Jorge García Seoane

Single pulse electrical stimulation for identification of structural abnormalities and prediction of seizure outcome after epilepsy surgery: a prospective study

BACKGROUND Abnormal late responses to single pulse electrical stimulation (SPES) in patients with intracranial recordings can identify epileptogenic cortex. We aimed to investigate the presence of neuropathological abnormalities in abnormal SPES areas and to establish if removal of these areas improved postsurgical seizure control. METHODS We studied abnormal responses to SPES during chronic intracranial recordings in 40 consecutive patients who were thereafter operated on because of refractory epilepsy and had a follow-up period of at least 12 months. FINDINGS 22 patients had abnormal responses to SPES exclusively located in resected regions (96% with favourable outcome), seven had abnormal responses to SPES located in resected and non-resected regions (71% with favourable outcome), three had abnormal responses to SPES exclusively outside the resected region (none with favourable outcome), and eight did not have abnormal responses to SPES (62.5% with favourable outcome). Surgical outcome was significantly better when areas with abnormal responses to SPES were completely resected compared with partial or no removal of abnormal SPES areas (p=0.006). Neuropathological examination showed structural abnormalities in the abnormal SPES areas in 26 of the 29 patients in whom these regions were resected, despite the absence of clear MRI abnormalities in nine patients. INTERPRETATION Abnormal responses to SPES are functional markers of epileptogenic structural abnormalities, and can identify epileptogenic cortex and predict surgical outcome, especially when a frontal or temporal focus is suspected.

Characteristics of scalp electrical fields associated with deep medial temporal epileptiform discharges

OBJECTIVE To determine scalp characteristics of epileptiform discharges arising from medial temporal structures (MT). METHODS Signal-to-noise ratio was increased by averaging simultaneous recordings from intracranial and scalp electrodes synchronised on discharges recorded by foramen ovale (FO) electrodes. The topography, amplitude and distribution of averaged scalp signals were analysed. RESULTS Four thousand three hundred and twenty-seven discharges from 20 patients were averaged into 77 patterns. Before averaging, only 9% of discharges were detectable on the scalp without the need of simultaneous FO recordings (SED). A further 72.3% of discharges fell into averaged patterns that could be detected on the scalp as small transients before or after averaging (STBA or STAA). In 18.7% of discharges, no scalp signal was seen after averaging. Whereas most SED patterns had largest amplitude on the scalp at anterior temporal electrodes, STBA and STAA patterns showed greater variability and more widespread scalp fields, suggesting a deeper source. Dipole source localisation modelled the majority of SED patterns as radial dipoles located just behind the eye. In contrast, dipoles corresponding to STBA or STAA patterns showed greater variability in location and orientation and tended to be located at MT. CONCLUSIONS SED patterns seem to arise from widespread subtemporal and/or superficial neocortical activation, generating EEG fields that are distorted by the high electrical conductivity of anterior cranial foramina. In contrast, STBA and STAA patterns represent electrical fields from neuronal activity more restricted to MT, that reach the scalp highly attenuated by volume-conduction and less distorted by cranial foramina. SIGNIFICANCE Low amplitude scalp signals can be related to MT activity and must be taken into consideration for the diagnosis of temporal lobe epilepsy, pre-surgical assessment and for valid modelling of deep sources from the scalp EEG and magnetoencephalogram.

Late EEG responses triggered by transcranial magnetic stimulation (TMS) in the evaluation of focal epilepsy

Purpose: To evaluate the use of EEG responses to transcranial magnetic stimulation (TMS‐EEG responses) as a noninvasive tool for the diagnosis of focal epilepsy.

In vivo neuronal firing patterns during human epileptiform discharges replicated by electrical stimulation.

Highlights ► The neuronal firing patterns during interictal epileptiform discharges (IEDs) and after single pulse electrical stimulation (SPES) can be described as burst-only, suppression-only, burst–suppression or no-change. ► Similar neuronal firing patterns can be observed during IEDs and after SPES. ► IEDs and responses to SPES appear to activate similar and generic cortical mechanisms, which may explain transient cognitive impairment.

Single-pulse electrical stimulation helps to identify epileptogenic cortex in children

Purpose:  The usefulness of single‐pulse electrical stimulation (SPES) during intracranial recordings was evaluated in a pediatric population. This method is useful in identifying epileptogenic cortex in adult subjects.

A hole in the skull distorts substantially the distribution of extracranial electrical fields in an in vitro model

Summary The purpose of this study was to quantify the distortion of electrical fields by skull foramina using an in vitro model. Extracranial voltage generated by current dipoles located inside a human calva immersed in saline were measured when a 4-mm hole was open and when it was blocked with paraffin wax. Dipoles were located either along the internal surface of the bone (superficial dipoles) or at increasing distances from the bone (deep dipoles). With the hole open, extracranial signals had a substantially greater amplitude than with the hole blocked. The locations of the largest voltage values recorded outside the skull depended on the distance of the recording electrode from the hole rather than on the location of the internal dipole. For superficial dipoles, voltage values with the hole open were as much as 116 times greater than when the hole was blocked. Furthermore, when the hole was open, the largest extracranial signals were seen at the hole even when the dipole was 5 to 6 cm away from the hole. The effects of skull holes were less prominent for deep dipoles than for superficial dipoles. Skull discontinuities can be major determinants for the distribution of extracranial EEG signals. These results have implications for EEG interpretation and for source localization.

Interictal estimation of intracranial seizure onset in temporal lobe epilepsy

OBJECTIVES To evaluate the lateralizing and localizing values of interictal focal slow activity (IFSA), single pulse electrical stimulation (SPES) and (18)FDG PET, in order to estimate their potential to complement ictal intracranial recordings and reduce prolonged monitoring in patients with temporal lobe epilepsy. METHODS The study includes 30 consecutive patients with bilateral temporal subdural electrodes and focal seizure onset. IFSA, SPES and (18)FDG PET when available, were visually assessed and their combined lateralization was based on the majority of the individual lateralizing tests. RESULTS In the 18 patients who had all three tests, lateralization was congruent with seizure onset areas in 15 (83%). When lateralized (15 patients), (18)FDG PET was always congruent with intracranial seizure onset. In all 12 patients without (18)FDG PET, lateralization combining IFSA and SPES was congruent with seizure onset, including two with bilateral independent seizure onset on subdural monitoring. 22 out of the 23 patients who had surgery enjoyed favorable outcome (Engel I or II). CONCLUSION Intracranial IFSA and SPES can reliably predict the side and site (mesial versus lateral temporal) of seizure onset when they lateralize to the same side. SIGNIFICANCE (18)FDG PET can be useful in planning electrode implantation. During intracranial recordings, IFSA and SPES have the potential to reduce telemetry time, risks and costs.

EEG latency analysis for hemispheric lateralisation in Landau-Kleffner syndrome

OBJECTIVE To determine the reliability of latency analysis in lateralising the origin of epileptiform discharges in pre-surgical assessment of Landau-Kleffner syndrome (LKS). METHODS A computer aided-method was developed to identify leading regions and measure inter-hemispheric latencies before and after averaging discharges. Scalp and intracranial EEG recordings were studied from seven patients undergoing surgical treatment. The laterality suggested by latency analysis was compared with that suggested by pharmacological tests. RESULTS Latency analysis of bilateral discharges showed a consistent leading hemisphere. The earliest low-amplitude deflections were located in temporal regions in all patients. Contralateral low-amplitude deflections, and ipsilateral and contralateral earliest large negative peaks were recorded in temporal and less frequently in parasagittal regions. Presurgical inter-hemispheric latencies ranged between 8 and 48 ms for the deflections and between 4 and 30 ms for the peaks. The leading hemisphere identified by latency analysis of the earliest low-amplitude deflections coincided with that suggested by pharmacological tests in all 7 patients, whereas latency of later components coincided in 6. CONCLUSIONS Latency analysis appears to be a reliable method to estimate the hemisphere driving bilateral discharges in LKS. SIGNIFICANCE It can be carried out non-invasively and could be used to confirm, and eventually replace, results from pharmacological tests.

A fast data acquisition system for neurophysiological signals based on a personal microcomputer

A general purpose analog-to-digital conversion system and its interface for a low-cost personal computer (Commodore 64) are described. Special emphasis has been put on achieving a high sampling frequency rate (up to 20,000 samples/s) and a high discrimination (12 bits). It has likewise been attempted to increase the number of input channels (up to 8, by means of a multiplexer) and averaging capability since such features may be useful in a great number of neurobiological studies. Commercially available hardware elements have been employed and software has been developed in BASIC and 6510 Machine Code.