Anti-LGI1 autoimmune encephalitis mimicking rapidly progressive dementia in an elderly patient: a case report and review of the literature

Autoimmune encephalitis is increasingly recognized as a major cause of potentially reversible encephalopathy and rapidly progressive dementia, yet it remains frequently underdiagnosed in elderly patients, particularly outside specialist neurological settings. Among these disorders, anti-leucine-rich glioma-inactivated 1 (LGI1) encephalitis represents a rare but distinctive entity, typically characterized by subacute cognitive decline, behavioral disturbances, seizures, and hyponatremia, often associated with mesial temporal lobe involvement on magnetic resonance imaging. We report the case of an 80-year-old man who presented with an acute confusional state and recurrent bilateral tonic-clonic seizures following a 2-month history of progressive cognitive and behavioral deterioration that had initially been misinterpreted as the onset of a neurodegenerative dementia. Laboratory testing revealed severe hyponatremia, whereas brain CT was unremarkable. Magnetic resonance imaging demonstrated bilateral mesial temporal hyperintensity, more pronounced on the right side, consistent with limbic encephalitis. Electroencephalography showed diffuse slowing of background activity, indicative of moderate global cerebral dysfunction. After exclusion of infectious etiologies, cerebrospinal fluid analysis was performed and was within normal limits; however, anti-LGI1 antibodies were detected in both serum and cerebrospinal fluid, confirming the diagnosis of anti-LGI1 autoimmune encephalitis. The patient was promptly treated with high-dose intravenous methylprednisolone followed by intravenous immunoglobulins, in combination with antiepileptic therapy, leading to progressive clinical and cognitive recovery and normalization of electroencephalographic abnormalities. At the 3-month follow-up, he had fully regained cognitive and functional independence. This case highlights the critical importance of early recognition of autoimmune encephalitis in elderly patients presenting with rapidly progressive cognitive decline and seizures, even in internal medicine settings where these conditions are often overlooked. Timely diagnosis and immunotherapy can dramatically improve prognosis and prevent irreversible neurological damage.