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Defining Meyers looptemporal lobe resections, visual field deficits and diffusion tensor tractography.
1656 - 1668.
Anterior temporal lobe resection is often complicated by superior quadrantic visual field deficits (VFDs). In some cases this can be severe enough to prohibit driving, even if a patient is free of seizures. These deficits are caused by damage to Meyers loop of the optic radiation, which shows considerable heterogeneity in its anterior extent. This structure cannot be distinguished using clinical magnetic resonance imaging sequences. Diffusion tensor tractography is an advanced magnetic resonance imaging technique that enables the parcellation of white matter. Using seed voxels antero-lateral to the lateral geniculate nucleus, we applied this technique to 20 control subjects, and 21 postoperative patients. All patients had visual fields assessed with Goldmann perimetry at least three months after surgery. We measured the distance from the tip of Meyers loop to the temporal pole and horn in all subjects. In addition, we measured the size of temporal lobe resection using postoperative T-1-weighted images, and quantified VFDs. Nine patients suffered VFDs ranging from 22 to 87 of the contralateral superior quadrant. In patients, the range of distance from the tip of Meyers loop to the temporal pole was 2443 mm (mean 34 mm), and the range of distance from the tip of Meyers loop to the temporal horn was 15 to 9 mm (mean 0 mm). In controls the range of distance from the tip of Meyers loop to the temporal pole was 2447 mm (mean 35 mm), and the range of distance from the tip of Meyers loop to the temporal horn was 11 to 9 mm (mean 0 mm). Both quantitative and qualitative results were in accord with recent dissections of cadaveric brains, and analysis of postoperative VFDs and resection volumes. By applying a linear regression analysis we showed that both distance from the tip of Meyers loop to the temporal pole and the size of resection were significant predictors of the postoperative VFDs. We conclude that there is considerable variation in the anterior extent of Meyers loop. In view of this, diffusion tensor tractography of the optic radiation is a potentially useful method to assess an individual patients risk of postoperative VFDs following anterior temporal lobe resection.
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