Using a standardized extraction form, the following data will be extracted independently by 2 authors: study name (along with the name of the first author and year of publication), country where the study was conducted, source from which patients or study participants were selected, study design, outcome definition, anesthetic used, age, gender, race, incidence or prevalence with 95% confidence intervals (CIs) or the number of patients with MH and the total number of subjects. If information is inadequate, we will attempt to contact study authors and request for additional information. If unsuccessful, missing information will be calculated from the available data if possible. In case only prevalence is reported, we will consider prevalence as incidence because MH has a short duration and a high mortality; we will perform sensitivity analysis by excluding data from those studies. The reference list will be divided into 2 halves. Two authors will complete data extraction, 1 for each half of the reference list. Data extraction forms will be cross-checked to verify accuracy and consistency of the extracted data.