Monday, 21 November 2011

Vertigo & Nystagmus with MRI

Level 3

Roberts et al. MRI magnetic field stimulates rotational sensors of the brain. Curr Biol. 2011 Oct 11;21(19):1635-40. 

Have you ever thought why patients and people get vertigo in and around MRI machines? 

Several mechanisms have been suggested to explain these sensations, yet without direct, objective measures, the cause has remained unknown, until now. 

This study has found that healthy human subjects developed a robust nystagmus while simply lying in the static magnetic field of an MRI machine. In comparison patients lacking labyrinthine function did not. 

They used the pattern of eye movements as a measure of vestibular stimulation to show that the stimulation is static (continuous, proportional to static magnetic field strength, requiring neither head movement nor dynamic change in magnetic field strength) and directional (sensitive to magnetic field polarity and head orientation). Their calculations and geometric model suggest that magnetic vestibular stimulation (MVS) derives from a Lorentz force resulting from interaction between the magnetic field and naturally occurring ionic currents in the labyrinthine endolymph fluid. This force pushes on the semicircular canal cupula, leading to nystagmus. 

They emphasize that the unique, dual role of endolymph in the delivery of both ionic current and fluid pressure, coupled with the cupula's function as a pressure sensor, makes magnetic-field-induced nystagmus and vertigo possible. 

Such effects could confound functional MRI studies of brain behavior, including resting-state brain activity.

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