Abstracts

Rationale: Percent pathology mapping (PPM) has previously been demonstrated to correlate with hippocampal cell counts associated with pathology in mesial temporal lobe epilepsy. Metabolic recovery in the contralateral hippocampus following temporal lobectomy has been reported with proton magnetic resonance spectroscopic imaging (1H-MRSI). The purpose of this study was to evaluate whether the PPM method would identify a similar and earlier normalization of cell counts. Such changes would presumably be reflected in the post-surgical contralateral hippocampal quantitative PPM (qPPM). Methods: 17 patients with medically intractable mesial temporal lobe epilepsy were studied both one week before and 2 months after anterior temporal lobectomy. Patients were on the same AEDs and dosages before and after surgery. 14 age-matched healthy controls were studied for comparison. A modified fast-spin echo-based FLAIR sequence was used with 7 TEs to generate R2-maps (R2=1/T2) for multiple coronal slices through the head and body of both hippocampi. PPMs were generated for each slice and qPPM values measured for both hippocampi before surgery and the unresected contralateral hippocampus after surgery. Paired and unpaired t-tests were used for statistical analysis. Results: Mean qPPMs: healthy controls 249.6 (SD 260.9), pre-surgical ipsilateral 3623.5 (SD 2266.4), pre-surgical contralateral 976.6 (SD 720.0), and post-surgical contralateral 403.4 (SD 319.7). The mean decrease of post-surgical contralateral PPM was 49.4%. The number of patients with a contralateral PPM in the normal range increased from 6 out of 17 pre-surgical to 12 out of 17 patients post-surgery. Three patients had an increase of the post-surgical contralateral qPPM, of which only one (33%) was seizure free. 12 of the 14 patients with a post-surgical decrease were seizure free (86%). Ipsilateral and contralateral pre-surgical qPPMs were significantly different (p<0.0001). Contralateral pre-surgical and post-surgical qPPMs were significantly different (p=0.001). Conclusions: The normalization of the post-surgical contralateral qPPM is consistent with the MRS literature. One potential mechanism is reversibility of a contralateral reactive gliotic process. Our finding that patients with an increase in post-surgical qPPM were less likely to have been seizure free supports this possibility, though the sample size is small. This mechanism is also supported by the fact that qPPM changes likely best reflect neuronal and glial cell counts and that the interval between surgery and the post-surgical scan was only two months - making a glial process more likely. This study supports the notion that contralateral hippocampal pathophysiology is partially reversible post-surgically and, to our knowledge, is the first study to show contralateral structural changes so soon after surgery.