A self-limited inflammatory dermatosis of unknown etiology. Subclassified into acute (pityriasis lichenoides et varioliformis acuta, PLEVA) and chronic (pityriasis lichenoides chronica, PLC) forms:
Pityriasis lichenoides et varioliformis acuta (PLEVA):
- Generalized eruption of lesions of varying ages which progress as follows: papule, central vesiculation, pustulation, erotion with ulceration, crusting and eventual healing with scar formation
- Predilection for flexural surfaces
- Systemic symptoms may occur.
- More common in children and young adults
- Parakeratotic scale, often containing neutrophils
- Interface change with necrosis of basal keratinocytes
- Marked upper and deep dermal perivascular lyphocytic inflammation (predominantly CD4+ T-cells)
- Frequently marked exocytosis of lymphocytes into the epidermis
- Fully developed lesions may show dermal and epidermal hemorrhage along with endothelial cell swelling and rarely vasculitis.
- Older lesions show confuent necrosis of keratinocytes and eventual vesicle formation, ulceration and granulation tissue, terminating with dermal scar formation.
Pityriasis lichenoides chronica (PLC):
- Generalized, chronic, small erythematous papules that evolve to form a pinpoint dusky center
- Lesions persist for several weeks, then heal without scarring.
- May evolve from PLEVA or arise de novo.
- Parakeratotic scale, rare necrotic keratinocytes, mild basal vacuolization (changes less pronounced than in PLEVA)
- Upper dermal perivascular lymphocytic infiltrate with exocytosis of lymphocytes into the epidermis
- Upper dermal and epidermal hemorrhage