Morphometric MRI Analysis of the Parahippocampal Region in Temporal Lobe Epilepsy

Morphometric MRI Analysis of the Parahippocampal Region in Temporal Lobe Epilepsy NEDA BERNASCONI, a ANDREA BERNASCONI, ZOGRAFOS CARAMANOS, FREDERICK ANDERMANN, FRANÇOIS DUBEAU, AND DOUGLAS L. ARNOLD Department of Neurology and Neurosurgery, McGill University and Montreal Neurological Institute and Hospital, Montreal, Quebec H3A 2B4, Canada ABSTRACT: Despite neuropathological and electrophysiological evidence for the involvement of parahippocampal structures in temporal lobe epilepsy (TLE), little attention has been paid to morphometric changes in these structures, and the relation of these changes to TLE. We performed high-resolution MRI volumetric analysis to examine in vivo the morphology of the parahippocampal region in 20 healthy subjects and 6 TLE patients with MRI evidence of unilateral hippocampal atrophy. In normal controls the standardized volume of the left entorhinal cortex (EC) was 1305 ± 138 mm 3 and that of the right EC was 1376 ± 170 mm 3 ; the left perirhinal cortex (PC) was 2900 ± 554 mm 3 and the right PC was 2771 ± 486 mm 3 ; the left posterior parahippocampal cortex (PPC) was 2499 ± 583 mm 3 and the right PPC was 2234 ± 404 mm 3. Using a 2 standard deviation cutoff from the mean of normal controls, we found ipsilateral to the seizure focus: (i) a reduction in the volume of the EC in all patients; (ii) a reduction of the PC in 2/6 (33%) patients; (iii) no reduction in the volume of the PPC in any patient. In 3/6 (50%) of patients, the EC was also abnormally small contralateral to the seizure focus. In patients with unilateral TLE, the EC is the most affected structure within the parahippocampal region. Whether this is due to a primary role of the EC in the genesis of TLE or is the consequence of its pivotal position in the reciprocal flow of information between the hippocampus and the neo- and limbic cortices remains to be explored. INTRODUCTION The human mesial temporal region is composed of the hippocampus, the amygdala, and the parahippocampal region. The parahippocampal region itself is composed of the entorhinal cortex (EC), the perirhinal cortex (PC), and the posterior parahippocampal (areas TH and TF of von Bonin and Bailey 1 ) cortex (PPC). In early studies of surgically resected specimens of patients with temporal lobe epilepsy (TLE), the term mesial temporal sclerosis was introduced to describe widespread pathological changes of the hippocampus, the amygdala, and the sura Address for correspondence: Dr. Neda Bernasconi, Brain Imaging Center, Montreal Neurological Hospital and Institute, 3801 University Street, Montreal, Quebec, Canada, H3A 2B4. Tel.: (514) 398-8185; fax: (514) 398-2975. e-mail: neda@bic.mni.mcgill.ca 495

496 ANNALS NEW YORK ACADEMY OF SCIENCES rounding cortical areas. 2 More recently, magnetic resonance imaging (MRI) studies in TLE have put the emphasis on the hippocampus. Hippocampal atrophy on MRI has been shown to correlate with the presence of hippocampal sclerosis. 3 We have recently shown a reduction in the volume of the EC ipsilateral to the seizure focus in patients with temporal lobe epilepsy 4 that is presumably an MRI correlate of neuronal loss and gliosis previously described. 5 The purpose of this study was to examine if in vivo volume changes of different components of the parahippocampal region are apparent on MRI and to determine the distribution of atrophy within the parahippocampal region in patients with TLE. METHODS We selected six patients with medically intractable TLE (mean age, 37; range, 20 56) and unilateral hippocampal atrophy on volumetric MRI. Patients were compared to 20 neurologically normal controls (mean age, 27; range, 20 45). Lateralization of Seizure Focus Seizure type and the site of seizure onset were determined by a comprehensive evaluation including detailed history, neurological examination, review of medical and EEG records, and neuropsychological evaluation. The seizure focus was determined by predominantly ipsilateral interictal epileptic abnormalities (70% cutoff), by unequivocal unilateral seizure onset recorded during prolonged video-eeg monitoring using sphenoidal electrodes, and by response to surgical treatment in all six cases. All patients underwent a selective amygdalo-hippocampectomy. Qualitative histopathologic examination 6 of the resected tissue revealed hippocampal sclerosis in all patients. Because of subpial gyral aspiration, histopathology of the parahippocampal region structures was not available. All patients have been seizure-free since surgery with a mean postoperative follow-up of 20 months (range, 12 months to 2.5 years). On the basis of these criteria, TLE patients were divided into those with a left-sided (n = 3) or a right-sided (n = 3) seizure focus. MRI Scanning MRI volumetric images were acquired on a 1.5 T Gyroscan (Philips Medical System, Eindhoven, The Netherlands), using a T1 fast-field echo, TR = 18, TE = 10, one acquisition average pulse sequence, 30 flip angle, matrix size, 256 256, FOV = 256, thickness = 1 mm. Approximately 170 isotropic images with a voxel size of 1mm 1 mm 1 mm were acquired. Image Processing Analysis was performed on a Silicon Graphics workstation (Mountain View, CA). Images were automatically registered into stereotaxic space 7 to adjust for differences in total brain volume and brain orientation and to facilitate the identification of boundaries by minimizing variability in slice orientation. 8 Each image underwent automated correction for intensity nonuniformity due to radiofrequency inhomogeneity of the MR scanner and intensity standardization. 9 This correction produces

BERNASCONI et al.: MRI ANALYSIS IN TEMPORAL LOBE EPILEPSY 497 FIGURE 1. Major anatomical boundaries of parahippocampal region structures on coronal MR images. A is the most rostral, and F is the most caudal MR section. Only MR images displaying critical landmarks are shown. (A) Anterior border of the perirhinal cortex (PC) at the level of limen insulae (LI); (B) anterior border of the entorhinal cortex (EC) at the level of temporal stem (TS); (C) posterior border of the EC at the level of posterior limit of gyrus intralimbicus (GI); (D) posterior border of the PC, situated 2 mm caudal to the posterior end of the EC; (E) anterior border of the posterior parahippocampal cortex (PPC), situated 1 mm caudal to the posterior end of the PC; (F) posterior border of the PPC, situated at the level of the posterior end of the hippocampal tail (Ht). Am = amygdala; Hb = hippocampal body; CS = collateral sulcus. consistent relative gray matter, white matter, and CSF intensities. The hippocampus, 10 the EC, 4,11 the PC, 11 and the PPC 12 were segmented manually using mousedriven software according to previously described protocols (FIG. 1). Statistical Analysis The normality of the distribution of volume measurements for the entorhinal, perirhinal, and posterior parahippocampal cortices was assessed using normal probability plots. The statistical significance of differences in mean volumes between right and left sides was assessed using paired t-test. In the analysis of individual patients, values two standard deviations (SD) below the mean of normal controls were considered as abnormal.

498 ANNALS NEW YORK ACADEMY OF SCIENCES FIGURE 2. Box-and-whisker plots of the volumes of the entorhinal, perirhinal, and posterior parahippocampal cortices in normal controls. The center horizontal line marks the median of the sample; the upper and lower edges of the box (the hinges) mark the 25th and 75th percentiles (i.e., the central 50% of the values fall within the box) the distance between these hinges being referred to as the Hspread, and the whiskers extend from the box and show the range of values that fall within 1.5 Hspread. The dots represent individual subjects. L and R refer to left and right. The mean and standard deviation (SD) is shown for each structure at the bottom of the figure. RESULTS In normal controls, the mean volume of the right EC (1376 ± 170 mm 3 ) was slightly greater than that of the left (1305 ± 138 mm 3 ; p <0.05). The mean volume of the right PC was 2771 ± 486 mm 3 and that of the left was 2900 ± 554 mm 3 (p >0.05). The mean volume of the left PPC (2499 ± 583 mm 3 ) was greater than that of the right (2234 ± 404 mm 3 ; p <0.01) (FIG. 2). Ipsilateral to the seizure focus, (i) the EC was abnormally small in all six patients; (ii) the PC was abnormally small in 2/6 patients; (iii) the PPC was normal in all six patients. Contralateral to the seizure focus, the EC was abnormally small in 3/6 patients. The PC and the PPC were normal in all patients (FIG. 3). DISCUSSION Our results show that in patients with intractable TLE and unilateral hippocampal atrophy, the EC is always abnormal, the PC is sometimes abnormal, and the PPC is

BERNASCONI et al.: MRI ANALYSIS IN TEMPORAL LOBE EPILEPSY 499 FIGURE 3. Box-and-whisker plots of the volumes of the entorhinal, perirhinal, and posterior parahippocampal cortices in TLE patients standardized relative to the normal controls. Scores are shown in units of standard deviations (SD) with 0 equaling the mean of normal controls. The dots represent individual subjects, and the filled squares represent the group means. The broken lines designate 2 SD below the mean of normal controls. Ipsi and contra refer to ipsilateral and contralateral to seizure focus. always normal ipsilateral to the seizure focus. Within the parahippocampal region, the EC is the only structure that is bilaterally small in 50% of cases. In an MRI volumetric analysis of the effect of long-lasting seizures in TLE patients, damage in the hippocampus, the amygdala, and the rostral portion of the parahippocampal gyrus has been described ipsilateral to the seizure focus. 13 In our study we divided the parahippocampal region into its three components. In doing so, we were able to show for the first time that atrophy is not equally distributed within the parahippocampal region. This information might be used in the planning of surgical treatment of pharmacologically intractable TLE. Investigations with stereotactic electrodes have shown that seizure onset may be in the amygdala, the hippocampus, or the entorhinal cortex. 14 In vitro studies of focal epileptogenesis in combined hippocampal entorhinal slices have demonstrated that the entorhinal cortex possesses an intrinsic capacity to generate epileptiform discharges. 15 After aminooxyacetic acid injection in the rat entorhinal cortex, there is extensive cell loss in layer III 16 of medial entorhinal cortex identical to the lesion found in human TLE. 5 The damage to the entorhinal cortex may contribute to longlasting changes in excitability in the entorhinal cortex and the hippocampus and play a primary role in the genesis and spread of temporal lobe seizures. 17 However, the reason for a preferential damage to the entorhinal cortex in our patients remains unclear. Our finding of a bilateral reduction in volume of the entorhinal cortex in 50% of cases concurs with the view that more widespread damage of mesial temporal lobe structures may occur even in patients with predominantly or strictly unilateral TLE. The fact that, within the parahippocampal region, only the entorhinal cortex is affected bilaterally may be due to its remarkable hyperexcitability. 17,18

500 ANNALS NEW YORK ACADEMY OF SCIENCES In this study we provide MRI corroboration of the pathological findings of neuronal loss and gliosis in the parahippocampal region, particularly in the entorhinal cortex. However, further studies are needed to define the roles of the different components of mesial temporal lobe in the genesis of temporal lobe epilepsy. REFERENCES 1. BAILEY, P. & G. VON BONIN. 1951. The Isocortex of Man. University of Illinois Press. Urbana, IL. 2. FALCONER, M.A., E.A. SERAFETINIDES & J.A.N. CORSELLIS. 1964. Etiology and pathogenesis of temporal lobe epilepsy. Arch. Neurol. 10: 233 248. 3. JACKSON, G.D., S.F. BERKOVIC, B.M. TRESS, et al. 1990. Hippocampal sclerosis can be reliably detected by magnetic resonance imaging. Neurology 40: 1869 1875. 4. BERNASCONI, N., A. BERNASCONI, F. ANDERMANN, et al. 1999. Entorhinal cortex in temporal lobe epilepsy: a quantitative MRI study. Neurology 52: 1870 1876. 5. DU, F., W.O. WHETSELL, B. ABOU-KHALIL & B. BLUMENKOPF. 1993. Preferential neuronal loss in layer III of the entorhinal cortex in patients with temporal lobe epilepsy. Epilepsy Res. 16: 223 233. 6. MEENCKE, H.J. & G. VEITH. 1991. Hippocampal sclerosis in epilepsy. In Epilepsy Surgery. H. Lüders, Ed. Raven Press. New York. 7. TALAIRACH, J. & P. TOURNOUX. 1988. Co-planar stereotaxic atlas of the human brain. Thieme Medical Publishers. New York. 8. COLLINS, D.L., P. NEELIN, T.M. PETERS & A.C. EVANS. 1994. Automatic 3D intersubject registration of MR volumetric data in standardized Talairach space. J. Comput. Assist. Tomogr. 18: 192 205. 9. SLED, J.G., A.P. ZIJDENBOS & A.C. EVANS. 1998. A nonparametric method for automatic correction of intensity non-uniformity in MRI data. IEEE Trans. Med. Imaging 17: 87 97. 10. WATSON, C., F. ANDERMANN, P. GLOOR, et al. 1992. Anatomic basis of amygdaloid and hippocampal volume measurement by magnetic resonance imaging. Neurology 42: 1743 1750. 11. INSAUSTI, R., K. JUOTTONEN, H. SOININEN, et al. 1998. MR volumetric analysis of the human entorhinal, perirhinal, and temporopolar cortices. AJNR Am. J. Neuroradiol. 19: 659 671. 12. INSAUSTI, R., A.M. INSAUSTI, M.T. SOBREVIELA, et al. 1998. Human medial temporal lobe in aging: anatomical basis of memory preservation. Microsc. Res. Tech. 43: 8 15. 13. SAUKKONEN, A., R. KÄLVIÄINEN, K. PARTANEN, et al. 1994. Do seizures cause neuronal damage? A MRI study in newly diagnosed and chronic epilepsy. Neuroreport 6: 219 223. 14. SPENCER, S.S. & D.D. SPENCER. 1994. Entorhinal hippocampal interactions in medial temporal lobe epilepsy. Epilepsia 35: 721 727. 15. BEAR, J. & E.W. LOTHMAN. 1993. An in vitro study of focal epileptogenesis in combined hippocampal parahippocampal slices. Epilepsy Res. 14: 183 193. 16. DU, F. & R. SCHWARCZ. 1992. Aminooxyacetic acid causes selective neuronal loss in layer III of the rat medial entorhinal cortex. Neurosci. Lett. 147: 185 188. 17. SCHARFMAN, H.E., J.H. GOODMAN, F. DU & R. SCHWARCZ. 1998. Chronic changes in synaptic responses of entorhinal and hippocampal neurons after amino-oxyacetic acid (AOAA)-induced entorhinal cortical neuron loss. J. Neurophysiol. 80: 3031 3046. 18. BEAR, J., N.B. FOUNTAIN & E.W. LOTHMAN. 1996. Responses of the superficial entorhinal cortex in vitro in slices from naive and chronically epileptic rats. J. Neurophysiol. 76: 2928 2940.