Intracranial Ictal Recordings Research Articles

Digital reconstruction of infraslow activity in human intracranial ictal recordings using a deconvolution-based inverse filter

Infraslow activity (ISA) is a biomarker that has recently become of interest in the characterization of seizure recordings. Recent data from a small number of studies have suggested that the epileptogenic zone may be identified by the presence of ISA. Investigation of low frequency activity in clinical seizure recordings, however, has been hampered by technical limitations. EEG systems necessarily include a high-pass filter early in the measurement chain to remove large artifactual drifts that can saturate recording elements such as the amplifier. This filter, unfortunately, attenuates legitimately seizure-related low frequencies, making ISA difficult to study in clinical EEG recordings. In this study, we present a deconvolution-based digital inverse filter that allows recovery of attenuated low frequency activity in intracranial recordings of temporal lobe epilepsy patients. First, we show that the unit impulse response (UIR) of an EEG system can be characterized by differentiation of the system’s step response. As proof of method, we present several examples that show that the low frequency component of a high-pass filtered signal can be restored by deconvolution with the UIR. We then demonstrate that this method can be applied to biologically relevant signals including clinical EEG recordings obtained from seizure patients. Finally, we discuss how this method can be applied to study ISA to identify and assess the seizure onset zone.

Comparing Noninvasive Dense Array and Intracranial Electroencephalography for Localization of Seizures

To compare the localization of the seizure onset zone estimated from ictal recordings with high spatial resolution, 256-channel scalp dense array electroencephalographic video long-term monitoring (LTM) with the aid of source analysis with that obtained from subsequent intracranial ictal recordings. Ten patients with medically refractory epilepsy, all surgical candidates, underwent intracranial LTM after standard noninvasive evaluation failed to provide adequate localizing information regarding ictal origins. Before invasive studies, all patients underwent dense array electroencephalographic LTM in which habitual clinical seizures were recorded for each patient. Source analysis was applied to ictal onsets. Intracranial electrode placement followed conventional guidelines, although the neurosurgeon was aware of the dense array electroencephalographic results. Patients ranged in age from 10 to 49 years (mean age, 24 y); 7 were male. Identified risk factors included closed head injury in 1 patient and childhood meningitis in another. No focal neurological signs were found in any patient. Magnetic resonance imaging findings were normal in 6 patients; 1 patient had cerebellar hypoplasia, 1 had right frontoparietal dysplasia, 1 had bilateral nonspecific white matter abnormalities, and 1 had bilateral cavernous angiomas. Ictal onsets, based on invasive recordings, were in the mesiotemporal lobe (3 patients), lateroparietal (2 patients), mesioparietal (1 patient), laterofrontal (1 patient), superolateral frontocentral (1 patient), frontopolar (1 patient), and posteroinferior temporo-occipital neocortex (1 patient). Dense array electroencephalography localized ictal onsets to the same region as intracranial monitoring in 8 of 10 cases; invasive studies disclosed an additional ictal focus in 2 of these patients. Surgical resections were based only on intracranial electroencephalographic findings. Dense array electroencephalography has the potential to assist in the noninvasive localization of epileptic seizures and to guide the placement of invasive electrodes for localizing seizure onset.

Time–frequency analysis using the matching pursuit algorithm applied to seizures originating from the mesial temporal lobe

Objectives: The ability to analyze patterns of recorded seizure activity is important in the localization and classification of seizures. Ictal evolution is typically a dynamic process with signals composed of multiple frequencies; this can limit or complicate methods of analysis. The recently-developed matching pursuit algorithm permits continuous time–frequency analyses, making it particularly appealing for application to these signals. The studies here represent the initial applications of this method to intracranial ictal recordings. Methods: Mesial temporal onset partial seizures were recorded from 9 patients. The data were analyzed by the matching pursuit algorithm were continuous digitized single channel recordings from the depth electrode contact nearest the region of seizure onset. Time–frequency energy distributions were plotted for each seizure and correlated with the intracranial EEG recordings. Results: Periods of seizure initiation, transitional rhythmic bursting activity, organized rhythmic bursting activity and intermittent bursting activity were identified. During periods of organized rhythmic bursting activity, all mesial temporal onset seizures analyzed had a maximum predominant frequency of 5.3–8.4 Hz with a monotonic decline in frequency over a period of less than 60 s. The matching pursuit method allowed for time–frequency decomposition of entire seizures. Conclusions: The matching pursuit method is a valuable tool for time–frequency analyses of dynamic seizure activity. It is well suited for application to the non-stationary activity that typically characterizes seizure evolution. Time–frequency patterns of seizures originating from different brain regions can be compared using the matching pursuit method.

Surgical outcomes in pure frontal lobe epilepsy and foci that mimic them

In this study we examined 37 subjects with a diagnosis of intractable frontal lobe epilepsy (FLE) based on non-invasive pre-surgical evaluation. Twenty-six underwent chronic intracranial ictal recordings (CIR) with video monitoring; 20 of these went on to surgical resection. Eleven underwent surgery without CIR. Retrospectively, we determined that 19 had pure FLE, 12 had frontal plus extrafrontal epileptogenic zones, and six others did not have FLE. We analysed the whole group and individual categories to evaluate the determinants of surgical outcome. Sixty percent of the pure frontal group is seizure free with all having > or = 75% reduction. The frontal-plus group had only 10% seizure free with 70% having > or = 75% reduction. Being in the pure frontal group was associated with better outcomes than the 'frontal-plus' group (P < 0.05; chi-square). Subjects with FSIQ > or = 85, focal pathologies and 18FDG-PET scans which were normal or had focal abnormalities (P < or = 0.05, all, chi-square) were more likely to have excellent outcomes. MRI abnormalities, surface EEG, and location and size of resection were not predictive of surgical outcomes. Rasmussen's encephalitis, incomplete surgical strategies and bilateral foci were apparent in those with poor outcomes, and surgical size predicted post-operative deficits (chi-square; P < 0.001). We conclude that careful, hypothesis-driven implants and operating procedures can result in good surgical outcomes for frontal lobe epilepsy subjects even when lesions are not apparent on routine neuroimaging.

Intracranial Ictal Recordings in Mesial Frontal Lobe Epilepsy

Localization of ictal onset in patients with medically refractory frontal lobe epilepsy is challenging even with intracranial monitoring. We present a series of nine patients with presumed mesial frontal lobe epilepsy in whom successful localization of ictal onset was achieved in most cases. Intracranial electrodes were placed over cingulate and supplementary motor cortex bilaterally, with additional electrodes placed over lateral and inferior frontal lobes as part of an evaluation for epilepsy surgery. Localization of the ictal onset was clearly defined in seven of nine patients and was characterized by a pattern of lower amplitude beta/gamma range frequencies noted in one to four adjacent electrodes arising from cingulate cortex or supplementary motor cortex in six patients. In the remaining patient, ictal onset was characterized by periodic high amplitude spike and slow-wave discharges evolving into a higher voltage faster rhythm. Electrographic onset occurred coincident with or preceded clinical findings. Ictal pattern also did not demonstrate a widespread propagation pattern in most of the recordings in which ictal onset was well localized. Precise localization of ictal onset within the mesial frontal lobe is possible. Rapid propagation to regions within and outside the frontal lobe does not always occur.

Subdural Recording of Ictal DC Shifts in Neocortical Seizures in Humans

Invasive ictal EEG recording is often necessary to delineate epileptogenic areas in patients with intractable partial epilepsy, but even intracranial ictal recordings often reveal ill-defined onset zones in neocortical epilepsy. We studied the physiologic significance of ictal direct current (DC) potentials recorded intracranially in human epilepsy. We made intracranial ictal EEG recordings in three patients with intractable partial seizures arising from frontal, lateral temporal, and parietal neocortical areas by using closely spaced subdural electrodes (platinum in two patients and stainless steel in one patient) with both standard (1.5 Hz) and open (0.016 Hz) low-frequency filter (LFF) settings. The initial ictal pattern was localized to two to nine subdural electrodes and characterized by very low voltage and high-frequency rhythmic activity ("electrodecremental pattern"). A slow-rising negative potential (DC potential) was seen in a slightly more restricted area (two to six electrodes) and occurred 1-10 s before the initial ictal EEG discharges in two patients. These results agree with those of previous studies of ictal DC shifts in animals and suggest that ictal DC shifts may be helpful in delineating the epileptogenic area more precisely in human epilepsy.

Three-dimensional localization of interictal epileptiform activity with dipole analysis: Comparison with intracranial recordings and SPECT findings

Seventeen patients with therapy-resistant partial epilepsy were studied in order to compare the localization of the epileptic focus provided by three-dimensional dipole analysis of scalp interictal epilepriform EEG activity to the localization of the ictal pacemaker zone obtained by intracranial recordings. The results were also compared with regional abnormalities in interictal 99mTc-hexamethylpropy-lenenamine oxime single-photon emission computed tomography (SPECT). Dipole analysis was performed using a well-known commercial computer program based on a three-shell model. For 12 of the 17 patients, the dipoles for all the individual spikes were located in the same area, in two cases, two separate locations were found, and in three cases the dipoles were scattered over a large area. In 8 of the 12 patients with a single dipole location, intracranial ictal recordings revealed a seizure initiation zone corresponding to the dipole location. In another two cases, intraoperative corticography confirmed the dipole findings. In the two remaining patients, the intracranial recordings revealed a different pattern of seizure initiation than had been anticipated from the dipole analysis. In one of the two patients with two dipole locations, intracranial recordings revealed a seizure initiation zone corresponding to one of the two dipole location sites, whereas in the other patient the seizure onset pattern was more complex. In two of the three patients with widely scattered dipole areas, seizure onset was complex, whereas in the third patient a localized seizure onset was found. In 10 of the 12 patients with a single dipole location site, there was good correlation between this site and regions of low flow in the interictal SPECT measurements. In both patients with two dipole areas, there was a low flow area corresponding to at least one of the dipole localizations. In two of the three patients with widespread dipoles, there were major SPECT low-flow areas, whereas in the third patient interictal SPECT was normal. Although our investigations so far are tentative, we think the results indicate that dipole analysis of interictal epileptiform EEG activity may become a useful supplementary method for the localization of the epileptogenic region in patients who are under consideration for surgical therapy, especially if combined with SPECT or other independent localizing techniques.