Cervical myogenic potentials and controlled postural responses elicited by a prototype vestibular implant

Abstract

Gaze stabilization and postural control are two key functions of the vestibular system. In consequence, oscillopsia and chronic imbalance are the two main complaints of patients presenting with a severe bilateral vestibular function loss. The vestibular implant is emerging as a promising treatment for this group of patients whose quality of life is significantly impaired. Although the final aim of the vestibular implant should be to restore vestibular function as a whole, until now the research has focused mainly on the restoration of the vestibulo-ocular reflex to improve gaze stabilization. In this study, we aimed to explore whether the vestibulo-collic and vestibulo-spinal pathways could be activated and controlled with the electrical stimuli provided by our vestibular implant prototype. This was first explored and demonstrated with recordings of electrically elicited cervical vestibular evoked myogenic potentials (ecVEMPs). ecVEMPs with characteristics similar to the classical acoustically elicited cervical vestibular evoked myogenic potentials (cVEMPs) were successfully evoked in five out of the eight tested patients. Amplitudes of the electrically elicited N-P complex varied, ranging from 44 to 120 µV. Mean latencies of the N and P waves were of 9.71(± 1.17) ms and 17.24 ms (± 1.74), respectively. We also evaluated the possibility of generating controlled postural responses using a stepping test. Here, we showed that controlled and consistent whole-body postural responses can be effectively obtained with rapid changes in the "baseline" (constant rate and amplitude) electrical activity delivered by the vestibular implant in two out of the three tested subjects. Furthermore, obtained amplitude of body rotations was significantly correlated with the intensity of stimulation and direction of body rotations correlated with the side of the delivered stimulus (implanted side). Altogether, these data suggest that the vestibular implant could also be used to improve postural control in patients with bilateral vestibulopathy.

Keywords: Bilateral vestibulopathy; Postural control; Stepping test; Unterberger/Fukuda test; Vestibular implant; Vestibulo-spinal reflex; cVEMPs.

Fig. 1

Illustration of the timeline for the experimental protocol of the stepping test. Experimental trials were first performed without any electrical stimulation (system OFF; yellow rectangle) and then in five different conditions of electrical stimulation (system ON; orange rectangle). Note that the system ON conditions included a baseline only condition, two “inhibitory” conditions, and two “excitatory” conditions

Fig. 2

cVEMPs waveforms recorded upon electrical stimulation. Successful recordings were obtained in five patients (9 out of 17 tested electrodes). Each line in the graph corresponds to recordings obtained in one patient, with one vestibular electrode, and one stimulus configuration (phase width and pulse amplitude). The gray dotted lines in the graphs mark the normal latencies of the “classical” acoustical VEMP signals. N.S. stands for no stimulation. Column a presents recordings obtained at increasing stimulation currents, and fixed pulse phase (200 µs). Column b presents recordings obtained with increasing phase width (i.e., duration), and fixed current amplitude. The detected N and P peaks for each waveform are marked with diamonds and triangles, respectively. Column c presents the resulting amplitude/charge relationships (e.g., growth function), both for fixed phase width trials (blue plot) and fixed current amplitude trials (red plot)

Fig. 3

Body rotations versus stimulation current amplitude measured for three implanted subjects (left side). For S1 and S2, the PAN electrode was active, for S4, the SAN electrode. Trunk (blue plots, left column) and head (red plots, right column) were measured during 20 s trials of the stepping test. Three trials were performed per experimental condition (blinded to the subject and the experimenters). “Inhibitory” conditions correspond to stimulation current amplitudes below “baseline” (350 µA, purple dotted lines). “Excitatory” conditions correspond to stimulation current amplitudes above “baseline”. Mean results in the system OFF conditions (without any electrical stimulation) are presented as solid gray lines (± SD, gray dotted lines)