Description, Causes and Risk Factors:
Pityriasis lichenoides affects all racial and ethnic groups in all geographic regions. It is more common in children and young adults but can affect all ages with seasonal variation in onset favoring fall and winter. There is a male predominance of 1.5:1 to 3:1.
The true etiology of pityriasis lichenoides is unknown. Some cases have been associated with infectious agents such as Toxoplasma gondii, Epstein-Barr virus, cytomegalovirus, parvovirus B19, and human immunodeficiency virus.
Immunohistologic studies have shown a reduction in CD1a* antigen-presenting dendritic (Langerhans) cells within the central epidermis of pityriasis lichenoides lesions. Keratinocytes and endothelial cells are HLADR*, which suggests activation by T-cell cytokines. CD8* T cells predominate in PLEVA (Pityriasis lichenoides et varioliformis acuta), whereas either CD8* or CD4* T cells predominate in PLC (pityriasis lichenoides chronica). Many of these T cells express memory proteins (CD45RO) and cytolytic proteins (TIA-1 and granzyme B). Dominant T-cell clonality has been demonstrated in about half of PLEVA cases and a minority of PLC cases. In aggregate, these findings raise the possibility that pityriasis lichenoides is a variably clonal cytolytic memory T-cell lymphoproliferative response to one or more foreign antigens. Deposition of immunoglobulin M, C3, and fibrin in and around blood vessels and along the dermal-epidermal junction in early acute lesions suggest a possible concomitant humoral immune response, although this could be a secondary phenomenon.
All cases of pityriasis lichenoides contain an interface dermatitis that is denser and more wedge shaped in the acute lesions. The infiltrate is composed mainly of lymphocytes with a variable admixture of neutrophils and histiocytes. There is exocytosis, parakeratosis, and extravasation of erythrocytes. Epidermal damage ranges from intercellular and extracellular edema in less severe cases to extensive keratinocyte necrosis, vesicles, pustules, and ulcers. The acute variants can exhibit lymphocytic vasculitis with fibrinoid degeneration of blood vessel walls.
Pityriasis lichenoides has a variable clinical course characterized by recurrent crops of lesions that spontaneously resolve. The disorder may resolve spontaneously within a few months or, less commonly, persist for years. PLEVA usually has a shorter duration than PLC. Although the conclusion was not confirmed by subsequent investigation, one report suggested that the duration of pityriasis lichenoides in children correlated better with its clinical distribution than with the relative abundance of acute and chronic lesions, which often coexisted. From longest to shortest duration, the distribution of lesions ranged from peripheral (distal extremities) to central (trunk) to diffuse.
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.
Initially a small pink papule occurs that turns a reddish-brown color. Usually a fine mica-like adherent scale attached to the central spot develops. This can be peeled off to reveal a shiny, pinkish brown surface.Over several weeks the spot flattens out spontaneously and leaves behind a brown mark, which fades over several months.
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.
Miscellaneous nonspecific abnormalities in blood testresults occur but are of little practical value. Leukocytosis and a decreased CD4/CD8 ratio can occur. The true diagnosis is based on clinical appearance and usually confirmed by skin biopsy. Other investigation are directed towards possible associated disease and the differential diagnosis including antistreptolysin O titers, ESR, Hepatitis B and C serology, HIV screening, infectious mononucleosis tests, syphilis serology, EB virus serology and Toxoplasma serology.
The mainstay of traditional therapy has been a combination of topical corticosteroids and phototherapy. Systemic antibiotics in the tetracycline and erythromycin families are used primarily for their anti-inflammatory rather than antibiotic effects. One newer option is azithromycin. Cases with minimal disease activity may not require any treatment. Photodynamic therapy has been used successfully for PLC. The more acute the clinical course and the more severe the individual lesions, the more systemic therapy is indicated. Methotrexate is often effective in relatively low doses. Calcineurin inhibitors and retinoids may also be beneficial. Severe cases of PLEVA often require systemic corticosteroids or similar drugs to gain control of systemic symptoms. Topical and systemic antibiotics may be needed to treat secondary infections complicating ulcerated skin lesions. These agents are often selected initially to cover Gram-positive pathogens, but subsequent use should be guided by culture results. Bromelain, a pineapple extract, cleared PLC lesions in 8/8 cases.
NOTE: The above information is educational purpose. The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition.
DISCLAIMER: This information should not substitute for seeking responsible, professional medical care.
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