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].