Facio-Brachial Dystonic Seizures as an Atypical Presentation of Anti-IgLON5 Disease.

Anti-IgLON5 disease is a rare neurological disorder with overlapping autoimmune and neurodegenerative mechanisms and a strong HLA haplotype association. It typically presents with sleep disturbances, though bulbar and neuropsychiatric symptoms contribute to clinical heterogeneity. The definitive diagnostic hallmark is the detection of anti-IgLON5 IgG antibodies. A 74-year-old female was initially admitted with pneumonia and persistent encephalopathy. Continuous video EEG monitoring captured multiple brief episodes of facial contraction with dystonic posturing of the left upper extremity, consistent with facio-brachial dystonic seizures (FBDS), arising from the right temporal region and evolution over the right temporo-parietal-occipital region. Magnetic resonance imaging of the brain showed T2-FLAIR and DWI hyperintensities in the right-temporoparietal and mesial temporal regions. The patient was initially treated with escalating anti-seizure therapy. Given concern for autoimmune-mediated encephalopathy and new-onset refractory status epilepticus (NORSE), along with an APE2 score of 10 and RITE2 score >7, high-dose corticosteroid therapy was initiated. Following the initiation of steroids, there was a notable rapid improvement in both clinical and electrographic parameters. Serum testing later returned positive for anti-IgLON5 antibodies. Epileptic seizures occur only in a minority of anti-IgLON5 cases, with focal or generalized events reported in approximately 5% of patients. Overall, the semiology observed is diverse, ranging from brief events of impaired consciousness, automotor seizures, to bilateral tonic-clonic seizures. FBDS, typically associated with LGI1 autoimmunity, has not previously been reported in this disorder. Neuroimaging is often nonspecific but may show brainstem or hippocampal involvement. Early immunotherapy, particularly intravenous immunoglobulin, is associated with improved outcomes, especially when initiated within six weeks of symptom onset. This case expands the known clinical spectrum of anti-IgLON5 disease by highlighting an atypical presentation characterized by facio-brachial dystonic seizures and response to immunotherapy.