Efficacy and Mechanism of Action of SCIg in Patients With Stiff Person Syndrome (SPS)

Inclusion Criteria: * Men or women aged \>18 years * Diagnosis of SPS based on standard criteria * IVIg Group: Receiving the equivalent of 1-2 g/kg IVIg every 4 weeks with dependence\* on IVIg to maintain clinical response \*Dependence is clinically determined either by symptomatic worsening of condition at the end of the inter-dose interval or by worsening after dose reduction or discontinuation within the previous 3 months. * IVIg-Naïve Group: Patients with symptomatic SPS and never treated with IVIg (IVIg-naïve group), poorly controlled with standard therapy Exclusion Criteria: * Pregnancy, planned pregnancy, breast-feeding or unwillingness to practice contraception * Severe concurrent medical conditions, which would prevent treatment or assessment, including significant hematological, renal or liver dysfunction or malignancies * Initiation of immunomodulatory treatment other than IVIg in the past 3 months * Participation in a trial of an investigational medicinal product in the past 12 weeks * Presence of any medical condition, which in the opinion of the investigator might interfere with performance or interpretation of this study.