Methods
Through September 2019, literature searches were performed using the MEDLINE, EMBASE, and Cochrane databases to identify eligible articles. This review was performed according to the Preferred Reporting Items for Systematic Reviews and meta-analysis guidelines (PRISMA) guidelines [18]. The exhaustive search strategy used for the databases is presented in Table 1. No date limits or language restrictions were applied. The references in the included articles were also extensively searched. The risk of bias and quality assessment scores were both performed based on the Newcastle–Ottawa Scale (NOS) to further evaluate the quality of the observational studies, and NOS ≥ 7 was regarded as a good quality score [19].