Received: 17 October 2018
Accepted: 26 April 2019
First Online: 14 May 2019
Ethics approval and consent to participate
: This study was approved by the Ethics Committee of Peking Union Medical College Hospital. All the participants were orally informed, and the parents gave their written consent.
: Not applicable.
: The authors declare that they have no competing interests.
: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.