Usefulness of proton MR spectroscopy in the evaluation of temporal lobe epilepsy

The purpose of our study was to compare the ability of MR spectroscopy with that of standard presurgical methods to accurately lateralize the abnormal hippocampus in a group of patients with complex partial seizures.

Ten healthy volunteers (five male, five female) without a history of seizures, significant head trauma, or other neurologic abnormalities were chosen to participate in the study. Twelve consecutive patients (three male, nine female) having intractable temporal lobe epilepsy and undergoing presurgical evaluation for temporal lobectomy were chosen to participate in the study. The condition of all patients was refractory to medications. All patients underwent presurgical examination with interictal and video ictal electroencephalography, ictal single-photon emission computed tomography, interictal positron emission tomography, MR imaging, and neuropsychologic testing. When noninvasive data were inconclusive, depth or grid recordings were performed. The results of MR spectroscopy were also compared with postsurgical seizure control as defined by the Engel classification.

Nine (90%) of 10 control subjects showed no significant difference in N-acetyl aspartate. One control subject showed 16% asymmetry in N-acetyl aspartate between sides. The control group showed no statistically significant differences in ratios of N-acetyl aspartate:creatine, N-acetyl aspartate:choline, or creatine:choline when comparing sides (p < .05). All 12 patients showed clearly lateralizing values identified by the index of asymmetry in N-acetyl aspartate (range, 24-93%), with a mean difference of 51% (SD, 22) (p = .01). Additionally, as a group, statistically significant indexes of asymmetry (p = .01) were seen in ratios of N-acetyl aspartate:choline (mean, 42; SD, 22%), N-acetyl aspartate:creatine (mean, 41; SD, 27), and N-acetyl aspartate:creatine + choline (mean, 42; SD, 22). Using an N-acetyl aspartate index of asymmetry of greater than 15%, which represents the mean index of asymmetry of the control subjects +/- two SDs, as our cutoff level for lateralization, the correct side was identified in all patients. When comparing both hippocampi using an asymmetry index of 15% for N-acetyl aspartate:choline + creatine, 11 (92%) of 12 were correctly lateralized. When comparing the unaffected temporal lobes between patients and control subjects, no statistically significant differences were detected in any metabolites or ratios.

Our study agrees with others in showing decreased N-acetyl aspartate in the hippocampus of seizure patients when compared with control subjects. Using N-acetyl aspartate, N-acetyl aspartate:creatine, N-acetyl aspartate:choline, and N-acetyl aspartate:creatine + choline as our parameters, patients with mesial temporal lobe epilepsy were correctly lateralized with MR spectroscopy when compared with clinical consensus criteria. We consider MR spectroscopy to be complementary to MR imaging. Both studies can be performed as a single integrated examination.