The goal of this clinical trial is to examine the proportion of patients with stiff person syndrome with no worsening in stiffness and/or spasms while on efgartigimod-hyaluronidase (EFD) when compared to their current treatment. The main questions it aims to answer are:

• proportion of patients with stiff person syndrome with no worsening in DSI and HSS scores
• whether there are changes in laboratory values, vital signs, electrocardiogram, or depression, and suicide risk during the trial, as well as the incidence of any adverse events.

Participants will come to the clinic up to 18 times and have up to 5 phone call visits during the course of the trial. During these visits a variety of measures will be collected, including blood samples, neurological and physical exams, patient reported outcomes on stiffness, spasms, depression, and suicide risk.

Inclusion Criteria:

1. Age 18-80, men and women, capable of giving informed consent.
2. Must have unexplained stiffness of limb and axial (trunk) muscles and involuntary striated muscle spasms.
3. DSI and HHS must be >=2 (to allow for detection of improvement).
4. Presence of serum (or CSF) anti-GAD antibodies regardless of titers.
5. Stable dosage for at least two months before screening visit with antispasmodics (muscle relaxants, benzodiazepines, baclofen).
6. Women with children bearing potential must be willing to use highly effective contraception method during and for a minimum of 90 days after the last dose of IP.

Exclusion Criteria:

1. Exclusionary conditions:

1a. type 1 DM

1b. Cancer, except basal cell skin cancer

1c. Infections: HIV, Hepatitis B or C, clinically significant active infection not sufficiently resolved in the investigator's opinion

1d. Serum IgG (<6 g/L) at screening

1e. pregnancy/breast feeding. 2. Exclusionary medications: rituximab within 6 months of screening. 3. IVIG or PLEX treatment 4 weeks prior to screening. 4. History of any type of solid organ or bone marrow transplant.

IPD will be shared between designated study sites during the trial. After the end of the trial data may be shared but this has yet to be determined.