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Abstract
We recently discovered that static magnetic fields from high-strength MRI machines induce nystagmus in all normal humans, and that a magneto-hydrodynamic Lorentz force, derived from ionic currents in the endolymph and pushing on the cupula, best explains this effect. Individuals with no labyrinthine function have no nystagmus. The influence of magnetic vestibular stimulation (MVS) in individuals with unilateral deficits in labyrinthine function is unknown and may provide insight into the mechanism of MVS. These individuals should experience MVS, but with a different pattern of nystagmus consistent with their unilateral deficit in labyrinthine function. We recorded eye movements in the static magnetic field of a 7 T MRI machine in nine individuals with unilateral labyrinthine hypofunction, as determined by head impulse testing and vestibular-evoked myogenic potentials (VEMP). Eye movements were recorded using infrared video-oculography. Static head positions were varied in pitch with the body supine, and slow-phase eye velocity (SPV) was assessed. All subjects exhibited predominantly horizontal nystagmus after entering the magnet head-first, lying supine. The SPV direction reversed when entering feet-first. Pitching chin-to-chest caused subjects to reach a null point for horizontal SPV. Right unilateral vestibular hypofunction (UVH) subjects developed slow-phase-up nystagmus and left UVH subjects, slow-phase-down nystagmus. Vertical and torsional components were consistent with superior semicircular canal excitation or inhibition, respectively, of the intact ear. These findings provide compelling support for the hypothesis that MVS is a result of a Lorentz force and suggest that the function of individual structures within the labyrinth can be assessed with MVS. As a novel method of comfortable and sustained labyrinthine stimulation, MVS can provide new insights into vestibular physiology and pathophysiology.
Highlights
Case reports of dizziness in and around high-strength (≥3 T) magnets have prompted investigations into the effects of high-strength magnetic fields on human balance and cognitive function
Roberts et al showed [1] that all normal human subjects examined have horizontal nystagmus while lying in a static magnetic field of 7 T and show the slow-phase velocities (SPV) can be as high as 40°/s; [2] the direction of nystagmus changes with head pitch and with direction of entry into the bore; [3] the effect persists throughout the time in the magnetic field; [4] the effect does not depend on rate of motion into or out of the field; [5] the effect scales with the intensity of the magnet field; and [6] the effect is absent in patients with bilateral vestibular loss [3]
There was no difference in change in SPV between right- and left-sided unilateral vestibular loss subjects for the horizontal component of nystagmus (P = 0.62), with all unilateral vestibular hypofunction (UVH) subjects demonstrating a nystagmus with leftward slow-phases on entering the magnetic field at the neutral, lying flat head position or with their heads pitched chin up
Summary
Case reports of dizziness in and around high-strength (≥3 T) magnets have prompted investigations into the effects of high-strength magnetic fields on human balance and cognitive function. Roberts et al showed [1] that all normal human subjects examined have horizontal nystagmus while lying in a static magnetic field of 7 T and show the slow-phase velocities (SPV) can be as high as 40°/s; [2] the direction of nystagmus changes with head pitch and with direction of entry into the bore; [3] the effect persists throughout the time in the magnetic field (at least to 25 min, the maximum tested far); [4] the effect does not depend on rate of motion into or out of the field; [5] the effect scales with the intensity of the magnet field; and [6] the effect is absent in patients with bilateral vestibular loss [3]. The MHD force produces a pressure in the endolymph that is sensed by the cupula of the lateral semicircular canal (SCC), producing a horizontal nystagmus [3] Quantitative analysis of this behavior suggests that resting utricular hair cell current is primarily responsible for generating the MHD force [4, 9]. The goal of the present study was to investigate MVS in humans with UVH to explore further the mechanisms involved in MVS and suggest a potential clinical use for MVS www.frontiersin.org
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