Autoimmune Encephalitis in Later Life: A Missed Opportunity in Routine Geriatric Care.

Autoimmune encephalitis (AE) is an increasingly recognized but frequently overlooked cause of subacute cognitive and behavioral decline in older adults, in whom delirium is typically attributed to metabolic, infectious, or vascular disorders. We describe an elderly man who presented with persistent encephalopathy following correction of hyponatremia. Despite normal structural neuroimaging and negative cerebrospinal fluid autoimmune markers, he remained delirious. Electroencephalography revealed diffuse slowing, and fluorodeoxyglucose positron emission tomography demonstrated characteristic hypermetabolic activity involving the temporal lobes, basal ganglia, thalami, and cerebellum, strongly suggestive of AE. Initiation of high-dose corticosteroids followed by therapeutic plasma exchange resulted in marked clinical improvement. This case highlights the diagnostic complexities of AE in geriatric practice, particularly the limitations of conventional testing and the value of clinical suspicion backed by functional neuroimaging. Greater clinical vigilance is required to avoid delayed diagnosis and treatment, which may substantially improve outcomes even in severe presentations.