Seronegative Autoimmune Encephalitis Presenting With Severe Neuropsychiatric Symptoms in an Older Adult: A Case of Diagnostic Complexity.

Autoimmune encephalitis (AE) represents a potentially reversible cause of subacute cognitive and psychiatric decline. When no autoantibodies are detected, seronegative AE remains diagnostically challenging, particularly among older adults presenting with new-onset neuropsychiatric symptoms. This report describes an older adult with recurrent episodes of confusion, aphasia, and psychosis whose diagnostic studies-including magnetic resonance imaging (MRI) and cerebrospinal fluid (CSF) analysis-were unremarkable. The individual had previously demonstrated substantial improvement after corticosteroid therapy and was again treated empirically with immunotherapy following psychiatric consultation. This case highlights the importance of maintaining a high index of suspicion for AE when standard serologic markers are absent and underscores the collaborative role of psychiatry and neurology in evaluating rapidly progressive cognitive and behavioral syndromes, which may present initially as primary psychiatric illness. Early consideration of autoimmune etiologies and prompt initiation of immunotherapy can significantly improve outcomes in patients with otherwise unexplained, rapidly progressive cognitive or behavioral syndromes.