All you need to do is a comprehensive search that is not filtered based on publications of any certain countries (then you can name it global!). Make sure to search in related databases (such as PubMed, etc.) so that you find all works in your area of interest. There are many books on this matter which give you some ideas how to search and process your findings. For instance:

http://www.amazon.com/s/ref=sr_nr_n_0?rh=n%3A283155%2Ck%3Ameta-analysis&keywords=meta-analysis&ie=UTF8&qid=1385597854&rnid=2941120011