Footnotes

• Contributors Both authors contributed to the conception and design of the work; acquisition of data and the analysis of the data; drafted, revised and edited this work; approve of the final version of the work and agree to be accountable for all aspects of the work in order to assure that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

• 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 for publication Not required.

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