It is estimated that the dopaminergic neuronal loss that causes the characteristic motor features of Parkinson’s disease (PD) has reached 50–70% by diagnosis and it is progressive. Clinical, imaging and pathological studies suggest that the cell loss begins 7–10 years before the onset of clinical features sufficient to allow the diagnosis of PD. During this period of pre-diagnostic neurodegeneration, compensatory mechanisms are developed to maintain normal basal ganglia function.1 Eventually, these fail and symptoms emerge, enabling a diagnosis which is usually based on the combination of bradykinesia, rigidity and tremor. The delay from symptom onset to diagnosis is variable and …