Experience of pallidal deep brain stimulation in dystonia at a tertiary care centre in India: An initial experience
Correspondence Address: Source of Support: None, Conflict of Interest: None DOI: 10.4103/0028-3886.217957
Source of Support: None, Conflict of Interest: None
Introduction: Dystonia is one of the most prevalent forms of movement disorders and is characterized by sustained or intermittent muscle contractions causing abnormal, often repetitive, movements, postures, or both. Dystonia causes significant morbidity with an adverse impact on the quality of life. When dystonia is medically refractory, causing severe pain and impairment in activities of daily living, deep brain stimulation (DBS) of the globus pallidus interna (GPi) is a potential option to reduce disability.
Keywords: Deep brain stimulation, dystonia, GPi, medically refractory
Dystonia is one of the most prevalent forms of movement disorders and is characterized by sustained or intermittent muscle contractions causing abnormal, often repetitive, movements, postures, or both. Dystonia is classified in many ways: According to the age at onset (early onset dystonia with age ≤ 26 years and late onset dystonia with age >26 years), according to the involved body distribution (focal, segmental, multifocal, generalized, and hemidystonia), and based on etiology, i.e., inherited dystonia of proven genetic origin, acquired dystonia with a known specific cause (e.g., perinatal brain injury, infection, drugs, toxicity, vascular etiology, neoplastic lesion, or brain injury), and isolated dystonia without a specific known cause.
Hallett proposed that dystonia could be due to a lack of inhibition of neurons not involved in the activation of the willed muscles. This leads to firing of surrounding neurons, which in turn causes coactivation of agonist and antagonist muscles. Anatomical location of dystonia caused by basal ganglia dysfunction has been supported by the dopa-responsiveness of some types of dystonia. The pathophysiology of dystonia remains uncertain., Two primary factors in the development of dystonic symptoms could be excessive cortical excitability and impaired sensorimotor processing. Kojovic et al., studied physiological differences between primary and secondary dystonia. Primary dystonia is caused by disorders of basal ganglia without morphological alterations whereas secondary dystonia is caused by anatomic lesions in the same region. Increased cortical plasticity was observed in patients with primary dystonia, whereas normal response to the plasticity protocol on both affected and non-affected sides was seen among patients with secondary dystonia. Dystonia causes significant morbidity with an adverse impact on the quality of life. Management of dystonia is targeted at symptomatic relief and improving abnormal posture. Evidence-based medicine on oral pharmacotherapy is limited. There are only few randomized controlled trials reported till date. All patients should begin with a trial of levodopa to look for dopa-responsive dystonia. Thereafter, various medications are tried individually and in combination including anticholinergics (trihexyphenidyl, benztropine), benzodiazepines, muscle relaxants (e.g., baclofen, tizanidine), carbamazepine, and dopamine antagonists/release inhibitors (e.g., tetrabenazine). Botulinum toxin injection remains the most effective treatment for focal dystonia. It prevents the release of acetylcholine into the neuromuscular junction; however, the utility of botulinum toxin is limited for focal dystonia and the development of antibodies precludes its long-term usage.
When dystonia is medically refractory, causing severe pain and impairment in activities of daily living, deep brain stimulation (DBS) of the globus pallidus internus (GPi) is a potential option to reduce disability., The first reported study of DBS for the treatment of dystonia was in 1977. In 1998, Alim Louis Benabid showed significant improvement of symptoms in a variety of movement disorders with DBS of the GPi and subthalamic nucleus (STN). Since then, DBS has been used as an effective treatment for various symptoms occurring across many disease states. The precise mechanism of DBS is still not understood but mimics the effects of ablative procedures. Success of this therapy has been in the use of DBS to treat refractory dystonia, a debilitating disease.
In recent years, among patients with both primary and secondary dystonias, GPi DBS has been employed as a safe, reliable, reversible, and adjustable treatment with a relatively low risk of adverse effects.,,,, In this article, we discuss our experience of pallidal DBS in mixed dystonia patients at a tertiary care centre in India. To date, this is the largest series reported from India.
This is a chart review of patients who underwent DBS for dystonia (from 2009 to 2015) at our center for Movement Disorders and Human Motor Physiology, Departments of Neurology and Neurosurgery, National Institute of Mental Health And NeuroSciences (NIMHANS), Bengaluru. India. A total of 11 patients (8 males, 3 females) underwent DBS for non-parkinsonian conditions. Among them, 10 patients underwent GPi-DBS for various types of dystonia and one for Tourette syndrome [Table 1]. Patients were selected after failure of adequate medical management, which included multiple drugs including anti-cholinergics, benzodiazepines, neuroleptics, baclofen, and botulinum toxin injections. All patients had a severe disability with normal cognitive (Mini-Mental State Examination) and psychiatric profile. They also had to have a suitable Gpi for DBS based on magnetic resonance imaging (MRI).
Clinical evaluation and outcomes measures
Patients who did not respond to adequate trials with appropriate oral medications and botulinum toxin injections were given an option of DBS. Those with no contraindications were considered for DBS after obtaining a written informed consent. Out of 10 patients (7 males, 3 females), 5 each had primary and secondary dystonia. The details along with clinical details are tabulated in [Table 1]. Two patients had mild contracture (#5, #6), and 1 patient had fixed skeletal deformities (#10, scoliosis).
The mean age of the cohort was 30.6 ± 14.46 years (range, 6–57 years). The youngest patient was 6 years old and the eldest was 57 years old. The mean age at onset was 20.6 ± 16.5 years (range, 3–52 years). Six patients had early-onset dystonia (<26 years: #1, #3, #5, #6, #7, and #10) and 4 had a late onset (>26 years) one. The mean duration of illness was 11.5 ± 7.2 years (range, 3–23 years). The mean age at surgery was 31.7 ± 14.8 years.
All patients were assessed and video recording was performed by two independent expert neurologists in movement disorders. The movement (M) and disability (D) subscores of the Burke–Fahn–Marsden dystonia (BFMDRS) scale  were used to assess the effects of neurostimulation.
The BFMDRS-M score (range, 0–120) is the sum of 9 body region subscores, which were grouped into 4 anatomical areas: face (eyes and mouth), speech and swallowing (SS), axial (neck and trunk) segment, and limbs. The total BFMDRS-D score (range, 0–30) is the sum of individual ratings for 7 activities: speech, handwriting, and the degree of dependence with respect to hygiene, dressing, feeding, swallowing, and walking. Higher scores indicate worse motor impairment and disability.
Clinical evaluations were performed preoperatively and postoperatively at 3 and 6 months and yearly intervals using the BFMDRS [Table 2]. Postoperatively, the evaluation was performed only in the “on” state. All patients were offered genetic testing but it could be done in only 1 patient due to financial issues. In this patient, DYT1 mutation in the torsin A gene was negative (#5). The mean follow-up period was 19.5 ± 14.34 months (range, 3–51 months) with only 2 patients (#2 and #8) having more than 3 years of follow up. For all patients, the medication intake was recorded at the baseline and at each follow-up.
Neurosurgical procedure and deep brain stimulation programming
All 10 patients underwent DBS by the same neurosurgeon (DS). Each patient underwent frame-based MRI and microelectrode-guided stereotactic implantation of DBS leads (model 3389; Medtronic Inc, Minneapolis, Minnesota). Preoperative MRI was performed for all patients to determine the posteroventral part of GPi bilaterally. After obtaining written informed consent, the patients underwent bilateral implantation of a quadripolar electrode (Medtronic, Minneapolis) in the GPi under local/general anesthesia. The intraoperative target refinement was obtained by neurophysiological monitoring, using multitrack pentapolar micro-drive recording system (FHC—Medtronic). In all cases, we used a standard assessment of three microelectrode tracks (anterior, medial, and posterior) in the sagittal plane. The postoperative cerebral MRI/CT confirmed the correct bilateral positioning of the electrodes within GPi.
The final functional target was based on the anatomical target in relation to the optic tract (at the top and middle of the optic tract at the mid- commisural point). All patients underwent a postoperative cerebral magnetic resonance imaging (MRI)/computed tomographic (CT) scan, which was fused with the preoperative images to check for the accuracy of electrode placement. During the initial programming, standard protocols were followed. Each contact was tested in a monopolar fashion (i.e., case positive and contact negative; setting frequency at 130 Hz and pulse width at 120/s). Once the putative best contact was chosen (with the highest threshold for adverse effects and the lowest threshold for clinical response), a fine augmentation of voltage was achieved up to 4 V. The ventral-most contacts were preferentially, but not exclusively, used for therapy. In the case of an unsatisfactory response, the pulse width was gradually increased to sub-threshold of adverse effects (e.g., slurred speech, optic sensations, dizziness). If the response was still suboptimal, the next contiguous contact (generally more dorsal) was used and the same steps were followed. During follow-up, the patients were thoroughly examined for hardware related issues.
Data were presented as mean and standard deviation (SD). To analyze the variations at 3 and 6 months and 1 year after the implant placement, for differences in movement and disability aspects assessed by BFMDRS, paired repeated-measures analysis of variance were performed. When analysis of variance was significant, the difference between means two by two were compared using the Bonferroni correction, as adjustment for multiple comparisons. A P < 0.05 was considered significant; all analyses were conducted utilizing Statistical Package for the Social Sciences (SPSS) 23 (IBM SPSS Statistics for Windows, Armonk, NY: IBM Corporation).
Clinical improvement at 3 and 6 months and 1 year
Dystonia rating scale
The mean baseline BFMDRS movement score of 10 patients selected for surgery was 60.3 ± 27.3 (range, 19–104). On repeated -measures analysis of variance, there was significant difference at different time points (pre-DBS, post-DBS at 3 months, 6 months, and 1 year) F (3, 5) =7.68; P = 0.026 [Figure 1]. All patients improved significantly after DBS, and the data showed that there was maximum improvement after 1 year of stimulation (pre-DBS vs 3 months 12.9 ± 1.9 vs 8.8 ± 2.1, P = 0.01, pre-DBS vs 6 months 12.9 ± 1.9 vs 7.4 ± 1.6; P = 0.043; pre-DBS vs 1 year, 12.9 ± 1.9 vs. 7 ± 2.4, P = 0.042 [Table 1] and 2].
On comparison of the subscores of the BFMDRS, there was a significant improvement in the axial scores after 1 year of DBS stimulation compared to the baseline scores after Bonferroni correction for multiple comparisons (pre-DBS vs 1 year, 18.7 ± 1.7 vs 8.7 ± 1.9, P = 0.003). There was also significant improvement after 6 months of stimulation compared to the baseline scores (pre-DBS vs 6 months, 18.7 ± 1.7 vs 10.3 ± 1.8; P = 0.013).
At the 3-month follow-up, the mean BFMDRS movement score was 39.2 ± 26.4 with a mean reduction of symptoms of 35.4% (range, 0–82.3%). Continuous improvement was noted over subsequent follow-up visits [Figure 2]. At the 6-month follow-up, the mean movement score of BFMDRS was 29.1 ± 17.2, with a mean improvement of 46.1 ± 27.9% (range, 0–82.3%), and at the 1-year follow-up, the mean movement score of BFMDRS was 28.0 ± 27.6 with a mean improvement of 50.2 ± 35.3% (range, 0–94.1%).
In the movement sub-items of BFMDRS analyzed, we reported different results in relation to the body segment involved. The most relevant improvement was obtained in the axial segment at 6 months and 1 year of stimulation compared to the baseline preoperative score (pre-DBS vs 6 months, 18.7 ± 1.7 vs 10.3 ± 1.8, P = 0.013; pre-DBS vs 1 year, 18.7 ± 1.7 vs 8.7 ± 1.9, P = 0.003; [Figure 1], [Figure 2], [Figure 3]).
The mean preoperative disability score of 10 patients selected for surgery was 18.7 ± 6.7 (range, 9–26). On repeated-measures analysis of variance, there was significant difference in the different time points (pre-DBS, post-DBS 3 months, 6 months, and 1 year) F (3, 6) =13.8; P = 0.004 [Figure 1]. All patients improved significantly after DBS, and the data showed that there was maximum improvement after 1 year of stimulation (pre-DBS vs 3 months 2.5 ± 0.3 vs 1.9 ± 0.3, P = 0.001; pre-DBS vs 6 months 2.5 ± 0.3 vs 1.6 ± 0.3, P = 0.003; pre-DBS vs 1 year 2.5 ± 0.3 vs 1.4 ± 0.3, P = 0.001).
On comparison of the subscores of disability, there was a significant improvement in the writing score F (3, 34) = 4.4, P = 0.009. Post hoc analysis results with Bonferroni correction revealed a significant improvement in writing after 1 year of stimulation compared to the preoperative state (pre-DBS vs 1 year, 2.9 ± 0.3 vs 1.1 ± 0.3, P = 0.009).
At the 3-month follow-up, the mean disability score was 14.4 ± 6.9 with a mean reduction of symptoms of 26.1% (range, 7.6–55.5%). A continuous improvement was obtained over the next follow-up visits [Figure 2]. At the 6-month follow-up, the mean disability score was 11.4 ± 6.9, with a mean improvement of 39.6 ± 19.1% (range, 0–66.6%), and at the 1-year follow-up, the mean disability score was 10.3 ± 7.4, with a mean improvement of 48.6 ± 27.32% (range, 12.5–88.8%; [Figure 3]).
To know whether or not movement improvement in dystonia patients improved their disability, correlation of the percentage improvement of BFMDS movement and disability scales were calculated at 3 months, 6 months, and 1 year. Corrections for multiple correlation tests were done for the three correlations and P value was considered significant at P < 0.01. There was a significant correlation between the percentage of improvement in the movement score of BFMDS scale and the disability score after 1 year of stimulation (r = 0.857, P = 0.007).
Preoperatively, all patients received treatment for dystonia. All 10 patients took anticholinergic agents (trihexyphenidyl, mean dose: 30 mg; range, 6–60 mg) and benzodiazepines; 8 patients received an antispastic medication (baclofen). Levodopa, carbidopa and dopaminergic drugs dosages were reduced by more than 50% for the patient with Parkinson's disease with dystonia complex (#4), with significant reduction in the dyskinesia following DBS. In all patients, botulinum toxin injections were suspended. Patient #9 underwent battery replacement early at a 2 -year post DBS follow-up. Patient # 2 is waiting for battery replacement at the end of 4th year following his DBS procedure. The youngest patient who had neurodegeneration with brain iron accumulation (NBIA) developed lead infection, for which the DBS leads were removed. There were no reported lead fractures reported in the available follow-up data.
In this study, the first of its kind from India, we have shown that patients with both primary and secondary dystonia had significant improvement in both movement and disability scores of BFMDRS following bilateral GPi DBS. The improvement was observed earliest at the 3rd month with a mean reduction of symptoms of 35.4%. The improvement observed was 46.1% at 6 months, and 50.2% at 1 year. There was significant difference in the different time points with a maximum improvement occurring after 1 year of stimulation. On subscores analysis, axial scores showed a statistically significant improvement at the 6th month and at the end of 1 year.
In the assessment of the BFMDRS disability scale, there was a significant difference in the different time points of evaluation with maximum improvement occurring at 1 year of stimulation (P = 0.001). The mean reduction of symptoms was 26.1% at the 3rd month, 39.61% at the 6th month, and 48.6% at the end of 1 year. Disability subscores analysis showed significant improvement in writing at 1 year.
This retrospective analysis confirms previous reports showing that GPi DBS is a safe and effective therapy for medication-refractory primary dystonia.,, These improvements can be measured as early as 3 months following the institution of DBS. The overall benefit accrues gradually after the first 6 months. Delayed and gradual improvement occurs more frequently in cases of generalized dystonia compared with other diseases treated with DBS. It demands that both the patient and the treating team exercise patience while awaiting the full benefit of the therapy.
A recent meta-analysis of patient outcomes revealed an average improvement of symptoms of 60.7% on the assessment of the BFMDRS subscore. It was similar to the improvement of 51% at 1 year and 58% at 3 years reported by Vidailhet et al., Tarsy et al., showed a mean improvement of 50% in the dystonia severity as well as in the patient's disability. Fifty percent reduction in the disability of patients suffering from primary generalized dystonia has been reported. The European Federation of the Neurological Societies (EFNS) guidelines  recommended DBS of the GPi as a good option for primary generalized and segmental dystonia (Level A) and cervical dystonia (Level B). For secondary dystonia, pallidal DBS is less effective (Level C).
The population examined in our study was quite heterogeneous for age, duration of the disease and its severity, the type of topical involvement, and the duration of follow-up. The small sample examined did not permit us to determine any clinical predictive factors that may be utilizable in future candidates for the assessment of the effectiveness of GPi-DBS in them. Long-term follow-up among primary generalized or segmental dystonia by Volkmann et al., showed that the severity of dystonia improved by 47.9% in 40 patients at 6 months, by 61.1% in 38 patients at 3 years, and by 57.8% in 32 patients at 5 years. They did not include secondary dystonia or complicated dystonia plus syndromes in contrast to the composition of our patient cohort. The genetic status for all patients was not systematically assessed in this group. These early interim observations support our results. However, our future long-term follow-up would throw more light on the results of this pilot study.
The variability of therapeutic effects could be related to the genetic heterogeneity of the disease or to the characteristics of abnormal movements. The complexity of the pathophysiological mechanisms underlying the expression of the disease could also account for the variability of response. However, it has been hypothesized that the choice of the target, the location of the electrodes, the location of dystonia, and the DYT1 status influence the therapeutic outcome.
We did not identify factors predictive of the magnitude of improvement; however, the ability to identify such predictors was limited by our small sample size. In our study, motor improvement was more with relation to the axial score. Motor improvement occurred in most segments of the body (neck, trunk, and limbs); the exceptions were facial movement and speech. These different effects of neurostimulation should be considered when one is selecting patients for surgery. In the previous reports, the beneficial effects of DBS on speech intelligibility have always been a subject of controversy and have often not been remarkable enough.,, In some cases, a deterioration of speech has been reported as an adverse effect due to the spread of current to the internal capsule.,, In fact, it is well-known that the benefits obtained following the application of DBS are often delayed and progressive, and this is particularly true for the function of speech.
Results in secondary dystonia have also been encouraging. In a study of 13 consecutive patients that included 9 patients with secondary dystonia, there were global subjective gains and a notable objective improvement; however, these benefits were variable and not completely predictable. Sixteen patients with dystonia secondary to Parkinson's disease who underwent GPi-DBS showed a significant improvement (P = 0.021); however, it is not clear whether or not the rating scale used was clinically validated. In 1 patient (#4) in our study with dystonia secondary to Parkinson's disease, there was good improvement. The evidence for the effectiveness of DBS for encephalitic secondary dystonia is limited. However, Zorzi et al., reported that status dystonicus resolved completely within a week of institution of DBS. A single patient (#5) did not show a long-term sustained good outcome in our study.
The limited evidence  available for the use of DBS to treat pantothenate kinase-associated neurodegeneration (PKAN) reflects the rarity of the disorder. It may not be possible to conduct randomized trials or even comparative studies. In 23 patients suffereing from neurodegeneration with brain iron accumulation (NBIA) who underwent GPi-DBS, at a follow-up of 9–15 months, 66.7% of patients had 20% or more improvement in the severity of dystonia. Case reports have reported improvement following the institution of DBS in dystonia associated with other complex acquired dystonic diseases including PKAN and Wilson's disease.
In our series, one NBIA patient (#3)) showed good and sustained improvement at 1-year follow-up, but another patient (#7) developed lead infection due to which the leads had to be removed. The patient with Wilson's disease (#10) did not show any improvement at the 3rd month and there was no further follow-up. Long-term clinical outcome of PKAN at 60 months was good though statistically not significant. Thus, studies with long-term follow-up are required. Better outcomes are seen in patients with normal brain MRIs, suggesting that benefit from DBS relies on a normal anatomical substrate. The effectiveness of DBS treatment for secondary dystonia varies according to the type of dystonia, and the evidence is limited by the small patient numbers.
The predictive role of the disease duration is of particular interest in the light of current models proposing that the therapeutic effect of DBS is mediated by a gradual brain reorganization or plasticity., Favorable outcome has been reported in patients with a shorter disease duration.,, A longer disease duration and the coexistence of the dystonia with fixed skeletal deformities are associated with less favorable outcome. DBS should be considered before musculoskeletal deformity and complications become fixed. There is no data in children younger than 7 years of age and there is no strict restriction on the upper age limit.
We used GPi as a target in all our patients. In some studies, STN has been reported as a useful target for dystonia., The motor thalamus, particularly the ventral oralis anterior and posterior, has also been used as a target of stimulation ,, for dystonic tremor, myoclonic dystonia, and writer's clamp., Schönecker et al., showed that the degree of clinical improvement declines exponentially as the electrode contact is displaced from the posterolateroventral subregion of the GPi. Research is ongoing to create GPi Atlas More Detailses that may further direct the electrode placement in DBS.
There were no intracerebral hemorrhages or adverse neurologic events in our series. No patient experienced an adverse neurologic event during surgery, which was similar to the findings seen in other studies. Hardware-related complications, including lead fractures, erosions, infections, and implantable pulse generator (IPG) malfunction, have been reported in long-term follow-up studies, with an incidence ranging from 13% to 40%.,, Only 1 patient (#7) developed infection that resulted in the removal of the electrodes. Stimulation-related adverse events were limited to speech abnormalities, similar to that reported by the other groups. The BFMDRS is extensively tested and validated. Rating scales for generalized dystonia, however, are not necessarily applicable to focal or segmental dystonia and may not capture all aspects of the disability, e.g., activities of daily living, the quality of life, and psychiatric or pain issues.
The available evidence for DBS in dystonia should be shared with patients and their families. When expectations of the patient and the family members regarding the outcome are realistic, the likelihood that the patient's expectations will be met by the procedure is increased. It should be stressed to patients and their families that motor benefits exceeding 80% are probably overestimated, and outcome for primary dystonia in the range of 50% improvement in motor symptoms is more realistic.
In medically refractory primary or secondary dystonia patients, bilateral GPi-DBS should considered as a viable option. Patients with disabling symptoms that significantly compromise activities of daily living may be considered for DBS before these symptoms become entrenched and irreversible. Further double-blind, prospective studies, and careful analysis of stimulation targets and postoperative results seem to be mandatory for better selection of patients for GPi-DBS.
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Conflicts of interest
There are no conflicts of interest.
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[Table 1], [Table 2]