Efgartigimod for Stiff Person Syndrome (ESPS)

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.