FROM: J Manipulative Physiol Ther 2001 (Sept); 24 (7): 457–466

Gert Bronfort, DC, PhD, Willem J.J. Assendelft, MD, PhD,
Roni Evans, DC, Mitchell Haas, DC, Lex Bouter, PhD

A previous systematic review assessing the effect of SMT on chronic headaches has suggested that SMT may be a worthwhile therapy for tension-type headache. [7] The findings of our review, which includes 3 additional relatively highquality RCTs, provide a basis for considering SMT in the therapeutic management of migraine, chronic tension-type and cervicogenic headaches. Although migraine, cervicogenic headache and tension-type headache generally are considered to be separate conditions, there is some support in the literature for the notion that they represent a continuum with several common underlying mechanisms, including cervical spine dysfunction. [46, 47] One possible explanation of the apparent effect of SMT in chronic headache comes from the results of several studies that have demonstrated that headache can be induced experimentally by noxiously stimulating tissues, including joint capsules, ligaments, and paraspinal muscles, enervated by the cervical nerve roots (C1–C3). [48] Headache pain caused by such stimulation may be possible because of the common neurological pathways shared by the trigeminal nucleus and the C1–C3 nerves. [48]

Methodological Limitations

Different methodologies have been advocated for the systematic review of studies addressing therapeutic efficacy. [15, 18, 49–52] Given the nature of RCTs available for this review, we chose to evaluate the strength of the evidence based on the best-evidence synthesis method rather than a formal meta-analysis. [9, 53] A number of meta-analytical methods have been advocated for combining results of RCTs. [15, 54] It is recognized by international experts that one of the most important limitations of published meta-analyses is inadequate control for clinical heterogeneity among synthesized studies. [8, 55, 56] There is currently little consensus on decision rules regarding statistical pooling of study results. [57] The clinical heterogeneity of the trials, in terms of headache type, patient characteristics, interventions, comparison therapies, and outcome measure prevented statistical pooling in this review.

A possible limitation of the current review is publication bias, of which there are several potential sources. [58] No effort was made to identify unpublished research, [59] which is more likely to have negative outcomes. [60] However, it is recognized that attempts to retrieve unpublished trial data may also bias studies. [60] The search strategy may have missed important studies not currently indexed, but by including citation tracking of non-indexed journals it is unlikely that many were overlooked. Optimally, reviews should include all trials regardless of language. [61–63] However, this review was initially restricted to the languages we spoke: English, German, French, Dutch, and the Scandinavian languages. Although an attempt was made to identify trials in other languages, this approach was not fully systematic; the possibility that some relevant trials may have been overlooked must be acknowledged.

The evidence for efficacy or inefficacy rests primarily on the results of a small number of RCTs of acceptable methodological quality. A few additional high-quality RCTs in the future could easily change the conclusions of our review. [62, 64] Little research has been done to determine what constitutes a minimal clinically-important difference in headache outcomes. The chosen cut-point of a medium effect-size (0.5) difference to determine inferiority/superiority of an intervention is somewhat arbitrary but similar to other reported estimates. [65, 66] Also, sensitivity analyses showed that the results and the overall study conclusions remained the same even when substantial changes in the prespecified assumptions/rules regarding the evidence determination were applied.

The reliability with which different reviewers use similar methodological scoring systems is a source of uncertainty. [67] Conclusions regarding the weight of evidence are largely dependent on the exact definition of the evidence classification system used. [64] An additional methodological assessment of the studies included in this review was performed by using a 5–point scoring system developed by Jadad et al. [18] This scale addresses 3 areas—randomization, double blinding, and description of dropouts—which, if not addressed adequately, may be important sources of bias. Studies that scored highly with our system also scored relatively high with the Jadad scale (correlation coefficient of .62). It is important to note that none of the studies could achieve higher than a 3–point score with the Jadad scale because none of them were double-blinded.

Another possible limitation of this review is that we who performed the methodological scoring were not blinded to the authors and results of the individual RCTs because of our familiarity with the SMT literature. Some maintain that blinding yields significantly lower methodological scores, [18] whereas others contend that it does not make a difference. [68] Berlin et al [69] have demonstrated that the overall results of meta-analyses are uninfluenced by blinding.

Limitations of the Individual Trials

Most of the headache trials, including those of acceptable quality, have substantial methodological limitations. In the trials by Boline et al [36] and Nelson et al, [3] 9 withdrawal of amitriptyline at the end of treatment is inconsistent with normal clinical practice. The return of these patients to near baseline values could be largely due to a medication rebound effect, making the apparent advantage of the SMT group less impressive. Longer periods of observation after treatment are necessary to adequately judge the value of SMT as a potential first line of therapy for tension-type headache.

In the trial by Nelson et al, [39] it appears that SMT has a magnitude of effect similar to the commonly used prophylactic medication amitriptyline. However, the trial was not designed to assess equivalence and did not have sufficient power to do so. Thus, whether the 2 therapies are equivalent is still unknown. Another concern regarding this study is the substantial loss of patients to follow up (28%). Although the study investigators performed missing data analyses, these can never fully compensate for the loss of data.

The authors of the trials by Bove and Nilsson [35] conclude that, as an isolated intervention, SMT does not have a positive effect on episodic tension-type headache. However, by its design the Bove and Nilsson trial did not assess the isolated effect of SMT; rather it looked at the combined effect of SMT with soft tissue massage. Whether there is an interaction that results from combining SMT with soft tissue massage is unknown. A more appropriate conclusion would have been that SMT, when combined with soft tissue massage, is no better than soft tissue therapy alone for episodic tension-type headache. This conclusion neither supports nor refutes the efficacy of SMT as a separate therapy.

In the trial by Parker et al, [38, 42] there is no description of the dropouts, increasing the likelihood of bias. The extended trial by Nilsson et al [37] on cervicogenic headache is somewhat unorthodox in that the decision to recruit more patients was made after the original analyses of the data. No prespecifications were made regarding separate analyses of the data, and one must be concerned about the possibility of a Type I error.

The results of the remainder of the trials, which were of lower methodological quality, all tend to suggest that SMT was better than the comparison therapies. This is consistent with studies in other fields that have shown that those of lower methodological quality tend to have positive outcomes. [52, 64, 70] Thus, one must interpret the results of these trials with caution.

None of the studies reviewed evaluated the cost-effectiveness of SMT for chronic headaches. Trials are needed to establish SMT’s relative cost-effectiveness to other commonly used therapies, and are particularly needed to address the potential for long-term effects. Finally, caution should be exercised when extrapolating from studies of SMT, because there is substantial diversity in terms of training and technique among providers.

CONCLUSION

SMT appears to have a better effect than massage for cervicogenic headache. It also appears that SMT has an effect comparable with commonly used first-line prophylactic prescription medications for tension-type headache and migraine headache. This conclusion rests on a few trials of adequate methodological quality. Before any firm conclusions can be drawn, further testing should be done in rigorously designed, executed, and analyzed trials with followup periods of sufficient length.

 

Source:  Chiro. org