Received: 16 April 2019
Accepted: 23 June 2019
First Online: 12 July 2019
Ethics approval and consent to participate
: Not applicable as this work is a data synthesis of existing systematic reviews in the literature and has involved no individuals, requiring no consent to participate.
: Not applicable as there are no data, pictures or illustrations relating to individuals that require consent to publish.
: The authors declare that they have no competing interests.