Isolated oculomotor dysfunction with downbeat nystagmus as a rare presentation of anti-GAD autoimmunity

Background Glutamic acid decarboxylase (GAD) antibodies are associated with a range of autoimmune neurological syndromes, including cerebellar ataxia (CA). Ocular motor disturbances such as nystagmus and diplopia are frequently observed in CA but rarely are seen as isolated manifestation. Case presentation We describe the case of a 60-year-old woman who presented with oculomotor dysfunction consisting of persistent diplopia and vertical nystagmus, in the absence of any other neurological manifestation. Brain MRI and PET-CT scans were unremarkable. Videonystagmography documented downbeat nystagmus (DBN) unaffected by visual fixation. High concentrations of GAD antibodies in serum and CSF were found, along with the presence of oligoclonal bands. The patient underwent treatment with intravenous immunoglobulin (IVIG), which led to partial clinical improvement, and was subsequently started on long-term immunosuppressive therapy with azathioprine. Follow-up videonystagmography revealed subclinical recovery, including reduced nystagmus severity and restored fixation-mediated suppression, likely reflecting functional improvement of cerebellar vestibular circuits. Conclusion This case illustrates an uncommon and highly localized presentation of GAD-related autoimmunity manifesting as isolated oculomotor dysfunction. It underscores the importance of considering GAD antibody testing in patients presenting with unexplained nystagmus and diplopia and supports the role of immunotherapy in achieving clinical improvement.