Efficacy of spinal manipulation for chronic headache: a systematic review

CRD COMMENTARY: The review question was clear in terms of the study design and intervention. Participants with chronic headache were eligible, but the criteria for this group were broadly defined. The inclusion criteria were not defined in terms of outcomes, but the preferred primary outcomes for use the review were stated. Several relevant databases were searched and papers in several languages were eligible. However, the search terms were not stated and the methods used to select the studies were not described. Unpublished studies were excluded and this, as the authors correctly stated, raises the possibility of publication bias. Two reviewers assessed study quality and extracted data, which reduces the potential for bias and error. Validity was assessed using defined criteria, some relevant information on the included studies was tabulated, and the characteristics of the individual studies were adequately summarised. The studies were appropriately combined by considering the level of evidence for specified interventions, and a sensitivity analysis was used to explore the influence of the cut-off point on the results. However, the level of evidence was not discussed in relation to the specific studies on which the classifications were based, and the conclusions were based predominantly on one study per intervention. Hence, as the authors stated, it is not possible to comment on whether the results from the review can be applied to all participants in all settings by all practitioners. The authors were correct to advise caution when interpreting the conclusions of this review.