This study was funded by Parkinson’s UK (RES/0211-7258: CN, AMO), Medical Research Council (CLR), the Wellcome Trust (088324: JBR) and the NIHR Biomedical Research Centre, Cambridge.
Conflicts of interest
The authors declare that they have no conflict of interest.
The study was approved by the local ethics committee, and performed in accordance with the 1964 Declaration of Helsinki. All participants signed an informed consent form before questionnaire completion.