DDC and Parkinson disease: The finding of similarities in terms of movement amplitude and velocity and intracortical excitability between the disorders with a more profound NT deficit (e.g., AADC, TH, and PTPS deficiency), and advanced PD and APs, may suggest that specific subtypes of primary monoamine disorders have comparable pathophysiological features to the most severe forms of parkinsonism observed in adulthood.