Received: 23 August 2017
Accepted: 29 January 2018
First Online: 7 February 2018
Ethics approval and consent to participate
: This study was approved by the Ethics Committee for Medical Care and Research at the University of Occupational and Environmental Health, Japan. Informed consent was waived because of the retrospective study design.
: 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.