In silico modeling of electric field modulation by transcranial direct current stimulation in stroke patients with skull burr holes: Implications for safe clinical application

Background: Transcranial direct current stimulation (tDCS) has emerged as a promising tool for stroke rehabilitation, supported by evidence demonstrating its beneficial effects on post-stroke recovery. However, patients with skull defects, such as burr holes, have been excluded from tDCS due to limited knowledge regarding the effect of skull defects on the electric field. Objective: We investigated the effect of burr holes on the electric field induced by tDCS and identified the electrode location that modulates the electric field. Methods: We generated mesh models of the heads of five patients with burr holes and five age-matched control patients who had never undergone brain surgery, based on magnetic resonance imaging. Then we conducted tDCS simulations, with the cathode fixed in one position and the anode in various positions. Regression analysis was employed to investigate the relationship between the electric field at the burr hole and the distance from the burr hole to the anode. Results: In patients with burr holes, the electric field intensity increased as the anode approached the burr hole, reaching a maximum electric field when the anode covered it, with this pattern remaining consistent across all patient models. Assuming the holes were filled with cerebrospinal fluid, the maximum electric field was 1.20 ± 0.20 V/m (mean ± standard deviation, SD). When the anode was positioned more than 60 mm away from the burr hole, the electric field at the burr hole remained low and constant, with an average value of 0.29 ± 0.04V/m (mean ± SD). In contrast, for all patients without burr holes, the electric field intensity stayed constant regardless of the anode's position, with a maximum amplitude of 0.36 ± 0.04 V/m (mean ± SD). Furthermore, when the burr hole was assumed to be filled with scar tissue, the mean peak electric field was 0.93 ± 0.16 V/m, indicating that the electric field strength varies depending on the conductivity of the tissue filling the burr hole. Conclusion: Based on the simulations, the minimum recommended distance from the burr hole to the anode is 60 mm to prevent unintended stimulation of the brain cortex during tDCS. These findings will contribute to the development of safe and effective tDCS treatments for patients with burr holes.