This caught our attention, and made us put on our gEEk hats over morning coffee: Starting at magnetic flux densities of 3 Teslas (3.0 Webers/m² or ≈2.4 million Ampere/meter with µ = µ0) the magneto-hydrodynamic Lorentz force, derived from ionic currents in the vestibular endolymph and pushing on the cupula, is enough to cause dizziness. This is called magnetic vestibular stimulation (MVS).
What’s more, eye movements were recorded in the static magnetic field of a 7T (≈5.6 million A/m) MRI machine in nine individuals with unilateral labyrinthine hypofunction, as determined by head impulse testing and vestibular-evoked myogenic potentials (VEMP).
From the Biomedical Engineering and Medical School crews at Johns Hopkins, we present Magnetic vestibular stimulation in subjects with unilateral labyrinthine disorders from Frontiers in Neurology:
Case reports of dizziness in and around high-strength (≥3T) magnets have prompted investigations into the effects of high-strength magnetic fields on human balance and cognitive function. The influence of strong magnetic fields on vestibular function in rats and mice has been identified by circling behavior after magnetic field exposure, an effect that does not occur after prior labyrinthectomy (1, 2). Roberts et al. showed (1) that all normal human subjects examined have horizontal nystagmus while lying in a static magnetic field of 7T 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 thus 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). This magnetic field-induced nystagmus requires only the presence of a static magnetic field; it is not a result of image acquisition.
The best explanation for these effects of magnetic vestibular stimulation (MVS) is that they are due to a static magneto-hydrodynamic (MHD) force, called the Lorentz force, which occurs in a magnetic field due to the presence of normal ionic currents into hair cells within the ion-rich endolymph of the labyrinth. 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). Because the utricle is close to the opening of the lateral SCC, the pressure from the Lorentz force it generates can deflect the cupula of the lateral SCC. Individuals with unilateral vestibular hypofunction (UVH) have asymmetric remaining vestibular function and should have a similar MVS response to those with intact function, but proportional to the residual function of their remaining utricle and SCCs. 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 in evaluating the function of individual structures within the labyrinth.
Figure 3: Click to enlarge in a new window, and to read a clearer version of the caption
The full journal article by Bryan K. Ward, Dale C. Roberts, Charles C. Della Santina, John P. Carey, and David S. Zee is here:
Magnetic vestibular stimulation in subjects with unilateral labyrinthine disorders.