Eye movement desensitization and reprocessing for mental health problems: a systematic review and meta-analysis

In randomized clinical trails (RCTs), effect sizes seen in earlier studies guide both the choice of the effect size that sets the appropriate threshold of clinical significance and the rationale to believe that the true effect size is above that threshold worth pursuing in an RCT. That threshold is used to determine the necessary sample size for the proposed RCT. Once the RCT is done, the data generated are used to estimate the true effect size and its confidence interval. Clinical significance is assessed by comparing the true effect size to the threshold effect size. In subsequent meta-analysis, this effect size is combined with others, ultimately to determine whether treatment (T) is clinically significantly better than control (C). Thus, effect sizes play an important role both in designing RCTs and in interpreting their results; but specifically which effect size? We review the principles of statistical significance, power, and meta-analysis, and commonly used effect sizes. The commonly used effect sizes are limited in conveying clinical significance. We recommend three equivalent effect sizes: number needed to treat, area under the receiver operating characteristic curve comparing T and C responses, and success rate difference, chosen specifically to convey clinical significance.

Psychotherapies may work through techniques that are specific to each therapy or through factors that all therapies have in common. Proponents of the common factors model often point to meta-analyses of comparative outcome studies that show all therapies have comparable effects. However, not all meta-analyses support the common factors model; the included studies often have several methodological problems; and there are alternative explanations for finding comparable outcomes. To date, research on the working mechanisms and mediators of therapies has always been correlational, and in order to establish that a mediator is indeed a causal factor in the recovery process of a patient, studies must show a temporal relationship between the mediator and an outcome, a dose–response association, evidence that no third variable causes changes in the mediator and the outcome, supportive experimental research, and have a strong theoretical framework. Currently, no common or specific factor meets these criteria and can be considered an empirically validated working mechanism. Therefore, it is still unknown whether therapies work through common or specific factors, or both.

Eye Movement Desensitisation and Reprocessing (EMDR) is now considered evidence based practice in the treatment of trauma symptoms. Yet in a previous meta-analysis, no significant effect was found for the eye movement component. However methodological issues with this study may have resulted in a type II error. The aim of this meta-analysis was to examine current published studies to test whether eye movements significantly affect the processing of distressing memories. A systematic review of the literature revealed two groups of studies. The first group comprised 15 clinical trials and compared the effects of EMDR therapy with eye movements to those of EMDR without the eye movements. The second group comprised 11 laboratory trials that investigated the effects of eye movements while thinking of a distressing memory versus the same procedure without the eye movements in a non-therapy context. The total number of participants was 849. The effect size for the additive effect of eye movements in EMDR treatment studies was moderate and significant (Cohen's d = 0.41). For the second group of laboratory studies the effect size was large and significant (d = 0.74). The strongest effect size difference was for vividness measures in the non-therapy studies (d = 0.91). The data indicated that treatment fidelity acted as a moderator variable on the effect of eye movements in the therapy studies. Results were discussed in terms of current theories that suggest the processes involved in EMDR are different from other exposure based therapies. Copyright © 2012 Elsevier Ltd. All rights reserved.