False-positive anti-neuronal nuclear antibody type 1 in a patient with RFC1 repeat expansion: Preventing “phenotype creep” in autoimmune neurology

A 61-year-old man was referred for evaluation of anti-neuronal nuclear antibody type 1 (ANNA-1, or “anti-Hu”)-associated sensory neuronopathy. Past medical history was significant for a right subcortical stroke involving the subthalamic nucleus three years prior. Family history was unremarkable. He reported insidiously progressive gait imbalance over 15 years, and worsening balance with head movement. He also noted mild dysarthria and left...