EEG: Current relevance and promising quantitative analyses.

Electroencephalography (EEG) remains an essential tool, characterized by an excellent temporal resolution and offering a real window on cerebral functions. Surface EEG signals are mainly generated by the postsynaptic activities of synchronously activated neural assemblies. EEG is also a low-cost tool, easy to use at bed-side, allowing to record brain electrical activities with a low number or up to 256 surface electrodes. For clinical purpose, EEG remains a critical investigation for epilepsies, sleep disorders, disorders of consciousness. Its temporal resolution and practicability also make EEG a necessary tool for cognitive neurosciences and brain-computer interfaces. EEG visual analysis is essential in clinical practice and the subject of recent progresses. Several EEG-based quantitative analyses may complete the visual analysis, such as event-related potentials, source localizations, brain connectivity and microstates analyses. Some developments in surface EEG electrodes appear also, potentially promising for long term continuous EEGs. We overview in this article some recent progresses in visual EEG analysis and promising quantitative analyses.

Jean Talairach (1911-2007). An untold story of the pioneer of stereotactic and functional neurosurgery.

CONTEXT: The Department of neurosurgery of the Sainte-Anne Hospital hosted Jean Talairach who created and developed stereotactic neurosurgery in France. Despite numerous neurosurgical and neuroscientific achievements, little is known about the life of Jean Talairach. METHODS: Systematic screening of Sainte-Anne Hospital Museum, Henry Ey Library, and Bibliothèque Inter-Universitaire de Santé funds, and medical databases using the term "Jean Talairach". RESULTS: Jean Talairach started his medical career at the Sainte-Anne Hospital in 1942 as a psychiatrist and became a neurosurgeon due to his interest in stereotactic neurosurgery. During World War II, Jean Talairach joined the French Resistance in Paris, then the French First Army. Jean Talairach created an original and specific stereotactic methodology with appropriate stereotactic frame and tools and performed one of the first human stereotactic surgeries in 1948. He described the reference lines passing by the anterior and posterior commissures in 1952 and developed a tridimensional co-planar stereotactic atlas of the human brain. With the collaboration of Jean Bancaud, he created stereo-electroencephalography to investigate patients suffering from drug-resistant epilepsy. The "Sainte-Anne school" trained French and foreign stereotactic and functional neurosurgeons ensuring the spread of Jean Talairach's innovative ideas. Jean Talairach retired in 1980. CONCLUSION: Jean Talairach's achievements encapsulate the evolution of neurosurgery in France during the 20th century. He developed an original stereotactic methodology including a tridimensional stereotactic atlas of the human brain and a stereotactic frame. He created stereo-electroencephalography, which remains the gold-standard to investigate patients suffering from drug-resistant epilepsy.

Deep brain activities can be detected with magnetoencephalography.

The hippocampus and amygdala are key brain structures of the medial temporal lobe, involved in cognitive and emotional processes as well as pathological states such as epilepsy. Despite their importance, it is still unclear whether their  neural activity can be recorded non-invasively. Here, using simultaneous intracerebral and magnetoencephalography (MEG) recordings in patients with focal drug-resistant epilepsy, we demonstrate a direct contribution of amygdala and hippocampal activity to surface MEG recordings. In particular, a method of blind source separation, independent component analysis, enabled activity arising from large neocortical networks to be disentangled from that of deeper structures, whose amplitude at the surface was small but significant. This finding is highly relevant for our understanding of hippocampal and amygdala brain activity as it implies that their activity could potentially be measured non-invasively.

Sources of interrater variability and prognostic value of standardized EEG features in post-anoxic coma after resuscitated cardiac arrest.

OBJECTIVES: To assess interrater variability and prognostic value of simple EEG features based on the recent American Clinical Neurophysiology Society (ACNS) classification in post cardiac arrest comatose patients. METHODS: All patients admitted for a resuscitated cardiac arrest in a university hospital were prospectively included in the study. EEG interpretation was made by 3 independent neurophysiologists (2 senior and 1 junior) blind to the outcome. Kappa score and prognostic performances were estimated for each EEG pattern and discrepancies were analyzed. RESULTS: 122 cardiac arrest patients were admitted of whom 48 went through a full neurologic evaluation. Eighty-one percent had an unfavorable outcome. Burst suppression, paroxystic seizure activity, and non-reactive EEG were strongly associated with an unfavorable evolution. Kappa score between the senior neurophysiologists was excellent or substantial while it was only fair or slight between the junior and senior neurophysiologists. Reactivity, discontinuity and electrographic seizure were patterns particularly subject to discrepancy. CONCLUSIONS: Bedside EEG is an excellent tool for predicting outcome of post-anoxic coma through simple EEG features. However, the interrater variability emphasizes the need to be well trained for the standardized methods of evaluating EEG parameters. SIGNIFICANCE: A second look at complex patterns seems mandatory. The development of automated tools could help to improve the reliability of EEG interpretation.

Technical solutions for simultaneous MEG and SEEG recordings: towards routine clinical use.

OBJECTIVE: The simultaneous recording of intracerebral EEG (stereotaxic EEG, SEEG) and magnetoencephalography (MEG) is a promising strategy that provides both local and global views on brain pathological activity. Yet, acquiring simultaneous signals poses difficult technical issues that hamper their use in clinical routine. Our objective was thus to develop a set of solutions for recording a high number of SEEG channels while preserving signal quality. APPROACH: We recorded data in a patient with drug resistant epilepsy during presurgical evaluation. We used dedicated insertion screws and optically insulated amplifiers. We recorded 137 SEEG contacts on 10 depth electrodes (5-15 contacts each) and 248 MEG channels (magnetometers). Signal quality was assessed by comparing the distribution of RMS values in different frequency bands to a reference set of MEG acquisitions. MAIN RESULTS: The quality of signals was excellent for both MEG and SEEG; for MEG, it was comparable to that of MEG signals without concurrent SEEG. Discharges involving several structures on SEEG were visible on MEG, whereas discharges limited in space were not seen at the surface. SIGNIFICANCE: SEEG can now be recorded simultaneously with whole-head MEG in routine. This opens new avenues, both methodologically for understanding signals and improving signal processing methods, and clinically for future combined analyses.

Preoperative and intraoperative neurophysiological investigations for surgical resections in functional areas.

Brain regions are removed to treat lesions, but great care must be taken not to disturb or remove functional areas in the lesion and in surrounding tissue where healthy and diseased cells may be intermingled, especially for infiltrating tumors. Cortical functional areas and fiber tracts can be localized preoperatively by probabilistic anatomical tools, but mapping of functional integrity by neurophysiology is essential. Identification of the primary motor cortex seems to be more effectively performed with transcranial magnetic stimulation (TMS) than functional magnetic resonance imaging (fMRI). Language area localization requires auditory evoked potentials or TMS, as well as fMRI and diffusion tensor imaging for fiber tracts. Somatosensory cortex is most effectively mapped by somatosensory evoked potentials. Crucial eloquent areas, such as the central sulcus, primary somatomotor areas, corticospinal tract must be defined and for some areas that must be removed, potential compensations may be identified. Oncological/functional ratio must be optimized, resecting the tumor maximally but also sparingly, as far as possible, the areas that mediate indispensable functions. In some cases, a transient postoperative deficit may be inevitable. In this article, we review intraoperative exploration of motricity, language, somatosensory, visual and vestibular function, calculation, memory and components of consciousness.

Intraoperative spinal cord monitoring: Lesional level diagnosis.

BACKGROUND: In spinal deformity surgery, iatrogenic spinal cord injury is the most feared complication. Intraoperative monitoring (IOM) of the spinal cord assesses its functional integrity and allows significant reduction of the rate of spinal cord injury. HYPOTHESIS: In case of severe IOM alert, lesional level diagnosis constitutes supplementary and useful information. MATERIAL AND METHODS: This study was retrospective and monocentric. In our institution, 1062 pediatric spinal deformity surgeries have been monitored since 2004. We review the records of the six patients who presented a severe and prolonged IOM alert with lesional level determination. Somatosensory evoked potentials (SSEP), neurogenic mixed evoked potentials (NMEP) and D-waves were performed. In cases of IOM alert, sequentially moving an epidural electrode along the spinal cord allows lesional level determination, using this electrode either for stimulation or recording. RESULTS: Six patients, aged 12 to 17 years, characterized by severe IOM alerts during spinal deformity surgery are reported. Postoperative neurological examination was normal for five out of six cases. For patient 2, lesional level diagnosis allowed to determine a bi-laminar claw between T2 and T3 as the etiology of IOM alert. This IOM alert was delayed in time, being detectable only 30minutes after the placement of this claw. Postoperative neurological examination was normal. For patient 6, a Stagnara wake-up test demonstrated paraplegia. Lesional level was established. Following corrective surgical maneuvers, postoperative neurologic deficit was limited to a pyramidal syndrome in one lower limb. Postoperative MRI demonstrated a spinal cord lesion at the determined lesional level. CONCLUSION: During an IOM alert, lesional level determination allows localization of spinal cord dysfunction. This data, obtainable whatever the IOM device, constitutes supplementary information in order to rapidly identify the etiology of IOM alert and thus to react in the most appropriate way. LEVEL OF EVIDENCE: IV, retrospective study.

Somatosensory evoked potentials in the assessment of peripheral neuropathies: Commented results of a survey among French-speaking practitioners and recommendations for practice.

BACKGROUND: Somatosensory evoked potentials (SSEPs) are increasingly performed for the assessment of peripheral neuropathies, but no practical guidelines have yet been established in this specific application. STUDY AIM: To determine the relevant indication criteria and optimal technical parameters for SSEP recording in peripheral neuropathy investigation. METHODS: A survey was conducted among the French-speaking practitioners with experience of SSEP recording in the context of peripheral neuropathies. The results of the survey were analyzed and discussed to provide recommendations for practice. RESULTS: SSEPs appear to be a second-line test when electroneuromyographic investigation is not sufficiently conclusive, providing complementary and valuable information on central and proximal peripheral conduction in the somatosensory pathways. CONCLUSIONS: Guidelines for a standardized recording protocol, including the various parameters to be measured, are proposed. CLINICAL RELEVANCE: We hope that these proposals will help to recognize the value of this technique in peripheral neuropathy assessment in clinical practice.

High-resolution EEG (HR-EEG) and magnetoencephalography (MEG).

High-resolution EEG (HR-EEG) and magnetoencephalography (MEG) allow the recording of spontaneous or evoked electromagnetic brain activity with excellent temporal resolution. Data must be recorded with high temporal resolution (sampling rate) and high spatial resolution (number of channels). Data analyses are based on several steps with selection of electromagnetic signals, elaboration of a head model and use of algorithms in order to solve the inverse problem. Due to considerable technical advances in spatial resolution, these tools now represent real methods of ElectroMagnetic Source Imaging. HR-EEG and MEG constitute non-invasive and complementary examinations, characterized by distinct sensitivities according to the location and orientation of intracerebral generators. In the presurgical assessment of drug-resistant partial epilepsies, HR-EEG and MEG can characterize and localize interictal activities and thus the irritative zone. HR-EEG and MEG often yield significant additional data that are complementary to other presurgical investigations and particularly relevant in MRI-negative cases. Currently, the determination of the epileptogenic zone and functional brain mapping remain rather less well-validated indications. In France, in 2014, HR-EEG is now part of standard clinical investigation of epilepsy, while MEG remains a research technique.

Sublaminar bands: are they safe?

BACKGROUND: Sublaminar bands (SB) are frequently used as implants in spine deviation correction. PURPOSE: Our purpose is to demonstrate their safety on a large series of patients. STUDY DESIGN: This is a retrospective study. METHODS: Our department treated 378 spine deviations on children and adolescents via a hybrid posterior technique (lumbar screws, hook and thoracic SB). Each surgery was undertaken using anesthesiologic and a neurophysiologic monitoring using somatosensory evoked potentials (SSEP) and neurogenic mixed evoked potentials (NMEP). An alert was described as an amplitude decrease of 50 % and/or a latency increase of 10 %. Data were analyzed using Student or Wilcoxon tests. RESULTS: We used 2,223 SB in 378 operative procedures. We described ten neurophysiologic alerts during the passage of the band under the lamina. There were no significative differences between the two groups concerning the age and the severity of the deformation (p > 0.05). These neurophysiologic alerts were associated with a dysautonomic trouble (hypertension and bradycardia). The lesional level was determined using a spinal electrode. In six cases, the responsible SB was removed. Three patients had post-operative neurologic deficiency (0.8 %) without complete recovery for one of them (localized incomplete sensitive deficiency). Within the group of 378 patients, 21 alerts were reported due to a screw or a hook, or during the correction manoeuver, without dysautonomic trouble. CONCLUSION: SB neurologic complications rate is as high as other implants' complication rate. Simultaneous hemodynamic and neurophysiologic change is an argument for vegetative response due to SB passage. Their optimal use requires a strict learning of their insertion under the lamina to be as less traumatic as possible. SB are as safe as any other spine implants.

[French guidelines on electroencephalogram]. / Recommandations françaises sur l'électroencéphalogramme.

Electroencephalography allows the functional analysis of electrical brain cortical activity and is the gold standard for analyzing electrophysiological processes involved in epilepsy but also in several other dysfunctions of the central nervous system. Morphological imaging yields complementary data, yet it cannot replace the essential functional analysis tool that is EEG. Furthermore, EEG has the great advantage of being non-invasive, easy to perform and allows control tests when follow-up is necessary, even at the patient's bedside. Faced with the advances in knowledge, techniques and indications, the Société de Neurophysiologie Clinique de Langue Française (SNCLF) and the Ligue Française Contre l'Épilepsie (LFCE) found it necessary to provide an update on EEG recommendations. This article will review the methodology applied to this work, refine the various topics detailed in the following chapters. It will go over the summary of recommendations for each of these chapters and underline proposals for writing an EEG report. Some questions could not be answered by the review of the literature; in those cases, an expert advice was given by the working and reading groups in addition to the guidelines.

EEG extended source localization: tensor-based vs. conventional methods.

The localization of brain sources based on EEG measurements is a topic that has attracted a lot of attention in the last decades and many different source localization algorithms have been proposed. However, their performance is limited in the case of several simultaneously active brain regions and low signal-to-noise ratios. To overcome these problems, tensor-based preprocessing can be applied, which consists in constructing a space-time-frequency (STF) or space-time-wave-vector (STWV) tensor and decomposing it using the Canonical Polyadic (CP) decomposition. In this paper, we present a new algorithm for the accurate localization of extended sources based on the results of the tensor decomposition. Furthermore, we conduct a detailed study of the tensor-based preprocessing methods, including an analysis of their theoretical foundation, their computational complexity, and their performance for realistic simulated data in comparison to conventional source localization algorithms such as sLORETA, cortical LORETA (cLORETA), and 4-ExSo-MUSIC. Our objective consists, on the one hand, in demonstrating the gain in performance that can be achieved by tensor-based preprocessing, and, on the other hand, in pointing out the limits and drawbacks of this method. Finally, we validate the STF and STWV techniques on real measurements to demonstrate their usefulness for practical applications.

Intraoperative neurophysiologic monitoring in spine surgery. Developments and state of the art in France in 2011.

Intraoperative spinal cord monitoring consists in a subcontinuous evaluation of spinal cord sensory-motor functions and allows the reduction the incidence of neurological complications resulting from spinal surgery. A combination of techniques is used: somatosensory evoked potentials (SSEP), motor evoked potentials (MEP), neurogenic motor evoked potentials (NMEP), D waves, and pedicular screw testing. In absence of intraoperative neurophysiological testing, the intraoperative wake-up test is a true form of monitoring even if its latency long and its precision variable. A 2011 survey of 117 French spinal surgeons showed that only 36% had neurophysiological monitoring available (public healthcare facilities, 42%; private facilities, 27%). Monitoring can be performed by a neurophysiologist in the operating room, remotely using a network, or directly by the surgeon. Intraoperative alerts allow real-time diagnosis of impending neurological injury. Use of spinal electrodes, moved along the medullary canal, can determine the lesion level (NMEP, D waves). The response to a monitoring alert should take into account the phase of the surgical intervention and does not systematically lead to interruption of the intervention. Multimodal intraoperative monitoring, in presence of a neurophysiologist, in collaboration with the anesthesiologist, is the most reliable technique available. However, no monitoring technique can predict a delayed-onset paraplegia that appears after the end of surgery. In cases of preexisting neurological deficit, monitoring contributes little. Monitoring of the L1-L4 spinal roots also shows low reliability. Therefore, monitoring has no indication in discal and degenerative surgery of the spinal surgery. However, testing pedicular screws can be useful. All in all, thoracic and thoracolumbar vertebral deviations, with normal preoperative neurological examination are currently the essential indication for spinal cord monitoring. Its absence in this indication is a lost opportunity for the patient. If neurophysiological means are not available, intraoperative wake-up test is a minimal obligation.

[Association of type 1 diabetes mellitus and epilepsy in children. A cohort of 10 cases]. / Association diabète de type 1et épilepsie chez l'enfant. À propos d'une série de 10 cas.

UNLABELLED: The association of type 1 diabetes mellitus (DM) and epilepsy has been previously reported. However, the physiopathology of this association remains misunderstood. OBJECTIVE: To describe epilepsy combined with type 1 DM in children. METHODS: Retrospective monocentric study of all the epileptic and type 1 diabetic children consulting at the Timone University Hospital, Marseille, France. For each patient, the type of epilepsy and its electroclinical and radiographic characteristics were studied as well as the type of diabetes (biological characteristics, glycemic control), and the onset of these 2 diseases. RESULTS: Ten patients are reported. Five suffered from generalized epilepsy (4 idiopathic, 1 nonidiopathic) and 5 from focal epilepsy (4 non-idiopathic, 1 idiopathic). For most of these cases, presence of GAD (glutamic acid decarboxylase) autoantibodies were confirmed and epilepsy followed diabetes. CONCLUSIONS: The 2 most common types of epilepsy in this association are idiopathic generalized epilepsy and non-idiopathic temporal epilepsy. Several mechanisms could be involved (immune, glycemia, and genetic disorders).

[Valproate-induced hyperammonemic encephalopathy]. / Encéphalopathie hyperammoniémique au valproate de sodium.

Valproate is a drug commonly prescribed in neurology and psychiatry. Hyperammonemic encephalopathy due to valproate is a severe complication. A 44-year-old man with a past medical history of partial symptomatic epilepsy and alcoholic cirrhosis was admitted for drowsiness. He was receiving valproate and gabapentin. A valproate-induced hyperammonemic encephalopathy was diagnosed on physical examination, generalized slow waves on electroencephalogram and hyperammonemia. Valproate withdrawal led to a progressive recovery of the consciousness, with a rapid normalisation of electroencephalogram and ammonium level. The pathogenesis of this encephalopathy is not clearly established. No correlation has been shown between the severity of encephalopathy, the plasma ammonium level, the valproate dose and its plasma concentration. Additional factors have been pointed out, such as carnitine deficiency or urea cycle enzyme defects. Furthermore, our case suggests an enhancing role of the liver disease in this encephalopathy.

Influence of source separation and montage on ictal source localization.

The aim of this paper is to evaluate the influence of the reference electrode (introduced to form an augmented average montage) and of the artifact elimination by blind source separation on the ictal electrical source imaging. We present here a preliminary study on one patient only. The results seem to indicate that the montage (and thus the reference handling method) has a limited but existent influence on the quality of the source localization. Artifact elimination highly improves this quality as well.

Neural networks underlying hyperkinetic seizures of "temporal lobe" origin.

PURPOSE: Hyperkinetic seizures are most often considered to originate from prefrontal cortex. Recently however, it has been suggested that hyperkinetic seizures can be found in patients with temporal lobe seizures. The objective of this study was to determine the features of temporal epilepsy with hyperkinetic seizures and the functional anatomy of involved brain networks. METHODS: We retrospectively identified patients investigated by depth electrodes (SEEG) in whom hyperkinetic manifestations were proved to be linked to initial temporal lobe involvement. Seizure organisation was determined according to the "Epileptogenicity Index" (EI), a new way to quantify rapid discharges at seizure onset. RESULTS: We found 7 patients among 130 SEEG investigations that fulfilled the inclusion criteria. Most of the patients presented with hyperkinetic occurring (or predominating) during sleep. SEEG signal analysis demonstrated a common temporo-frontal network in which the temporal pole played a central role. Major involvement of the orbito-frontal cortex and to a lesser extent the cingulate gyrus was also a particular feature of these seizures. DISCUSSION: Seizures originating in the temporal lobe must be recognized as an important cause of hyperkinetic seizures. The temporal pole and its connexions with medio-basal prefrontal cortex represent the main structures involved in epileptogenic networks.

[High-resolution EEG (HR-EEG) and magnetoencephalography (MEG)]. / EEG haute résolution (EEG-HR) et magnétoencéphalographie (MEG).

HR-EEG (high resolution EEG) and MEG (magnetoencephalography) allow the recording of cerebral electromagnetic activities with excellent temporal resolution. These tools have also considerably progressed in spatial resolution and now constitute real methods of Electric and Magnetic Source Imaging. Their limits and the precision of the results obtained are discussed in distinct types of partial epilepsy. HR-EEG and MEG allow localization of scalp-EEG interictal spikes and more rarely ictal activities. They now contribute to the presurgical evaluation of pharmacoresistant partial epilepsies. These investigations appear to be of particular importance in presurgical assessment of MRI-negative epilepsy.