Abstracts

Visual field defects are a well-described complication of temporal lobe epilepsy (TLE) surgery and are believed to result from injury to the optic radiations in the anterior temporal lobe (i.e. Meyer[apos]s Loop). Diffusion tensor imaging (DTI) is a novel imaging modality that allows the in vivo delineation of white matter tracts (DTI tractography) along with quantification of their axonal integrity. The purpose of this study was to assess whether TLE patients who undergo surgery have detectable DTI abnormalities in the optic radiations. The authors used DTI to measure the structure, directionality, and organization of the optic radiation in patients both pre-operatively and post-operatively. Six patients with medically intractable TLE referred for neurosurgery were studied. All patients had pre and post op DTI and formal visual field assessment. DTI scans were obtained on a Siemens Sonata 1.5T scanner (2x2x2mm voxel resolution). The optic radiations were delineated using DTI tractography. The tractography-defined voxels were then used for quantitative analysis of fractional anisotropy (FA), a measure of the structural integrity of white matter fasciculi and a marker of Wallerian degeneration. Pre and post-op values were compared using a paired t-test. All patients were found to have upper homonymous quadrantanopsia on post-op assessment (mean=15.6[plusmn]7.3% loss of visual field, range:9.2-25%). Tractography data overlaid on post-op high resolution scans failed to demonstrate an intersection of the tractography defined optic radiation and the surgical resection. FA analysis of the tractography defined optic radiations did not demonstrate a significant difference between healthy and pathologic lobes in patients pre-operatively (n=6 healthy:0.49[plusmn]0.038, pathologic:0.50[plusmn]0.031, t-test:p=0.58) or post-operatively (n=3 healthy:0.50[plusmn]0.041, pathologic:0.47[plusmn]0.033, t-test:p=0.44). The fact that all patients exhibited post-op visual field deficits suggests that they all experienced injury to the optic radiations and presumably Wallerian axonal degeneration downstream of the lesion. As we were unable to delineate Meyer[apos]s Loop with tractography, we suspect that our failure to detect FA abnormalities in these patients reflects inadequate resolution of our technique. DTI obtained with higher angular resolution (i.e. more diffusion-sensitizing gradient directions), better spatial resolution, and larger static magnetic field may allow the detection of subtle changes that were undetected with our current protocol. (Supported by Canadian Institutes of Health Research, Alberta Heritage Foundation for Medical Research and the Savoy Foundation.)