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Pityriasis Lichenoides et Varioliformis Acuta (tier 2)
a general reaction pattern in the category of lichenoid lymphocytic vasculitis.
Pityriasis lichenoides represents a third major division in the general category of lichenoid lymphocytic vasculitis. The category of pityriasis
lichenoides can be divided into two sub-categories: pityriasis lichenoides et varioliformis acuta and pityriasis lichenoides chronica. The latter category probably is heterogeneous with some examples representing a
chronic form of pityriasis lichenoides et varioliformis; other examples may be a variant of the premalignant T-cell dysplasias.
Pityriasis lichenoides et varioliformis acuta (PLVA) is characterized by hyperplasia of the basal unit of the epidermis; the superficial unit may
show hyperplasia, but often is thin (S4C17P9-1-3, & S4C18P10-1). The cells
of the basal unit show cytoplasmic basophilia; those of the superficial unit tend to be pale and acidophilic. The keratinized product at the surface tends to be parakeratotic; there may be uniform parakeratosis
(this contrasts with a keratinized product that is often normal at the surface of a lesion of EM). Neutrophils often collect in the parakeratotic cap in Munro microabscess-like patterns (S4C18P10-1). There are prominent infiltrates of lymphocytes, and histiocytes in the dermis; they tend to be band-like in the papillary dermis and
perivascular in the reticular dermis. The lymphoid cells migrate into the basal unit of the epidermis (S4C17P9-4-6). In an expanded basal unit, the
lymphoid infiltrates may be prominent and extensive. Scattered necrotic keratinocytes are common, but clusters of such cells, as seen in EM, are uncommon. In some examples, lytic defects form at the dermal-epidermal
interface (S4C17P9-7, & S4C19P11-1). Extravasation of red blood cells into
the dermis and the overlying epidermis is a common feature (S4C17P9-5, S4C18P10-1, & S4C19P11-1)
The lymphoid infiltrates also involve skin appendages; the resulting patterns are similar to those of the papillary dermis and the epidermis (S4C17P9-7, & S4C19P11-4-5).
Th perivenular infiltrates of lymphoid cells often are a prominent feature; they extend along vessels well into the reticular dermis. This
perivenular component is often dismissed as simply an infiltrate but, from another perspective, the vascular changes qualify as a vasculitic component. In some example, the vascular changes include fibrinoid
degeneration of the walls of some of the dermal vessels (S4C18P10-2, & S4C19P11-6).
Occasionally, the epidermis shows degeneration with cytoplasmic pallor and lysis of nuclei. Changes of this type may be an expression of the reaction of the epidermis to ischemia (S4C19P11-3).
The lymphoid infiltrates of PLVA may show some degree of atypia; this is particularly true of the epidermal infiltrates. The lymphoid cells may show
enlarged nuclei with convoluted outlines; the cytologic features may suggest the possibility of a progressive T-cell dysplasia (S4C20P12-1-4). If
the changes are those of PLVA with only cytologic atypia as a deviation, the lesion qualifies as atypical PLVA (mild to moderate T-cell dysplasia of indeterminate malignant potential). Such cases have been
included, by some observers, in the category of lymphomatoid papulosis. Other observers question whether a relationship exists between PLVA and lymphomatoid papulosis.
Gelmetti C, et al: Pityriasis lichenoides in children: a long-term follow-up of eighty-nine cases. J Am Acad Dermatol 1990: 473-8.
Zirbel GM, et al: Lymphomatoid papulosis in children. J Am Acad D ermatol 1995;33: 741-8.
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