Markedly Elevated IgG Index: A Key to Differentiating Neurosyphilis from Autoimmune Limbic Encephalitis.

We herein report a 48-year-old man with neurosyphilis manifesting as limbic encephalitis, initially suspected to be autoimmune limbic encephalitis. The patient exhibited rapid behavioral changes, and magnetic resonance imaging showed high-intensity lesions in both medial temporal lobes. The diagnosis was based on symptoms, cerebrospinal fluid abnormalities, and positive serum tests for syphilis.