Introduction: Bilateral vestibular hypofunction causes balance deficits, increases falls risk and decreases quality of life . Recent work has demonstrated that vestibulo-ocular reflexes  and postural responses  can be elicited by electrical stimulation via vestibular implants.
Research question: This pilot study aimed to determine if electrical stimulation of the vestibular nerve would result in an immediate change in stride length and time.
Methods: Two female adults (71 and 51 years; P1 and P2 respectively) with bilateral vestibular hypofunction equipped with a vestibular implant  participated. The implants electrically stimulate the posterior ampullary branch of the vestibular nerve. Participants were tested under three conditions: after restoration of a constant baseline stimulation ; during baseline stimulation modulated by a gyroscope attached to the head, eliciting eye movements in the opposite direction to head movements (positive modulation); and with the gyroscope modulating the baseline stimulation eliciting eye movements in the direction of head movement (negative modulation). Experiments were conducted with the CAREN Extended system (Motekforce Link, Amsterdam, The Netherlands) using a reduced kinematic model (6 markers). The display was fixed (no optical flow) and an object was displayed for visual fixation. The participants firstly completed multiple three-minute familiarisation treadmill walking trials with no stimulation, starting at 0.3 m/s, followed by higher speeds based on ability. Three no-stimulation measurements of three minutes each at self-selected walking speeds (P1: 0.5 m/s; P2: 0.7 m/s) were then conducted. Participants then completed multiple three-minute walking bouts under baseline stimulation and stimulation modulated by positive or negative gain. The final 60 s of the last no stimulation trial and the first of each stimulation conditions were analysed. Results are expressed as mean SD.
Results: Participants showed an increase in stride length under vestibular stimulation, especially in the positive gain condition, compared to no stimulation (P1: 0.54 0.03 m, 0.56 0.03 m, 0.58 0.03 m and 0.57 0.03 m; P2: 0.8 0.04 m, 0.81 0.05 m, 0.91 0.08 m and 0.88 0.04 m for no-stimulation and baseline, positive and negative gain stimulation respectively). Stride time increased in a similar manner to stride length for both patients, with positive gain showing the greatest difference to no stimulation (P1: 1.11 0.03 s, 1.15 0.03 s, 1.18 0.04 s and 1.16 0.04 s; P2: 1.17 0.06 s, 1.17 0.06 s, 1.34 0.11 s and 1.28 0.06 s).
Discussion: These data suggest that vestibular nerve stimulation via an artificial vestibular implant can acutely alter stride length and time in these patients. Further research is needed to generalise the findings to other patients and to investigate if stability improves when using the vestibular implants.
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