The population of interest will be patients undergoing general anesthesia across all countries. If the studies report the incidence or prevalence of MH in patients undergoing surgery or any type of anesthesia without specifying that the patients underwent general anesthesia, or if the studies report these cases in hospitalized inpatients as opposed to patients undergoing general anesthesia, we will attempt to contact the authors to ascertain the incidence of MH in patients undergoing general anesthesia. If this is unsuccessful, we will regard these studies as reports with an unspecified type of anesthesia, and will perform both pooled analysis and sensitivity analysis excluding the data from those studies. We will only include studies that report the incidence of MH and cases of MH; reports of neuroleptic malignant syndrome or hyperpyrexia of unclear etiology will be excluded.