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SS has an annual incidence rate of 1/10,000,000 and represents 3% of all cutaneous lymphomas.

SS develops most frequently in men, in most cases during their fifth decade of life and progresses rapidly. SS correspond to stages IVA2 and IVB of T-cell cutaneous lymphoma (see this term). Patients present with a scaling erythroderma and infiltration often manifesting with leonine facies and severe pruritus. Alopecia, ectropium, mild palmoplantar keratoderma and nail onychodystrophy may be present. Lymphadenopathy and hepatosplenomegaly are observed. Patients often shiver and complain of chills and general fatigue.

SS has been linked to a wide range of chromosomal anomalies, in particular rearrangements in the 6q23-27 region leading to alterations in the MYB proto-oncogene and the interleukin-22 receptor subunit alpha-2 gene (IL22RA2), but its etiology remains unclear.

Criteria that define SS are currently the following: an absolute Sézary cell count of 1,000 cells/mm3 or greater (B2 stage); an increase in CD3 or CD4 positive cells resulting in a CD4/CD8 ratio of 10 or greater; aberrant expression of pan T cell markers (i.e. deficient CD7 expression on T cells); increased relative or absolute lymphocyte counts with evidence of an identical T cell clone in the blood and skin by Southern blot or PCR technique. Skin biopsy may be not conclusive.

Differential diagnosis includes adverse drug reactions, classical mycosis fungoides and other forms of primary cutaneous T cell lymphoma (see these terms) as well as other causes of erythroderma such as psoriasis, atopic dermatitis and pityriasis rubra pilaris (see this term).

Assessment includes chest X-ray, computed tomography scan, magnetic resonance imaging and PET scan and an initial lymph node biopsy. Bimonthly extracorporeal photopheresis treatment may be combined with low doses of methotrexate, bexarotene or interferon-alpha. In advanced or non-responsive cases, chemotherapy with liposomal doxorubicine, gemcitabine or alemtuzumab may be considered. In cases of relapse, treatment may include total skin electron beam therapy and allogeneic stem cell transplantion.

Prognosis is poor, with median survival of patients being approximately 5 years, and is dependent upon initial presentation and evolution.