Anti-LGI-1 autoimmune encephalitis: Insights from literature and a very late-onset case report

Anti-leucine-rich glioma-inactivated protein 1 (LGI-1) autoimmune encephalitis is a rare but increasingly recognized cause of seizures and cognitive decline. Most cases occur in middle-aged or elderly adults, whereas very late-onset presentations above 80 years are exceptionally uncommon and may mimic neurodegenerative, metabolic or cerebrovascular conditions, contributing to diagnostic delays.

We report an 84-year-old woman who presented with progressive somnolence, speech disturbance, and focal motor seizures. MRI demonstrated bilateral temporal lobe T2 hyperintensities, while EEG revealed temporal epileptiform discharges. Fluorodeoxyglucose positron emission tomography (FDG-PET) showed hypermetabolism in the temporal lobes and basal ganglia. Serum testing confirmed the presence of LGI-1 antibodies. The patient received a 7-day course of intravenous methylprednisolone followed by clinical monitoring. Rapid neurological improvement was observed within the first week, and no escalation to second-line immunotherapy was required. Over a four-year follow-up period, she remained clinically stable without relapse.

This case expands the upper age spectrum of anti-LGI-1 encephalitis and emphasizes the importance of considering autoimmune etiologies in elderly patients presenting with new-onset seizures and cognitive impairment. Neuroimaging and electrophysiological findings play a crucial role in diagnosis, particularly when classical faciobrachial dystonic seizures are absent. Timely corticosteroid therapy may provide sustained remission even in very late-onset cases, highlighting the potential efficacy of conservative immunotherapeutic strategies.