Magnetoencephalography in partial epilepsy: Clinical yield and localization accuracy

The goals of this study were to determine (1) the yield of magnetoencephalography (MEG) according to epilepsy type, (2) if MEG spike sources colocalize with focal epileptogenic pathology, and (3) if MEG can identify the epileptogenic zone when scalp ictal electroencephalogram (EEG) or magnetic resonance imaging (MRI) fail to localize it. Twenty-two patients with mesial temporal (10 patients), neocortical temporal (3 patients), and extratemporal lobe epilepsy (9 patients) were studied. A 37-channel biomagnetometer was used for simultaneously recording MEG with EEG. During the typical 2–3–hour MEG recording session, interictal epileptiform activity was observed in 16 of 22 patients. MEG localization yield was greater in patients with neocortical epilepsy (92%) than in those with mesial temporal lobe epilepsy (50%). In 5 of 6 patients with focal epileptogenic pathology, MEG spike sources were colocalized with the lesions. In 11 of 12 patients with nonlocalizing (ambiguous abnormalities or normal) MRI, MEG spike sources were localized in the region of the epileptogenic zone as ultimately defined by all clinical and EEG information (including intracranial EEG). In conclusion, MEG can reliably localize sources of spike discharges in patients with temporal and extratemporal lobe epilepsy. MEG sometimes provides noninvasive localization data that are not otherwise available with MRI or conventional scalp ictal EEG.     

Neuromagnetic recordings in temporal lobe epilepsy.

The introduction of whole-head magnetoencephalography (MEG) systems facilitating simultaneous recording from the entire brain surface has established MEG as a clinically feasible method for the evaluation of patients with temporal lobe epilepsy (TLE). In mesial TLE, two types of MEG spike dipoles could be identified: an anterior vertical and an anterior horizontal dipole. Dipole orientations can be used to attribute spike activity to temporal lobe subcompartments. Whereas the anterior vertical dipole is compatible with epileptic activity in the mediobasal temporal lobe, the anterior horizontal dipole can be explained by epileptic activity of the temporal tip cortex. In nonlesional TLE, medial and lateral vertical dipoles were found which could distinguish between medial and lateral temporal seizure onset zones as evidenced from invasive recordings. In lesional TLE, MEG could clarify the spatial relationship of the structural lesion to the irritative zone. Evaluation of patients with persistent seizures after epilepsy surgery may represent another clinical important application of MEG because magnetic fields are less influenced than electric fields by the prior operation. Simultaneous MEG and invasive EEG recordings indicate that epileptic activity restricted to mesial temporal structures cannot reliably be detected on MEG and that an extended cortical area of at least 6 to 8 cm2 involving also the basal temporal lobe is necessary to produce a reproducible MEG signal. In lateral neocortical TLE MEG seems to be more sensitive than scalp-EEG which further underlines the potential role of MEG for the study of nonlesional TLE. Whole-head MEG therefore can be regarded as a valuable and clinically relevant noninvasive method for the evaluation of patients with TLE.     

Controversies in clinical neurophysiology. MEG is superior to EEG in the localization of interictal epileptiform activity: Con.

OBJECTIVE: To assess whether MEG is superior to scalp-EEG in the localization of interictal epileptiform activity and to stress the 'con' part in this controversy. METHODS: Advantages and disadvantages of the two techniques were systematically reviewed. RESULTS: While MEG and EEG complement each other for the detection of interictal epileptiform discharges, EEG offers the advantage of long-term recording significantly increasing its diagnostic yield which is not feasible with MEG. Localization accuracies of EEG and MEG are comparable once inaccuracies for the solution of the forward problem are eliminated. MEG may be more sensitive for the detection of neocortical spike sources. EEG and MEG source localizations show comparable agreement with invasive electrical recordings, can clarify the spatial relationship between the irritative zone and structural lesions, guide the placement of invasive electrodes and attribute epileptic activity to lobar subcompartments in temporal lobe epilepsy and to a lesser extent in extratemporal epilepsy. CONCLUSIONS: A clear superiority of MEG over EEG for the localization of interictal epileptiform activity cannot be derived from the studies presently available. SIGNIFICANCE: The combination of EEG and MEG provides information for the localization of interictal epileptiform activity which cannot be obtained with either technique alone.     

Magnetoencephalography is more successful for screening and localizing frontal lobe epilepsy than electroencephalography

PURPOSE: The diagnosis of frontal lobe epilepsy may be compounded by poor electroclinical localization, due to distributed or rapidly propagating epileptiform activity. This study aimed at developing optimal procedures for localizing interictal epileptiform discharges (IEDs) of patients with localization related epilepsy in the frontal lobe. To this end the localization results obtained for magnetoencephalography (MEG) and electroencephalography (EEG) were compared systematically using automated analysis procedures. METHODS: Simultaneous recording of interictal EEG and MEG was successful for 18 out of the 24 patients studied. Visual inspection of these recordings revealed IEDs with varying morphology and topography. Cluster analysis was used to classify these discharges on the basis of their spatial distribution followed by equivalent dipole analysis of the cluster averages. The locations of the equivalent dipoles were compared with the location of the epileptogenic lesions of the patient or, if these were not visible at MRI with the location of the interictal onset zones identified by subdural electroencephalography. RESULTS: Generally IEDs were more abundantly in MEG than in the EEG recordings. Furthermore, the duration of the MEG spikes, measured from the onset till the spike maximum, was in most patients shorter than the EEG spikes. In most patients, distinct spike subpopulations were found with clearly different topographical field maps. Cluster analysis of MEG spikes followed by dipole localization was successful (n = 14) for twice as many patients as for EEG source analysis (n = 7), indicating that the localizability of interictal MEG is much better than of interictal EEG. CONCLUSIONS: The automated procedures developed in this study provide a fast screening method for identifying the distinct categories of spikes and the brain areas responsible for these spikes. The results show that MEG spike yield and localization is superior compared with EEG. This finding is of importance for the diagnosis and preoperative evaluation of patients with frontal lobe epilepsy.     

Benign epileptiform discharges in Rolandic region with mesial temporal lobe epilepsy: MEG, scalp and intracranial EEG features

AIM OF THE STUDY: To report benign epileptiform discharges (BEDs) in the Rolandic region, coexisting in a pediatric patient with intractable localization-related epilepsy, secondary to hippocampal sclerosis. METHODS: We describe the clinical features, MRI, scalp video EEG, magnetoencephalography (MEG) and intracranial video EEG findings, and surgical outcome in a 9-year-old boy with BEDs and intractable complex partial seizures. RESULTS: MRI showed left hippocampal sclerosis. Scalp video EEG interictally demonstrated left temporal spike and sharply contoured slow waves, and right fronto-centro-temporal spike and waves. Ictal scalp video EEG showed left temporal rhythmic sharp waves after the clinical onset of epigastric aura, followed by staring. MEG showed interictal dipoles in the bilateral Rolandic regions with a uniform orientation and right hemispheric predominance. Intracranial video EEG, with bilateral mesial temporal depth and fronto-temporo-parietal strip electrodes, interictally showed polyspikes and slow waves with superimposed low-amplitude fast waves in the left mesial and posterior lateral temporal regions, and spike and waves in the bilateral fronto-parietal regions. Ictal onset was marked by low-amplitude fast waves in the left mesial and posterior lateral temporal regions. He underwent left anterior temporal lobectomy with hippocampectomy. Pathology was hippocampal sclerosis. Predominant right fronto-centro-temporal spike and waves and MEG right Rolandic dipoles persisted after surgery. He was seizure-free 14 months after surgery. CONCLUSION: This is the first report on MEG and intracranial video EEG features of BEDs in the Rolandic region, coexisting with hippocampal sclerosis. Persistence of contralateral benign MEG Rolandic dipoles after surgery indicates that BEDs are coincidental in mesial temporal lobe epilepsy. MEG identified Rolandic dipoles, although was unable to localize the deep and focal epileptogenic dipoles from the hippocampal sclerosis.     

MEG and EEG in epilepsy.

Both EEG and magnetoencephalogram (MEG), with a time resolution of 1 ms or less, provide unique neurophysiologic data not obtainable by other neuroimaging techniques. MEG has now emerged as a mature clinical technology. While both EEG and MEG can be performed with more than 100 channels, MEG recordings with 100 to 300 channels are more easily done because of the time needed to apply a large number of EEG electrodes. EEG has the advantage of the long-term video EEG recordings, which facilitates extensive temporal sampling across all periods of the sleep/wake cycle. MEG and EEG seem to complement each other for the detection of interictal epileptiform discharges, because some spikes can be recorded only on MEG but not on EEG and vice versa. Most studies indicate that MEG seems to be more sensitive for neocortical spike sources. Both EEG and MEG source localizations show excellent agreement with invasive electrical recordings, clarify the spatial relationship between the irritative zone and structural lesions, and finally, attribute epileptic activity to lobar subcompartments in temporal lobe and to a lesser extent in extratemporal epilepsies. In temporal lobe epilepsy, EEG and MEG can differentiate between patients with mesial, lateral, and diffuse seizure onsets. MEG selectively detects tangential sources. EEG measures both radial and tangential activity, although the radial components dominate the EEG signals at the scalp. Thus, while EEG provides more comprehensive information, it is more complicated to model due to considerable influences of the shape and conductivity of the volume conductor. Dipole localization techniques favor MEG due to the higher accuracy of MEG source localization compared to EEG when using the standard spherical head shape model. However, if special care is taken to address the above issues and enhance the EEG, the localization accuracy of EEG and MEG actually are comparable, although these surface EEG analytic techniques are not typically approved for clinical use in the United States. MEG dipole analysis is approved for clinical use and thus gives information that otherwise usually requires invasive intracranial EEG monitoring. There are only a few dozen whole head MEG units in operation in the world. While EEG is available in every hospital, specialized EEG laboratories capable of source localization techniques are nearly as scarce as MEG facilities. The combined use of whole-head MEG systems and multichannel EEG in conjunction with advanced source modeling techniques is an area of active development and will allow a better noninvasive characterization of the irritative zone in presurgical epilepsy evaluation. Finally, additional information on epilepsy may be gathered by either MEG or EEG analysis of data beyond the usual bandwidths used in clinical practice, namely by analysis of activity at high frequencies and near-DC activity.     

Two magneto-encephalographic epileptic foci did not coincide with the electrocorticographic ictal onset zone in a patient with temporal lobe epilepsy.

To evaluate the usefulness and limitations of magneto-encephalography (MEG) for epilepsy surgery, we compared 'interictal' epileptic spike fields on MEG with ictal electrocorticography (ECoG) using invasive chronic subdural electrodes in a patient with intractable medial temporal lobe epilepsy (MTLE) associated with vitamin K deficiency intracerebral hemorrhage. A 19-year-old male with an 8-year history of refractory complex partial seizures, secondarily generalized, and right hemispheric atrophy and porencephaly in the right frontal lobe on MRI, was studied with MEG to define the interictal paroxysmal sources based on the single-dipole model. This was followed by invasive ECoG monitoring to delineate the epileptogenic zone. MEG demonstrated two paroxysmal foci, one each on the right lateral temporal and frontal lobes. Ictal ECoG recordings revealed an ictal onset zone on the right medial temporal lobe, which was different from that defined by MEG. Anterior temporal lobectomy with hippocampectomy was performed and the patient has been seizure free for two years. Our results indicate that interictal MEG does not always define the epileptogenic zone in patients with MTLE.     

Magnetoencephalography source localization and surgical outcome in temporal lobe epilepsy.

OBJECTIVE: We prospectively investigated the role of magnetoencephalography (MEG) in localizing the seizure focus and in predicting outcome to surgical resections for intractable temporal lobe epilepsy (TLE). METHODS: We performed simultaneous interictal EEG and MEG recording (two 37-channel system) in 26 TLE patients followed by MEG source localization. We correlated early modeling dipoles with intracranial EEG, temporal surgical resection and surgical outcome. RESULTS: There were 12 patients who had anterior temporal horizontal or tangential dipoles to the anterior infero-lateral temporal tip cortex. Two patients underwent selective amygdalo-hippocampectomy (SAH) and nine patients had antero-medial temporal lobectomy (AMTL). All patients had successful outcome except for one patient who initially failed SAH, but became seizure-free after AMTL. There were 11 patients who demonstrated anterior temporal vertical or tangential oblique dipoles. Five patients had AMTL and three had SAH; all became seizure free. Five of above 23 patients had invasive EEG and demonstrated mesial seizure onset. Three TLE patients had lateral vertical dipoles that were concordant with intracranial EEG and these became seizure free after temporal neocortical resections. CONCLUSIONS: MEG source analysis produces distinct source patterns that provide useful localizing information, predict surgical outcome, and may aid in planning limited surgical resection in TLE.     

Ictal magnetoencephalographic discharges from elementary visual hallucinations of status epilepticus

PURPOSE: To report the rare opportunity to study ictal magnetoencephalography (MEG) in a 26 year old man with simple partial status epilepticus that presented as elementary visual hallucinations (EVHs) in the right upper visual field. METHODS: The patient described his EVHs as "snowing on TV," "flickering lights," and "rotating coloured balls" that continued for several days. MEG and simultaneous EEG were recorded twice: during an episode of EVHs (ictal recordings) and after EVHs were controlled by medications (interictal recordings). RESULTS: During EVHs, MEG showed continuous periodic epileptiform discharges over the left posterior superior temporal region, while simultaneous EEG showed rhythmic theta waves and sporadic spikes over the left temporal region. The MEG discharge consisted of a three phase spike complex. Equivalent current dipoles (ECDs), modelled from spike complexes, localised in the left superior temporal area. After drug treatment controlled the EVHs, interictal MEG and EEG showed rare spikes over the same left temporal region. The average ictal ECD moment (mean (SD)) (128.7 (32.8 nAm)) was significantly weaker than the average interictal ECD moment (233.0 (63.9) nAm) (p<0.05). CONCLUSIONS: The continuous, periodic, and clustered discharges seen on ictal MEG were the sources of EVH. The weaker ictal ECD sources were frequently not detected by scalp EEG, while the stronger interictal sources, presumably originating from an extensive interictal zone, were sufficiently large to be seen as EEG spikes.     

Ictal magnetoencephalography in temporal and extratemporal lobe epilepsy

PURPOSE: We evaluated visual patterns and source localization of ictal magnetoencephalography (MEG) in patients with intractable temporal lobe epilepsy (TLE) and extratemporal epilepsy (ETE). METHODS: We performed spike and seizure recording simultaneously with EEG and MEG on two patients with TLE and five patients with ETE. Scalp EEG was recorded from 21 channels (10-20 international system), whereas MEG was recorded from two 37-channel sensors. We compared ictal EEG and MEG onset, frequency, and evolution and performed MEG dipole source localization of interictal spikes and early ictal discharges and co-registered dipoles to brain magnetic resonance imaging (MRI). We correlated dipole characteristics with intracranial EEG, surgical resection, and outcome. RESULTS: Ictal MEG lateralized seizure onset in both TLE patients and demonstrated ictal onset, frequency, and evolution in accordance with EEG. Ictal MEG source analysis revealed tangential vertical dipoles in the anterolateral angle in one patient, and anterior dipoles with anteroposterior orientation in the other. Intracranial EEG revealed regional entorhinal seizure onset in the first patient. Both patients became seizure free after temporal lobectomy. In ETE, ictal MEG demonstrated visual patterns similar to ictal EEG and had concordant localization with interictal MEG in all five patients. Two patients underwent surgery. Ictal MEG localization was concordant with intracranial EEG in both cases. One patient had successful outcome after surgery. The second patient did not improve after limited resection and multiple subpial transections. CONCLUSIONS: Ictal MEG can demonstrate ictal onset frequency and evolution and provide useful localizing information before epilepsy surgery.     

Magnetoencephalographic and electroencephalographic source localization of the interictal spike activities in patients with medial temporal lobe epilepsy]

Magnetoencephalographic (MEG) and electroencephalographic (EEG) source localization of the interictal spike activities was performed in 5 patients with medial temporal lobe epilepsy (MTLE) to clarify the usefulness of MEG as the preoperative noninvasive examination. According to the Ebersole's classification based on the pattern of spike source localization and orientation, three patients were classified to posterior temporal vertical type, one anterior temporal horizontal type, and one anterior temporal vertical type. In all cases, the MEG and EEG spike did not completely coincide in waveform with each other. The latency of the best correlated equivalent current dipole (ECD) with MEG was slightly shorter than the peak latency of EEG spike. Although the EEG-source montage images at this latency were failed to reveal the potential to be localized to the medial temporal lobe, they demonstrated the potential to extend to the lateral temporal lobe at the peak latency of EEG spike. All patients became seizure free following anterior temporal lobectomy with hippocampectomy. The MEG spike source localization based on the Ebersole's classification may be calculated by the potential extended to the lateral temporal lobe, and, thus, was not useful in the diagnosis of MTLE and for predicting postoperative seizure outcome.     

Ictal magnetic source imaging as a localizing tool in partial epilepsy

OBJECTIVE: To determine the feasibility and usefulness of ictal magnetoencephalography (MEG) recordings in the presurgical evaluation of patients with epilepsy. METHODS: Twenty patients with frequent or predictable seizures were studied with the intent to capture seizures using a large array single-probe 37-channel or dual-probe 74-channel biomagnetometer. RESULTS: Successful ictal MEG recordings were made in 6 of 20 patients with neocortical epilepsy. In one other patient, a seizure was captured but movement artifact made MEG recordings impossible. As determined by invasive EEG recording and postsurgical outcome, ictal MEG provided localizing information that was superior to interictal MEG in three of the six patients. Localization of ictal onset by MEG was at least equivalent to invasive EEG in five of the six patients, and was superior in two patients as determined by postsurgical outcome. CONCLUSION: Larger studies are necessary to confirm that ictal MEG recordings in patients with frequent or easily provoked neocortical seizures can contribute localizing information equivalent or superior to invasive EEG recording.     

Abstracts of the 5th joint meeting of the German, Austrian, and Swiss Sections of the International League Against Epilepsy, Basle, May 16-19, 2007

Presidential Symposium ¹ E. Pataraia
¹ University Hospital of Neurology (Vienna, A) Goals, methodology: Epilepsy surgery is defined as any neurosurgical intervention with the primary goal to relieve intractable epilepsy. On the other hand essential brain regions like primary motor and sensory cortex as well as brain areas supporting language and memory functions have to be spared to avoid neurological deficits caused by the operation. Thus, the exact localization of the epileptogenic zone and of essential brain regions is crucial for the successful surgical treatment of seizures that can only be accomplished during a thorough presurgical work‐up. A new noninvasive brain mapping procedure magnetoencephalography (MEG) was employed during the presurgical evaluation for localization of the epileptogenic zones and for the determination of hemispheric dominance and intrahemispheric localization of linguistic functions in patients with drug‐resistant focal epilepsies. Results: The role of MEG for the localization of the epileptogenic zone in the noninvasive evaluation of patients with focal drug‐resistant epilepsies: We evaluated the sensitivity and selectivity of interictal MEG versus prolonged ictal and interictal scalp video‐ EEG in order to identify patient groups that would benefit from preoperative MEG testing. One hundred thirteen consecutive patients with medically refractory epilepsy who underwent surgery were included. The epileptogenic region predicted by interictal and ictal Video‐EEG (V‐EEG) and MEG was defined in relation to the resected area as perfectly overlapping with the resected area, partially overlapping, or nonoverlapping. Using MEG, we were able to localize the resected region in a greater proportion of patients (72.3%) than with noninvasive V‐EEG (40%). MEG contributed to the localization of the resected region in 58.8% of the patients with a non‐localizing V‐EEG study and 72.8% of the patients for whom V‐EEG only partially identified the resected zone. Overall, MEG and V‐EEG results were equivalent in 32.3% of the cases, and additional localization information was obtained using MEG in 40% of the patients. MEG was most useful for presurgical planning in patients who had either partially or nonlocalizing V‐EEG results. Functional organization of interictal spike complex in medial temporal lobe epilepsies: Thirty patients with mesial temporal lobe epilepsy (MTLE) using combined MEG and EEG recordings were icluded. Spikes could be recorded in 14 patients (47%) during the 2‐ to 3‐h MEG/EEG recording session. The MEG and EEG spikes were subjected to separate dipole analyses and the spike dipole localizations were superimposed on MRI scans. All spike dipoles could be localized to the temporal lobe with a clear preponderance in the medial region. Based on dipole orientations in MEG, patients could be classified into two groups: patients with anterior medial vertical (AMV) dipoles, suggesting epileptic activity in the mediobasal temporal lobe and patients with anterior medial horizontal (AMH) dipoles, indicating involvement of the temporal pole and the anterior parts of the lateral temporal lobe. Whereas patients with AMV dipoles had strictly unitemporal interictal and ictal EEG changes during prolonged video‐EEG monitoring, 50% of patients with AMH dipoles showed evidence of bitemporal affection on interictal and ictal EEG. Nine patients underwent epilepsy surgery so far: all five patients with AMV dipoles became completely seizure‐free postoperatively (Class Ia) and two out of four patients with AMH dipoles experienced persistent auras (Class Ib). Plasticity of the brain mechanisms for receptive language in patients with mesial temporal lobe epilepsy and structural lesions: We examined brain activation profiles for receptive language function in patients with left hemisphere space occupying lesions and patients with left temporal lobe epilepsy due to mesial temporal sclerosis (MTS) to assess whether cross‐ and intrahemispheric plasticity for language varied as a function of lesion type or location. We evaluated 44 patients: 21 patients with MTS and 23 lesional patients. All patients underwent preoperative language mapping while performing a word recognition task. The location of the activity sources was subsequently determined by co‐registering them with MRIs. The number of clustered, contiguous activity sources located in temporal and inferior parietal regions (excluding sources in somatosensory cortices) was then assessed. Hemispheric lateralization of language‐specific magnetic activity was determined as left hemispheric, right hemispheric and bilateral according to relation of the acceptable late activity sources in left and right hemispheres. Patients were classified into two groups based on the location of the cluster(s) of language‐specific activity sources within the dominant hemisphere: typical localization of receptive language‐specific cortex (if the cluster of activity sources fell within the cortical region that is commonly identified as Wernicke's area) and atypical localization of receptive language‐specific cortex (if the cluster of activity sources did not overlap with Wernicke's area). A higher incidence of atypical language lateralization was noted among patients with MTS compared with lesional patients (43% vs. 13%). The majority of MTS patients with early seizure onset (before 5 years of age) showed atypical language lateralization. In contrast, the precise location of receptive language‐specific cortex within the dominant hemisphere was found to be outside of Wernicke's area in 30% of lesional patients and only 14% of MTS patients. There is an increased probability of a partial or total displacement of key components of the brain mechanisms responsible for receptive language function to the nondominant hemisphere in MTS patients. Early onset of seizures was strongly associated with atypical language lateralization. Lesions in the dominant hemisphere tend to result in an intrahemispheric reorganization of linguistic function. Organization of receptive language‐specific cortex before and after left temporal lobectomy: In the present study we documented the reorganization of brain areas mediating receptive language function in patients with left temporal lobe epilepsy after a standard anterior temporal lobe resection. We evaluated which patients were most likely to show a change in the lateralization and localization of the mechanisms supporting receptive language and if such changes were associated with neuropsychological function. The results of preoperative Wada‐testing and pre‐ and post‐operative neuropsychological testing and MEG language mapping were compared. Patients with atypical (bilateral) hemispheric dominance pre‐operatively were significantly more likely than patients with (typical) left‐hemisphere dominance to show evidence of increased right hemisphere participation in language functions after surgery. Patients with left hemispheric dominance preoperatively were more likely to show intra‐hemispheric changes involving a slight inferior shift of the putative location of Wernicke's area. Patients with bilateral representation tended to perform worse on neuropsychological test measures obtained both pre‐ and postoperatively. Interhemispheric functional reorganization of language‐specific areas may occur in patients undergoing left anterior temporal lobectomy. Intrahemispheric reorganization may take place even when the resection does not directly impinge upon Wernicke's area. Conclusions: Combined MEG/EEG dipole modeling can identify subcompartments of the temporal lobe involved in epileptic activity and may be helpful to differentiate between subtypes of mesial temporal lobe epilepsy noninvasively. MEG is most useful for presurgical planning in patients who have either partially or nonlocalizing V‐EEG results in the noninvasive evaluation phase. We predict the replacement of the more invasive procedure with MEG in the near future for temporal lobe epilepsies, subsequent to the optimization of the conditions under which preoperative MEG is performed. MEG can be especially helpful in the localization of language‐critical cortex in sites other than those expected within the dominant hemisphere. Our findings also suggest that not only structural elements, but also functional factors have an effect on receptive language organization in the brain. Factors influencing atypical language lateralization have theoretical importance for understanding the organization and reorganization of higher cognitive functions, as well as practical implications, especially in brain surgery and neurological rehabilitation. MEG is a useful method in clinical practice, as it has the capacity to provide reliable images of the working brain of individual subjects, and it is capable of capturing relevant aspects of brain activation by reflecting the actual participation of a particular area in the function under investigation. Finally, it is capable of capturing both the spatial as well as the temporal features of that activation.     

Combined use of sphenoidal electrodes and the dipole localization method for the identification of the mesial temporal focus

We attempted to sub-classify four cases who show temporal spikes on standard scalp electroencephalogram (EEG), using sphenoidal electrodes and the dipole localization METHOD: In a case with mesial temporal epilepsy, spikes showed phase reversal in a sphenoidal electrode, and the spike dipoles were estimated to be in the mesial temporal lobe. In a case with lateral temporal epilepsy, spikes showed no phase reversal in a sphenoidal electrode, and the spike dipoles were estimated to be in the lateral temporal lobe. In two cases out of four, spikes showed phase reversal in sphenoidal electrodes, whilst the dipoles were estimated to be in the frontal lobe. Clinical features also suggested a diagnosis of frontal lobe epilepsy. In one of the two cases in which frontal lobe epilepsy was suspected, ictal dipoles as well as interictal spike dipoles indicated participation of the frontal lobe in the genesis of seizures. Nevertheless, only mesial temporal lobectomy was performed based on results obtained by invasive subdural electrodes. As a result, seizures were not controlled. Although sphenoidal electrodes were useful for differentiating between mesial and lateral temporal lobe foci, it is advisable to use them in combination with the dipole localization method to identify frontal lobe foci.     

Neuromagnetic separation of secondarily bilateral synchronized spike foci: report of three cases.

To demonstrate the high spatiotemporal resolution of magnetoencephalography (MEG), we report three cases with focal epilepsy that exhibited bilateral synchronized spikes on simultaneous scalp EEG and MEG recording. Constant time lags (19.4 +/- 3.0 ms and 20.0 +/- 5.5) between the leading and the following contralateral spikes were noted on MEG and the current dipole sources were localized in the bilateral homotopic regions symmetrically in Cases 1 and 3. In Case 2, MEG indicated leading spikes in the left frontal region, with a time lag of 42.3 +/- 8.4 ms to reach the contralateral frontal and bilateral temporal regions as well. Chronic subdural EEG recording in Cases 1 and 2 confirmed that the leading spike focus in MEG was close to the seizure onset zone in cortical EEG. Spatio-temporal analysis of MEG spikes may be useful to identify the primary epileptic region in patients with synchronized bilateral epileptiform discharges.     

Two magneto-encephalographic epileptic foci did not coincide with the electrocorticographic ictal onset zone in a patient with temporal lobe epilepsy

Abstract

To evaluate the usefulness and limitations of magneto-encephalography (MEG) for epilepsy surgery, we compared 'interictal' epileptic spike fields on MEG with ictal electrocorticography (ECoG) using invasive chronic subdural electrodes in a patient with intractable medial temporal lobe epilepsy (MTLE) associated with vitamin K deficiency intracerebral hemorrhage. A 19-year-old male with an 8-year history of refractory complex partial seizures, secondarily generalized, and right hemispheric atrophy and porencephaly in the right frontal lobe on MRI, was studied with MEG to define the interictal paroxysmal sources based on the single-dipole model. This was followed by invasive ECoG monitoring to delineate the epileptogenic zone. MEG demonstrated two paroxysmal foci, one each on the right lateral temporal and frontal lobes. Ictal ECoG recordings revealed an ictal onset zone on the right medial temporal lobe, which was different from that defined by MEG. Anterior temporal lobectomy with hippocampectomy was performed and the patient has been seizure free for two years. Our results indicate that interictal MEG does not always define the epileptogenic zone in patients with MTLE. [Neurol Res 2001; 23: 830-834]     

"Cavernous Sinus EEG": A New Method for the Preoperative Evaluation of Temporal Lobe Epilepsy

Summary: Purpose: In presurgical evaluation of temporal lobe epilepsy (TLE), invasive methods are necessary if results of various noninvasive methods are not sufficiently convergent enough to identify the epileptogenic area accurately. To detect the epileptiform discharges originating specifically from the mesial temporal lobe, we applied the cavernous sinus catheterization technique. Methods: We placed Seeker Lite-10 guide wire electrodes into bilateral cavernous sinus through the internal jugular veins to record EEG (cavernous sinus EEG) in 6 patients with intractable TLE. Scalp EEG was simultaneously recorded in all 6 and electrocorticogram (ECoG) was also recorded in 4. Results: The cavernous sinus EEG demonstrated clear epileptiform discharges, sometimes even when they were absent on the simultaneously recorded scalp EEG. The epileptiform discharges recorded from the cavernous sinus electrodes were specifically associated with those in the mesial temporal region.on ECoG. Ictal EEG pattern originating from mesial temporal lobe was also clearly documented on the cavernous sinus EEG. Conclusions: This new, semi-invasive method of identifying epileptogenic areas can detect the epileptiform discharges specifically arising from the mesial temporal lobe; it is as useful as or complements the invasive techniques such as foramen ovale or depth recording.     

Interictal and ictal magnetoencephalographic study in patients with medial frontal lobe epilepsy

PURPOSE: To determine whether magnetoencephalography (MEG) has any clinical value for the analysis of seizure discharges in patients with medial frontal lobe epilepsy (FLE). METHODS: Four patients were studied with 74-channel MEG. Interictal and ictal electroencephalographic (EEG) and MEG recordings were obtained. The equivalent current dipoles (ECDs) of the MEG spikes were calculated. RESULTS: In two patients with postural seizures, interictal EEG spikes occurred at Cz or Fz. The ECDs of interictal MEG spikes were localized around the supplementary motor area. In the other two patients with focal motor or oculomotor seizures, interictal EEG spikes occurred at Fz or Cz. The ECDs of interictal MEG spikes were localized at the top of the medial frontal region. The ECDs detected at MEG ictal onset were also localized in the same area as those of the interictal discharges. CONCLUSIONS: In medial FLE patients, interictal and ictal MEG indicated consistent ECD localization that corresponded to the semiology of clinical seizures. Our findings demonstrate that MEG is a useful tool for detecting epileptogenic focus.     

Scalp EEG in temporal lobe epilepsy surgery

Electroencephalography (EEG) with standard scalp and additional noninvasive electrodes plays a major role in the selection of patients for temporal lobe epilepsy surgery. Recent studies have provided data supporting the value of interictal and postictal EEG in assessing the site of ictal onset. Scalp ictal rhythms are morphologically complex but at least one pattern (a five cycles/second rhythm maximum at the sphenoidal or anterior temporal electrode) occurs in >50% of patients and has a high predictive value and interobserver reliability for temporal lobe originating seizures. Thorough interictal and ictal scalp EEG evaluation, in conjunction with modern neuroimaging, is sufficient for proceeding to surgery without invasive recordings in some patients. Further studies are required to define the scalp ictal characteristics of mesial vs. lateral temporal lobe epilepsy.     

The magnetic field of epileptic spikes agrees with intracranial localizations in complex partial epilepsy

The magnetoencephalogram (MEG) and electroencephalogram (EEG) were measured during interictal epileptic spikes in nine patients with complex partial seizures. The MEG localization estimates were compared with localizations by intraoperative cortical electrodes, subdural electrodes, stereotaxic depth electrodes, anatomic imaging, postoperative pathologic analysis, and postoperative follow-up. In all patients, MEG localization estimates were in the same lobe as the epileptic focus determined by invasive methods and EEG. In two patients, it was possible to quantify precisely the accuracy of MEG localization by mapping a spike focus that was visually indistinguishable on MEG and cortical recordings. In both patients, MEG localization was approximately 12 mm from the center of the cortical spike focus on intracranial recordings. In eight patients, MEG showed tangential dipolar field patterns on the spontaneous record, but EEG did not. In one patient, a cortical epileptic discharge was detected only on MEG for some discharges and only on EEG for other discharges. The MEG did not detect deep spikes with present levels of environmental noise.