Pityriasis lichenoides

Author: Vanessa Ngan, Staff Writer, 1998.

What is pityriasis lichenoides?

Pityriasis lichenoides is an uncommon rash of unknown cause. It can range from a relatively mild chronic form to a more severe acute eruption. The mild chronic form, pityriasis lichenoides chronica (PLC), is characterised by the gradual development of symptomless, small, scaling papules that spontaneously flatten and regress over a period of weeks. At the other end of a continuous clinical spectrum of pityriasis lichenoides is the acute form, characterised by the abrupt eruption of small scaling papules that develop into blisters and crusted red-brown spots. This form is also known as pityriasis lichenoides et varioliformis acuta (PLEVA).

Who gets pityriasis lichenoides and why?

Pityriasis lichenoides most often affects adolescents and young adults, usually appearing before the age of 30. It appears to be slightly more common in males. It is rare in infants and in old age.

The cause of pityriasis lichenoides is not yet known but 3 major theories exist:

Infections that have been associated with both PLC and PLEVA include:

Lymphoproliferative disorders are conditions in which there excessive numbers of lymphocytes (T- and B-cells), including lymphoma (a malignant growth of lymphocytes). The idea that pityriasis lichenoides may be a lymphoproliferative disorder arises because lesions of patients with pityriasis lichenoides showed the presence of immune T-cells with specific CD30+ markers or antigens in PLEVA, and loss of CD7 antigens on T-cells in PLC. These characteristics of T cells are indicators of lymphoproliferative disorders.

The third theory behind the cause of pityriasis lichenoides is the detection of circulating immune complexes (aggregations of antigens and antibodies) deposited in the skin in some patients with the condition.

What are the clinical features of pityriasis lichenoides?

Pityriasis lichenoides chronica (PLC)

PLC has a more low-grade clinical course than PLEVA. PLC lesions may appear over the course of several days, weeks or months. Lesions at various stages may be present at any one time.

PLC most commonly occurs over the trunk, buttocks, arms and legs, but may also occur on the hands, feet, face and scalp. Unlike PLEVA, lesions are not painful, itchy or irritable. Often patients with PLC have exacerbations and relapses of the condition, which can last for months or years.

Pityriasis lichenoides et varioliformis acuta (PLEVA)

PLEVA is characterised by red patches that quickly evolve into papules 5-15 mm in diameter. They are often covered with a fine mica-like adherent scale. The centre of the papules often becomes filled with pus and blood, or eroded with overlying red-brown crust.

PLEVA most often occurs on the trunk and extremities but sometimes may also be diffuse and widespread, covering any part of the body. Unlike PLC, which has no apparent symptoms, patients with PLEVA experience burning and itchiness.

A subtype of PLEVA is febrile ulceronecrotic Mucha-Habermann disease, in which black or necrotic papules rapidly develop into large coalescent crusted ulcers, blood-filled blisters, and pustules. Mucha-Habermann lesions are usually very painful. The ulcers may become infected. In addition, systemic symptoms may include high fever, sore throat, diarrhoea, abdominal pain, central nervous system symptoms, lung disease, enlarged spleen, arthritis, sepsis, anaemia, and conjunctival ulcers. In some cases Mucha-Habermann disease can lead to death.

How is pityriasis lichenoides diagnosed?

Pityriasis lichenoides is diagnosed by its clinical history and appearance, and the diagnosis is usually confirmed by skin biopsy. The pathology of PLEVA and of PLC are distinctive.

What is the treatment of pityriasis lichenoides?

Pityriasis lichenoides may not always respond to treatment and relapses often occur when treatment is discontinued. If the rash is not causing symptoms, treatment may not be necessary. Large ulcerations found in febrile ulceronecrotic Muchas-Habermann disease require local wound care.

In cases where treatment is necessary, there are several different therapies available. Current recommended first-line therapies include:

Second-line therapies include:

Phototherapy – artificial ultraviolet radiation treatment with UVB or PUVA has been used with varying success both in patients with PLEVA and in those with PLC.

Third-line therapies include:

Pityriasis lichenoides may persist for some years but is generally fairly harmless, although there have been rare reports of malignant transformation. Because of this, regular follow-up is recommended.

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