Pityriasis lichenoides: pathophysiology, classification, and treatment.

Pityriasis lichenoides (PL) is an uncommon, acquired spectrum of skin conditions that poses various challenges to patients as well as clinicians. It is a difficult and debatable disorder to diagnose, categorize, and treat. Besides these inherent obstacles, PL merits awareness because of its potential to progress to cutaneous lymphoma or an ulceronecrotic presentation, both of which carry a significant risk of mortality. The scope of PL presentations is delineated along a continuum of multiple variants including pityriasis lichenoides et varioliformis acuta (PLEVA), pityriasis lichenoides chronica (PLC), and febrile ulceronecrotic Mucha-Habermann disease (FUMHD). Classification of these presentations as separate subsets is debatable in view of their overlapping clinical, histopathologic, and etiologic features. PLEVA generally presents as an acute-to-subacute skin eruption of multiple, small, red papules that develops into polymorphic lesions and vacillates with periods of varying remissions as well as possible sequelae of hyper/hypopigmentation and varicella-like scars. PLC has a more gradual manifestation of very small red-to-brown flat maculopapules with mica-like scale; it also follows a relapsing course but with long periods of remission. FUMHD is an acute and severe generalized eruption of purpuric and ulceronecrotic plaques with associated systemic involvement and a mortality rate of up to 25%; hence, it should be approached as a dermatologic emergency.Histopathological evaluation of PL usually reveals dermal, wedge-shaped, lymphocytic infiltrate, epidermal spongiosis, parakeratosis, and variable necrosis of keratinocytes. PLC demonstrates more subtle histology whereas, at the other end of the spectrum, febrile ulceronecrotic FUMHD exhibits the most exaggerated histological features. The pathogenic mechanism behind PL is unclear although infectious or drug-related hypersensitivity reactions versus premycotic lymphoproliferative disorder are the mainstay theories. The foremost therapies for PLEVA and PLC are phototherapy, systemic antibacterials, and topical corticosteroids. Aggressive treatment with immunosuppressant and/or immunomodulating agents as well as intensive supportive care are recommended for FUMHD. We first describe a representative case of a 14-year-old boy with PLC who was successfully treated with narrow-band UVB. We then review the pathophysiology, classification, and treatment of PL.