What is pityriasis lichenoides?

Introduction

According to MedScape, pityriasis lichenoides is “a rare cutaneous disorder of unknown etiology,” characterized by “a spectrum of clinical presentations ranging from acute papular lesions that rapidly evolve into pseudovesicles and central necrosis (pityriasis lichenoides et varioliformis acuta or PLEVA) to small, scaling, benign-appearing papules (pityriasis lichenoides chronica or PLC).”

According to a 2007 article published in the American Journal of Clinical Dermatology, pityriasis lichenoides is “an uncommon, acquired spectrum of skin conditions that poses various challenges to patients as well as clinicians.”

All this is a fancy way of saying that PL/PLC/PLEVA represent a family of inflammatory skin rashes without a clear-cut cause or cure; sometimes it goes away on its own, and sometimes it doesn’t.

Pityriasis Lichenoides Fast Facts

  • Characterized by the acute onset of red, raised papules on the torso and proximal extremeties (arms, legs) that are otherwise asymptomatic in the case of PLC, and that may itch and ulcerate in the case of PLEVA
  • Can affect both genders but slightly more prevalent in males
  • Most often affects adolescents and young adults, usually appearing before the age of 30
  • Generally not harmful to general health (with the notable exception ofof febrile ultranecrotic mucha-haberman disease), although rare cases of malignant transformation have been reported
  • Not directly contagious and usually self-limiting, although it may persist over the course of months and years

PLC vs PLEVA

Pityriasis lichenoides chronica (PLC) tends to present as a more mild, less symptomatic rash (smaller papules, little to no itching) that continues for a longer period of time, from months to years.

Distinguishing criteria of PLC: 

  • Small papules that spotaneously appear, flatten and regress over the course of weeks, leaving behind hyperpigmented spots
  • Usally present on torso and proximal extremities
  • Papules tend not to itch
  • Papules tend not to ulcerate (turn into an open sore)

On the other hand, pityriasis lichenoides et varioliformis acuta (PLEVA) usually presents with angrier, redder lesions that are 5-15mm in diameter and that may exhibit other symptoms like ulceration and itching:

Distinguishing criteria of PLEVA: 

  • Somewhat larger papules of around 5-15mm
  • Present on torso and proximal extremetires
  • Itching may be present
  • Ulceration may be present

Both types of lesions can exist on the same patient at the same time; a biopsy is required to conclusively distinguish these subtypes, as they exhibit different microscopic structures, although debate still exists as to whether they are truly distinct conditions or exist on a unified spectrum.

A third, extremely rare condition called febrile ultranecrotic Mucha-Haberman disease (FUMHD) exists within the pityriasis lichenoides family and is characterized by large, highly irritated papules that rapidly transform into painful blood blisters and pustules. This condition is considered a medical emergency and anyone who exhibits a severe worsening of their PL condition should seek urgent care immediately.

Underlying Mechanism Behind PLC/PLEVA

I’ll discuss this in greater depth in a future post, but there are three generally accepted hypotheses on the mechanism behind the chronic skin inflammation that characterizes pityriasis lichenoides:

  • Inflammation caused by an infection: An inflammatory reaction caused by infectious agents; suspects include members of the herpes family like the Epstein-Barr virus (EBV) and cytomegalovirus (CMV), toxoplasma gondii, parovirus and staph.
  • Benign T-cell lymphoproliferative disorder: Biopsies of patients with PLC and PLEVA have shown an excessive number of immune T-cells present in the lesions, leading some studies to conclude that the condition is on the benign end of a spectrum that includes cutaneous T-cell lymphomas like mycosis fungoides.
  • Hypersensitivity vasculitis: Circulating immune complexes may deposit themselves in the skin and encourage lesion formation.

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