This review was conducted to assess the effectiveness of repetitive transcranial magnetic stimulation (rTMS) in the treatment of major depressive disorder (MDD).
rTMS is a noninvasive way to stimulate nerve cells in areas of the brain. During rTMS, an electrical current passes through a wire coil placed over the scalp. The current induces a magnetic field that produces an electrical field in the brain that then causes nerve cells to depolarize, resulting in the stimulation or disruption of brain activity.
Researchers have investigated rTMS as an option to treat MDD, as an add-on to drug therapy, and, in particular, as an alternative to electroconvulsive therapy (ECT) for patients with treatment-resistant depression.
The advantages of rTMS over ECT for patients with severe refractory depression are that general anesthesia is not needed, it is an outpatient procedure, it requires less energy, the simulation is specific and targeted, and convulsion is not required. The advantages of rTMS as an add-on treatment to drug therapy may include hastening of the clinical response when used with antidepressant drugs.
The Medical Advisory Secretariat used its standard search strategy to locate international health technology assessments and English-language journal articles published from January 1996 to March 2004.
Summary of Findings
Some early meta-analyses suggested rTMS might be effective for the treatment of MDD (for treatment-resistant MDD and as an add-on treatment to drug therapy for patients not specifically defined as treatment resistant). There were, however, several crucial methodological limitations in the included studies that were not critically assessed. These are discussed below.
Recent meta-analyses (including 2 international health technology assessments) have done evidence-based critical analyses of studies that have assessed rTMS for MDD. The 2 most recent health technology assessments (from the Oxford Cochrane Collaboration and the Norwegian Centre for Health Technology Assessment) concluded that there is no evidence that rTMS is effective for the treatment of MDD, either as compared with a placebo for patients with treatment-resistant or nontreatment-resistant MDD, or as an alternative to ECT for patients with treatment-resistant MDD. This mainly due to the poor quality of the studies.
The major methodological limitations were identified in older meta-analyses, recent health technology assessments, and the most recently published trials (Level 2–4 evidence) on the effectiveness of rTMS for MDD are discussed below.
Small sample size was a limitation acknowledged by many of the authors. There was also a lack of a priori sample size calculation or justification.
Biased randomization may have been a problem. Generally, the published reports lacked detailed information on the method of allocation concealment used. This is important because it is impossible to determine if there was a possible influence (direct or indirect) in the allocation of the patients to different treatment groups.
The trials were single blind, evaluated by external blinded assessors, rather than double blind. Double blinding is more robust, because neither the participants nor the investigators know which participants are receiving the active treatment and which are getting a placebo. Those administering rTMS, however, cannot be blinded to whether they are administering the active treatment or a placebo.
There was patient variability among the studies. In some studies, the authors said that patients were “medication resistant,” but the definitions of resistant, if provided, were inconsistent or unclear. For example, some described “medication resistant” as failing at least one trial of drugs during the current depressive episode. Furthermore, it was unclear if the term “medication resistant” referred to antidepressants only or to combinations of antidepressants and other drug augmentation strategies (such as neuroleptics, benzodiazepine, carbamazepine, and lithium). Also variable was the type of depression (i.e., unipolar and/or bipolar), if patients were inpatients or outpatients, if they had psychotic symptoms or no psychotic symptoms, and the chronicity of depression.
Dropouts or withdrawals were a concern. Some studies reported that patients dropped out, but provided no further details. Intent-to-treat analysis was not done in any of the trials. This is important, because ignoring patients who drop out of a trial can bias the results, usually in favour of the treatment. This is because patients who withdraw from trials are less likely to have had the treatment, more likely to have missed their interim checkups, and more likely to have experienced adverse effects when taking the treatment, compared with patients who do not withdraw. (1)
Measurement of treatment outcomes using scales or inventories makes interpreting results and drawing conclusions difficult. The most common scale, the Hamilton Depression Rating Scale (HDRS) is based on a semistructured interview. Some authors (2) reported that rating scales based on semistructured interviews are more susceptible to observation bias than are self-administered questionnaires such as the Beck Depression Inventory (BDI). Martin et al. (3) argued that the lack of consistency in effect as determined by the 2 scales (a positive result after 2 weeks of treatment as measured by the HDRS and a negative result for the BDI) makes definitive conclusions about the nature of the change in mood of patients impossible. It was suggested that because of difficulties interpreting results from psychometric scales, (4) and the subjective or unstable character of MDD, other, more objective, outcome measures such as readmission to hospital, time to hospital discharge, time to adjunctive treatment, and time off work should be used to assess rTMS for the treatment of depression.
A placebo effect could have influenced the results. Many studies reported response rates for patients who received placebo treatment. For example, Klein et al. (5) reported a control group response rate as high as 25%. Patients receiving placebo rTMS may receive a small dose of magnetic energy that may alter their depression.
Short-term studies were the most common. Patients received rTMS treatment for 1 to 2 weeks. Most studies followed-up patients for 2 to 4 weeks post-treatment. Dannon et al. (6) followed-up patients who responded to a course of ECT or rTMS for up to 6 months; however, the assessment procedure was not blinded, the medication regimen during follow-up was not controlled, and initial baseline data for the patient groups were not reported. The long-term effectiveness of rTMS for the treatment of depression is unknown, as is the long-term use, if any, of maintenance therapy. The cost-effectiveness of rTMS for the treatment of depression is also unknown. A lack of long-term studies makes cost-effectiveness analysis difficult.
The complexity of possible combinations for administering rTMS makes comparing like with like difficult. Wasserman and Lisanby (7) have said that the method for precisely targeting the stimulation in this area is unreliable. It is unknown if the left dorsolateral prefrontal cortex is the optimal location for treatment. Further, differences in rTMS administration include number of trains per session, duration of each train, and motor threshold.
Clinical versus statistical significance. Several meta-analyses and studies have found that the degree of therapeutic change associated with rTMS across studies is relatively modest; that is, results may be statistically, but not necessarily clinically, significant. (8-11). Conventionally, a 50% reduction in the HDRS scores is commonly accepted as a clinically important reduction in depression. Although some studies have observed a statistically significant reduction in the depression rating, many have not shows the clinically significant reduction of 50% on the HDRS. (11-13) Therefore, few patients in these studies would meet the standard criteria for response. (9)
Clinical/methodological diversity and statistical heterogeneity. In the Norwegian health technology assessment, Aarre et al. (14) said that a formal meta-analysis was not feasible because the designs of the studies varied too much, particularly in how rTMS was administered and in the characteristics of the patients. They noted that the quality of the study designs was poor. The 12 studies that comprised the assessment had small samples, and highly variable inclusion criteria and study designs. The patients’ previous histories, diagnoses, treatment histories, and treatment settings were often insufficiently characterized. Furthermore, many studies reported that patients had treatment-resistant MDD, yet did not listclear criteria for the designation. Without this information, Aarre and colleagues suggested that the interpretation of the results is difficult and the generalizability of results is questionable. They concluded that rTMS cannot be recommended as a standard treatment for depression: “More, larger and more carefully designed studies are needed to demonstrate convincingly a clinically relevant effect of rTMS.”
In the Cochrane Collaboration systematic review, Martin et al. (3;15) said that the complexity of possible combinations for administering rTMS makes comparison of like versus like difficult. A statistical test for heterogeneity (chi-square test) examines if the observed treatment effects are more different from each other than one would expect due to random error (or chance) alone. (16) However, this statistical test must be interpreted with caution because it has low power in the (common) situation of a meta-analysis when the trials have small sample sizes or are few. This means that while a statistically significant result may indicate a problem with heterogeneity, a nonsignificant result must not be taken as evidence of no heterogeneity.
Despite not finding statistically significant heterogeneity, Martin et al. reported that the overall mean baseline depression values for the severity of depression were higher in the treatment group than in the placebo group. (3;15) Although these differences were not significant at the level of each study, they may have introduced potential bias into the meta-analysis of pooled data by accentuating the tendency for regression to the mean of the more extreme values. Individual patient data from all the studies were not available; therefore, an appropriate adjustment according to baseline severity was not possible. Martin et al. concluded that the findings from the systematic review and meta-analysis provided insufficient evidence to suggest that rTMS is effective in the treatment of depression. Moreover, there were several confounding factors (e.g., definition of treatment resistance) in the studies, thus the authors concluded, “The rTMS technique needs more high quality trials to show its effectiveness for therapeutic use.”
Due to several serious methodological limitations in the studies that have examined the effectiveness of rTMS in patients with MDD, it is not possible to conclude that rTMS either is or is not effective as a treatment for MDD (in treatment-resistant depression or in nontreatment-resistant depression).