Heterogeneity in meta-analyses of treatment of acute postoperative pain: a review.

BACKGROUND Heterogeneity and its causes must be assessed using meta-analyses (meta-analysis). Especially in meta-analysis dealing with treatment of acute postoperative pain, the type of surgery is a source of heterogeneity. We aimed to assess whether the type of surgery is considered a source of heterogeneity in meta-analysis and how it is taken into account in meta-analysis evaluating the efficacy of treatment of acute postoperative pain. We further compared meta-analysis that pooled trials of surgeries with highly heterogeneous postoperative pain levels, the heterogeneous group, with meta-analysis that pooled trials involving surgeries with homogeneous pain levels, the homogenous group. METHODS The meta-analysis reports available in Issue 3, 2011 of the electronic database of the Cochrane library and pooling results of randomized or quasi-randomized controlled trials that assessed the efficacy of treatment of acute postoperative pain alone were considered. A survey of experts established a rating of the postoperative pain levels for the type of surgery. For each meta-analysis, the different pain level ratings associated with the trials included in the meta-analysis were considered and the standard deviation (sd) of these ratings calculated. From the distribution of sd values, we defined the heterogeneous and homogeneous groups. RESULTS Sixty-one meta-analyses were included; all assessed heterogeneity. Twenty-six meta-analyses considered the type of surgery as a subgroup (50% vs 38% in the homogeneous group vs heterogeneous group). Forty-four reports discussed the type of surgery as a source of clinical heterogeneity (85% vs 62% for the homogeneous vs heterogeneous group). Twenty-nine meta-analyses compared 'postoperative pain from dental surgery' to 'other type of surgery'. CONCLUSIONS Meta-analyses evaluating treatment of postoperative pain should explore clinical heterogeneity associated with the type of surgery for better implications for practice.