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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.2,8–10 Lymphomatoid papulosis is discussed in Chapter 145 as part of the spectrum of CD30+ cutaneous lymphoproliferative disorders.
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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.
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Etiology and Pathogenesis
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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)....