Overlapping Autoimmune Syndrome: A Case of Concomitant Anti- GABABR Encephalitis and Ri and Yo Antibodies

Anti-gamma-aminobutyric acid-B receptor (GABABR) encephalitis represents an autoimmune disorder of the central nervous system that presents as a complex neuropsychiatric syndrome. Antibodies targeting Ri and Yo are associated with limbic encephalitis, cerebellar ataxia, and cancer. The co-occurrence of GABABR encephalitis along with two paraneoplastic syndrome antibodies is infrequent. We present the case of a 79-year-old male diagnosed with GABABR encephalitis and two paraneoplastic syndrome antibodies, without the presence of a primary tumor lesion. Two years later, the patient exhibited enlarged lymph nodes in the right axilla. After a puncture biopsy, it was neuroendocrine carcinoma, potentially originating from the lungs. Whole-body PET-CT suggested that the primary tumor in the left upper lung. This report aims to remind clinicians that patients with GABABR autoimmune encephalitis who test positive for Ri and Yo antibodies may not currently have a primary lung lesion, but such a lesion could be detected in the future with careful follow-up. Additionally, prolonged high-dose steroid therapy may benefit anti-GABABR encephalitis patients with Ri and Yo antibodies.