Current steering to control the volume of tissue activated during deep brain stimulation

Abstract

Background: Over the last two decades, deep brain stimulation (DBS) has become a recognized and effective clinical therapy for numerous neurological conditions. Since its inception, clinical DBS technology has progressed at a relatively slow rate; however, advances in neural engineering research have the potential to improve DBS systems. One such advance is the concept of current steering, or the use of multiple stimulation sources to direct current flow through targeted regions of brain tissue. The goals of this study were to develop a theoretical understanding of the effects of current steering in the context of DBS, and use that information to evaluate the potential utility of current steering during stimulation of the subthalamic nucleus.

Methods: We used finite element electric field models, coupled to multi-compartment cable axon models, to predict the volume of tissue activated (VTA) by DBS as a function of the stimulation parameter settings.

Results: Balancing current flow through adjacent cathodes increased the VTA magnitude, relative to monopolar stimulation, and current steering enabled us to sculpt the shape of the VTA to fit a given anatomical target.

Conclusions: These results provide motivation for the integration of current steering technology into clinical DBS systems, thereby expanding opportunities to customize DBS to individual patients, and potentially enhancing therapeutic efficacy.

Keywords: electric field; electrode; model; neuromodulation; neurostimulation; subthalamic nucleus.

Figure 1

DBS model. The subthalamic nucleus (green volume) represents a common anatomical target for the DBS lead (grey shaft, pink electrode contacts) shown passing through the thalamus (yellow volume). The stimulation protocol specified the waveform and the amount of current injected through each contact. The voltage distribution in the tissue medium generated during current-controlled stimulation was determined from a finite element model with the total current amplitude divided between adjacent electrode contacts.

Figure 2

Axonal activation prediction. A) Axons were distributed in a grid around the DBS electrode (black dots). The voltage within the tissue medium was determined relative to the active DBS contact(s) and the threshold for action potential generation in each axon was defined. B, C) The second spatial difference of the extracellular voltage at threshold was calculated for each axon as a function of distance from the electrode for a range of current amplitudes and percentage mixtures between adjacent contacts. A predictor curve was generated by aggregating the data and fitting it to an exponential function.

Figure 3

Current steering VTA. Contour plot of the volume of tissue activated as a function of stimulus amplitude and the percentage of current delivery through each electrode contact.

Figure 4

Current steering in the STN. A diffusion-tensor based 3D FEM of DBS was used to predict the VTA (red volume) from various percentage mixtures of current delivery through two DBS contacts in the STN. The stimulation parameters were held constant with 0.1 ms pulses delivered at 130 Hz and at a total current of 2 mA. VTAs are shown as a function of different current mixtures from 100%/0% (monopolar on contact 2) to 50%/50% (through contacts 1 and 2) to 0%/100% (monopolar on contact 1).

Figure 5

STN constrained stimulation. A) Monopolar stimulation through contact 2 was increased from 0 to −0.7 mA before the VTA spread beyond the borders of the STN. Note that the VTA first spreads beyond the STN on the medial side (not visible from this view). B) Monopolar stimulation through contact 1 could be increased to −1.6 mA before spreading beyond the borders of the STN. C) Controlled mixing of −1.8 mA through contact 1 (80%) and contact 2 (20%) stimulated 63 mm³ of the STN without spreading into neighboring structures.

Figure 6

Effects of electrode contact spacing on voltage distribution. The effects of contact spacing are shown by comparing current injection through one contact (leftmost panel) to injection through two contacts. The current injected is indicated on each electrode contact, and the resulting distribution is represented by isovoltage contours according to the colorbar at right. The voltage distributions from adjacent active contacts show substantial overlap (2nd panel from left), while the presence of one or more intervening dormant contacts results in separate distributions that approach the single contact configuration (rightmost two panels).