are recognized.
  • Large and small “plaque” lesions actually present as flat patches rather than infiltrated plaques.
  • Lesions are chronic and favor nonsun-exposed skin; LPP may be poikilodermatous.
  • Pathology consists of superficial, mostly CD4+ T-cell infiltrate; dominant clonality is more common in LPP than in SPP.
  • LPP appears to exist on a continuum with patch-stage mycosis fungoides (MF) and progresses to overt MF at a rate of approximately 10% per decade.
  • SPP has minimal risk of progression to overt MF in the experience of most experts.
  • Treatment options include topical corticosteroids; ultraviolet B (UVB) irradiation, and psoralen and ultraviolet A (UVA) irradiation; excimer laser; and topical cytotoxic drugs.

Historical Aspects

The term parapsoriasis was coined originally by Brocq in 1902.1 As shown in Table 25-1, the currently accepted classification of parapsoriasis includes large- and small-plaque forms of parapsoriasis en plaques (often referred to simply as parapsoriasis) as well as acute and chronic forms of pityriasis lichenoides [known today as pityriasis lichenoides et varioliformis acuta (PLEVA) and pityriasis lichenoides chronica (PLC), respectively].2 Pityriasis lichenoides was first described in 1894 by Neisser3 and Jadassohn.4 In 1899, Juliusberg delineated the chronic form and named it PLC.5 Mucha redescribed the acute form in 1916 and distinguished it from the chronic form.6 Habermann named the acute variant PLEVA in 1925.7 Mucha–Habermann disease is synonymous with PLEVA. Some authors regard lymphomatoid papulosis as a variant of pityriasis lichenoides, whereas others consider it to be a separate disease.2,810 Lymphomatoid papulosis is discussed in Chapter 145 as part of the spectrum of CD30+ cutaneous lymphoproliferative disorders.

Table 25-1 Classification of Parapsoriasis

Epidemiology

Large-plaque parapsoriasis (LPP) and small-plaque parapsoriasis (SPP) are, in general, diseases of middle-aged and older people, with a peak incidence in the fifth decade. Occasionally, lesions arise in childhood and may be associated with pityriasis lichenoides. SPP shows a definite male predominance of approximately 3:1. LPP is probably more common in males, but the difference is not as striking as in SPP. Both occur in all racial groups and geographic regions.

Etiology and Pathogenesis

It is likely that a complete understanding of the pathogenesis of parapsoriasis will develop with our understanding of the pathogenesis of both chronic dermatitis and mycosis fungoides (MF), because parapsoriasis appears to bridge these disorders. The T cells that mediate most inflammatory skin diseases belong to the skin-associated lymphoid tissue (SALT)....

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