We propose to search MEDLINE, EMBASE, and Google scholar using search terms related to the incidence of MH. Search terms to be used for MEDLINE and EMBASE are presented in the appendix. Two authors will screen titles and abstracts of the retrieved articles. Reference lists will be imported to Endnote software 8.1 (Thompson Reuters, CA) and duplicate articles will be removed. Additional relevant articles will be identified by scanning reference lists of articles obtained from the original search.