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. Notably, the cerebrospinal fluid immunoglobulin G index was markedly elevated (4.91). This case highlights the diagnostic challenges in differentiating neurosyphilis from autoimmune limbic encephalitis, given the increasing atypical presentation. A markedly elevated immunoglobulin G index may serve as a valuable diagnostic indicator for neurosyphilis in such cases.