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

Scalp attached tangential magnetoencephalography using tunnel magneto-resistive sensors

Non-invasive human brain functional imaging with millisecond resolution can be achieved only with magnetoencephalography (MEG) and electroencephalography (EEG). MEG has better spatial resolution than EEG because signal distortion due to inhomogeneous head conductivity is negligible in MEG but serious in EEG. However, this advantage has been practically limited by the necessary setback distances between the sensors and scalp, because the Dewar vessel containing liquid helium for superconducting quantum interference devices (SQUIDs) requires a thick vacuum wall. Latest developments of high critical temperature (high-T_c) SQUIDs or optically pumped magnetometers have allowed closer placement of MEG sensors to the scalp. Here we introduce the use of tunnel magneto-resistive (TMR) sensors for scalp-attached MEG. Improvement of TMR sensitivity with magnetic flux concentrators enabled scalp-tangential MEG at 2.6 mm above the scalp, to target the largest signal component produced by the neural current below. In a healthy subject, our single-channel TMR-MEG system clearly demonstrated the N20m, the initial cortical component of the somatosensory evoked response after median nerve stimulation. Multisite measurement confirmed a spatially and temporally steep peak of N20m, immediately above the source at a latency around 20 ms, indicating a new approach to non-invasive functional brain imaging with millimeter and millisecond resolutions.

Epilepsy Surgery is a Viable Treatment for Lennox Gastaut Syndrome

Lennox Gastaut Syndrome (LGS) is a severe developmental epileptic encephalopathy with onset in childhood characterized by multiple seizure types and characteristic electroencephalogram findings. The majority of patients develop drug resistant epilepsy, defined as failure of 2 appropriate anti-seizure medications used at adequate doses. Epilepsy surgery can reduce seizure burden, in some cases leading to seizure freedom, and improve neuro-developmental outcomes and quality of life. Epilepsy surgery should be considered for all patients with drug resistant LGS. Herein, we review current surgical treatment options for patients with LGS, both definitive and palliative, including: focal cortical resection, vagus nerve stimulation and corpus callosotomy. Newer neuromodulation techniques will be explored, as well as the concept of LGS as a secondary network disorder.

Magnetoencephalography in presurgical epilepsy diagnosis

Magnetoencephalography (MEG) is a functional modality to register magnetic brain activity with high spatiotemporal resolution. Since distortion of magnetic fields by the skin, skull and cerebrospinal fluids is negligible, the technique offers an almost undistorted view on brain activity. While MEG systems are still expensive and complex, the technique's characteristics offer promising possibilities for the investigation of epilepsy patients, for example, for focus localization and presurgical functional mapping. This review gives an overview of the method and discusses advantages and limitations in the clinical context of presurgical epilepsy diagnosis.

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.

Magnetoencephalography in Neurosurgery

OBJECTIVE:

To present applications of magnetoencephalography (MEG) in studies of neurosurgical patients.

METHODS:

MEG maps magnetic fields generated by electric currents in the brain, and allows the localization of brain areas producing evoked sensory responses and spontaneous electromagnetic activity. The identified sources can be integrated with other imaging modalities, e.g., with magnetic resonance imaging scans of individual patients with brain tumors or intractable epilepsy, or with other types of brain imaging data.

RESULTS:

MEG measurements using modern whole-scalp instruments assist in tailoring individual therapies for neurosurgical patients by producing maps of functionally irretrievable cortical areas and by identifying cortical sources of interictal and ictal epileptiform activity. The excellent time resolution of MEG enables tracking of complex spaciotemporal source patterns, helping, for example, with the separation of the epileptic pacemaker from propagated activity. The combination of noninvasive mapping of subcortical pathways by magnetic resonance imaging diffusion tensor imaging with MEG source localization will, in the near future, provide even more accurate navigational tools for preoperative planning. Other possible future applications of MEG include the noninvasive estimation of language lateralization and the follow-up of brain plasticity elicited by central or peripheral neural lesions or during the treatment of chronic pain.

CONCLUSION:

MEG is a mature technique suitable for producing preoperative “road maps” of eloquent cortical areas and for localizing epileptiform activity.

Evaluation of Interhemispheric Time Difference by Magnetoencephalography Before and After Total Callosotomy-Two Case Reports-

Interhemispheric time difference (ITD) measured by electroencephalography (EEG) and magnetoencephalography (MEG) was compared to seizure outcome after callosotomy. Two patients with frequent drop attacks underwent simultaneous EEG and MEG before and after total callosotomy. ITDs in 30 bilateral synchronized (BS) discharges were calculated independently by EEG and MEG. As minimum transcallosal conduction time was suggested to be approximately 20 msec, BS discharges were classified into five categories according to ITD and side: left- or right-leading long (300 to 80 msec), left- or right-leading moderate (80 to 20 msec), and negligible (<20 msec). In Case 1 before callosotomy, EEG detected 77% negligible and 23% right-leading moderate BS discharges, whereas MEG detected 30% and 63%, respectively. After callosotomy, drop attacks reduced remarkably and EEG and MEG detected no BS discharges. In Case 2 before callosotomy, EEG detected 77% negligible and 23% moderate BS discharges, whereas MEG detected 80% and 20%, respectively. After callosotomy, drop attacks recurred 2 months later and EEG and MEG detected left- and right-leading long BS discharges (63% by EEG and 56% by MEG). MEG detected a large number of BS discharges with moderate ITD before surgery in Case 1, suggesting that the transcallosal pathway was the main pathway for the synchronization, whereas the negligible ITD in Case 2 excludes transcallosal propagation. BS discharges with longer ITD after surgery in Case 2 suggest a persistent poly-synaptic non-transcallosal pathway. MEG with higher spatial resolution than EEG may provide surgical indications for callosotomy.

Neuromagnetic measurement of unilateral temporo-parietal theta rhythm in patients with internal carotid artery occlusive disease

Rhythmic theta activity detected by electroencephalography (EEG) may be correlated with cerebrovascular brain diseases. Magnetoencephalography (MEG) has higher sensitivity and spatial resolution than conventional scalp EEG, so may be a better method to detect theta rhythm in patients with internal carotid artery (ICA) occlusive disease. Simultaneous EEG and MEG were performed in the awake state in 48 patients with unilateral (n = 42) or bilateral (n = 6) stenotic lesions (more than 60% occlusion) of the ICA (n = 47) or middle cerebral artery (n = 7), and in 27 age-matched healthy normal subjects. No subject had severe neurological deficits. MEG detected the theta rhythm (6-8 Hz) in 14 of 48 patients: ipsilateral to the stenotic or occluded side in 13 hemispheres and bilaterally in one patient with unilateral lesion. The source of the MEG theta rhythm was estimated in the dorsolateral temporo-parietal area, regardless of the location of infarct foci or the stenotic portion of the ICA system. The temporo-parietal theta rhythm was separated from the occipital alpha rhythm by frequency and distribution in MEG. The theta rhythm was found in only two patients by EEG, as well as by MEG. MEG provided better separation of this theta rhythm from the occipital alpha rhythm. Neither MEG nor EEG detected this theta rhythm in the normal subjects. Unilateral temporo-parietal theta rhythm is correlated with the hemisphere with ICA occlusive disease. This rhythm may indicate mild or subclinical abnormalities in the ICA system. MEG is superior to EEG for the detection and localization of theta rhythm.