Co-occurrence of Anti-GAD65 Syndrome, Type 1 Diabetes Mellitus, and Focal Seizures With Impaired Awareness

Glutamic acid decarboxylase (GAD) is an intracellular enzyme found in the presynaptic end of nerve terminals that functions to synthesize ​​gamma-aminobutyric acid (GABA) via decarboxylation. Autoantibodies to the GAD65 isoform have been found in high levels in neurological disorders including stiff person syndrome (SPS), autoimmune encephalitis, and refractory epilepsy. Low levels of anti-GAD65 have also been noted in type 1 diabetes mellitus. We present the unusual case of a woman with a longstanding history of focal seizures with impaired awareness and type 1 diabetes mellitus who was found to have extremely high titers of anti-GAD65 and clinical presentation suggestive of stiff person syndrome. This case highlights the increasing significance of autoimmune etiologies within neurologic disorders, as well as the importance of maintaining a high index of suspicion for rare anti-GAD65 syndromes. Although uncommon and with an unclear pathophysiology, we discuss the importance of establishing SPS diagnostic criteria to facilitate timely diagnosis and quickly direct patient management towards immunotherapy.