Risk of bias: In the selection process bias risk was managed by using a double check of the selection criteria by senior reviewers, by scoring each study by STARD methodology using its checklist and a rigid criteria of >60% to be considered for further analysis, any disagreements between the junior and/ senior review authors over the risk of bias in particular studies were solved by discussion during an extra research meeting of two senior authors, with involvement of a third reviewer when necessary. The publication bias was evaluated in graphic manner using Funnel plots and in mathematical form with Begg and Egger tests. A value of p<0.05 was defined as significant.