Abstracts

Rationale: Studies suggest that visual field defects (VFD) of varying magnitude can occur in up to 80% of patients (Hughes et al, Neurology 1999 53: 167-172) following anterior temporal lobe resection (ATLR). This is most commonly a superior homonymous quadrantanopia, and is caused by disruption of Meyer’s loop. The anterior extent of Meyer’s loop is not visualised on conventional structural magnetic resonance imaging, and there is much inter-individual variation. For this reason the likelihood of a postoperative VFD cannot be accurately predicted in patients undergoing surgery. Tractography is a diffusion based magnetic resonance imaging technique, which can be used to parcellate out white matter structures. We used this method to delineate the anterior extent of the optic radiation pre-operatively, and assess whether it could predict the risk of visual field defects following ATLR. Methods: We studied 20 healthy controls and 19 patients who underwent ATLR for mesial temporal lobe epilepsy (12 left 7 right). All subjects were scanned pre-operatively at 3 Tesla, and both anatomical T1 and diffusion tensor images were acquired. Probabilistic tractography (Parker et al, Phil. Trans. R. Soc. B 2006 360: 893-902) was then applied, using a seed region at the lateral geniculate nucleus. All resulting tracts were thresholded at the same level, and distances from the anterior edge of Meyer’s loop to the temporal pole (ML-TP) and tip of the temporal horn (ML-TH) were calculated. Pre-operative tracts were also overlaid on post-operative, high resolution, distortion matched, echo planar images. All patients underwent postoperative Goldmann perimetry at least 3 months after surgery, and both VFDs and the extent of resection were quantified using previously described methods. Results: Tractography of the optic radiation was consistent with previous anatomical and imaging studies. The anterior extent of Meyer’s loop ranged from 24 to 43 mm from the temporal pole across all subjects. In 8 patients (6 left ATLR, 2 right ATLR) VFDs, ranging from 20 to 80% of a quadrant averaged across both eyes, were present. In these patients Meyer’s loop tended to cap the temporal horn, and disruption of Meyer’s loop was apparent after ATLR. A mixed design AONVA analysis revealed a trend towards the left ML-TP being smaller than the right ML-TP across all groups. A linear regression analysis revealed that ML-TP, followed by extent of resection, were significant predictors of post-operative visual field deficit. Conclusions: Pre-operative tractography is a promising method to visualize Meyer’s loop, and assess an individual patient’s risk of a VFD as a result of ATLR. [Funded by the Wellcome Trust and National Society for Epilepsy]