Pityriasis Lichenoides Chronica Presenting With Bilateral Palmoplantar Involvement

Etiology

The cause of pityriasis lichenoides is unknown, but there are 3 popular theories regarding its pathogenesis: a hypersensitivity response due to an infectious agent, an inflammatory response to a T-cell dyscrasia, or an immune complex–mediated hypersensitivity vasculitis.² The theory of an infectious cause has been proposed due to reports of disease clustering in families and communities.^2,3 Elevated titers of certain pathogens and clearing of the disease after pathogen-specific treatment also have been reported. Possible triggers cited in the literature include the Epstein-Barr virus, Toxoplasma gondii, parvovirus B19, adenovirus, human immunodeficiency virus, freeze-dried live attenuated measles vaccine, Staphylococcus aureus, and group A β-hemolytic streptococci.^2,3

Some reported cases of pityriasis lichenoides have demonstrated T-cell clonality. Weinberg et al⁴ found a significantly higher number of clonal T cells in PLEVA than in PLC (P=.008) and hypothesized that PLEVA is actually a benign clonal T-cell disorder arising from a specific subset of T cells in PLC. Malignant transformation of pityriasis lichenoides has been reported but is rare.³

Differential Diagnosis

Historically, pityriasis lichenoides has been confused with many other dermatoses. With palmoplantar involvement, consider other papulosquamous disorders such as palmoplantar psoriasis, lichen planus, cutaneous T-cell lymphoma, lymphomatoid papulosis, vasculitis, and secondary syphilis. Rule out alternative diagnoses with histologic examination; assessments of nails, oral mucosa, joints, and constitutional symptoms; and laboratory testing.

Histopathology

Pityriasis lichenoides et varioliformis acuta and PLC are similar with subtle and gradually evolving differences, supporting the notion that these disorders are polar ends of the same disease spectrum.² Pityriasis lichenoides et varioliformis acuta typically produces a dense wedge-shaped dermal infiltrate composed of CD8+ T cells and histiocytes most concentrated along the basal layer with lymphocytic exocytosis into the epidermis and perivascular inflammation. The epidermis also demonstrates spongiosis, necrosis and apoptosis of keratinocytes, neutrophilic inclusions, vacuolar degeneration, intraepidermal vesicles and ulceration, and focal parakeratosis with scale and crust. In contrast, PLC is less exaggerated than PLEVA with a superficial bandlike lymphocytic infiltrate in which CD4+ T cells predominate with minimal perivascular involvement. Immunohistochemical studies reveal that CD8+ cells predominate in PLEVA, while CD4+ cells predominate in PLC. Staining for HLA-DR–positive keratinocytes yields stronger and more diffuse findings in PLEVA than in PLC and is considered a marker for the former.²

Treatment

There is no standard treatment of pityriasis lichenoides. However, combination therapy is considered the best approach. To date, phototherapy has been the most effective modality and is considered a first-line treatment of PLC. Variants of phototherapy include UVB, NB-UVB, psoralen plus UVA, and UVA1.⁵ One study showed UVA1 (340–400 nm) treatment to be effective and well tolerated at a medium dose of 60 J/cm2.⁶ Narrowband UVB has become a well-used phototherapy for a variety of skin conditions including pityriasis lichenoides. In a study by Aydogan et al,⁵ NB-UVB was safe and effective for the management of PLEVA and PLC. The authors also argue that it has added advantages over other phototherapies, including a more immunosuppressive effect on lymphoproliferation that causes a greater depletion of T cells in skin lesions, possibly due to its deeper dermal penetration compared with broadband UVB. Narrowband UVB also is safe in children.⁵ Tapering of phototherapy has been recommended to prevent relapses.³

If infection is a suspected contributor to the problem, treat as needed. The antibiotics tetracycline, erythromycin, and dapsone have been used with success, as well as the antiviral acyclovir. Tetracycline and erythromycin also may confer anti-inflammatory benefits. A gradual taper of these agents is advised to prevent recurrences. Topical corticosteroids and coal tar may help alleviate pruritus and inflammation; however, they do not affect the course of the disease.³ In one report, the topical immunomodulator tacrolimus markedly reduced lesions, most likely due to its anti-inflammatory effect. After discontinuation of the medication, lesions recurred but were less severe.⁷

Clinical Recommendations

Early diagnosis and management of pityriasis lichenoides is essential. At this time, screening for pathogens is not advised unless the patient has specific symptoms of infection. Due to the history of recurrence with this disease, combination therapy is recommended with a gradual taper of all modalities. Because of the rare but possible transformation to malignancy, careful follow-up and repeated biopsies have been advised in chronic intermittent disease.³