Footnotes

• Contributors WK contributed to the study design, statistical analysis, interpretation of the data, drafting and revision of the manuscript. LL contributed to the data collection and revision of the manuscript. AL contributed to the study design and the revision of the manuscript. BW contributed to the data collection and revision of the manuscript. LL contributed to the data collection and revision of the manuscript. ES contributed to the data collection and revision of the manuscript. KPN contributed to the study design, drafting and revision of the manuscript. NK contributed to the study design, statistical analysis, interpretation of the data, drafting and final approval of the manuscript.

• Funding This study was supported by the National Neuroscience Institute, Singapore. The study was funded by the Singhealth Foundation Research Grant (SHF/FG483P/2012). Audrey Low is funded by the Lee Kuan Yew Fitzwilliam Scholarship and the Tan Kah Kee Postgraduate Scholarship. The funders had no role in the initiation or design of the study, collection of samples, analysis, interpretation of data, writing of the paper or the submission for publication.

• Competing interests None declared.

• Patient consent for publication Not required.

• Ethics approval Ethics approval was provided by the SingHealth Centralised Institutional Review Board (2012/750/A).

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