Footnotes

• Contributors NV: study concept and design, acquisition and interpretation of data, drafting and revision of manuscript. AJL: interpretation of data, drafting and revision of manuscript. HRM: interpretation of data, drafting and revision of manuscript.

• Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.

• Competing interests None declared.

• Patient consent Not required.

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