A 45-year-old woman presents with a 5-year history of progressive gait unsteadiness and bilateral hand tremor. She has no family history of ataxia. Examination shows gait ataxia, limb dysmetria, and downbeat nystagmus. MRI shows cerebellar vermis atrophy. Vitamin B12, vitamin E, thyroid function, and anti-TTG antibodies are all normal. The most appropriate next diagnostic step is: