Parapsoriasis, Large Plaque
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- Parapsoriasis describes a group of cutaneous diseases characterized by scaly patches or plaques that resemble psoriasis.
- There are three main types of parapsoriasis:
- Pityriasis lichenoides
- Small plaque parapsoriasis (see separate chapter)
- Large plaque parapsoriasis (LPP)
- LPP is a cutaneous disease of scaly patches and plaques caused by a T-cell lymphoproliferative disorder, which may precede cutaneous T-cell lymphoma (CTCL). The superficial dermal infiltrate is composed primarily of CD4+ T cells often with a dominant T-cell type (clonality).
- It is characterized by psoriasiform epidermal hyperplasia and areas of poikiloderma (a combination of mottled pigmentation, telangiectasia, and epidermal atrophy).
- Histology shows vacuolization in the basal keratinocyte layer, capillary dilation, and a lymphocytic infiltrate with a band-like distribution along the dermoepidermal junction.
- Plaques may become secondarily infected (impetiginized) secondary to scratching and excoriation.
There are no good data reflecting the true incidence of LPP.
Etiology and Pathophysiology
- LPP is a chronic inflammatory disorder likely caused by long-term stimulation of a specific subset of T cells by a corresponding antigen. This causes particular T cells to proliferate, resulting in a dominant T-cell clone that may represent up to 50% of the T-cell infiltrate. If atypical lymphocytes are present, classification as patch stage CTCL may be warranted.
- 10% of LPP cases per year progress to CTCL, and some believe LPP is actually the earliest stage of CTCL.
- Human herpesvirus type 8 has been detected in >85% of LPP lesions.
- Several theories have been postulated regarding etiology, but all studies have been inconclusive.
Specific risk factors have not been identified.
There are no known preventative measures.
Commonly Associated Conditions
10% of LPP patients per year will progress to mycosis fungoides (MF), a type of CTCL.