Vagus Nerve Stimulation (VNS) vs. Deep Brain Stimulation (DBS) Treatment for Major Depressive Disorder and Bipolar Depression: A Comparative Meta-analytic Review

Khan, Ahmed, Kotapati, Dar, Qamar, Jafri, Ibrahim, Kumar, and Begum: Vagus Nerve Stimulation (VNS) vs. Deep Brain Stimulation (DBS) Treatment for Major Depressive Disorder and Bipolar Depression: A Comparative Meta-analytic Review



Major depressive disorder (MDD) is a syndrome that deals with both behavior and biology; symptoms include abnormal mood, disturbances in neuro-vegetative functions and a decrease in cognitive abilities and psychomotor functions. It is also referred to as Unipolar Depressive Disorder when it is not accompanied by a history of manic episodes; in cases where there is a history of manic episodes, the condition is referred to as Bipolar Disorder. Manic episodes are characterized by a distinct period of abnormally and persistently elevated, expansive, or irritable mood and abnormally and persistently increased activity or energy, lasting at least four consecutive days and present most of the day, nearly every day. People with the disorder will also sometimes experience mixed episodes in which they experience both high and low moods at the same time or rapidly cycling in the same episode. However, both patients with MDD and those with bipolar disorders are similar in terms of mixed depression states, dysphoric hypomania and family history patterns. Major Depressive Disorder shares some similarities with Bipolar Disorder. Bipolar patients experience includes both mania and depression, whereas persons with major depression (who are not bipolar) will not experience mania. In comparisons of MDD and depressed Bipolar Disorder patients, atypical features, such as hypersomnia or leaden paralysis, psychotic symptoms, psychomotor retardation, shorter depressive episodes, a higher number of depressive recurrences, family history of mood disorders, comorbidity with substance abuse and earlier age at onset are reported more frequently in Bipolar Disorder, whereas somatic disturbances, anxiety, sleep loss and appetite loss are reported more frequently in MDD. They also evince some similarities: major depressive disorder may shift to bipolar disorder and bipolar disorder shares the feature of depressive states with Major Depressive Disorder.1-2

Treatment approach

The similarities between MDD and bipolar depression presents some difficulty in determining which treatment modality should be utilized. The first line of treatment for patients suffering from depression includes pharmacotherapies and psychotherapies. The pharmacological treatments include the administration of antidepressant drugs such as selective serotonin reuptake inhibitors and tricyclic anti-depressants, which target specific neurobiological targets.3-4 These medications are indicated once the patient begins experiencing depressive states.5 Usually the treatment of depression begins with first-line antidepressants such as selective serotonin reuptake inhibitors (fluoxetine, citalopram); if this treatment fails, switching strategies are considered, If that doesn’t work; combination strategies are considered (using more than one medication in concert) or augmentation strategies such as psychotherapy, adding other medications (such as bupropion, buspirone, mirtazapine, antipsychotics, T3 thyroid hormone and lithium) and dietary supplements (such as omega-3 fatty acids, S-Adenosyl-L-methionine [SAMe] and folic acid).6-7 However, these treatment strategies may not always reap desired results, such as in the case of treatment-resistant disorders, resistance which often shows itself with major depressive disorders and the depressive state of bipolar disorder. These disorders are characterized by the failure of antidepressant and augmentation therapies to bring relief to the patient, resulting in demand for better treatment methods for the management of the condition.8

The advanced field of neurobiology has associated the occurrence of depressive states, such as MDD and bipolar depression, with abnormalities in the functioning of neural networks. The neural network theory states that a normal mood is regulated by means of a coordinated neural structural that is functional normally. The occurrence of depressive states has been speculated to arise from communication of a dysfunctional nature between the nodes of different brain regions.9-10 Neuroimaging studies have indicated certain consistent and overlapping brain abnormalities in MDD and bipolar patients, such as an enlargement of the limbic regions, ventricle volume and increased and hyper-intensive rates of sub-cortical gray matter functioning. Thus, the presentation of biological models concerning depression has led to the advent of novel treatment methodologies that target the neural systems concerning the functional clusters of cognition, motivation, homeostasis and emotional regulation.11

The term neuromodulation techniques (NTs) refers to one such group of methods that involve targeting specific neural structures to facilitate improvement in functionality and they have proven effective in treating treatment-resistant disorders.9 These techniques involve invasive, minimally invasive, or non-invasive techniques that stimulate cortical or sub-cortical regions of the brain for therapeutic purposes.12 There are a number of NTs adopted for treating MDD such as Transcranial magnetic stimulation (rTMS), Vagus nerve stimulation (VNS), Deep brain stimulation (DBS), Transcranial direct current stimulation (rTDS), Direct cortical stimulation (DCS) and Magnetic seizure therapy.13-14 VNS and DBS constitute the invasive category, whereas rTMS, tDCS, DCS and MST constitute the group of techniques in the non-invasive category.

VNS involves the electrical stimulation of the tenth cranial nerve with a miniature implantable neurostimulator such as a Bionic Neuron that can be implanted (with a minor surgical procedure) adjacent to one or more portions of the vagus nerve and that manipulate the pathological substrate so as to achieve the desirable therapeutic effect.15 The VNS generator contains a small battery in the device that generates an intermittent electrical stimulation to the vagus nerve. A surgeon implants the generator subcutaneously over the chest and attaches the electrodes to the left vagus nerve. Intermittent signals from the VNS device travel up the vagus nerve and enter the medulla.16

DBS, on the other hand, involves the electrical stimulation of particular regions of the brain like the subgenual anterior cingulate cortex via electrodes, most commonly using four-contact stimulating electrodes.17 However, both techniques include the stimulation of specific neurological centers to provide relief from symptoms. The non-invasive techniques involve the application of electromagnetic waves to relevant brain regions to modulate activity and provide relief.18

Need for study

Treatment-resistant depression could lead to deleterious consequences for patients because not treating the disorder can contribute to the global burden of disease, impairing the structural and functional capacity of the brain and rendering the individual incapable of social functioning.8 There are a number of treatment modalities available for providing relief to patients suffering from treatment-resistant MDD and bipolar depression. Proposed theories regarding the neurobiological basis of disease progression has led to the incorporation of several neuromodulation techniques such as transcranial magnetic stimulation, Vagus nerve stimulation, deep brain stimulation, magnetic seizure therapy and others in the treatment of these conditions. The present study is motivated by increasing evidence of the efficacy and safety of VNS and the similar treatment methodology of DBS in improving depression.14,19 The present study is undertaken to provide a comparative view of the two stimulation techniques by evaluating the relative difference in the efficacy of these two methodologies. Thus, the aim of the present study is to perform a meta-analysis of treatment results pertaining to VNS and DBS. The objective of the study is to identify the superior method among two (VNS vs. DBS) for treating patients suffering from MDD and bipolar depression based on analytic review of the relevant research.


Search strategy

For the purpose of extracting relevant literature for the present meta-analysis, different electronic databases were searched. The investigated databases included PubMed, EMBASE, Cochrane Central Register of Controlled Trials and PubMed Clinical Queries; the time frame referenced for this study is last 20 years (until the time of the search in June 2017). Additionally, a manual search of references from previously published meta-analyses focused upon either DBS or VNS was also performed. The set of keywords used are listed in Table 1.

Table 1

Keywords used for searching the databases for relevant literature.

Individual KeywordsCombined Keywords
Vagus Nerve Stimulation
Deep Brain Stimulation
Vagus Nerve Stimulation Treatment-Resistant
Deep Brain Stimulation Treatment-Resistant

Inclusion Criteria

The criteria listed below were designated to collect relevant studies fulfilling the aim of the present research:

  • Studies published in the English language.

  • Studies involving only treatment-resistant depression, either/both Unipolar (MDD) and Bipolar Treatment-Resistant Depression.

  • The studies contained clear descriptions of clinical outcomes utilizing validated outcome measures.

  • Studies reporting treatment with respect to Studies reporting pre- and post-treatment scores.

Exclusion Criteria

The exclusion criteria were designated to minimize the inclusion of non-pertinent information sources. The following list details exclusion criteria:

  • Studies which included brain stimulation techniques other than VNS and DBS.

  • Studies where DBS or VNS used as adjunctive therapies.

  • Studies reporting patients with other mental illnesses such as bipolar illness (manic or rapid cyclic), schizoaffective disorders, or others.

  • Case reports, gray literature

Selection of studies

Initially, searches were conducted to find studies using VNS and DMS in patients with MDD/Unipolar depression. The initial search produced only two and seven studies for VNS and DBS, respectively. As the search can only be built to be inclusive and not exclusive, the search results included participants with bipolar depression. To better understand the effectiveness of VNS and DMS in affective disorders, the decision was made to incorporate studies that included patients with bipolar depression. The final search resulted in a total of 25 studies for the systematic review. Bipolar depression and MDD are studied together in published literature in the form of meta-analyses to better understand the effectiveness of treatment modalities.20 Out of the 25 studies, nine were selected for conducting a meta-analysis, of which five examined VNS and four examined DBS. The relevant studies were selected on the basis of homogeneity of the treatment outcome (pre-post scores) presented by the studies, over the three-month acute period, to facilitate extraction of valid statistical results.

Data extraction

The papers selected included significant information regarding the two different treatment methods; hence, uniformity was maintained in extracting the data from the different studies. The following structure was followed to extract the information:

  1. Patient characteristics: Age, gender, duration of illness, criteria for diagnosis and definition of treatment-resistant depression.

  2. Measure of primary outcome: The Hamilton Depression Rating Scale (HDRS) and Montgomery–Åsberg Depression Rating Scale (MADRS) scores, both pre- and post-neuro-modulation treatment were recorded and provided an operational measure of mood improvement. Besides HDRS and MADRS scores, the number of participants who were identified as improving, based on an efficacy measure of ≥ 50% reduction in HDRS score post-treatment, were noted.

  3. Measures of secondary outcome: The pre- and post-neuro-modulation treatment scores for the secondary measures were also recorded, at different time points, by the researchers (CGI-I, GAF, BDI, BAI, IDS-SR, YMRS).

  4. The stimulation parameters and adverse events due to treatment procedure were also extracted.

Data synthesis and analysis

The systematic review was conducted in agreement with the PRISMA guidelines,21 accompanied by a statistical meta-analysis procedure performed using the Comprehensive meta-analysis software (CMA v3, Englewood, NJ, USA). The meta-analysis was conducted using random effects modeling, as it accounts for the assumption of difference in effect sizes between the different studies and the summary effect size is the estimate of the mean of a random sample of effect sizes. The standard difference in means was calculated using the continuous data of treatment scores reported on HDRS and MADRS scales. For the studies reporting two groups, the data from the group receiving active VNS or DBS treatment was used.

The heterogeneity among the different studies was reported using the I-square test statistic, with heterogeneity among the studies evaluated as a measure of the statistic value.22 The studies were also analyzed for publication bias by using funnel plots as well as Begg and Mazumdar’s and Egger’s linear regression test statistics. The outlying effect sizes, if any, were identified using sensitivity analysis, which involved removing the studies having greatest effect size values.

Literature search and screening

The studies included in the final meta-analysis were searched and screened following the methodology depicted in Figure 1.

Figure 1

Systematic review flowchart.

Systematic review

The systematic review involved the collection of studies for Vagus nerve stimulation and Deep brain stimulation techniques of neuro-modulation administered to the patients demonstrating treatment-resistant depression. Table 1 describes the characteristics of the studies included in the present systematic review; the VNS review included twelve studies for the period from year 2000 to 2017 and the DBS review included thirteen studies for the period between the years of 2005 and 2017. The studies pertaining to both the NTs presented different study designs. For VNS, six open-label trials and a single study each of long-term follow-up study, comparative study, randomized control trial, prospective study, double-blind study and observational study design were included in the systematic review. For DBS, seven open-label trials, one randomized control trial and one prospective study each were included. Additionally, there were four DBS studies using combined research designs.

Characteristics of studies

  1. VNS : As shown in Table 2, the VNS studies were largely conducted in the US and Germany.29-30 The studies included the early post- operative follow-up and the follow-up period. The early post-operative follow-up included all of the patients participating in the study and they were monitored by investigators. The follow-up period varied from study to study. During this follow-up period, improvement in condition was monitored, but not all participants could be followed. The VNS studies included patients suffering from bipolar and unipolar depressive disorders; care was taken to ensure to include only bipolar patients not currently in a rapid cycling stage and not currently suffering a major depressive episode.

    Table 2

    Characteristics of studies included in the systematic review.

    TechniqeAuthorPlace of studyNature of studyStudy period (months)UnipolarBipolar
    Early post-operative follow upFollow up
    VNSRush et al.23USAOpen label trial312217
    Sackeim et al.24USAOpen pilot study2.55.54416
    Marangell et al.25USALong-term follow-up312219
    George et al.26USAComparative study312VNS + TAU = 90 TAU = 88VNS+TAU=10 TAU = 12
    Nahas et al.27USAOpen label trial3244316
    Rush et al.28USARandomized control trial2.512Treatment = 99 Control = 100Treatment = 13 Control = 10
    Sperling et al.29GermanyProspective study2002-200518NA
    Bajbouj et al.30GermanyOpen label trial3245420
    Christancho et al.31USAOpen label trial2005-2006105
    Aaronson et al.32USADouble-blind study5.51224466
    Tisi et al.33ItalyOpen label trial2006-200927NA
    Aaronson et al.34USAObservational study2006-2015VNS = 360 TAU = 230VNS = 134 TAU = 71
    DBSMayberg et al.35USAOpen label trial651
    Lozano et al.36CanadaOpen label trial1220NA
    Malone et al.37USAOpen label trial12141
    Bewernick et al.38GermanyOpen label trial1210NA
    Bewernick et al.39GermanyOpen label trial1211NA
    Puigdemont et al.40SpainRandomized control and Cross over clinical trial128NA
    Doughtery et al.41USARandomized control trial430NA
    Holtzheimer et al.9USAProspective study24107
    Puigdemont et al.42SpainDouble-blind, randomized, sham-controlled, crossover study65NA
    Ramasubbu et al.43CanadaBiphasic study: Double-blind optimization phase and Open-label phase64NA
    Riva-Posse et al.44USAOpen label trial1211
    Bewernick et al.45GermanyOpen label trial1271
    Bergfeld et al.46NetherlandsBiphasic study: Open-label trial and Double-blind, randomized, crossover phase1225

    The Vagus nerve stimulation technique was clinically tested on patients suffering from both MDD and bipolar disorder and has proven to be a powerful treatment option.47

  2. DBS: Studies of DBS have been conducted with fair geographical distribution, spread across the US, Canada,35-36,43 Germany, Spain and the Netherlands.46 The DBS studies were also characterized by different acute study and follow-up periods. After device implantation, the individual studies continued for varying periods of time. Also, the majority of DBS trials took into account only those patients suffering from major depressive or unipolar disorder, except for four studies which included the only bipolar patients in the total sample.

Patient Characteristics

Table 3 shows the characteristics of patients included in the VNS and DBS studies. The VNS studies included 1743 patients, which included 1213 females and 530 males. Their mean age was 47.75 years and the average duration of illness was 22.86 years. The DBS studies included a total of 165 patients, with 98 females and 67 males, a mean age of 33.11 years and an average duration of illness of 9.9 years. The DBS sample size was very small compared to the sample size of the VNS trials, which could possibly be attributed to the relatively recent emergence of this technique for treating depression.48

Table 3

Patient characteristics of the studies included in the systematic review.

TechniqueAuthorAgeGender (F/M)Duration of illness/current episodeDuration of intervention
VNSRush et al.2347.5 ± 7.520/1019.3 ± 13.13 months
Sackeim et al.2446.8 ±8.739/2118.1±10.93 months
Marangell et al.25NANANA12 months
George et al.26VNS + TAU = 21.8 ± 11.9 TAU = 20.8±11.5VNS + TAU = 64/36 TAU = 69/32VNS + TAU = 25.5 ± 11.9 TAU = 25.8 ± 13.2NA
Nahas et al.2746.8 ±8.738/2146.8 ±8.724 months
Rush et al.28Treatment = 47± 9 Control = 45.9± 9Treatment = 66/46 Control = 73/37Treatment = 26.10± 11 Control = 24.9± 13.03 months
Sperling et al.29Treatment = 50.2±8.5 Control = 50±8.85F/4M (both)Treatment = 7.2 ± 1.9 Control = 6.9 ± 0.8NA
Bajbouj et al.3047.4 ± 11.750/2419.1 ± 10.524 months
Christancho et al.3149 ± 109F/6M31.7 ± 11.1NA
Aaronson et al.32Low dosage = 49.1 ± 10.5 Medium dosage = 47.2 ± 11 High dosage = 47.4 ± 10.8Low dosage = 68/34 Medium dosage = 69/32 High dosage = 73/34Low dosage = 29.8 ± 12.1 Medium dosage = 26.3 ± 10.9 High dosage = 27.0 ± 12.1NA
Tisi et al.3357.5 ± 149F/18M18.5 ± 13.324 months
Aaronson et al.34VNS = 48.9±10.12 TAU = 49.9 ±11.07VNS = 350/144 TAU = 211/90NANA
DBSMayberg et al.3529.5 ± 123F/3M29.5 ± 126 months
Lozano et al.3627.1 ± 8.311F/9M27.1 ± 8.312 months
Malone et al.3725.3 ± 10.511F/4M25.3 ± 10.512 months
Bewernick et al.3848.6 ± 11.74F/6M48.6 ± 11.724 months
Bewernick et al.3948.36 ± 11.084F/7M48.36 ± 11.0824 months
Puigdemont et al.4024.9±5.36F/2M24.9±5.36 months
Doughtery et al.41Active=46.6 Control = 48.917/13Active = 46.6 Control = 48.9NA
Holtzheimer et al.942±8.910F/7M42±8.9NA
Puigdemont et al.42OFF-ON = 27.5±0.7 ON-OFF = 20.3±2.5OFF-ON = 27.5±0.7 ON-OFF = 20.3±2.56 months
Ramasubbu et al.4317.25±5.03F/1M17.25±5.06 months
Riva-Posse et al.4448.73±10.109F/2M48.73±10.1012 months
Bewernick et al.4541.9±8.703F/5M41.9±8.70NA
Bergfeld et al.4653.2±8.417/853.2±8.4NA

Treatment Outcomes

Table 4 shows the primary treatment outcomes for both the VNS and DBS trials. The Hamilton Depression Rating Scale (HDRS) and Montgomery–Asberg Depression Rating Scale (MADRS) were designated as the primary outcome measures, whereas IDS-C, YMRS, CGI, GAF all reported secondary outcome measures. Both HDRS and MADRS are the most commonly used rating scales for evaluating depression, using established clinical criteria to differentiate between severity levels of the condition and to measure the evolution and recovery from the depressive episode.49 The two scales have also been found to be correlated to each other to a significant degree, suggesting the two scales measure depression similarly, with only slight differences.50-51 Although some studies criticize the HDRS scale as being biased and argue that the MADRS is superior in evaluating the condition, HDRS has continued to be the gold standard rating scale for the past 40 years.30

Table 4

Treatment outcomes as reported in the studies included in the systematic review.

TechniqueAuthorStimulation ParametersTreatment effectNumber/Percentage of patients reporting side effects
Amplitude (Volt, V)Pulse width (µs)Frequency (Hertz)Primary outcomes (HDRS, MADRS)Secondary outcomes (IDS-C, MADRS, YMRS, CGI, GAF)
VNSRush et al.28NA250-50020-30HDRS38.± 6 5.523.0 ± 10.8CGI5.3±0.73.7 ±1.4≥ 7%
MADRS33.8 (5.6)20.1 ±12.2GAF40.6 ±6.861.9 ±16.8
YAMRS2.3 ±1.31.9 ± 3.4
Sackeim et al.24NA250-50020-30HDRS36.8 ± 5.824.7 ±10.9CGI5.2 ± 0.73.9 ± 1.3≥ 5%
MADRS33.4±5.222.9 ± 11.7GAF40.6 ± 6.057.4 ± 16.2
BDI34.9 ± 7.723.0 ± 11.1
YMRS2.1 ± 1.52.1 ± 3.3
Marangell et al.25NANANAHDRS38 ± 5.523 ± 10.8CGI5.3±0.73.7 ±1.4≥ 5%
MADRS33.8 ± 5.620.1±12.2GAF40.6 ± 6.861.9 ±16.8
YMRS2.3±1.31.9 ±3.4
George et al.26NANANAHDRSVNS+TAU = 28 ± 5.7 TAU = 27.5 ± 5.1VNS+TAU = 28 ± 5.7 TAU = 27.5 ± 5.1
Nahas et al.27NANANAHDRS46.8 ±8.724.9 ± 11.2CGI4.1 ± 0.72.9±1.1≥ 10%
MADRS33.4±5.722.9±11.7GAF40.6 ± 6.057.4±16.2
Rush et al.23NA50020HDRSTreatment = 28.80±5.3 Control = 29.7± 5.2Treatment = 45.1± 33.4 Control = 45± 30.7IDS-SRTreatment = 44.30± 59.1 Control = 45.4± 8.5Treatment = 21.2± 25.4 Control = 16.3± 26.2≥ 10%
MADRSTreatment = 31.40± 6.3 Control = 31.9± 6.3Treatment = 17.1± 31.2 Control = 12.4± 27.1
Sperling et al.29NANA15-30HDRS23.7 ± 2.410.2 ± 2.4NA
Bajbouj et al.30NANANAHDRS34.0 ± 5.822.1 ±11.5IDS-SR47.6 ± 9.133.2 ±15.9≥ 3%
MADRS32.9 ±6.420.5 ±11.7CGI5.5 ±0.93.9 ±1.6
Christancho et al.31NA22025HDRS29.36 ± 4.5815.84 ±8.8CGINANA≥ 6.7%
Aaronson et al.32NA13020NANANANA≥ 1%
Tisi et al.33NANANAHDRS25.6 ± 4.015.3 ± 7.7NANA
Aaronson et al.34NANANAMADRSResponse rate: VNS arm = 67.6%; TAU = 40.9% QIDSCGIResponse rate: VNS arm = 79.9%%; TAU = 48.6%NA
Response rate: VNS arm = 64.7%; TAU = 41.7%NA
DBSMayberg et al.35460130HDRS34.6±1.918.8±10.6CGI6.2(0.4)4.2(0.6)3
Lozano et al.363.5-590130HDRS24.4 ± 3.511.8 ± 5.9CGI5.1 (0.7)3.4(1.2)20
BAI14.1 (9.2)12.8(10.2)
BDI27.5 (9.2)22.3(11.8)
Malone et al.376.7 ± 1.8113.0± 45.0100-130HDRS33.1± 5.517.5± 8.2GAF43.4 (2.8)58.4 (8.4)≥ 4%
MADRS34.8 ± 7.316.1 ± 9.2
Bewernick et al.382V-4V90130HDRS32.523.8NA--26
MADRS32.3 ±3.723.4 ±4.7
Bewernick et al.391.5–1060-120100-150HDRS32.2 ± 5.520.2 ± 7.5MADRS, HAMA, Activities, SF-36, Physical healthNANA41
Puigdemont et al.403.5-590135MADRS28.5 ± 6.031.08 ± 11.3NA--3
Doughtery et al.410-8V90-120NAMADRSActive = 37.7 Control= 36.4Active =29.7 Control= 27.4NA--25
Holtzheimer et al.9NA91130HDRS23.9 ± 0.77.3 ±0.7GAF, BDI-IINANA9
Puigdemont et al.423.5-5120-240130-135HDRS3.5 ± 2.18.5 ± 12.0NA--NA
3.7 ± 2.37.0 ± 5.6
MADRS2.0 ± 2.88.5 ± 12.0
3.4 ± 2.57.7 ± 4.5
Ramasubbu et al.432V-5V90-4505-185HDRS30.75±2.919.75±5NA--2
Riva-Posse et al.442V-5V150-450130HDRS28.13± 4.6710.5 ± 10.39MADRS, BDI, HAMA, SF-36, Positive activities, GAF--NA
Bewernick et al.45NA91130HDRS22.2±4.915.9±9.2IDS-S--NA
Bergfeld et al.462.5 – 690130-180HDRS22.2 (4.9)Active = 13.6 (7.8) Sham = 23.1 (5.1IDS-S49.3 (10.1)Active = 32.6 (19.1) Sham = 46.6 (11.3)Optimization = 14 Stimulation = 18 Crossover phase =12
MADRS13.6 (7.8)Active = 34.1b (7.7) Sham = 21.3 (13.5)


  1. VNS: The VNS trials reported improvement in the depressive states of patients as seen in the reduction in post-test scores. The average values of simulation parameters bringing about relief in the condition of patients could not be calculated, as some of the studies did not report the stimulation parameters. Reported side effects were observed in all of the studies, side effects associated with the implantation and operation of the VNS device. Voice alteration or hoarseness was reported to be the most common side effect of VNS, both of which could be caused by compromised airways in the larynx.52 Other reported side effects associated with device implantation were pain at the site of incision, infection, throat pain, neck and general pain, shortness of breath, headache, dyspnea, pharyngitis, dysphagia, asystole, bradycardia and discomfort. The VNS stimulation led to a significant degree of improvement, yet some studies reported suicidal ideation as well as suicides and suicide attempts in some patients. The research conducted by Aaronson et al.32 studied the effects of VNS at three different settings: low (Current= 0.25 mA, Pulse width = 130 μsec), medium (Current= 0.5-1.0 mA, Pulse width = 250 μsec) and high (Current= 1.25-1.5 mA, Pulse width = 250 μsec). Nine to eleven percent of patients in the medium and high groups showed remission compared to a five to six percent remission rate in the group using low settings.

  2. DBS: The DBS trials, despite small sample sizes, demonstrated significant improvement in the depressive states of patients. The average amplitude of the administered treatment was calculated to be 4.03V, the average pulse width was 127.69 ms and the average frequency was 118.26 Hz. Besides the different stimulation parameters, the DBS studies focused on stimulating different areas of the brain, areas that are thought to play an important role in the development and maintenance of depression. The different brain regions stimulated by researchers in the studies to date are the subgenual cingulate white matter, the ventral capsule/ventral striatum,14,53 the nucleus accumbens,38-39 the subgenual cingulate gyrus and the superolateral branch of the forebrain bundle.45 However, determining the most effective brain region will require more research and trials. DBS was found to have a higher number of side effects, with unique effects such as psychomotor slowing at high settings, hand numbness, aconuresis and cephalalgia. There were other general side effects including infection. The most severe reported side effect was suicidal ideation.

The study conducted by Riva-Posse et al.44 aimed at validating the subcallosal cingulate region as an effective site to stimulate in reducing depressive states by using a four-bundle tractography ‘connectome blueprint’ to plan surgical targeting in participants. Ramasubbu et al.43 discusses identifying the optimal procedure for stimulating parameters and evaluating optimized stimulation parameters by using a research design consisting of a double-blind stimulus optimization phase and an open-label post-optimization phase. Puigdemont et al.40 successfully demonstrated full remission in four patients out of eight in total after a complete year of stimulation. They also noted that the localization of electrodes is an important parameter in eliciting a response, with electrodes localized in the BA24 region, the corpus callosum and the head of caudate producing the highest response. All of the studies agreed that the positioning of electrodes and the stimulation parameters play a crucial role in treating depression.


Effect Size

The present meta-analytic review evaluates the treatment efficacy of two neuromodulation techniques, VNS and DBS, as established by published research. The treatment designs recorded improvement in terms of pre- and post-test scores, with treatment administered to a single group of patients. To observe the reported efficacy of individual treatment arms, the meta-analysis was conducted separately for each technique and the summary effect sizes were compared to determine the efficacy and sensitivity of each treatment.54 A higher value of summary effect size indicates a higher degree of improvement in scores, thus the associated efficacy of the treatment methodology.

HDRS: The summary effect size (standard difference in means) for VNS and DBS groups was 1.247 and 2.063 respectively, as shown in Figure 2. These figures indicate that DBS is more effective than VNS, as reported by the analyzed studies, during the early postoperative follow-up (three months) in treating depression. With respect to heterogeneity, the VNS group of studies had the value of I2 statistic of 87.46, p=0.00, indicating (84.76%) a high level of significant heterogeneity among the studies, whereas the DBS group of studies had the value of I2 statistic of 0.00, but p=0.740, indicating insignificant heterogeneity among the studies.22

Figure 2

Forest plot for VNS and DBS treatment scores with respect to the HDRS rating scale.

MADRS: The summary effect size (standard difference in means) for VNS and DBS groups was 1.110 and 1.996 respectively, as shown in Figure 3. This indicated the higher efficacy of DBS over VNS, as reported by studies, during the early post-operative follow-up (three months), in treating depression. With respect to heterogeneity, the VNS group of studies had the value of I2 statistic of 80.75, p=0.00, indicating (80.75%) a high level of significant heterogeneity among the studies, whereas the DBS group of studies had the value of I2 statistic of 0.00, but p = 0.761, indicating an insignificant heterogeneity among the studies.22

Figure 3

Forest plot for VNS and DBS treatment scores with respect to MADRS rating scale.

Publication Bias

The data suffered from inherent bias pertaining to study designs, as the age and duration of the illness of the subjects in VNS and DBS groups were not comparable. Also, the VNS publications were older than the DBS publications.

  1. VNS: The publication bias was tested and the funnel plot was obtained, depicted in Figure 4. The plot was asymmetrical, with Egger’s rank test showing an intercept = 6.43, p = 0.04, indicating the presence of bias.

    Figure 4

    Funnel plot for VNS studies.

  2. DBS: We tested the publication bias was tested obtained the funnel plot, which is depicted in Figure 5. The plot was asymmetrical, with Egger’s rank test showing an intercept of -2.59, p = 0.31, indicating symmetry in funnel plot and an insignificant publication bias.

Figure 5

Funnel plot for DBS studies.


Vagus Nerve Stimulation (VNS) and Deep Brain Stimulation (DBS) both involve electrical stimulation of the neural target via an implanted device.55 The constant electrical stimulation results in the alteration of the activity of the targeted brain area; depending upon the location of the implant, these NTs have proven to be effective in bringing relief to patients suffering from MDD and bipolar depression.56

VNS involves the placement of a pulse-generating device subcutaneously wrapped around the left Vagus nerve, with the stimulatory effect resulting in changes in neurotransmitters such as serotonin, glutamate, norepinephrine and GABA, all of which are indicated in depression pathogenesis and improving mood. This treatment also brings about changes in the anatomy of brain regions and blunts mild stress events that may eventually result in stress sensitization in the individual.57

DBS involves invasive surgery and the electrode placement varies. This variation has been a topic of research: different studies have targeted different areas of brain, such as the Subcingulate–Broadmann area 25, the Ventral anterior internal capsule/ventral striatum (VC/VS), the Nucleus accumbens (NAcc), the Inferior thalamic peduncle and the Lateral habenula.14 Though DBS has been known to improve mood in depression, the technique is still being tested to determine which brain areas obtain optimal results.

The stimulation of Nucleus accumbens (NAcc) has been observed to play a significant role in the abnormal reward process, whereas VC/VS stimulation has been shown to improve concomitant symptoms of depression in patients treated with OCD. Even though VNS and DBS have proved themselves to significantly improve the symptoms of MDD and bipolar depression, these methods are also associated with certain shortcomings. The foremost drawback associated with both is the expense of treatment, the special aftercare required, the invasive nature of procedure and the risk of hemorrhages.58 These techniques are also associated with certain diverse events such as infections due to implants, neck pain, dyspnea, dysphagia, vomiting, voice alteration, headache and other conditions.59

The two techniques vary significantly in their mode of action: VNS has been demonstrated to act upon the prefrontal cortex and limbic structures, whereas the mode of action of DBS is still under investigation. The deeply invasive nature of this technique has been associated with its action on areas far beyond the targeted region. Thus, it has been associated with putative modeling of the complex neural networks and is perceived to influence larger volumes of neural tissue, depending upon the stimulation parameters.60 The mode of action of the two techniques is yet to be understood completely, even using advanced neuroimaging techniques.

The clinical studies are still in their nascent stage and researchers continue to establish the safety and efficacy of these treatment methods. The results of the individual VNS and DBS meta-analyses has shown that these treatment methods indeed provide benefit, even in patients who previously exhibited high refractory responses with previous drug and ECT treatments. The long periods of follow-up in VNS treatment provide evidence for the stability of the technique in bringing long-term relief to patients, as the study outcomes showed continued improvement in scores even after a period of 12-24 months. Although DBS has shown promising results, the results suffer limitation in terms of sample size and randomized controlled data.61

The invasive nature of the treatment methodologies causes some patients to be reluctant to undergo such treatments; however, in spite of recorded side effects, patients generally tolerate the surgery and continued stimulation well. A major effort is being directed towards the optimization of the stimulation parameters, a consideration that is even more important using DBS because different areas of the brain are structured differently and have relatively different impedance profiles. The stimulation of different brain areas involves impacting different surface areas and thus different charge densities are required. Overall, VNS and DBS are suitable treatment methods for providing relief to patients with mental illnesses.

The present meta-analytic review has compared the relative efficacies of the two emerging neurostimulation treatment methodologies for depression, namely VNS and DBS. The present study is the first one to attempt such a comparison of these techniques. The comparison of the summary effect sizes showed the superiority of DBS over VNS in ameliorating depression.

The summary effect size for VNS was lower than that of DBS groups, indicating that DBS method of brain stimulation is more effective than VNS. The finding is corroborated by the tests of heterogeneity: while the VNS group of studies indicated a high level of significant heterogeneity among the studies, the DBS group indicated insignificant level of heterogeneity. Thus, it may be that DBS is more efficacious than VNS. Additionally, the presence of publication bias in the case of VNS and the insignificant publication bias in the DBS studies further supports the claim that DBS is more efficient than VNS. Thus, the current meta-analysis demonstrates that Deep Brain Stimulation (DBS) is a better treatment modality for Major Depressive Disorder and Bipolar Depression than Vagus Nerve Stimulation (VNS).

However, as the clinical profile of patients in the VNS group and DBS group were very different in terms of age for treatment of MDD and bipolar depression, the duration of the illness and the duration of follow-up or intervention, it is relatively difficult to make direct comparison between these two treatment interventions. To be able to make these comparisons, it is necessary to have reasonably matched studies (both in terms of participants and in terms of methodologies). Since such matched studies do not exist in published literature, it may not be practical to interpret the difference in effect size reported from the meta-analysis as being evidence for one being a better treatment.


The present study suffers from certain inherent limitations in terms of the studies included and in terms of statistical procedures. The foremost limitation is that the results generated from the present study could not be extrapolated to a general, large-scale population, as studies that were analyzed included research with very small sample sizes, especially the DBS studies. The clinical profile of patients in the VNS group and DBS group were very different in terms of age, duration of illness and duration of follow up or intervention. Another limitation is that these techniques are relatively new in the domain of psychiatric procedures and the complete implications are not yet known. Additionally, the VNS trials included bipolar patients (though they were also in a depressive state) with those suffering from major depression; therefore, it is unclear how effective VNS is for treating unipolar depressive states. (The present review includes only two studies29,33 in which the VNS technique was used with only one clinical group.) Therefore, the comparison was, in some ways, comparing apples to oranges: studies of unipolar patients were compared with studies that included both unipolar and bipolar patients. And while the results of the statistical analysis and subsequent comparison of summary effect size have indicated that DBS is the superior treatment methodology, the disparity between the sample sizes of VNS and DBS also posed a limitation to the conclusions drawn from the meta-analysis. This led to the biggest limitation: because the studies included different sample sizes (and the samples themselves were so different), because the types and durations of the treatments varied and because one group was more homogenous than the other, direct comparisons could not be made. Thus, while the summary effects were different, it is not clear that these are very conclusive.

Implications of research

The treatment of mental illnesses is a fertile area of research and the increase in refractory responses in patients with depression calls for improved treatment methods. This meta-analysis seems to indicate that DBS is a more effective treatment. However, the present review is a pilot review subject to limitations. Those limitations can and should be utilized as a framework for outlining problems in the research and suggesting the types of research that should be done, calling future researchers to delve deeper into the problem and present concrete evidence. The analysis has suggested that DBS can improve major depressive states through the stimulation of brain regions. However, an investigation into the region of the brain producing the highest level of response is required, with simultaneous optimization of stimulation parameters. Also, there is a need for controlled trials to confirm the efficacy of treatment in diagnosed patients as compared to control groups. The stimulation parameters may also function as criteria for customized therapy for non-responders, allowing the adjustment of the parameters in order to obtain suitable responses. The invasive nature of these stimulation techniques requires close monitoring of patient, however, as these techniques have not been tested on a larger scale with large numbers of people and the associated cognitive and physiological effects have yet to be identified.


The VNS and DBS techniques have been found to be suitable for treating major depression. The long-term follow-up periods of some of these studies have indicated that these methods may provide long-term, sustained and stable relief for patients. The stimulation parameters and positioning of electrodes were also seen as predictors of response; in other words, some techniques and positioning proved more effective than others. However, studies with larger sample sizes and synchronous experimental and control groups are required. There is a need for studies whose participants are more similar in age with similar durations of the illness and a need for studies which match in terms of the type of stimulation used and the duration of the treatment. However, these methods are showing promising results and taken together with suitable medications, they could be used in the management of treatment-resistant depression.


The authors declare no conflict of interest.



Vagus Nerve Stimulations


Deep Brain Stimulation


Major Depressive Disorder


Treatment Resistant Depression


Electro Convulsive Therapy


Obsessive Compulsive Therapy


Treatment As Usuals


Generalized Anxiety


Transcrania Magnetic Stimulation


Neuromodulation Techniques


Direct Cortical Stimulation


Transcranial Direct Current stimulation


Gamma-Aminobutyric Acid


Transcranial Direct-current Stimulation


Magnetic secure Transmission


Beck Anxiety Inventory


Inventory of Depressive Symptomatology- Clinician Rated


Montgomery Asberg Depression Dating Scale


Young Mania Rating Scale


Clinical Global Impression- Improvement


Global Assessment of Functioning


Hamilton Anxiety Rating Scale


Beck Depression Inventory


Inventory of depressive symptomatology- self-rated


Quick Inventory of Depressive Symptomatology.


In this study we assessed the relative difference in the efficacy of VNS versus DBS for treatment of MDD and bipolar depression and to provide evidence for the superior technique. 26 studies were selected, consisting of 1160 patients who were treated with either VNS or DBS treatment arms and analyzed them to determine the amount of improvement in mood and primary outcome measures were evaluated in terms of change between pre-test and post-test scores over a period of three months, as measured by HDRS and MADRS rating scales. Results comparing effect size produced by VNS (HDRS = 1.247, MADRS = 1.110) to that produced by DBS (HDRS = 2.063, MADRS = 1.996) seems to demonstrate that DBS is the more effective treatment, while the VNS group of studies indicated a high level of heterogeneity Vs. DBS group indicated insignificant level of heterogeneity. However, as the VNS and DBS groups differed concerning the clinical profiles of the patients both in terms of age and regarding the duration of the illness. Research studies with larger, synchronous sample sizes and control groups are required for a meta-analysis to draw a steadfast conclusion.



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