Introduction: Directional deep brain stimulation (DBS) enables targeted stimulation of desired brain areas. However, the impact of stimulation amplitude on volume of tissue activated (VTA) for directional and nondirectional (conventional) stimulation is unclear. A computational model was used to evaluate the effect of stimulation parameters on VTA in the subthalamic nucleus (STN).

Methods: The first stage of the model used finite element analysis (FEA) to calculate electrical potentials generated in the brain with 90µs cathodic DBS pulses. The FEA model incorporated an imaging-based anatomical model of the human head, and the directional DBS lead (1-3-3-1 configuration) was placed in the STN and surrounded by a 0.5mm thick encapsulation layer. The second stage used biophysical cellular models of 5.7µm diameter myelinated axons. A total of 6,888 axons were distributed around contact 2, with the axons oriented perpendicular to the lead axis. FEA electrical potentials were interpolated along each axon, and delivered as extracellular stimulation to determine the VTA for one-contact directional (segment 2B), two-contact directional (2A-B), and non-directional (2A-C) DBS. Finally, the extent of directionality was calculated as the percentage of VTA volume on the side of the lead with segment 2B.

Results: VTA volume increased non-linearly with stimulation amplitude, and was typically largest for one-contact directional stimulation and smallest for non-directional stimulation (Figure 1). Moreover, directional stimulation but not non-directional stimulation allowed for spatial steering of the VTA (Figure 2), with the extent of directionality being higher for one-contact directional DBS (0.5mA: 94.5%, 3mA: 80.4%) than two-contact directional DBS (range: 57.7-59.8%) and non-directional DBS (49.8-51.7%).

Conclusions: A model of STN-DBS demonstrated that VTA volume varies non-linearly with stimulation amplitude, and that the VTA is larger with directional stimulation. In addition, directional DBS allows for spatial steering of the VTA, and the extent of directionality increased with lower stimulation amplitudes.

Patient Care: This research improves our understanding of how directional deep brain stimulation leads can be programmed to target stimulation towards desired brain areas, and thereby improve the therapeutic benefit for patients with movement disorders.

Learning Objectives: By the conclusion of this session, participants should be able to: 1) Describe how directional DBS generates a targeted stimulation field, 2) Discuss how changes in stimulation parameters impact the size and shape of the VTA, 3) Understand the basis of computational modeling for investigating DBS.

References: McIntyre CC, Richardson AG, Grill WM. Modeling the excitability of mammalian nerve fibers: influence of afterpotentials on the recovery cycle. J Neurophysiol 2002 Feb;87(2): 995–1006. Butson CC and McIntyre CC. Current steering to control the volume of tissue activated during deep brain stimulation. Brain Stimul 2008 Jan;1(1): 7-15.