Parkinson's disease subtypes: lost in translation?

Like many neurodegenerative disorders, Parkinson's disease (PD) is clinically highly heterogeneous. A number of studies have proposed and defined subtypes of PD based on clinical features that tend to cluster together. These subtypes present an opportunity to refine studies of aetiology, course and treatment responsiveness in PD, as clinical variability must represent underlying biological or pathophysiological differences between individuals. In this paper, we review what subtypes have been identified in PD and the validation they have undergone. We then discuss what the subtypes could tell us about the disease and how they have been incorporated into studies of aetiology, progression and treatment. Finally, with the knowledge that they have been incorporated very little into PD clinical research, we make recommendations for how subtypes should be used and make some practical recommendations to address this lack of knowledge translation.