Parapsoriasis, Large Plaque

Basics

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.

Incidence
There are no good data reflecting the true incidence of LPP.

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.

10% of LPP patients per year will progress to mycosis fungoides (MF), a type of CTCL.

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