Paraneoplastic anti-GAD65 extralimbic encephalitis presented with epilepsy: A case report

Autoimmunity targeting glutamic acid decarboxylase 65 (GAD65) is associated with type 1 diabetes mellitus as well as various neurological diseases. In the central nervous system, GAD65 autoimmunity usually presents with limbic encephalitis, whereas extralimbic encephalitis (ELE) has only been reported in a few cases. Moreover, anti-GAD65 ELE in the paraneoplastic context has not yet been reported.A 60-year-old man presented with intermittent cough and sputum for 10 years, with no other diseases. The patient presented with recurrent seizures that were resistant to antiepileptic drugs (AEDs). Chest computed tomography and pathological results confirmed the diagnosis of small cell lung cancer. Paraneoplastic testing found a high level of GAD65 antibodies in his serum, and cerebrospinal fluid analysis revealed lymphocytic pleocytosis, indicating autoimmune encephalitis. Brain magnetic resonance imaging showed multifocal T2 fluid-attenuated inversion recovery hyperintensities in the extralimbic areas including the subcortex and deep white matter of the bilateral frontal lobe, parietal lobe, and insula lobes.Finally, a diagnosis of anti-GAD65 autoimmune ELE with a paraneoplastic etiology from the small cell lung cancer was suspected.The patient refused any tumor-suppressive treatment or immunotherapy for potential side effects and only received AEDs levetiracetam, sodium valproate, and diazepam.The epilepsy of the patient was resistant to AEDs, and the patient died a week after discharge due to disease progression.Anti-GAD65 autoimmune encephalitis can be extralimbic, can present with isolated epilepsy, and extralimbic anti-GAD65 encephalitis can occur with an underlying malignancy.