H-coil: Induced electric field properties and input/output curves on healthy volunteers, comparison with a standard figure-of-eight coil

Objective: To acquire information about the physical properties and physiological effects of the H-coil. Methods: We used a robotized system to measure the electric field (E-field) generated by a H-coil prototype and compared it with a standard figure-of-eight coil. To explore the physiological properties of the coils, input/output curves were recorded for the right abductor digiti minimi muscle (ADM) as target muscle. To explore focality of stimulation, simultaneous recordings were performed for the left ADM, right abductor pollicis brevis (APB), extensor digitorum communis (EDC) and biceps brachii (BB) muscles. Results: Physical measurements of the H-coil showed four potentially stimulating foci, generating different electric field intensities along two different spatial orientations. RMT was significantly lower for H-coil- as compared to figure-of-eight coil stimulation. When stimulation intensity for the input-output curve was determined by percent of maximum stimulator output, the H-coil produced larger MEPs in the right ADM, as compared to the figure-of-eight coil, due to the larger relative enhancement of stimulation intensity of the H-coil. When stimulation intensity was adjusted to RMT, MEPs elicited at the right ADM were larger for figure-of-eight coil than for H-coil stimulation, while this relation was reversed for distant non-target muscles, with low stimulation intensities. With high stimulation intensities, the H-coil elicited larger MEPs for all tested muscles. Onset latency of the MEPs was never shorter for H-coil than for figure-of-eight coil stimulation of the target muscles. Conclusions: These results are in favor for a non-focal, but not deeper effect of the H-coil, as compared to a figure-of-eight coil. Significance: This is the first neurophysiological study exploring the focality and depth of stimulation delivered by the H-coil systematically in humans. We found no advantage of this coil with regard to depth of stimulation in comparison to the figure-of-eight coil. Future studies have to show if the non-focality of this coil differs relevantly from that of other non-focal coils, e.g. the round coil.