Brief reportEpilepsy surgery, resection volume and MSI localization in lesional frontal lobe epilepsy
Introduction
Epilepsy surgery in principle aims at the control of seizures in patients with pharmacoresistant epilepsies while avoiding neurological or neuropsychological deficits. Identification and delineation of the epileptogenic zone, defined as the brain region that generates a patient's habitual focal seizures and has to be removed to abolish seizures completely (Lüders, 1991), represents the main task in presurgical epilepsy evaluation. For this purpose, a variety of noninvasive and invasive diagnostic tools are applied. Noninvasive investigations include intensive video-EEG monitoring, high-resolution magnetic resonance imaging (MRI), single photon emission computerized tomography (SPECT), positron emission tomography (PET) and neuropsychological testing. In many cases, these techniques provide ample evidence of brain regions involved in seizure generation. Nevertheless, in a significant number of patients, the epileptogenic zone cannot be identified conclusively by these methods, and invasive EEG recordings are required to detect focal onset of epileptic activity (Engel and Ojemann, 1993). Invasive procedures, however, not only bear considerable risks of intracranial infection or bleeding and often result in prolonged hospitalization and postponement of surgery (Engel and Ojemann, 1993) but the limited “tunnel view” of invasive recordings also requires detailed planning of electrode placement.
Therefore, delineation of the epileptogenic zone with noninvasive techniques should be optimized. Magnetoencephalography (MEG), a neurophysiological method with high temporal and spatial resolution, is increasingly applied to improve noninvasive presurgical focus localization and delineation of functionally important areas Baumgartner et al., 2000, Ebersole et al., 1995, Iwasaki et al., 2002, Pataraia et al., 2002. Magnetic source imaging (MSI), the combination of MEG source localization with co-registered MRI data, does not only permit localizations at the surface of the brain, as obtained by electrocorticography, but provides a three-dimensional view of source localizations, even in deeper brain structures or in association with structural lesions.
Delineation of the epileptogenic zone in extratemporal epilepsies (ETE), especially frontal lobe epilepsies, is considered to be much more demanding than in temporal lobe epilepsies (TLE), as the frontal cortex covers 40% of the brain, and propagation of epileptic activity can be fast (Kotagal and Arunkumar, 1998). Despite these difficulties, MEG has been proven to be particularly useful in localization of the epileptogenic zone in ETE Nakasato et al., 1994, Smith et al., 1995, Stefan et al., 2000.
The aim of this study was to validate MEG localizations in frontal lobe epilepsy by analyzing congruency among presurgical focus localization by MEG, resection volume and postoperative outcome. Moreover, MSI results were compared to results of prolonged video-EEG monitoring in all patients, to recordings of invasive strip and depth electrodes (case 4) and intraoperative electrocorticogram (cases 2 and 3).
Section snippets
Patients and methods
So far, data of five patients suffering from frontal lobe epilepsy was analyzed. All patients underwent presurgical epilepsy evaluation in the Erlangen Epilepsy Center (ZEE).
Case 1: Four weeks before admission to the hospital for epilepsy assessment, the 20-year-old patient developed simple partial seizures in the right arm, initiating with loss of control followed by clonic jerks. Once, speech arrest and a tonic clonic seizure occurred. MRI scans showed a calcified mass in the left frontal
Results
Case 1: MSI yielded dipole sources at the border of the lesion in the left frontal lobe. During prolonged video-EEG monitoring, ictal recordings showed high-frequency seizure pattern in electrodes F3 and Fz. Lesionectomy revealed a calcified cavernoma. Two years after surgery, the patient was still completely seizure-free. Superimposed preoperative MSI results and postoperative MRI scans showed that preoperative dipole localizations clustered well within the resection cavity (Fig. 1).
Case 2:
Discussion
Definition of the exact resection volume is a major difficulty in postoperative evaluation of epilepsy surgery. As the extent of resection may be overestimated by the operating surgeon (Schwartz et al., 2002), evaluation of postoperative MRI scans indicating the correct resection volume is crucial. In this study, good correlation of MEG localizations with pre- and postoperative MRI was enabled by using a common spatial coordinate system for electrophysiological and imaging data. By matching MSI
Acknowledgements
Part of this research was supported by Deutsche Forschungsgemeinschaft (DFG) Grant STE 380/9.
References (15)
- et al.
Magnetoencephalography in focal epilepsy
Epilepsia
(2000) - et al.
Applications of magnetic source imaging in evaluation of candidates for epilepsy surgery
Neuroimaging Clin. N. Am.
(1995) - et al.
The next step
- et al.
Functional neuronavigation with magnetoencephalography: outcome in 50 patients with lesions around the motor cortex
J. Neurosurg.
(1999) - et al.
Surgical implications of neuromagnetic spike localization in temporal lobe epilepsy
Epilepsia
(2002) - et al.
Magnetoencephalography in neocortical epilepsy
Adv. Neurol.
(2000) - et al.
Lateral frontal lobe seizures
Epilepsia
(1998)
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