Contribution to authorship:
All authors had access to all data and can take responsibility for the integrity and accuracy of the analysis. RL and HB designed the study. RL and WH developed the search strategy with assistance of a medical librarian. RL and WH acquired the data from all the studies. HB en RL performed the data-analysis. RL developed the first draft of the manuscript and HB en WH revised all manuscript drafts and approved the final version.