PET-positive extralimbic presentation of anti-glutamic acid decarboxylase antibody-associated encephalitis

Anti-glutamic acid decarboxylase (GAD) antibody-associated autoimmune encephalitis has been reported mostly as limbic encephalitis. Only few cases with extralimbic involvement are reported with limited investigation. Here, we report an extensive investigation with MRI, PET, and pathological examination. A 66-year-old Japanese female with a history of hypothyroidism, colon cancer, pheochromocytoma, and thymoma-associated myasthenia gravis presented with generalised tonic-clonic seizures. MRI showed multiple hyperintense lesions and PET showed hypermetabolic lesions in the brain. Biopsy showed non-specific gliosis, microglial proliferation, and perivascular lymphohistiocytic infiltrates. Various neuronal antibodies were negative, except for anti-GAD antibody. Anti-GAD antibody-associated encephalitis is an increasingly recognised CNS disease. Pathophysiology of this encephalitis is unclear. While PET showed hypermetabolic lesions, the biopsy showed non-specific changes. The treatments may include immunosuppressants, IVIg, and plasma exchange. One should consider to measure this antibody, in addition to others, when autoimmune encephalitis is suspected [Published with video sequences].