Development of a multichannel vestibular prosthesis prototype by modification of a commercially available cochlear implant

Abstract

No adequate treatment exists for individuals who remain disabled by bilateral loss of vestibular (inner ear inertial) sensation despite rehabilitation. We have restored vestibular reflexes using lab-built multichannel vestibular prostheses (MVPs) in animals, but translation to clinical practice may be best accomplished by modification of a commercially available cochlear implant (CI). In this interim report, we describe preliminary efforts toward that goal. We developed software and circuitry to sense head rotation and drive a CI's implanted stimulator (IS) to deliver up to 1 K pulses/s via nine electrodes implanted near vestibular nerve branches. Studies in two rhesus monkeys using the modified CI revealed in vivo performance similar to our existing dedicated MVPs. A key focus of our study was the head-worn unit (HWU), which magnetically couples across the scalp to the IS. The HWU must remain securely fixed to the skull to faithfully sense head motion and maintain continuous stimulation. We measured normal and shear force thresholds at which HWU-IS decoupling occurred as a function of scalp thickness and calculated pressure exerted on the scalp. The HWU remained attached for human scalp thicknesses from 3-7.8 mm for forces experienced during routine daily activities, while pressure on the scalp remained below capillary perfusion pressure.

Fig. 1

Modified cochlear implant circuit block diagram. Our MCI prototype consists of (1) the belt-worn unit (BWU), (2) the head-worn unit (HWU) and (3) the implanted stimulator (IS). The BWU samples sensors on the HWU every 5ms and calculates instantaneous rate of stimulation. Pulse commands are sent to the HWU and relayed via radio frequency to the IS, which delivers frequency-modulated biphasic charge-balanced pulses to the vestibular nerve. Power is provided by a +3.3V supply, drawn from a 3.6V Li-ion battery pack housed in the BWU.

Fig. 2

(A) The implanted stimulator (IS), which delivers electrical pulses to the vestibular nerve, is shown with supplementary fixation magnets. Its location and position on skull should approximately align gyroscope axes on the head-worn unit (HWU) with each semicircular canal. (B) Top of HWU (without lid) containing circuitry for 3D head motion measurement and pulse command relay to the IS. (C) Bottom of HWU, curved at 7.8cm radius to adapt to the curvature of the average human skull. (D) and (E) show the sides of the BWU (without lids) where batteries and circuitry are held, respectively.

Fig. 3

Setup used to test performance of the internal system. A computer sinusoidally modulated virtual head rotations and calculated timing of pulse commands, which were sent to the implanted stimulator via the research interface box's (RIBII) inductive link. Frequency-modulated biphasic charge-balanced pulses were delivered to each of the monkey's semicircular canals.

Fig. 4

Time plots of real-time pulse-command rate modulation by the modified cochlear implant's external system in response to on-axis sinusoidal yaw, pitch, and roll rotations, at 1Hz with a peak velocity of ±50°/s. Recordings were taken on three channels concurrently.

Fig. 5

The head-worn unit (HWU) was attached to the IS using various magnet combinations across different tissue thicknesses and pulled in the (A) anterior, (B) inferior, and (C) outward directions. Filled and empty shapes denote measurements taken using temporal bone (TB) and cork pads, respectively. Filled circles on left of each plot denote forces on the head during walking (w), jogging (j), climbing stairs (s) and voluntary yaw rotations (v); rectangles denote range. Pressure exerted by all magnet combinations is indicated on the axis at the right of the figure. Mean, arteriolar and venous end pressures according to Landis [1] are denoted by the circle and filled rectangles. Capillary pressure ranges according to Steinmetz and Langemo [2] are denoted by the empty rectangles. Occipital capillary pressure according to Ryan et al. [3] is denoted by the filled triangle.

Fig. 6

Comparison of 3D vestibulo-ocular reflex peak responses elicited in rhesus monkey A by delivering stimulation pulses to each canal using our lab-built MVP2 (A-C) and the Pulsar IS (D-F). Stimulus was sinusoidally modulated at 1Hz from 68-130pps peak (corresponding ±50°/s in a normal animal). Standard deviation was ≤ 3.8°/s for all traces.

Fig. 7

Excitatory and inhibitory 3D vestibulo-ocular reflex responses elicited in rhesus monkey ‘A’ by stimulating each left canal using our lab-built MVP2 (solid line) and the MCI (dotted line). Stimulus was modulated at 1-5Hz from 68-130pps peak (equivalent to ±50°/s in a normal animal), using the head-velocity to pulse rate mapping curve with C=2, baseline pulse rate 96pps, maximum pulse rate 350pps, and current at 150, 170, 200uA for the horizontal, left-anterior, and left-posterior canals, respectively. Mean responses ranged from 2.9-16.5°/s with the MVP2 and from 2.5-14.6°/s with the MCI.