Effect of epilepsy magnetic source imaging on intracranial electrode placement.

OBJECTIVE: Intracranial electroencephalography (ICEEG) with chronically implanted electrodes is a costly invasive diagnostic procedure that remains necessary for a large proportion of patients who undergo evaluation for epilepsy surgery. This study was designed to evaluate whether magnetic source imaging (MSI), a noninvasive test based on magnetoencephalography source localization, can supplement ICEEG by affecting electrode placement to improve sampling of the seizure onset zone(s). METHODS: Of 298 consecutive epilepsy surgery candidates (between 2001 and 2006), 160 patients were prospectively enrolled by insufficient localization from seizure monitoring and magnetic resonance imaging results. Before presenting MSI results, decisions were made whether to proceed with ICEEG, and if so, where to place electrodes such that the hypothetical seizure-onset zone would be sampled. MSI results were then provided with allowance of changes to the original plan. RESULTS: MSI indicated additional electrode coverage in 18 of 77 (23%) ICEEG cases. In 39% (95% confidence interval, 16.4-61.4), seizure-onset ICEEG patterns involved the additional electrodes indicated by MSI. Sixty-two patients underwent surgical resection based on ICEEG recording of seizures. Highly localized MSI was significantly associated with seizure-free outcome (mean, 3.4 years; minimum, >1 year) for the entire surgical population (n = 62). INTERPRETATION: MSI spike localization increases the chance that the seizure-onset zone is sampled when patients undergo ICEEG for presurgical epilepsy evaluations. The clinical impact of this effect, improving diagnostic yield of ICEEG, should be considered in surgery candidates who do not have satisfactory indication of epilepsy localization from seizure semiology, electroencephalogram, and magnetic resonance imaging.

Left mesial temporal sclerosis and verbal memory: a magnetoencephalography study.

The purpose of this study is to examine the pattern of mesial temporal lobe activity in patients with left mesial temporal sclerosis (LMTS) and normal control subjects during a verbal memory task using magnetoencephalography. Six LMTS patients and seven normal control subjects performed a word recognition task while event-related neuromagnetic responses were recorded for 248 trials in each of two runs. An equivalent current dipole model was used to localize activity sources and determine if the right or left mesial temporal lobe was active. All six (100%) LMTS patients but only two (28%) normal control subjects had right mesial temporal activity (P < 0.02). Two (33%) LMTS patients and 6 (86%) normal control subjects had left mesial temporal activity (P = 0.27). No significant differences in latency or duration of activity were seen between left and right sides or between groups. Our data suggest that patients with left hippocampal dysfunction are more likely to recruit the right mesial temporal lobe during verbal memory tasks than normal control subjects. A trend toward less left mesial temporal activity in LMTS patients is seen as well. Further study is needed to determine the predictive value of this technique regarding postresection memory outcome.

Magnetic source imaging (MSI) in children with neocortical epilepsy: surgical outcome association with 3D post-resection analysis.

OBJECTIVE: To investigate the validity of magnetic source imaging (MSI) to localize seizure-onset zone using 3D analysis of pre-operative MSI source imaging coregistered to post-resection MRI following neocortical epilepsy surgery. METHODS: Twenty-two children who had MSI and epilepsy surgery were studied (median age=11 years, 1 year 2 months-22 years). Only seven (31.8%) had localized lesions on pre-operative conventional brain MRIs. Sixteen (72.7%) underwent intracranial EEG monitoring. Mean post-operative follow-up was 4.7 years (1 year 3 months-8 years 2 months). Fifteen patients (68%) were seizure-free. MEG spike dipole sources were superimposed onto post-operative MRIs. The number and proportion of spike dipoles within resection volume were calculated and compared between seizure free and non-free groups. RESULTS: Both number of dipole clusters and proportion of dipoles in resection volume were not associated with seizure-free outcome (p>0.05). In seven cases with MRI lesions, six of these with a ≥70% dipoles within the resection margin were seizure-free, while one with the proportion <70% was not seizure-free. Further, among the 15 cases with non-localized or normal MRI, five with both the proportion <70% and multiple dipoles clusters were post-operatively seizure free. CONCLUSION: Number and density of clustered spike dipole sources within the surgical resection volume is not associated with postoperative seizure-free outcome. MSI successfully localized the perilesional epileptogenic zone in cases with localized MRI lesions, but not in cases with normal MRI in this study. Even if MEG localizes spikes to a single focal region, confirmation of epilepsy localization with intracranial EEG is still recommended in cases with non-lesional MRI.

Reliability of language mapping with magnetic source imaging in epilepsy surgery candidates.

The external validity of a noninvasive language mapping protocol with magnetoencephalography (MEG) has been established through direct comparisons with invasive functional mapping techniques. This study examines the test-retest and interrater reliability of this protocol under realistic testing conditions in 21 epilepsy surgery candidates. Brain activation maps were obtained in the context of an auditory word recognition task and represented by temporally contiguous dipolar activity sources. Both the duration and strength of the associated magnetic flux were used as measures of the magnitude of regional brain activity. Hemispheric asymmetry indices based on these measures showed good interrater reliability and intraparticipant reproducibility. Similar findings were obtained with respect to the location of the geometric center of receptive language-specific cortex (Wernicke's) area in the dominant hemisphere. The results further support the adequacy of this MEG-based brain mapping protocol as a noninvasive tool for receptive language localization in epilepsy surgery candidates.