Magnetoencephalography for pediatric epilepsy: how we do it

Pediatric Anesthesia Outside the Operating Room: Case Management

Anesthesiology teams care for children in diverse locations, including diagnostic and interventional radiology, gastroenterology and pulmonary endoscopy suites, radiation oncology units, and cardiac catheterization laboratories. To provide safe, high-quality care, anesthesiologists working in these environments must understand the unique environmental and perioperative considerations and risks involved with each remote location and patient population. Once these variables are addressed, anesthesia and procedural teams can coordinate to ensure that patients and families receive the same high-quality care that they have come to expect in the operating room. This article also describes some of the considerations for anesthetic care in outfield locations.

Neuroimaging in Pediatric Epilepsy

Pediatric epilepsy presents with various diagnostic challenges. Recent advances in neuroimaging play an important role in the diagnosis, management and in guiding the treatment of pediatric epilepsy. Structural neuroimaging techniques such as CT and MRI can identify underlying structural abnormalities associated with epileptic focus. Functional neuroimaging provides further information and may show abnormalities even in cases where MRI was normal, thus further helping in the localization of the epileptogenic foci and guiding the possible surgical management of intractable/refractory epilepsy when indicated. A multi-modal imaging approach helps in the diagnosis of refractory epilepsy. In this review, we will discuss various imaging techniques, as well as aspects of structural and functional neuroimaging and their application in the management of pediatric epilepsy.

Advances in pediatric neuroimaging

Conventional MRI protocols are an integral part of routine clinical imaging in pediatric patients. The advent of several newer MRI techniques provides crucial insight into the structural integrity and functional aspects of the developing brain, especially with the introduction of 3T MRI systems in clinical practice. The field of pediatric neuroimaging continues to evolve, with greater emphasis on high spatial resolution, faster scan time, as well as a quest for visualization of the functional aspects of the human brain. MR vendors are increasingly focusing on optimizing MR technology to make it suitable for children, in whom as compared to adults the head size is usually smaller and demonstrates inherent neuroanatomical differences relating to brain development. The eventual goal of these advances would be to evolve as potential biomarkers for predicting neurodevelopment outcomes and prognostication, in addition to their utility in routine diagnostic and therapeutic decision-making. Advanced MR techniques like diffusion tensor imaging, functional MRI, MR perfusion, spectroscopy, volumetric imaging and arterial spin labeling add to our understanding of normal brain development and pathophysiology of various neurological disease processes. This review is primarily focused on outlining advanced MR techniques and their current and potential pediatric neuroimaging applications as well as providing a brief overview of advances in hardware and machine design.

Improving the excess kurtosis (g2) method for localizing epileptic sources in magnetoencephalographic recordings

OBJECTIVE

To suggest ways to apply the excess kurtosis estimator g2, in the detection of epileptic activity with magnetoencephalography, while avoiding its bias towards detecting high-amplitude, infrequent events.

METHODS

Synthetic aperture magnetometry (SAM), combined with g2, was applied using window lengths ranging from 0.125 s to 32 s and with sum and maximum metrics on simulated data and recordings of two focal epilepsy patients.

RESULTS

Comparing sources with different spike rates (two per second and one per 2s), the sum metric was most efficient when using a window of 0.25s. Simulations showed that the sum metric is insensitive to spike frequency when the window includes more than one spike. SAM(g2) images from long segments with maximum metric resulted in misleading images, showing the strongest activity away from the lesions.

CONCLUSIONS

Using a sliding window and the sum metric is beneficial when imaging interictal spikes and status epilepticus. Windows should be short enough not to include more than one interictal event. For continuous events such as electrographic seizures windows should contain baseline data and the epileptic event.

SIGNIFICANCE

The sliding window and metric should be set according to the suggested guidelines when using SAM(g2) for presurgical evaluation.

Somatosensory evoked field in response to visuotactile stimulation in 3- to 4-year-old children

A child-customized magnetoencephalography system was used to investigate somatosensory evoked field (SEF) in 3- to 4-year-old children. Three stimulus conditions were used in which the children received tactile-only stimulation to their left index finger or visuotactile stimulation. In the two visuotactile conditions, the children received tactile stimulation to their finger while they watched a video of tactile stimulation applied either to someone else’s finger (the finger-touch condition) or to someone else’s toe (the toe-touch condition). The latencies and source strengths of equivalent current dipoles (ECDs) over contralateral (right) somatosensory cortex were analyzed. In the preschoolers who provided valid ECDs, the stimulus conditions induced an early-latency ECD occurring between 60 and 68 ms mainly with an anterior direction. We further identified a middle-latency ECD between 97 and 104 ms, which predominantly had a posterior direction. Finally, initial evidence was found for a late-latency ECD at about 139–151 ms again more often with an anterior direction. Differences were found in the source strengths of the middle-latency ECDs among the stimulus conditions. For the paired comparisons that could be formed, ECD source strength was more pronounced in the finger-touch condition than in the tactile-only and the toe-touch conditions. Although more research is necessary to expand the data set, this suggests that visual information modulated preschool SEF. The finding that ECD source strength was higher when seen and felt touch occurred to the same body part, as compared to a different body part, might further indicate that connectivity between visual and tactile information is indexed in preschool somatosensory cortical activity, already in a somatotopic way.

[Neuroimaging in pediatric epilepsy]

The main causes of epilepsy in children are cortical malformations (hemimegalencephaly, cortical dysplasia, lissencephaly, etc.) and phakomatosis (tuberous sclerosis, Sturge-Weber disease, neurofibromatosis type 1, etc.), perinatal ischemia, traumatisms, infections, mesial temporal sclerosis, metabolic diseases, and tumors. Imaging indications are precise, including partial seizures and a pathological electroencephalogram. Twenty-five percent of these epilepsy cases are pharmacoresistant. Indeed, MRI is essential to consider surgical treatment, allowing one to localize potential epileptogenic anatomic lesions. The protocol includes sequences in three planes of space, weighted in T1, T2, Flair, T1 inversion-recovery, and T1 after gadolinium injection. MRI findings are characteristic for some tumors, but most malformations are subtle. Consequently recent techniques (spectroscopy, diffusion, etc.) are crucial when conventional MRI is not sufficient. The aim of this article is to illustrate, with a substantive image revue, this wide diversity of etiologies in pediatric epilepsy, in order to help the attendee recognize MRI findings, also discussing the role of newer imaging modalities in this field.

The role of magnetoencephalography in children undergoing hemispherectomy

OBJECT

Hemispherectomy is an established neurosurgical procedure for medication-resistant epilepsy in children. Despite the effectiveness of this technique, there are patients who do not achieve an optimum outcome after surgery; possible causes of suboptimal results include the presence of bilateral independent epileptogenic foci. Magnetoencephalography (MEG) is an emerging tool that has been found to be useful in the management of lesional and nonlesional epilepsy. The authors analyzed the relative contribution of MEG in patient selection for hemispherectomy.

METHODS

The medical records of children undergoing hemispherectomy at the Hospital for Sick Children were reviewed. Those patients who underwent MEG as part of the presurgical evaluation were selected.

RESULTS

Thirteen patients were included in the study. Nine patients were boys. The mean age at the time of surgery was 66 months (range 10-149 months). Seizure etiology was Rasmussen encephalitis in 6 patients, hemimegalencephaly in 2 patients, and cortical dysplasia in 4 patients. In 8 patients, video-EEG and MEG results were consistent to localize the primary epileptogenic hemisphere. In 2 patients, video-EEG lateralized the ictal onset, but MEG showed bilateral spikes. Two patients had bilateral video-EEG and MEG spikes. Engel Class I, II, and IV outcomes were seen in 10, 2, and 1 patients, respectively. In 2 of the patients who had an outcome other than Engel Class I, the MEG clusters were concentrated in the disconnected hemisphere. The third patient had bilateral clusters and potentially independent epileptogenic foci from bilateral cortical dysplasia.

CONCLUSIONS

The presence of unilateral MEG spike waves correlated with good outcomes following hemispherectomy. In some cases, MEG provides information that differs from that obtained from video-EEG and conventional MR imaging studies. Further studies with a greater number of patients are needed to assess the role of MEG in the preoperative assessment of candidates for hemispherectomy.

Magnetoencephalography in neonatology

Magnetoencephalography (MEG) is a noninvasive method to study brain activity. In the previous decade the advantages of MEG -- good temporal resolution combined with good spatial resolution allowing separation of activated brain areas -- have been successfully used in gaining new information about the neonatal brain functioning. In this review, we discuss the findings from studies of spontaneous magnetoencephalogram and evoked responses to somatosensory, auditory, and visual stimulation. Our group has shown that stimulation of the upper limb in neonates evokes a response sequence reflecting activation of both primary (S(I)) and secondary somatosensory (S(II)) cortices. Like in mature brains, the earliest cortical response to median nerve stimulation reflects the arrival of afferent information to S(I). However, source modeling of the subsequent activation from S(I)suggests immature cortical functioning in neonates. Another feature typical for neonates is that the S(II)response is prominent in quiet sleep, unlike in adults in whom it diminishes in sleep. Interestingly, in very prematurely-born infants, we found alterations of the somatosensory responses at both group and individual levels. MEG provides a novel way to look at brain activity in neonates and can be used to increase knowledge of the development of brain processing and its disturbances.

Source localization in magnetoencephalography to identify epileptogenic foci

RATIONALE

Magnetoencephalography (MEG) is useful to localize epileptic foci in epilepsy as MEG has higher spatio-temporal resolution than conventional diagnostic imaging studies; positron emission computed tomography, single photon emission computed tomography and magnetic resonance imaging (MRI).

METHODS

We use 204-channel helmet-shaped MEG with a sampling rate of 600 Hz. A single dipole method calculates equivalent current dipoles to localize epileptic sources. The equivalent current dipoles are superimposed onto MRI as magnetic source imaging (MSI). Ictal MEG data are analyzed using time-frequency analysis. The power spectrum density is calculated using short-time Fourier transform and superimposed onto MRI results.

RESULTS

Clustered equivalent current dipoles represent epileptogenic zones in patients with localization-related epilepsy. The surgical plan is reliably developed from source localizations of dipoles and power spectrum of interictal spike discharges, and ictal frequency.

CONCLUSION

MEG is indispensable in diagnosis and surgical resection for epilepsy to accurately localize the epileptogenic zone.

Magnetoencephalography

Although magnetoencephalography (MEG) may not be familiar to many pediatric radiologists, it is an increasingly available neuroimaging technique both for evaluating normal and abnormal intracranial neural activity and for functional mapping. By providing spatial, temporal, and time-frequency spectral information, MEG affords patients with epilepsy, intracranial neoplasia, and vascular malformations an opportunity for a sensitive and accurate non-invasive preoperative evaluation. This technique can optimize selection of surgical candidates as well as increase confidence in preoperative counseling and prognosis. Research applications that appear promising for near-future clinical translation include the evaluation of children with autism spectrum disorder, traumatic brain injury, and schizophrenia.