Pityriasis Lichenoides

Pityriasis lichenoides (PL) is a papulosquamous inflammatory skin disorder. Two clinical variants can be distinguished, pityriasis lichenoides et varioliformis acuta (PLEVA) and pityriasis lichenoides chronica (PLC) (Fig. 1). Although PL is regarded as reactive dermatosis, some cases may precede or evolve into large plaque parapsoriasis and mycosis fungoides (MF). Histologically, epidermotropism of lymphocytes, necrotic keratinocytes, and parakeratosis are characteristic features of PL. Some larger and occasionally atypical lymphoid cells are admixed. Immunohisto-chemically, the lymphocytes are CD4+, CD8+ or CD4- CD8- double negative. Clonal T cells can be found in the majority of PLEVA and PLC by PCR-based techniques (8-12). Although most cases of PL show a benign, self-limiting course, aggressive cases of PLEVA with a foudroyant and fatal course with disseminated erosive skin lesions have been observed (13,25). Such rare patients add support to the hypothesis that PL belong to a spectrum of cutaneous T-cell lymphoproliferative disorders with potential for malignant transformation (14).

Figure 1 PLEVA: Disseminated hemorrhagic and crusted papules and macules in a child. Occasional detection of clonal T-cell receptor gamma gene rearrangement by PCR has been described.

Figure 3 PPD with monoclonality: dense lichenoid lymphocytic infiltrate with occassional atypical cells and extravasated erythrocytes. Detection of clonal T-cell receptor gamma gene rearrangement by PCR.
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