Case Report: Seronegative rhombencephalitis presenting with ocular flutter and ataxia as a potential sentinel manifestation of gastric adenocarcinoma.

Objectives: This case report aims to highlight the rare clinical phenotype of seronegative autoimmune rhombencephalitis presenting with ocular flutter and ataxia, and to emphasize its potential association with underlying paraneoplastic neurological syndromes. Methods: The patient was identified through comprehensive clinical neurological evaluation, video-assisted oculomotor analysis, and diagnostic testing including brain MRI, cerebrospinal fluid (CSF) analysis, and an extensive screening of serum/CSF autoantibodies using cell-based assays (CBAs) and immunoblot assay. To rule out other etiologies, extensive infectious panels were assessed. A tissue-based assay (TBA) was employed to identify potential autoimmune reactivity when standard antibody panels were negative. Long-term follow-up was conducted after the patient was discharged. Results: A 60-year-old male presented with persistent ocular flutter, mild head-neck-tremor, and cerebellar ataxia. While standard autoantibody panels were negative, TBA demonstrated weak neuronal IgG reactivity in serum, whereas the corresponding CSF TBA was negative. The patient demonstrated a favorable response to immunotherapy (intravenous methylprednisolone and IVIG), with near-complete resolution of symptoms. Notably, gastric adenocarcinoma was diagnosed ten months after the onset of neurological symptoms following the emergence of new gastrointestinal symptoms. Discussion: This case highlights an exceedingly rare presentation of autoimmune rhombencephalitis characterized by ocular flutter as the sentinel symptom. Although no characterized neural autoantibody was identified, weak serum neuronal reactivity on TBA, mild CSF pleocytosis, the clinical phenotype, and the response to immunotherapy supported a suspected immune-mediated process. The subsequent diagnosis of gastric adenocarcinoma ten months later raised the possibility of a paraneoplastic association, although causality could not be established. Our findings underscore that ocular flutter-ataxia can be a diagnostic clue for underlying malignancy; therefore, clinicians should maintain high vigilance and perform longitudinal tumor screening in such patients, even when initial evaluations are negative.