Gaze-Evoked Nystagmus & Slow Saccades Due to Anti-GAD Antibodies in a Patient with Stiff Person Syndrome

This is a 70-year-old woman with a several year long history of imbalance and stiffness. Exam demonstrated axial and lower extremity stiffness, and ocular motor exam demonstrated gaze-evoked nystagmus (e.g., right-beating in right gaze, left-beating in left gaze, up-beating in up gaze), and mild to moderate slowing of saccades vertically and horizontally. Vestibular function was normal. Evaluation for a posterior fossa syndrome given gaze-evoked nystagmus (GEN, implies dysfunction of cerebellum or its connections) and slow saccades (implies impairment of brainstem burst neurons) demonstrated normal neuroimaging, although she had a very high level of anti-glutamic acid decarboxylase (GAD-65) serum IgG (hers was >300,000 IU/ml where the upper limit of normal is 5 IU/ml). The patient was diagnosed with probable stiff person syndrome, and ocular motor abnormalities were also attributed to the elevated anti-GAD-65 antibody levels. She has not yet received intravenous immunoglobulin. An anti-GAD syndrome with posterior fossa involvement should be considered in any patient with dizziness/vertigo and/or imbalance with central ocular motor signs including GEN and/or slow saccades (both have been reported). It is an important consideration, especially since it can be treated with immunotherapy and benzodiazepines (for stiffness and spasm).