Inclusion Criteria:

• Patient must be 18 years of age or older.
• Must have symptoms of SPS for less than 3 years.
• If taking corticosteroids, the patients must be on a stabile dose for 30 days prior to enrolment.
• Patients will have a diagnosis of SPS spectrum disorder based on both of:
  • Clinical: rigidity with muscle co-contraction affecting 2 or more body parts (lower trunk, upper trunk, one or both legs, one or both arms, craniocervical region, and abdomen) and episodic spasms precipitated by unexpected noises, tactile stimuli, or emotional upset; the absence of any other neurologic disease that could explain the stiffness; and
  • Serological: GlyRα1 antibody positive in serum, CSF (or both). The presence of coexisting antibodies is not exclusionary, as these are commonly encountered.

Exclusion Criteria:

• Patients on immune suppressants initiated/dose increased in the prior 6 months.
• History of thrombotic episodes within the 2 years prior to enrolment.
• Known allergic or other severe reactions to blood products including intolerability to previous IVIG.
• Previous adequate trial of IVIG as determined by the Principal Investigator.
• IgA deficiency.
• Reproductive status:
  • Women who are pregnant;
  • Women who are breastfeeding;
  • Women and men of childbearing potential who are unwilling or unable to use an acceptable method of birth control to avoid pregnancy for the entire study period, as evaluated by the investigator (women of non-childbearing potential are those that have a history of hysterectomy, bilateral oophorectomy, or are postmenopausal with no history of menstrual flow for > 12 months prior to screen visit).
• Any surgical procedure (except for minor surgeries) within 4 weeks prior to baseline.
• Evidence of serious uncontrolled concomitant diseases that may preclude patient participation (physician determined) as described.
• Other nervous system disease, cardiovascular disease, hematologic/hematopoiesis disease, respiratory disease, muscular disease, endocrine disease, renal/urologic disease, digestive system disease, congenital or acquired severe immunodeficiency.
• Known active infection (excluding fungal infections of nail beds or caries dentium) within 4 weeks prior to baseline.
• Evidence of chronic active hepatitis B or C.
• Active ischemic heart disease in the past year prior to baseline.
• Patients should not have severe renal or hepatic disease (determined by treating physician).
• Severe hypertension (diastolic pressure >120 mmHg or systolic >170 mmHg).