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Abstract
Humans often experience dizziness and vertigo around strong static magnetic fields such as those present in an MRI scanner. Recent evidence supports the idea that this effect is the result of inner ear vestibular stimulation and that the mechanism is a magnetohydrodynamic force (Lorentz force) that is generated by the interactions between normal ionic currents in the inner ear endolymph and the strong static magnetic field of MRI machines. While in the MRI, the Lorentz force displaces the cupula of the lateral and anterior semicircular canals, as if the head was rotating with a constant acceleration. If a human subject’s eye movements are recorded when they are in darkness in an MRI machine (i.e., without fixation), there is a persistent nystagmus that diminishes but does not completely disappear over time. When the person exits the magnetic field, there is a transient aftereffect (nystagmus beating in the opposite direction) that reflects adaptation that occurred in the MRI. This magnetic vestibular stimulation (MVS) is a useful technique for exploring set-point adaptation, the process by which the brain adapts to a change in its environment, which in this case is vestibular imbalance. Here, we review the mechanism of MVS, how MVS produces a unique stimulus to the labyrinth that allows us to explore set-point adaptation, and how this technique might apply to the understanding and treatment of vestibular and other neurological disorders.
Highlights
Specialty section: This article was submitted to Neuro-otology, a section of the journal Frontiers in Neurology
Recent evidence supports the idea that this effect is the result of inner ear vestibular stimulation and that the mechanism is a magnetohydrodynamic force (Lorentz force) that is generated by the interactions between normal ionic currents in the inner ear endolymph and the strong static magnetic field of MRI machines
We review the mechanism of magnetic vestibular stimulation (MVS), how MVS produces a unique stimulus to the labyrinth that allows us to explore set-point adaptation, and how this technique might apply to the understanding and treatment of vestibular and other neurological disorders
Summary
Recognized by Sherrington as a key principle of postural control, tone (set point) is optimally achieved via sustained balance level of opposing tonic neural activity in a push–pull fashion [16, 17]. Any relatively constant sustained nystagmus is a low-frequency stimulus and is presumably interpreted by the brain as resulting from disease since sustained nystagmus almost never occurs naturally unless there is a lesion producing a persistent tone imbalance This experimental paradigm using sustained rotation of the head, while producing a sustained nystagmus, does not perfectly mimic a naturally occurring lesion in one labyrinth because in normal subjects there is a tonic level of activity coming from both labyrinths and one side is excited and the other inhibited. The persistent nystagmus from a caloric irrigation appears to be independent of any direct thermal effect on activity in the hair cells and the vestibular nerve [22] In spite of these limitations, a caloric stimulus has one advantage over MVS since it is unilateral and, in this way, better mimics a lesion of one lateral semicircular canal. Slower dynamic properties shift the system in stages toward a new 0 set point by gradually eliminating the unwanted bias
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