Ward Bryan K, Zee David S, Roberts Dale C, Schubert Michael C, Pérez-Fernández Nicolas, Otero-Millan Jorge
Department of Otolaryngology-Head and Neck Surgery, The Johns Hopkins University, Baltimore, MD, United States.
Department of Neurology, The Johns Hopkins University, Baltimore, MD, United States.
Front Neurol. 2019 Jan 22;9:1197. doi: 10.3389/fneur.2018.01197. eCollection 2018.
Strong static magnetic fields such as those in an MRI machine can induce sensations of self-motion and nystagmus. The proposed mechanism is a Lorentz force resulting from the interaction between strong static magnetic fields and ionic currents in the inner ear endolymph that causes displacement of the semicircular canal cupulae. Nystagmus persists throughout an individual's exposure to the magnetic field, though its slow-phase velocity partially declines due to adaptation. After leaving the magnetic field an after effect occurs in which the nystagmus and sensations of rotation reverse direction, reflecting the adaptation that occurred while inside the MRI. However, the effects of visual fixation and of head shaking on this early type of vestibular adaptation are unknown. Three-dimensional infrared video-oculography was performed in six individuals just before, during (5, 20, or 60 min) and after (4, 15, or 20 min) lying supine inside a 7T MRI scanner. Trials began by entering the magnetic field in darkness followed 60 s later, either by light with visual fixation and head still, or by continuous yaw head rotations (2 Hz) in either darkness or light with visual fixation. Subjects were always placed in darkness 10 or 30 s before exiting the bore. In control conditions subjects remained in the dark with the head still for the entire duration. In darkness with head still all subjects developed horizontal nystagmus inside the magnetic field, with slow-phase velocity partially decreasing over time. An after effect followed on exiting the magnet, with nystagmus in the opposite direction. Nystagmus was suppressed during visual fixation; however, after resuming darkness just before exiting the magnet, nystagmus returned with velocity close to the control condition and with a comparable after effect. Similar after effects occurred with continuous yaw head rotations while in the scanner whether in darkness or light. Visual fixation and sustained head shaking either in the dark or with fixation inside a strong static magnetic field have minimal impact on the short-term mechanisms that attempt to null unwanted spontaneous nystagmus when the head is still, so called VOR set-point adaptation. This contrasts with the critical influence of vision and slippage of images on the retina on the dynamic (gain and direction) components of VOR adaptation.
强静磁场，如磁共振成像（MRI）机器中的磁场，可诱发自我运动感觉和眼球震颤。提出的机制是，强静磁场与内耳内淋巴中的离子电流相互作用产生洛伦兹力，导致半规管壶腹移位。在个体暴露于磁场的整个过程中，眼球震颤持续存在，不过由于适应性，其慢相速度会部分下降。离开磁场后会出现一种后效应，即眼球震颤和旋转感觉方向相反，这反映了在MRI内发生的适应性变化。然而，视觉注视和摇头对这种早期前庭适应性类型的影响尚不清楚。对6名个体在仰卧于7T MRI扫描仪内之前、期间（5、20或60分钟）和之后（4、15或20分钟）进行了三维红外视频眼动描记术。试验开始时，先在黑暗中进入磁场，60秒后，要么在有视觉注视且头部静止的情况下进入光亮环境，要么在黑暗或有视觉注视的光亮环境中以连续的偏航头部旋转（2Hz）进行。在退出磁体孔之前10或30秒，受试者总是被置于黑暗中。在对照条件下，受试者在整个时间段内都保持头部静止地处于黑暗中。在头部静止的黑暗环境中，所有受试者在磁场内都出现了水平眼球震颤，其慢相速度随时间部分降低。退出磁体后会出现后效应，眼球震颤方向相反。在视觉注视期间，眼球震颤受到抑制；然而，在即将退出磁体之前恢复黑暗后，眼球震颤以接近对照条件的速度恢复，且具有类似的后效应。在扫描仪内，无论在黑暗还是光亮环境中，连续偏航头部旋转时都会出现类似的后效应。在黑暗中或在强静磁场内有注视时，视觉注视和持续摇头对试图消除头部静止时不需要的自发性眼球震颤的短期机制（即所谓的前庭眼反射（VOR）设定点适应性）影响极小。这与视觉以及视网膜上图像的滑动对VOR适应性动态（增益和方向）成分的关键影响形成对比。
