Abstracts

Rationale: The role of Diffusion Tensor imaging (DTI) in the presurgical evaluation of patients with epilepsy is not entirely clear. Some studies have suggested that this technique might be helpful in lateralization of the seizure focus. However only limited data is available regarding DTI in the white matter of the parahippocampal region. We hypothesize that DTI of this region may be helpful in lateralization of the seizure focus in patients with intractable temporal lobe epilepsy (TLE). Methods: We retrospectively analyzed DTI images of 15 consecutive patients who had temporal lobectomy (8 left, 7 right) between 2007 and 2009 as well as those of healthy normal controls. Other inclusion criteria for patients were: 1) Engel Class 1a outcome after surgery, and 2) preoperative MRI scans with 25 directional DTI. Tractography was performed on each patient and control in the white matter of both parahippocampal regions using DTI studio software. Using this software, measures of Fractional Anisotropy (FA), Relative Anisotropy (RA) and Volume Ratio (VR) were obtained. In addition, Apparent Diffusion Coefficients (ADC) were also measured in all subjects. For statistical comparisons we used the t-test to examine the significance of differences in DTI measurements (FA, RA, VR). Results: In 14 of 15 patients, all measures of anisotropy (FA, RA, VR) of white matter tracts in the parahippocampal region were significantly reduced (p<0.001) on the side of the seizure focus. One patient with left TLE had reduced anisotropy measures in the contralateral parahippocampal region. In the control group, anisotropy measures could be lower on either side. Computation of ADC values revealed equal ADC values bilaterally in 10 patients. Four patients had increased ADC values in the ipsilateral parahippocampal region and 1 patient had an increased ADC value in the contralateral parahippocampus. ADC values were equal on both sides in the control group. Review of preoperative brain MRI revealed the presence of unilateral hippocampal sclerosis (HS) on the operated side in 12 patients. One patient had evidence of bilateral hippocampal abnormalities and 2 patients had normal brain MRIs. Five of 15 patients had subdural electrodes implanted prior to resection because of uncertainty in localization of the epileptogenic zone. Neuropathology was available for 14 patients and revelealed HS in 8 patients, gliosis in 2, mild cortical dysplasia in 1 and neuronal heterotopia in 1 patient. Nonspecific changes were seen in 2 patients including the 1 patient with incongruent anisotropy findings. DTI was also helpful in lateralizing the epileptogenic zone in all 3 patients who had normal MRI or bilateral MRI changes. In addition, all 5 patients who needed implantation of intracranial electrodes were correctly lateralized using this technique. Conclusions: DTI of the white matter tracts of the parahippocampal gyrus may be helpful in lateralization of the epileptogenic zone in intractable TLE including in cases where standard noninvasive studies have yielded ambiguous results.