Etiology
The etiology of pityriasis lichenoides is unclear. One of the leading etiological hypotheses is that pityriasis lichenoides is a form of atypical immune response in individuals genetically susceptible to a foreign agent(s).[1] A variety of infectious pathogens have been linked to this disease, including HIV, varicella-zoster virus, Epstein-Barr virus, cytomegalovirus, parvovirus B19, adenovirus, Staphylococcus, Streptococcus, Mycoplasma, Toxoplasma, and hepatitis C.[1][6] Use of medications such as tegafur, estrogen-progesterone, astemizole, kampo (Japanese herbal medicine), radiocontrast iodine, statins, and measles vaccine have also been associated with pityriasis lichenoides.[1][7][8] There are several reports of patients on tumor necrosis factor (TNF)-alpha inhibitors (infliximab and adalimumab) who developed pityriasis lichenoides, possibly due to the use of these treatments.[9][10] In addition, there are reports indicating pityriasis lichenoides as a form of immune-complex or a cell-mediated hypersensitivity reaction.[3][4] Some patients with pityriasis lichenoides exhibit elevated serum immune-complexes and deposition of IgM and C3 along the junctional zone of their skin lesions.[11] Meanwhile, other immunopathologic studies of pityriasis lichenoides lesions describe the extensive cytotoxic T cell dermal proliferation exocytosing into epidermis with decrease in Langerhans cells, correlating with a cell-mediated type of response.[12]
The second major etiologic hypothesis is that pityriasis lichenoides is a T cell lymphoproliferative disorder due to a monoclonal lymphocytic proliferation noted in the skin.[13] Furthermore, all subtypes of pityriasis lichenoides have shown rare transformation into skin lymphomas, which reinforces the hypothesis of atypical lymphocytic propagation.[1]
Pathophysiology
The pathophysiology of pityriasis lichenoides is unknown. It may represent a benign and abnormal immune response to infection or medication or development toward a cutaneous T cell neoplastic process. One study demonstrated production of nitric oxide in the skin lesions of pityriasis lichenoides, which may play a role in pathogenesis.[14] Furthermore, pityriasis lichenoides may be regarded as a paraneoplastic sign, due to its occasional association with indolent lymphoma-like conditions such as lymphomatoid papulosis, parakeratosis variegata, and mycosis fungoides.[4] It has likewise been associated with multiple autoimmune conditions, including rheumatoid arthritis, hypothyroidism, idiopathic thrombocytopenic purpura, and pernicious anemia.
Several reports described pityriasis lichenoides in pregnant women, which is likely due to the immune modification that occurs during pregnancy.[15] Koebner’s phenomenon has also been noted in pityriasis lichenoides, therefore, minimizing trauma and scratching of the skin is important with therapy.[16] Pityriasis lichenoides has also been associated with periodic fever, aphthous stomatitis, pharyngitis, and adenitis syndrome.[17]
Classification
Clinical variants[1]
Pityriasis lichenoides et varioliformis acuta: acute, generalized, pink-to-violet macules and papules that rapidly evolve into pseudovesicles with central necrosis.
Pityriasis lichenoides chronica: subacute, widespread, small, pink-to-light-brown, flat papules with adherent and shiny scales. A rare localized acral presentation of pityriasis lichenoides chronica has been reported.[2]
Ulceronecrotic Mucha-Habermann disease: emergent, very rare, diffuse, febrile, and ulceronecrotic variant.
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