Reversing the polarity of bipolar stimulation in deep brain stimulation for essential tremor: a theoretical explanation for a useful clinical intervention

Tremor habituation to deep brain stimulation: Underlying mechanisms and solutions

DBS of the ventral intermediate nucleus is an extremely effective treatment for essential tremor, although a waning benefit is observed after a variable time in a variable proportion of patients (ranging from 0% to 73%), a concept historically defined as "tolerance." Tolerance is currently an established concept in the medical community, although there is debate on its real existence. In fact, very few publications have actually addressed the problem, thus making tolerance a typical example of science based on "eminence rather than evidence." The underpinnings of the phenomena associated with the progressive loss of DBS benefit are not fully elucidated, although the interplay of different-not mutually exclusive-factors has been advocated. In this viewpoint, we gathered the evidence explaining the progressive loss of benefit observed after DBS. We grouped these factors in three categories: disease-related factors (tremor etiology and progression); surgery-related factors (electrode location, microlesional effect and placebo); and stimulation-related factors (not optimized stimulation, stimulation-induced side effects, habituation, and tremor rebound). We also propose possible pathophysiological explanations for the phenomenon and define a nomenclature of the associated features: early versus late DBS failure; tremor rebound versus habituation (to be preferred over tolerance). Finally, we provide a practical approach for preventing and treating this loss of DBS benefit, and we draft a possible roadmap for the research to come. © 2019 International Parkinson and Movement Disorder Society.

Therapeutic Window of Deep Brain Stimulation Using Cathodic Monopolar, Bipolar, Semi-Bipolar, and Anodic Stimulation

OBJECTIVES

To compare the therapeutic window (TW) of cathodic monopolar, bipolar, anodic monopolar, and a novel "semi-bipolar" stimulation in ten Parkinson's disease patients who underwent deep brain stimulation of the subthalamic nucleus.

MATERIALS AND METHODS

Patients were assessed in the "OFF" L-dopa condition. Each upper limb was tested separately for therapeutic threshold, TW and side-effect threshold (SET). Battery consumption index (BCI) also was documented.

RESULTS

Compared to cathodic stimulation, therapeutic threshold was significantly higher for anodic, bipolar, and semi-bipolar stimulation (3.8 ± 1.6 vs. 4.9 ± 2.1, 5.0 ± 1.9, and 5.2 ± 1.9 mA, p = 0.0006, 0.0002, and 0.008, respectively). SET was significantly higher for bipolar stimulation (10.9 ± 2.5 mA) vs. cathodic (6.8 ± 2.2 mA, p < 0.0001) and anodic stimulation (9.2 ± 2.6 mA, p = 0.005). The SET of anodic and semi-bipolar stimulation was significantly higher vs. cathodic stimulation (p < 0.0001). TW of cathodic stimulation (2.5 ± 1.5 mA) was significantly narrower vs. bipolar (5.4 ± 2.0 mA, p < 0.0001), semi-bipolar (4.6 ± 2.6 mA, p = 0.001) and anodic stimulation (4.3 ± 2.3 mA, p < 0.0001). Bipolar (p = 0.005) and semi-bipolar (p = 0.0005) stimulation had a significantly wider TW vs. anodic stimulation. BCI of cathodic stimulation (5.9 ± 1.3) was significantly lower compared to bipolar (13.7 ± 6.8, p < 0.0001), semi-bipolar (11.0 ± 4.3, p = 0.0005), and anodic stimulation (8.1 ± 3.0, p < 0.0001). Anodic BCI was significantly lower than bipolar (p = 0.005) and semi-bipolar (p = 0.0002) stimulation while semi-bipolar BCI was lower than bipolar stimulation (p = 0.0005).

CONCLUSIONS

While awaiting further studies, our findings suggest that cathodic stimulation should be preferred in light of its reduced battery consumption, possibly followed by semi-bipolar in case of stimulation-induced side-effects.

Mapping the current flow in sacral nerve stimulation using computational modelling

Sacral nerve stimulation (SNS) is an established treatment for faecal incontinence involving the implantation of a quadripolar electrode into a sacral foramen, through which an electrical stimulus is applied. Little is known about the induced spread of electric current around the SNS electrode and its effect on adjacent tissues, which limits optimisation of this treatment. The authors constructed a 3-dimensional imaging based finite element model in order to calculate and visualise the stimulation induced current and coupled this to biophysical models of nerve fibres. They investigated the impact of tissue inhomogeneity, electrode model choice and contact configuration and found a number of effects. (i) The presence of anatomical detail changes the estimate of stimulation effects in size and shape. (ii) The difference between the two models of electrodes is minimal for electrode contacts of the same length. (iii) Surprisingly, in this arrangement of electrode and neural fibre, monopolar and bipolar stimulation induce a similar effect. (iv) Interestingly when the active contact is larger, the volume of tissue activated reduces. This work establishes a protocol to better understand both therapeutic and adverse stimulation effects and in the future will enable patient-specific adjustments of stimulation parameters.

A Network Model of Local Field Potential Activity in Essential Tremor and the Impact of Deep Brain Stimulation

Essential tremor (ET), a movement disorder characterised by an uncontrollable shaking of the affected body part, is often professed to be the most common movement disorder, affecting up to one percent of adults over 40 years of age. The precise cause of ET is unknown, however pathological oscillations of a network of a number of brain regions are implicated in leading to the disorder. Deep brain stimulation (DBS) is a clinical therapy used to alleviate the symptoms of a number of movement disorders. DBS involves the surgical implantation of electrodes into specific nuclei in the brain. For ET the targeted region is the ventralis intermedius (Vim) nucleus of the thalamus. Though DBS is effective for treating ET, the mechanism through which the therapeutic effect is obtained is not understood. To elucidate the mechanism underlying the pathological network activity and the effect of DBS on such activity, we take a computational modelling approach combined with electrophysiological data. The pathological brain activity was recorded intra-operatively via implanted DBS electrodes, whilst simultaneously recording muscle activity of the affected limbs. We modelled the network hypothesised to underlie ET using the Wilson-Cowan approach. The modelled network exhibited oscillatory behaviour within the tremor frequency range, as did our electrophysiological data. By applying a DBS-like input we suppressed these oscillations. This study shows that the dynamics of the ET network support oscillations at the tremor frequency and the application of a DBS-like input disrupts this activity, which could be one mechanism underlying the therapeutic benefit.

Essential tremor (ET) is acknowledged to be the most common movement disorder affecting 1% of the population. Although the underlying mechanisms remain elusive, the thalamus, cortex and cerebellum are implicated in the underlying pathology. More recently, it has been shown that ET can be successfully treated by deep brain stimulation (DBS). This clinical treatment involves the surgical implantation of electrodes into the brain, through which current is applied. However, the mechanisms of how DBS achieves clinical benefit continue to be debated. A key question is whether ET can be modeled as a pathological network behavior as has been suggested previously. If so, we can then ask how DBS would modulate this brain activity. Our study combines: (i) simultaneous electrophysiological recordings from the brain and muscle; (ii) computational modelling; (iii) mathematical analysis. We found that the network supports oscillations in the tremor range, and the application of high frequency DBS switches this to low amplitude, high-frequency activity. We propose that our model can be used to predict DBS parameter settings that suppress pathological network activity and consequently tremor. In summary, we provide the first population level model of essential tremor including the effect of DBS on network behaviour.

Measurement of evoked potentials during thalamic deep brain stimulation

BACKGROUND

Deep brain stimulation (DBS) treats the symptoms of several movement disorders, but optimal selection of stimulation parameters remains a challenge. The evoked compound action potential (ECAP) reflects synchronized neural activation near the DBS lead, and may be useful for feedback control and automatic adjustment of stimulation parameters in closed-loop DBS systems.

OBJECTIVES

Determine the feasibility of recording ECAPs in the clinical setting, understand the neural origin of the ECAP and sources of any stimulus artifact, and correlate ECAP characteristics with motor symptoms.

METHODS

The ECAP and tremor response were measured simultaneously during intraoperative studies of thalamic DBS, conducted in patients who were either undergoing surgery for initial lead implantation or replacement of their internal pulse generator.

RESULTS

There was large subject-to-subject variation in stimulus artifact amplitude, which model-based analysis suggested may have been caused by glial encapsulation of the lead, resulting in imbalances in the tissue impedance between the contacts. ECAP recordings obtained from both acute and chronically implanted electrodes revealed that specific phase characteristics of the signal varied systematically with stimulation parameters. Further, a trend was observed in some patients between the energy of the initial negative and positive ECAP phases, as well as secondary phases, and changes in tremor from baseline. A computational model of thalamic DBS indicated that direct cerebellothalamic fiber activation dominated the clinically measured ECAP, suggesting that excitation of these fibers is critical in DBS therapy.

CONCLUSIONS

This work demonstrated that ECAPs can be recorded in the clinical setting and may provide a surrogate feedback control signal for automatic adjustment of stimulation parameters to reduce tremor amplitude.

[The use of neuromodulation for the treatment of tremor]

BACKGROUND

Tremor may be a disabling disorder and pharmacologic treatment is the first-line therapy for these patients. Nevertheless, this treatment may lead to a satisfactory tremor reduction in only 50% of patients with essential tremor. Thalamotomy was the treatment of choice for tremor refractory to medical therapy until deep brain stimulation (DBS) of the ventral intermedius nucleus (Vim) of the thalamus has started being used. Nowadays, thalamotomy is rarely performed.

METHODS

This article is a non-systematic review of the indications, results, programming parameters and surgical technique of DBS of the Vim for the treatment of tremor.

RESULTS

In spite of the fact that it is possible to achieve similar clinical results using thalamotomy or DBS of the Vim, the former causes more adverse effects than the latter. Furthermore, DBS can be used bilaterally, whereas thalamotomy has a high risk of causing disartria when it is performed in both sides. DBS of the Vim achieved an adequate tremor improvement in several series of patients with tremor caused by essential tremor, Parkinson's disease or multiple sclerosis. Besides the Vim, there are other targets, which are being used by some authors, such as the zona incerta and the prelemniscal radiations.

CONCLUSION

DBS of the Vim is a useful treatment for disabling tremor refractory to medical therapy. It is essential to carry out an accurate patient selection as well as to use a proper surgical technique. The best stereotactic target for tremor is still unknown, although the Vim is the most used one.

Neural origin of evoked potentials during thalamic deep brain stimulation

Closed-loop deep brain stimulation (DBS) systems could provide automatic adjustment of stimulation parameters and improve outcomes in the treatment of Parkinson's disease and essential tremor. The evoked compound action potential (ECAP), generated by activated neurons near the DBS electrode, may provide a suitable feedback control signal for closed-loop DBS. The objectives of this work were to characterize the ECAP across stimulation parameters and determine the neural elements contributing to the signal. We recorded ECAPs during thalamic DBS in anesthetized cats and conducted computer simulations to calculate the ECAP of a population of thalamic neurons. The experimental and computational ECAPs were similar in shape and had characteristics that were correlated across stimulation parameters (R(2) = 0.80-0.95, P < 0.002). The ECAP signal energy increased with larger DBS amplitudes (P < 0.0001) and pulse widths (P < 0.002), and the signal energy of secondary ECAP phases was larger at 10-Hz than at 100-Hz DBS (P < 0.002). The computational model indicated that these changes resulted from a greater extent of neural activation and an increased synchronization of postsynaptic thalamocortical activity, respectively. Administration of tetrodotoxin, lidocaine, or isoflurane abolished or reduced the magnitude of the experimental and computational ECAPs, glutamate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and D(-)-2-amino-5-phosphonopentanoic acid (APV) reduced secondary ECAP phases by decreasing postsynaptic excitation, and the GABAA receptor agonist muscimol increased the latency of the secondary phases by augmenting postsynaptic hyperpolarization. This study demonstrates that the ECAP provides information about the type and extent of neural activation generated during DBS, and the ECAP may serve as a feedback control signal for closed-loop DBS.