Pityriasis lichenoides

Pityriasis lichenoides represents a group of uncommon skin disorders that tend to affect children and young adults, and are divided into two main conditions: pityriasis lichenoides chronica (PLC) and pityriasis lichenoides et varioliformis acuta (PLEVA). PLC is the most common form and presents with small red-brown papules with an adherent 'mica-like' scale. PLEVA presents more abruptly with grouped pink papules, which become vesicular and develop haemorrhagic crusts. A third, much rarer and aggressive condition in this group is a febrile ulcero-necrotic variant.

This chapter is set out as follows:

• The cause is unknown

• Both PLC and PLEVA present predominantly in children and young adults, although PLEVA is the most common type in young children
• Both types are rare in infancy and old age
• PLC has a male predominance

Pityriasis lichenoides et varioliformis acuta (PLEVA) 

• History
• Presents abruptly 
• Occasionally there are mild constitutional symptoms eg fever, headache, malaise and arthralgia
• Patients may complain of burning or itch

• Distribution
• Although the rash can be generalised, the trunk, buttocks and proximal limbs are the most commonly affected sites. There tends to be sparing of the head, palms and soles

• Morphology
• Lesions are numerous, tend to arise in crops, and are discrete 
• The initial lesion is a pink papule, which rapidly become vesicular and develops a haemorrhagic crust
• Lesions often heal with chickenpox-like scarring

• Other features
  • There can be mucous membrane involvement with erythematous or necrotic lesions 

A variant of PLEVA, referred to as febrile ulceronecrotic PLEVA, presents mainly in young male adults with a diffuse eruption of coalescent necrotic papules, and sometimes pustules. PLEVA is associated with high fever, other marked constitutional upset, and is occasionally fatal

Pityriasis lichenoides chronica (PLC)

• History
• ​Lesions are asymptomatic and tend to present more gradually than with PLEVA

• Distribution
• Similar to PLEVA, although acral and segmental variants have been reported
• Lesions are discrete, and more numerous than in PLEVA

• Morphology
• The characteristic lesion is a small, firm, red-brown papule with a single adherent scale, described as 'mica-like', which can be detached by gently scraping to reveal a shiny brown surface. The scale is usually not seen initially, and the eruption is often polymorphic, with lesions at different stages of evolution
• Over several weeks lesions flatten and the scale separates to leave a hyperpigmented macule, which gradually fades. Post-inflammatory hypopigmentation may also be seen and can be quite persistent

Pityriasis lichenoides - a spectrum

PLEVA lesions can evolve into lesions of PLC, consequently some patients have simultaneous lesions of both 

• Histology (reference: Rook's Textbook of Dermatology)

• PLEVA lesions are characterised by a wedge-shaped superficial and deep dermal lymphohistiocytic infiltrate with intravascular margination of neutrophils, a confluent parakeratotic crust, thinning of the granular layer, basilar necrosis of keratinocytes, vacuolar interface dermatitis with a lymphocyte in nearly every vacuole, erythrocyte extravasation, and dermal oedema

• PLC lesions are characterised by a superficial dermal infiltrate, focal parakeratosis, preservation of the granular layer, and focal disappearance of the dermal-epidermal interface

• The course of pityriasis lichenoides varies with an average duration of 18 months during which time there can be exacerbations and relapses. In some cases the condition can last for many years, with one small study suggesting this to be more likely the younger the patient and the greater the extent of the lesions

• There have been no randomised controlled trials for the treatment of pityriasis lichenoides

• Conservative management
• Provide a patient information leaflet
• Since the conditions tends towards self-resolution, a watch and wait policy may be adopted
• Emollients and/or topical steroids can be used if the skin feels itchy or uncomfortable, but are not thought to affect the course of the condition

• In cases where treatment is required, there are several options, which have been shown to benefit some patients 
  • Oral antibiotics - the ones most commonly used are the tetracyclines (eg lymecycline 408 mg OD or doxycycline 100 mg OD - avoid under the age of 12 years and in pregnancy), or the macrolides (clarithromycin or erythromycin). The minimum treatment period should be four weeks
  • Phototherapy - UVA, UVB, or even natural sunlight

• For more severe disease treatments used include methotrexate, ciclosporin, dapsone and systemic steroids