Previous clinical studies evaluating the cross-sectional areas of vagus nerves using ultrasonography, found evidence of smaller vagus nerves in patients with PD compared to healthy controls, reflecting vagal neuronal loss due to α-synuclein aggregates [55–57], which has been confirmed by a postmortem histopathological study analyzing the distribution of α-synucleinopathy in the brain and peripheral autonomic nervous system [58]. Here, SNCA is linked to Parkinson disease.