Multiple electronic databases (PubMed, Scopus, and Google Scholar) were searched for observational studies published until August 2016. 
PubMed
Multiple electronic databases (PubMed, Scopus, and Google Scholar) were searched for observational studies published until August 2016. 
Scopus
Multiple electronic databases (PubMed, Scopus, and Google Scholar) were searched for observational studies published until August 2016. 
Google Scholar