Clinical Background
Researchers have sought to confirm the accuracy of MEG using both direct and indirect approaches. According to Robert Knowlton of the University of Alabama at Birmingham writing in Epilepsia, MEG can detect spikes that EEG does not and vice versa. In addition, Knowlton cites that given “ the classic estimate of 6cm cubic required for EEG to detect spikes, MEG may be more sensitive for convexity neocortical sources. Finally, MEG is intrinsically better at recording and detecting signals from sources that are primarily oriented tangentially to the convexity, such as intrasylvian cortex”. (1)
Direct methods of MEG accuracy in epilepsy localization mainly reflect work done with special intracranial electrodes and simultaneous intracranial/MEG recordings. Data from DF Rose published in Epilepsia from implanted dipoles with the intracranial electrodes in lateral, basal, and mesial regions of the temporal lobe indicated that MEG predicted localizations were respectively within 4, 2, and 1 mm of the actual locations.(2)
Knake of the Martinos/MGH/MIT/HMS Bioimaging Center presented during the Biomag 2004 meeting a study meant to evaluate the clinical impact of a combined 306 channel MEG and a 70 channel EEG recording array on the presurgical evaluation of epilepsy patients. The conclusion of the study was that MEG and simultaneously acquired EEG are complementary in the presurgical evaluation of the patient. The combined evaluation improved the selection of candidates for epilepsy surgery. (3) In addition, there was a suggestion that simultaneous MEG/EEG investigation could induce a change in the clinical management of the epilepsy patient.
In addition to using MEG as a noninvasive tool for spike localization, there is a general feeling that MEG could also be used to aid in the placement of intracranial electrodes for surgical epilepsy evaluations. It is hoped that MEG spikes would correlate well with intracranial electrodes. Sutherling in a study published in the journal Neurology looked at how well MEG predicted, in neocortical epilepsy, localization of discharges when compared with subdural grids used in presurgical epilepsy evaluation. (4)
In all of the patients evaluated by Sutherling, MEG localization estimates were in the same lobe as the epileptic focus determined by invasive methods and EEG.
MEG has great clinical utility in temporal lobe epilepsy (TLE) when one compares the distribution of spikes and surgical outcome related with anterior medial temporal lobectomy. Iwasaki in the Journal of Clinical Neuroscience compared anterior MEG spike localization with non anterior temporal spike localization, showing that patients with anterior localization became seizure free following anterior temporal lobectomy. (5)
In addition, Iwasaki suggested that preoperative MEG detected additional epileptogenic foci outside a surgically resected region in patients who were not seizure free post epilepsy surgery.
During the Biomag 2004 meeting, Ossenblok presented a study from the Netherlands aimed at demonstrating that advanced source analysis of interictal MEG yields an additional tool for preoperative localization in frontal lobe epilepsy (FLE) when compared to EEG alone. (6)
Clinical Case
Clinical History
The patient is a 38 year old female with a history of epileptic seizures occurring at a frequency of 3 to 4 per week. The patient also has a history of multiple head traumas.
Prior ambulatory EEG monitoring and EEG video monitoring showed electrographic seizures discharges over the right fronto-temporal areas, but no discrete localization was seen. Many of the observed clinical attacks were not associated with any electrographic seizure. Interictal transients were seen over the right temporal area. One video EEG session indicated right fronto-temporal activity associated with some clinical events, but precise localization was not clear.
Imaging Studies
Prior MRI at another institution excluded mesial temporal sclerosis.
MRI was performed using a 1.5 T MRI with the following pulse sequences:
- Sagittal and axial 3D RF-Fast whole brain images
- Axial FSE proton density and T2-weighted images
- Axial and coronal FLAIR images
- Axial EXPRESS images
MRI results
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MRI Results
There is an area of thickening and irregularity of the gray matter mantle involving the right perisylvian cortex which extends around the posterior margin of the sylvian fissure. This is an area of cortical dysplasia. In addition, there is diffuse cortical atrophy, more pronounced in the left hemisphere.
The MRI Impresssion: Right perisylvian cortical dysplasia and diffuse cortical atrophy
MEG
Magnetic Source Imaging (MSI) evaluation consisting of MEG and MRI was performed.
MEG Protocol
Data was acquired with a whole head MEG instrument. 70 minutes of continuous data were recorded during which the patient reported that she had a seizure. Following sedation with oral Chloralhydrate, approximately 50 minutes of continuous data were recorded while the patient slept. All data were analyzed off-line for epileptiform transients. Somatosensory functioning was assessed using electrical stimulation of right and left index fingers. Fingers were used due to the fact that the patient indicated prior wrist surgery bilaterally with no thumb twitch being achieved with stimulation above the median nerve.
MEG Results
Stimulation of the left index finger elicited a cortical response with age appropriate latency and distribution over the right hemisphere. The source of the 20 millisecond component ( which has been shown in normal subjects to localize to the primary somatosensory cortex) localized to an appropriate region of the post central gyrus. Stimulation of the right index finger elicited only a very weak left hemisphere cortical response and source localization was not possible.
The spontaneous data were abnormal, showing right fronto-temporal slowing and frequent bursts of sharp transients over the right hemisphere. These bursts appear epileptiform and are likely to reflect spike trains. The epileptiform activity occurred with only one distribution. The burst discharges were over the right perisylvian region and occurred at an overall rate of 2-3 per minute, but sometimes there were bursts every 3-4 seconds. Source modeling showed this activity to originate from the right inferior parietal/frontal junction, just above the sylvian plane, but with rapid spread and reciprocal to the right superior and middle temporal gyri. The implicated superior peri-sylvian region is the region identified as dysplastic on the MRI. There was no clear evidence for interhemispheric propagation. At least one electrographic seizure was recorded. This seizure was characterized by an extended 25 second long burst of sharp transients. Like the interictal bursts, sources for these localized above the sylvian plane at the inferior aspect of the central frontal region.
Summary
An abnormal MSI exam showing right hemisphere slowing and epileptiform activity appearing as bursts of sharp transients. Sources for these localized just above the sylvian plane at the inferior parietal frontal region with propagation to the superior and middle temporal gyri. Epileptic activity originates from the area of dysplasia and from the adjacent peri-rolandic cortex.
An overall conclusion of this clinical case of the MEG used to evaluate an epilepsy patient is one of an abnormal cranial MSI with epileptiform acivity involving the right perisylvian region corresponding to an area of cortical dysplasia and the adjacent cortex.
References
- Knowlton, R, Shih, J Magnetoencephalography in Epilepsy. Epilepsy 2004 Volume 45 Supplement 4 61-71.
- Rose DF, Sato S Magnetoencephalographic localization of subdural dipoles in a patient with temporal lobe epilepsy. Epilepsia 1991; 32 635-41.
- Knake S, Stufflebeam, S. et al. Whole Head MEG and EEG in the Presurgical Evaluation on Epilepsy Patient: A Prospective Study. Biomag 2004
- Sutherling WW, Crandall PH, Cahan LD et al. The magnetic field of epileptic spikes agrees with intracranial localizations in complex partial epilepsy. Neurology 1988; 38 778-86.
- Iwasaki M, Nakasato N, Shamoto H et al. Surgical implications by neuromagnetic spike localization in temporal lobe epilepsy Epilepsia 2002 43: 415-24.
- Ossenblok P, de Munck, et al. MSI yields an additional tool for successful presurgical evaluation of frontal lobe epilepsy Biomag 2004.
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