Extracorporeal Photopheresis (ECP) in the Treatment of Stiff Person Syndrome (OPTION Study)

Inclusion Criteria: * Documented history of classical SPS with serum and CSF anti-GAD antibodies. * Age ≥ 18 ≤ 75 years. * Weight ≥ 50 kg. * Hematocrit ≥ 30 % (with or without transfusion support). * Platelet count ≥ 100 x 10\^3/uL (with or without transfusion support). * Willingness to use at least one reliable method of birth control (e.g., abstinence, oral contraceptives, intrauterine devices, barrier method with spermicide, or surgical sterilization) throughout the study for all men and women of childbearing potential. * Willingness to participate in all study tests, visits, and procedures (including ECP), as outlined in the informed consent. * Adequate peripheral venous access to initiate ECP therapy. * The patient agrees to participate in the trial and signs the informed consent form. Exclusion Criteria: * Progressive encephalomyelitis with rigidity and myoclonus (PERM). * Paraneoplastic variants of SPS. * Concurrent diagnosis of a neurological condition that would interfere with the assessment of SPS. * Women who are pregnant and/or lactating. * Absolute medical contraindication to receive ECP. * Hypersensitivity or allergy to psoralen (methoxalen). * Hypersensitivity or allergy to heparin. * Previous history of heparin-induced thrombocytopenia. * Aphakia or photosensitive disease (systemic lupus erythematosus, porphyrias, etc.). * Laboratory evidence of any of the following: * Mononuclear cell (MNC) count \<2.0 x 10\^3 cells/uL. * Serum transaminase l…