The authors have clearly stated their research question but not their inclusion and exclusion criteria. The literature search is restricted in the databases used the search strategy is not stated. It is also not reported whether the authors restricted their search to English language publications. For these reasons, the authors may have missed additional relevant studies.
The quality of the included studies was not formally assessed but the authors have discussed the limitations of their review. The authors have not reported on how the articles were selected, or how many of the reviewers were involved in the process of data extraction.
The data extraction is reported in tables and text and the narrative review was appropriate since there was no basis for a statistical pooling of the data.
Although no formal tests for homogeneity were reported the authors have discussed the quality of the data and the effects of bias and differences between studies on the outcomes of the review. Their conclusions appear to follow from the results but should be viewed with caution because of the stated methodological limitations of the review.