Footnotes

• Contributors DPB carried out the analysis and interpretation of the data, and prepared the manuscript. CHWG, SLM and TF carried out patient assessments and edited the manuscript. RAB conceived the study and edited the manuscript. All authors gave final approval for the manuscript to be published.

• Funding This work was supported by a NIHR Biomedical Research Award to Addenbrooke's Hospital/University of Cambridge, as well as grants from Parkinson's UK/Big Lottery Fund, Wellcome Trust and Patrick Berthoud Charitable Trust. DPB and CHWG have each been recipient of a Raymond and Beverly Sackler Studentship.

• Competing interests None.

• Patient consent Obtained.

• Ethics approval Cambridgeshire Research Ethics Committee.

• Provenance and peer review Not commissioned; externally peer reviewed.