Anatomic correlates of deep brain stimulation electrode impedance

Impedance Characteristics of Stimulation Contacts in Deep Brain Stimulation of the Anterior Nucleus of the Thalamus and Its Relationship to Seizure Outcome in Patients With Refractory Epilepsy

BACKGROUND

Deep brain stimulation (DBS) of the anterior nucleus of the thalamus (ANT) is an emerging form of adjunctive therapy in focal refractory epilepsy. Unlike conventional DBS targets, the ANT is both encapsulated by white matter layers and located immediately adjacent to the cerebrospinal fluid (CSF) space. Owing to the location of the ANT, implantation has most commonly been performed using a transventricular trajectory. Previous studies suggest different electrical conductivity between gray matter, white matter, and CSF.

OBJECTIVES

In this study, we asked whether therapeutic impedance values from a fully implanted DBS device could be used to deduce the actual location of the active contact to optimize the stimulation site. Secondly, we tested whether impedance values correlate with patient outcomes.

MATERIALS AND METHODS

A total of 16 patients with ANT-DBS for refractory epilepsy were evaluated in this prospective study. Therapeutic impedance values were recorded on regular outpatient clinic visits. Contact locations were analyzed using delayed contrast-enhanced postoperative computed tomography-3T magnetic resonance imaging short tau inversion recovery fusion images previously shown to demonstrate anatomical details around the ANT.

RESULTS

Transventricularly implanted contacts immediately below the CSF surface showed overall lower and slightly decreasing impedances over time compared with higher and more stable impedances in contacts with deeper parenchymal location. Impedance values in transventricularly implanted contacts in the ANT were significantly lower than those in transventricularly implanted contacts outside the ANT or extraventricularly implanted contacts that were typically at the posterior/inferior/lateral border of the ANT. Increasing contact distance from the CSF surface was associated with a linear increase in therapeutic impedance. We also found that therapeutic impedance values were significantly lower in contacts with favorable therapy response than in nonresponding contacts. Finally, we observed a significant correlation between the left- and right-side averaged impedance and the reduction of the total number of seizures.

CONCLUSIONS

Valuable information can be obtained from the noninvasive measurement of therapeutic impedances. The selection of active contacts to target stimulation to the anterior nucleus may be guided by therapeutic impedance measurements to optimize outcome.

The Safety to Switch from Constant Voltage to Constant Current with a Mixed Internal Pulse Generator in Deep Brain Stimulation

Background

Deep brain stimulation (DBS) is an efficient modality for the treatment of movement disorders. Differing from the constant voltage (CV)-DBS devices, constant current (CC)-DBS devices may allow more precise stimulation of the target brain regions since they are less influenced by impedance. If internal pulse generators (IPGs) of DBS devices are required to be connected with electrodes of different brands, employing proper adapters is necessary. Such connected DBS devices are called mixed or hybrid devices.

Objectives

As there is sparse information about the clinical mixed devices, we studied their safety and efficacy.

Materials and Methods

Clinical scores of 13 patients implanted with mixed DBS devices were determined with the Unified Parkinson's Disease Rating Scale (UPDRS) in Parkinson's disease (PD) (n = 10) and with the Burke-Fahn-Marsden Dystonia Rating Scale (BFMDRS) in dystonia (n = 3). Patient satisfaction was assessed with the Timmerman questionnaire. The Clinical Global Impression Improvement (CGI-I) Scale was also evaluated.

Results

Patients' overall satisfaction was considerably higher with mixed devices. The UPDRS and BFMDRS clinical scores did not significantly differ after switching to a mixed DBS device. Three patients before the DBS switch suffered from side effects under the CV mode. These patients got rid of the side effects in their follow-up with a reduction in pulse width values.

Discussion

Mixed devices working in CC mode are well tolerated with high patient satisfaction.

Conclusion

Besides patient satisfaction, mixed IPGs are also considered safe.

Efficacy and Safety of Electroconvulsive Therapy in Patients With Deep Brain Stimulation: Literature Review, Case Report for Patient With Essential Tremor, and Practical Recommendations

AIM

Deep brain stimulation (DBS) has proven to be an effective therapy of some treatment-resistant psychiatric disorders and movement disorders. Comorbid depressive symptoms are common and difficult to manage. Treatment with electroconvulsive therapy (ECT) may be required. There are few published cases describing the safety and efficacy of ECT for patients with DBS implants, and there are no available guidelines for administration of ECT in patients with DBS and mood disorders. The current study had 3 aims: (i) to conduct a systematic review of case reports on patients with DBS implants who received ECT; (ii) to report the case of a 69-year-old man with a DBS implant for essential tremor, who required ECT; and (iii) to provide practical recommendations for ECT in patients with DBS implants.

METHODS

We conducted a systematic review, in accordance with Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines, of existing case reports on patients with DBS implants administered ECT for psychiatric disorders.

RESULTS

Our search yielded 25 cases of ECT in patients implanted with DBS systems. In addition, we here describe successful ECT management of major depressive disorder in a patient treated by DBS. We also set forth ECT management guidelines based on points of consensus. The 2 most important practical recommendations are to make sure the DBS system is set to 0 V and turned off before ECT, and to avoid sites near the DBS electrodes.

CONCLUSIONS

Electroconvulsive therapy may be an effective and safe treatment for DBS patients with MDD.

Local anatomy, stimulation site, and time alter directional deep brain stimulation impedances

Directional deep brain stimulation (DBS) contacts provide greater spatial flexibility for therapy than traditional ring-shaped electrodes, but little is known about longitudinal changes of impedance and orientation. We measured monopolar and bipolar impedance of DBS contacts in 31 patients who underwent unilateral subthalamic nucleus deep brain stimulation as part of a randomized study (SUNDIAL, NCT03353688). At different follow-up visits, patients were assigned new stimulation configurations and impedance was measured. Additionally, we measured the orientation of the directional lead during surgery, immediately after surgery, and 1 year later. Here we contrast impedances in directional versus ring contacts with respect to local anatomy, active stimulation contact(s), and over time. Directional contacts display larger impedances than ring contacts. Impedances generally increase slightly over the first year of therapy, save for a transient decrease immediately post-surgery under general anesthesia during pulse generator placement. Local impedances decrease at active stimulation sites, and contacts in closest proximity to internal capsule display higher impedances than other anatomic sites. DBS leads rotate slightly in the immediate postoperative period (typically less than the angle of a single contact) but otherwise remain stable over the following year. These data provide useful information for setting clinical stimulation parameters over time.

The Active Electrode in the Living Brain: The Response of the Brain Parenchyma to Chronically Implanted Deep Brain Stimulation Electrodes

BACKGROUND

Deep brain stimulation is an established symptomatic surgical therapy for Parkinson disease, essential tremor, and a number of other movement and neuropsychiatric disorders. The well-established foreign body response around implanted electrodes is marked by gliosis, neuroinflammation, and neurodegeneration. However, how this response changes with the application of chronic stimulation is less well-understood.

OBJECTIVE

To integrate the most recent evidence from basic science, patient, and postmortem studies on the effect of such an "active" electrode on the parenchyma of the living brain.

METHODS

A thorough and in-part systematic literature review identified 49 papers.

RESULTS

Increased electrode-tissue impedance is consistently observed in the weeks following electrode implantation, stabilizing at approximately 3 to 6 mo. Lower impedance values are observed around stimulated implanted electrodes when compared with unstimulated electrodes. A temporary reduction in impedance has also been observed in response to stimulation in nonhuman primates. Postmortem studies from patients confirm the presence of a fibrous sheath, astrocytosis, neuronal loss, and neuroinflammation in the immediate vicinity of the electrode. When comparing stimulated and unstimulated electrodes directly, there is some evidence across animal and patient studies of altered neurodegeneration and neuroinflammation around stimulated electrodes.

CONCLUSION

Establishing how stimulation influences the electrical and histological properties of the surrounding tissue is critical in understanding how these factors contribute to DBS efficacy, and in controlling symptoms and side effects. Understanding these complex issues will aid in the development of future neuromodulation systems that are optimized for the tissue environment and required stimulation protocols.

Deep Brain Stimulation Impedance Decreases Over Time Even When Stimulation Settings Are Held Constant

Objectives: To study whether and to what extent the therapeutic impedance and current change under long-term deep brain stimulation (DBS) with constant stimulation settings, which could inform the role of constant current stimulation.

Methods: Therapy impedance and current measurements were retrospectively collected from patients with Parkinson’s disease (PD) undergoing DBS of the subthalamic nucleus (STN) or essential tremor (ET) undergoing ventral intermediate nucleus (VIM). Baseline and follow-up measurements were obtained for intervals of at least 6 months without changes in stimulation settings. The single longest interval of constant stimulation for each electrode was included. Temporal trends in impedance and current were analyzed as absolute and relative differences and as the rate of change.

Results: Impedance and current data from 79 electrodes (60 in STN, 19 in VIM) in 44 patients (32 with PD, 12 with ET) met inclusion criteria. The duration between baseline and follow-up measurements with constant stimulation settings was 17 months (median, with an interquartile range of 12–26 months) in the mixed group. Therapy impedance decreased by 27 ± 12 Ω/year (mean ± 2 standard errors; p < 0.0001), and therapy current increased at a rate of 0.142 ± 0.063 mA/year (p < 0.0001). Similar results were observed in the STN and VIM subgroups.

Conclusions: Impedance decreases gradually over time, even when stimulation settings are kept constant. The rate of decrease is smaller than previously reported, suggesting that changes in stimulation settings contribute to impedance drift. Stimulation-independent impedance drift is gradual but relevant to constant-current programming.

Intraoperative High Impedance Levels During Placement of Deep Brain Stimulating Electrode

BACKGROUND AND IMPORTANCE

Deep brain stimulation (DBS) is a well-established treatment for medically refractory Parkinson's disease (PD), essential tremor (ET), and dystonia. The field of DBS is expanding and techniques are under investigation for the treatment of several neurological disorders. A critical component of the success of these procedures depends significantly on the reliability and durability of devices implanted. Immediate feedback during surgery often gives the surgeon and patient a sense of confidence of long term success. When impedances are found to be elevated during the implantation of the DBS leads, appropriate trouble shooting measures are critical.

CLINICAL PRESENTATION

We present a 73-yr-old male undergoing awake subthalamic DBS with microelectrode recordings for severe PD. Once the optimal trajectory and depth were ascertained, the permanent DBS electrode was placed. High impedances were recorded. Troubleshooting procedures were performed and were all negative as to the cause of the values. Correct impedance levels of the DBS electrode was confirmed with extracranial testing, but continued high values were found again with intracranial positioning of the electrode. A postoperative computerized tomography (CT) scan confirmed intracranial air surrounding all of the contacts. The patient went on to outpatient programming with excellent clinical results.

CONCLUSION

The presence of pneumocephalus surrounding the DBS lead contacts at the target nucleus may have accounted for the intraoperative impedance findings. When all troubleshooting checks have not identified an explanation for the high impedances, intraoperative imaging may demonstrate pneumocephalus around the lead contacts, which should resolve and impedances return to normal values.

Longitudinal Follow-up of Impedance Drift in Deep Brain Stimulation Cases

Background

Impedance is an integral property of neuromodulation devices that determines the current delivered to brain tissue. Long-term variability in therapeutic impedance following deep brain stimulation (DBS) has not been extensively investigated across different brain targets. The aim was to evaluate DBS impedance drift and variability over an extended postoperative period across common DBS targets.

Methods

Retrospective data from 1,764 electrode leads were included and drawn from 866 DBS patients enrolled in the University of Florida Institutional Review Board-approved INFORM database and analyzed up to 84 months post implantation. An exploratory analysis was conducted to identify trends in impedances using a Mann–Kendall test of trend.

Results

There were 866 patients and 1,764 leads available for analysis. The majority of subjects had Parkinson’s disease (60.7%). The mean age at implantation was 58.7 years old and the mean follow-up time was 36.8 months. There were significant fluctuations in the mean impedance of all electrodes analyzed that largely stabilized by 6 months except for the subthalamic nucleus (STN) target, in which fluctuations persisted throughout the duration of follow-up with a continued downward trend (p < 0.001).

Discussion

The drift in impedance observed primarily within the first 6 months is in keeping with prior studies and is likely due to surgical micro-lesioning effects and brain parenchyma remodeling at the electrode–tissue interface, typically at values approximating 1,000 Ω. The differences in impedance trends over time in the various DBS targets may be due to underlying differences in structure and tissue composition.

Management of Elevated Therapeutic Impedances on Deep Brain Stimulation Leads

CLINICAL VIGNETTE

A 64-year-old male with a history of essential tremor with bilateral thalamic ventralis intermedius deep brain stimulation implants had elevated therapeutic impedance values despite normal lead integrity impedances and good response to stimulation.

CLINICAL DILEMMA

Do elevated therapeutic impedance values indicate a sign of hardware malfunction? What are the guidelines to approach deep brain stimulation hardware malfunction?

CLINICAL SOLUTION

Lead integrity impedance values are a better evaluation of hardware integrity. The discrepancy between therapeutic and lead-integrity impedance values can arise when using low voltage settings.

GAPS IN KNOWLEDGE

There are no established guidelines for the management of possible hardware malfunction in deep brain stimulation. The recommended approach is to distinguish between open and short circuit problems followed by an "inching" evaluation, assessing the structures from the implantable and programmable generator to the intracranial leads. Constant-current devices will deliver a more stable stimulation but the effect of their adoption is still not clear.

EXPERT COMMENTARY

This case emphasizes the need for clinicians to understand fundamental differences in lead integrity and therapeutic impedance while utilizing a methodical approach in treating hardware malfunction. It highlights future avenues of investigation regarding the utility of constant current DBS technology.

Localization of Deep Brain Stimulation Contacts Using Corticospinal/Corticobulbar Tracts Stimulation

Background

Successful deep brain stimulation (DBS) in Parkinson’s disease (PD) requires optimal electrode placement. One technique of intraoperative electrode testing is determination of stimulation thresholds inducing corticospinal/corticobulbar tracts (CSBT) motor contractions.

Objective

This study aims to analyze correlations between DBS electrode distance to CSBT and contraction thresholds, with either visual or electromyography (EMG) detection, to establish an intraoperative tool devoted to ensure safe distance of the electrode to the CSBT.

Methods

Twelve PD patients with subthalamic nucleus DBS participated. Thresholds of muscular contractions were assessed clinically and with EMG, for three different sets of stimulation parameters, all monopolar: 130 Hz high-frequency stimulation (HFS); 2 Hz low-frequency stimulation with either 60 or 210 µs (LFS-60, LFS-210). The anatomical distance of electrode contacts to CSBT was measured from fused CT-MRI.

Results

The best linear correlation was found for thresholds of visually detected contractions with HFS (r² = 0.63, p < 0.0001) when estimated stimulation currents rather than voltages were used. This correlation was found in agreement with an accepted model of electrical spatial extent of activation (r² = 0.50). When using LFS, the correlation found remained lower than for HFS but increased when EMG was used. Indeed, the detection of contraction thresholds with EMG versus visual inspection did allow more frequent detection of face contractions, contributing to improve that correlation.

Conclusion

The correlation between electrode distance to the CSBT and contraction thresholds was found better when estimated with currents rather than voltage, eliminating the variance due to electrode impedance. Using LFS did not improve the precision of that evaluation, but EMG did. This technique provides a prediction band to ensure minimum distance of the electrode contacts to the CSBT, integrating the variance that can be encountered between prediction of models and practice.

In-vivo measurements of human brain tissue conductivity using focal electrical current injection through intracerebral multicontact electrodes

In-vivo measurements of human brain tissue conductivity at body temperature were conducted using focal electrical currents injected through intracerebral multicontact electrodes. A total of 1,421 measurements in 15 epileptic patients (age: 28 ± 10) using a radiofrequency generator (50 kHz current injection) were analyzed. Each contact pair was classified as being from healthy (gray matter, n = 696; white matter, n = 530) or pathological (epileptogenic zone, n = 195) tissue using neuroimaging analysis of the local tissue environment and intracerebral EEG recordings. Brain tissue conductivities were obtained using numerical simulations based on conductivity estimates that accounted for the current flow in the local brain volume around the contact pairs (a cube with a side length of 13 mm). Conductivity values were 0.26 S/m for gray matter and 0.17 S/m for white matter. Healthy gray and white matter had statistically different median impedances (P < 0.0001). White matter conductivity was found to be homogeneous as normality tests did not find evidence of multiple subgroups. Gray matter had lower conductivity in healthy tissue than in the epileptogenic zone (0.26 vs. 0.29 S/m; P = 0.012), even when the epileptogenic zone was not visible in the magnetic resonance image (MRI) (P = 0.005). The present in-vivo conductivity values could serve to create more accurate volume conduction models and could help to refine the identification of relevant intracerebral contacts, especially when located within the epileptogenic zone of an MRI-invisible lesion. Hum Brain Mapp 38:974-986, 2017. © 2016 Wiley Periodicals, Inc.

Three-dimensional SPACE fluid-attenuated inversion recovery at 3 T to improve subthalamic nucleus lead placement for deep brain stimulation in Parkinson's disease: from preclinical to clinical studies

OBJECTIVE Deep brain stimulation (DBS) of the subthalamic nucleus (STN) is a well-established therapy for motor symptoms in patients with pharmacoresistant Parkinson's disease (PD). However, the procedure, which requires multimodal perioperative exploration such as imaging, electrophysiology, or clinical examination during macrostimulation to secure lead positioning, remains challenging because the STN cannot be reliably visualized using the gold standard, T2-weighted imaging (T2WI) at 1.5 T. Thus, there is a need to improve imaging tools to better visualize the STN, optimize DBS lead implantation, and enlarge DBS diffusion. METHODS Gradient-echo sequences such as those used in T2WI suffer from higher distortions at higher magnetic fields than spin-echo sequences. First, a spin-echo 3D SPACE (sampling perfection with application-optimized contrasts using different flip angle evolutions) FLAIR sequence at 3 T was designed, validated histologically in 2 nonhuman primates, and applied to 10 patients with PD; their data were clinically compared in a double-blind manner with those of a control group of 10 other patients with PD in whom STN targeting was performed using T2WI. RESULTS Overlap between the nonhuman primate STNs segmented on 3D-histological and on 3D-SPACE-FLAIR volumes was high for the 3 most anterior quarters (mean [± SD] Dice scores 0.73 ± 0.11, 0.74 ± 0.06, and 0.60 ± 0.09). STN limits determined by the 3D-SPACE-FLAIR sequence were more consistent with electrophysiological edges than those determined by T2WI (0.9 vs 1.4 mm, respectively). The imaging contrast of the STN on the 3D-SPACE-FLAIR sequence was 4 times higher (p < 0.05). Improvement in the Unified Parkinson's Disease Rating Scale Part III score (off medication, on stimulation) 12 months after the operation was higher for patients who underwent 3D-SPACE-FLAIR-guided implantation than for those in whom T2WI was used (62.2% vs 43.6%, respectively; p < 0.05). The total electrical energy delivered decreased by 36.3% with the 3D-SPACE-FLAIR sequence (p < 0.05). CONCLUSIONS 3D-SPACE-FLAIR sequences at 3 T improved STN lead placement under stereotactic conditions, improved the clinical outcome of patients with PD, and increased the benefit/risk ratio of STN-DBS surgery.

DBSproc: An open source process for DBS electrode localization and tractographic analysis

Deep brain stimulation (DBS) is an effective surgical treatment for movement disorders. Although stimulation sites for movement disorders such as Parkinson's disease are established, the therapeutic mechanisms of DBS remain controversial. Recent research suggests that specific white-matter tract and circuit activation mediates symptom relief. To investigate these questions, we have developed a patient-specific open-source software pipeline called 'DBSproc' for (1) localizing DBS electrodes and contacts from postoperative CT images, (2) processing structural and diffusion MRI data, (3) registering all images to a common space, (4) estimating DBS activation volume from patient-specific voltage and impedance, and (5) understanding the DBS contact-brain connectivity through probabilistic tractography. In this paper, we explain our methodology and provide validation with anatomical and tractographic data. This method can be used to help investigate mechanisms of action of DBS, inform surgical and clinical assessments, and define new therapeutic targets.