A comparison of EEG seizure patterns recorded with surface and depth electrodes in patients with temporal lobe epilepsy

Intensive neurodiagnostic monitoring in psychiatry

A series of technological advances have made it possible to closely monitor electrophysiological and behavioural manifestations of episodic clinical events over prolonged periods of time, with the ability to review the records at leisure or to submit them to computer analysis. The more promising techniques are time-locked video/EEG monitoring, cable telemetry, radiotelemetry, ambulatory cassette recording, intensive plasma anti-epileptic drug monitoring and continuous neuropsychological monitoring. The greatest promise of these techniques is for the diagnosis, research and management of epilepsy. For psychiatry, they offer additional help in the differential diagnosis of non-epileptic events from epilepsy, the most important of which are psychogenic seizures and episodes of aggression. This paper discusses the potential role of these techniques in the assessment of non-epileptic events and transient cognitive impairment in clinical psychiatry.

Single photon emission computed tomography (SPECT) using N-isopropyl-p-(123I) iodoamphetamine (IMP) in the evaluation of patients with epileptic seizures

To evaluate the clinical usefulness of IMP SPECT in the diagnosis of epilepsy, 6 normals and 52 patients in the interictal phase were studied. Thirty min after an intravenous injection of 111 MBq IMP, SPECT was performed using a rotating gamma camera. Of 21 patients with simple partial seizures, a localized decrease of uptake was shown in 16, and an increase in 3. Topologically, these findings corresponded well to the ictal symptoms. Nine of 13 patients with localized epileptic EEG had a good correspondence between the findings on EEG and IMP SPECT. In 20 of 23 with complex partial seizures, the coronal images showed laterality of uptake in the temporal lobes, whereas the CT was normal in 14. However, these findings on IMP SPECT agreed with the EEG in the temporal leads in only 5 cases. Of 8 patients with primary generalized seizures, a diffuse cerebral decrease was shown in 3 of 4 patients with convulsive seizures (grand mal), and a normal uptake in 3 of 4 patients with non-convulsive seizures (petit mal). However, 2 patients showed a localized decrease, therefore, we determined that they suffered from partial seizures evolving to secondarily generalization. From these data, we concluded that IMP SPECT could be a useful method in the diagnosis of epilepsy.

High-frequency components in epileptiform EEG

We analyzed the spectral content of interictal and ictal EEG of rats treated with metrazol. Significant power was present in the EEG between 100 and 800 Hz ictally but not interictally. Since high-frequency activities appeared to accompany spiking episodes, a computer algorithm was developed to recognize rapid voltage fluctuations or high-frequency components in EEG. The algorithm turned out to be effective in the identification of seizure spikes. The significance of high frequencies in EEG spikes and the advantage of the spike selection algorithm are discussed.

Evidence for a chronic loss of inhibition in the hippocampus after kindling: electrophysiological studies

Rats were kindled with either of 2 protocols: (1) a rapidly recurring hippocampal seizure (RRHS) paradigm in which 10 sec stimulus trains were delivered every 5 min through hippocampal electrodes; and (2) a traditional approach in which 1 sec stimulus trains were given to the amygdala once daily. Three groups of kindled rats were prepared: (1) one of amygdala-kindled rats that had experienced 9-15 seizures; (2) one of RRHS-kindled rats that had experienced 96 seizures; and (3) one of RRHS-overkindled rats that had experienced 144-336 seizures. After a 1 month seizure-free period, the animals were anesthetized with urethane and measurements were made on the potency of paired pulse inhibition in the CA1 region of the hippocampus. All groups of kindled animals were found to have significantly less paired pulse inhibition than control rats. This decrement was confined to interpulse intervals less than or equal to 70 msec. The amount of inhibition lost correlated with the number of seizure that had occurred. The GABAergic agonist muscimol restored paired pulse inhibition in kindled animals for interpulse intervals less than or equal to 70 msec towards normal values. These results indicate that not only RRHS, but also other modes of kindling, reduced GABAergic inhibition in the CA1 region of the hippocampus and that this diminution was long-lasting, if not permanent.

Neocortical propagation in temporal lobe spike foci on magnetoencephalography and electroencephalography

Propagation of the neuronal population of the interictal epileptic spike was quantified in 5 patients with complex partial epilepsy arising from temporal lobe using electroencephalography and magnetoencephalography. During the spike complex in each patient there was a spike at the deep sphenoidal electrode and a spike at the superficial scalp electrode on spontaneous electroencephalography. In each patient the sphenoidal spike had a different peak latency than the scalp spike, consistent with spike propagation. Electroencephalography was used to trigger two magnetoencephalographic averages of stereotyped spikes during the sphenoidal peak and the scalp peak. Magnetoencephalography discriminated the centers of two cortical spike populations at different latencies, showing deeper localization with sphenoidal trigger and more superficial localization with scalp trigger in each patient (p less than 0.05). Latency differences and propagation distances of spikes were consistent with the conduction velocity of corticocortical fibers. Noninvasive estimates of the cortical surface area of the spikes agreed with estimates obtained by electrocorticography over temporal neocortex. These findings indicate propagation of neuronal populations active during human interictal spikes between deep and superficial cortex of temporal lobe, likely by monosynaptic or oligosynaptic pathways. This interictal system appears to be partly independent of the hippocampal interictal system in complex partial epilepsy.

Human hippocampal seizure spread studied by depth and subdural recording: the hippocampal commissure

Eleven patients had seizures with unilateral temporal lobe onset recorded with simultaneous bilateral medial temporal depth electrodes and neocortical (subdural) electrodes at least on the side of seizure onset. Of a total of 55 seizures, four had simultaneous onset in neocortex and hippocampus, and 51 had onset in unilateral hippocampus. None originated solely in temporal neocortex. Three reproducible patterns of seizure spread from hippocampus were defined in which seizures spread initially to ipsilateral neocortex (32 seizures), spread first to contralateral hippocampus (13 seizures), or spread simultaneously to ipsilateral neocortex and contralateral hippocampus. Although the region of hippocampus in which seizures arose was constant, patterns of spread sometimes varied in the same patient. When contralateral neocortical involvement occurred, it was after or with contralateral hippocampus but never before. These results suggest the existence of an operational hippocampal commissure in humans.

Inter-hemispheric propagation of human mesial temporal lobe seizures: a coherence/phase analysis

Intra- and inter-hemispheric propagation of ictal discharges was analyzed with computer techniques in 10 patients with complex partial seizures of mesial temporal lobe origin in whom depth electrodes had been stereotaxically implanted. Coherence and phase analysis of seizure discharges was used to detect the emergence of linear relationships between all possible pairs of surface and depth recording derivations both between and within hemispheres. This analysis included mesial temporal, lateral temporal, and frontal lobe sites during both the onset and inter-hemispheric propagation of 28 ictal episodes. Although strong intra-hemispheric coherences and linear phase spectra reliably emerged in both the epileptogenic and non-epileptogenic hemispheres during seizure onset and contralateral spread, these relationships were usually not observed for inter-hemispheric comparisons. Only 3 of 10 patients demonstrated some degree of consistency in the emergence of significant wideband coherences and linear phase spectra between left and right mesial temporal sites during the inter-hemispherics propagation of ictal discharges. Mesial temporal lobe sites which demonstrated such a relationship included the amygdala, pes hippocampi, and parahippocampal gyrus. In 7 of 10 patients, lateral temporal derivations were sampled during ictal events; the emergence of linear relationships between left and right lateral temporal derivations during inter-hemispheric propagation was observed for only two. Various frontal lobe sites were monitored in 3 of the 10 patients; the emergence of linear relationships was observed only between left and right orbitofrontal derivations in the one patient for whom this region was sampled. These results suggest that the hippocampal commissure, parts of the corpus callosum, and parts of the anterior commissure may be relatively unimportant for the inter-hemispheric propagation of mesial temporal seizures in man. Future studies in non-human primates may reveal that ictal discharges which originate in the mesial temporal region propagate preferentially via brain-stem pathways to contralateral homologous regions.

Twenty-four hour ambulatory EEG monitoring: development and applications

Since 1975 four-channel ambulatory monitoring has been available as a technique for prolonged EEG recording in an unrestricted environment. This increases EEG sampling time so that attacks can be recorded and enables the differentiation of epileptic and non-epileptic attacks. In recent years an eight-channel system has become available which provides greater scalp coverage and allows better localization of attacks and EEG abnormalities. Four-channel recording has been widely used to assess the efficacy of anticonvulsant medication in patients with absence seizures. It has also been used to investigate the effect of the environment on discharges, as well as any circadian variations in discharges. Ambulatory monitoring provides a useful alternative to sleep recording in the laboratory, both for the detection of abnormalities during sleep and for experimental sleep studies. Automated analysis techniques have so far been confined to the analysis of spike and wave activity and to the scoring of sleep stages.

Interhemispheric propagation time of human hippocampal seizures: II. Relationship to pathology and cell density

The time required for hippocampal seizure activity to propagate to the contralateral hippocampal formation was evaluated in 57 patients with complex partial seizures and related to histological findings following temporal lobectomy. Interhemispheric propagation times were found to be significantly longer (greater than 20 s) in those patients diagnosed as having hippocampal sclerosis than in those patients without hippocampal sclerosis. Quantitative analysis of cell densities was made in 28 patients. Dentate fascia granule densities and hippocampal pyramidal cell densities were found to be lower in those patients with longer interhemispheric propagation times. However, this relationship was not observed when only patients with hippocampal sclerosis were considered. Thus, a tendency for hippocampal seizures to exhibit interhemispheric propagation times greater than 20 s is suggestive of underlying hippocampal sclerosis. However, information about interhemispheric propagation time does not appear useful for estimating the actual degree of cell loss within the sclerotic hippocampus. The association of longer interhemispheric propagation times with hippocampal sclerosis supports the hypothesis that direct commissural connections between the hippocampal formations in humans are either unimportant or missing.

Cardiac manifestations of complex partial seizures

Six patients with complex partial seizures in whom the first or most prominent manifestation was cardiovascular are described. "Anginal pain," which was usually atypical, developed in five patients (mean age 34), although three were admitted to coronary care units; sinus tachycardia developed in one, sinus bradycardia leading to syncope developed in one, and a cluster of symptoms suggestive of pheochromocytoma developed in one. The causal relationship between complex partial seizures and these symptoms is based on clinical history, electrodiagnostic studies indicating epilepsy (in five of six cases) without changes of myocardial ischemia, and response to anticonvulsant but not antianginal medications. Complex partial seizures that present with cardiovascular and other visceral symptoms represent an underdiagnosed, treatable disorder.

Spatio-temporal EEG measures and their application to human intracranially recorded epileptic seizures

We describe analysis methods which involve the computation of band-integrated, intrachannel spectral measures from the EEG. These measures require only very general assumptions, have a simple interpretation and achieve significant data reduction, while providing a quantitative assessment of significant EEG modification at seizure or onset or in the course of seizure evolution. In developing these measures, we experimented with various measures derived from spatio-temporal power spectra. Bandpass (8-30 Hz) integrated relative energy measures appear to be the most useful since they objectively measure paroxysmal modification. Two such measures are illustrated in this report: the band-integrated power z-score (BIPZ), which is a quantitative measure designed to allow significance testing, and the band-integrated power ratio (BIPR), which is a simply calculated qualitative measure that behaves very similarly to the BIPZ measure and may be useful for real-time data screening. In addition to developing these intrachannel measures, we have also developed methods of displaying these measures in a format appropriate to the analysis of interchannel relationships. In a general sense, the displays provide objective information regarding the spatio-temporal dynamics of the EEG in a condensed format which is still readily understandable to the electroencephalographer. This analysis was applied to 29 spontaneous, depth-recorded seizures in 14 patients with the aim of identifying the initial locus of seizure activity in each seizure episode. The initial locus was defined as that location (or set of locations) to first exhibit statistically significant values of the BIPZ measure. The method was very effective in this regard, as shown by correspondence between the conclusions of this automated analysis and conventional visual analysis.

Distribution of pyramidal cell density and hyperexcitability in the epileptic human hippocampal formation

Pyramidal cell densities in various regions of the anterior and posterior hippocampal formation were measured from en bloc temporal lobe resections and compared with presurgical stereoelectroencephalography (SEEG) data derived from depth electrodes in 12 patients with temporal lobe epilepsy. These data were compared with cell densities observed in four nonepileptic control patients. Patients who consistently exhibited anterior focal changes in the SEEG accompanying onset of ictus had cell densities that were selectively reduced in the anterior hippocampal formation but normal with respect to controls in the posterior hippocampal formation. Patients who exhibited more regional changes in the SEEG at onset of ictus had reduced cell densities in both the anterior and posterior hippocampal formation. Patients who exhibited focal spike activity in the anterior hippocampal formation as their predominant interictal SEEG pattern also had selectively reduced cell densities in the anterior hippocampal formation, while patients with widespread spiking throughout the hippocampal formation had reduced cell densities both anteriorly and posteriorly. These data support the concept that epileptogenesis occurs in or near those areas of epileptic hippocampus that are most damaged. Hippocampal sclerosis must be viewed as related to adjacent hyperexcitable or epileptogenic neurons and not solely as a passive result of repeated anoxia or ischemia.

Functional anatomy of limbic seizures: focal discharges from medial entorhinal cortex in rat

Focal seizure discharges were induced in the ventral aspect of the medial entorhinal cortex of awake, freely moving rats, either with cannula injections of penicillin or picrotoxin (0.02 microliters every 10-15 min) or by repetitive tetanic electrical stimulation. [14C]Deoxyglucose autoradiography (DG) was performed when animals were in a 'steady-state' with respect to electrographic discharges and/or behavioral changes. During simple interictal spikes behavior remained normal and DG labeling was increased only in the entorhinal focus and stratum moleculare of the ventral dentate gyrus. With complex spikes and short seizures animals exhibited staring, decreased responsiveness, and occasional wet dog shakes. DG labeling was increased in all layers of the dentate gyrus, Ammon's horn (ipsilateral greater than contralateral) and, to a lesser degree, in ipsilateral amygdala, and the accumbens-ventral pallidum area. During strong seizures, rearing and forelimb clonus occurred and metabolism was strongly activated bilaterally in the hippocampal formation, amygdala, accumbens, substantia nigra, and the anterior and periventricular thalamic nuclei. These studies indicate that the dentate gyrus initially restricts the entry of seizures from entorhinal cortex into the rest of hippocampus. As this is overcome there is rapid bilateral spread through the hippocampal formation with passive interruption of normal behavior. With prolonged seizure discharges there is further capture of amygdala and subcortical extrapyramidal and thalamic nuclei associated with behavioral convulsions.

The localizing value of depth electroencephalography in 32 patients with refractory epilepsy

Clinical seizure manifestations, physical examination, radiological studies, neuropsychological tests, and scalp and depth electroencephalographic (EEG) studies were done to localize seizure foci in 32 patients, 23 of whom have undergone surgery with more than one year of follow-up. Of 16 patients with unlocalized scalp EEGs, depth EEG detected a consistent focal seizure onset in 3. Of 15 patients with localized scalp EEGS, depth EEG revealed multiple foci in 3 and inaccurate localization of the focus by scalp EEG in 4. Therefore, localization by scalp EEG was inaccurate in 10 of 31 patients. When depth EEG revealed a consistent focal seizure onset and this localization was used to determine to surgical resection site, good to excellent results were obtained in 12 of 13 patients. When depth EEG revealed additional foci of seizure origin, surgical results were fair or poor in all patients. No other localizing criteria predicted depth EEG results or surgical success with such accuracy. Furthermore, some criteria, including neuropsychological tests and radiological abnormalities, were falsely localizing at times. Therefore, of the presently available localizing criteria, depth EEG appears to be the most accurate.

Changes in intelligence following temporal lobectomy: relationship to EEG activity, seizure relief, and pathology

Pre- and posttemporal lobectomy measures of intelligence and memory in 36 patients with medically refractory complex partial seizures were compared with (1) various aspects of presurgical ictal and interictal EEG activity derived from surface and deep electrodes, (2) postlobectomy seizure relief, and (3) pathological findings in the resected lobe. With respect to interictal EEG data, bilaterally synchronous surface spikes (accompanied or unaccompanied by simultaneous deep spikes) and sharp waves were significantly correlated with lower prelobectomy intelligence scores and a drop in these scores following lobectomy. With respect to ictal EEG data, bilaterally synchronous and multifocal onsets were significantly correlated with a postlobectomy drop in intelligence scores. Patients with poor postlobectomy seizure relief tended to have lower presurgical intelligence scores and a drop in intelligence scores following lobectomy. The patients most likely to show a postlobectomy drop in intelligence were those demonstrating some combination of poor seizure relief, an absence of pathology in the resected specimen, or EEG signs indicative of poor seizure relief. Postlobectomy changes in intellectual status are therefore not necessarily exclusively attributable to the amount of postlobectomy seizure relief experienced by these patients, but might be due to a combination of factors.

Surface and deep EEG correlates of surgical outcome in temporal lobe epilepsy

Interictal and ictal EEG characteristics derived from limited surface montages and medial temporal lobe sites were compared with long-term seizure relief following anterior temporal lobectomy in 52 epileptics. Patients were classified into one of four surgical outcome groups, ranging from seizure free to no clinical improvement. For each patient, interictal records were analyzed according to deep and surface spike characteristics and background activity. Ictal records were analyzed according to the proportion of episodes initiated in a unilateral or bilaterally synchronous fashion, the proportion of surface or deep onsets, the variability of onset location, and the morphology of seizures onsets. Interictal EEG variables that correlates with surgical outcome included: (a) various types of bilaterally synchronous surface/deep spikes; (b) diffuse background slowing; (c) sharp waves; and (d) the presence of multiple independent deep spike patterns in the lobe chosen for resection. Relevant ictal EEG variables included: (a) episodes initiated in a bilaterally synchronous fashion; (b) variability in seizure onset location; (c) the proportion of precisely focal onsets from deep sites; (d) the proportion of surface onsets; and (e) the proportion of onsets from the side chosen for resection. Multivariate analysis of these data with linear, stepwise, discriminate analysis and adaptive, nonlinear, distribution-free pattern recognition demonstrated that: (a) both interictal and ictal EEG characteristics can independently predict surgical outcome at levels significantly better than chance; (b) ictal and interictal EEG data contain nonredundant information for making such predictions; and (c) nonlinear pattern recognition techniques are capable of deriving the most accurate rules for predicting the effects of surgery.

Depth electroencephalography in selection of refractory epilepsy for surgery

Depth electroencephalography (EEG) is sometimes used to evaluate medically refractory epileptic patients for surgical treatment. Surgical excision of well-defined epileptogenic foci has been shown repeatedly to cause a substantial reduction of seizure frequency in 60 to 80% of these patients; however, because surgical success is no better at centers that employ depth EEG in patient evaluation, the procedure remains controversial. Review of the available literature shows that depth EEG results reported to date, when compared to scalp EEG results in 178 patients, could have enabled selection of 36% more patients for surgery by defining otherwise unidentifiable single epileptogenic foci. Furthermore, depth EEG could have prevented surgery in another 18% by demonstrating different or additional epileptogenic foci in patients otherwise thought to have a single discharging focus amenable to resection. Thus depth EEG had the potential to alter the surgical decision in more than 50% of patients reported. Centers that employ depth EEG may evaluate a different population of patients, which could account for their lack of increased surgical success.

Neurotensin: first report of a cortical pathway

Neurotensin is one of a growing number of putative peptidergic neurotransmitters common to both gut and brain. It has been localised in many areas of the central nervous system including the amygdala, hypothalamus and brain stem, although its physiological significance is unknown. Using a specific antiserum to neurotensin and immunocytochemical techniques, we have investigated the distribution of neurotensin and neurotensin containing pathways in the rat limbic system. Immunoreactive neurotensin was found in all areas of the limbic system and within the limbic cortex. In addition a previously unreported group of neurotensin-containing cell bodies was found in the subicular region of the hippocampus.. In serial sections we were able to trace a pathway which appears to be the first peptide-containing pathway to be reported in the cerebral cortex from the dorsal hippocampus to the anterior cingulate cortex, following the path of the cingulate bundle. The presence of neurotensin (a putative excitatory transmitter) in this new hippocampal efferent pathway indicates that it may have an important role in cortical "association" mechanisms and limbic cortex functions.

Sleep state and seizure foci related to depth spike activity in patients with temporal lobe epilepsy

Depth spike activity was evaluated from medial temporal lobe sites using computer spike recognition techniques in all-night sleep records derived from 10 patients with medically refractory complex partial seizures. Sleep stages were classified into 1 of 4 groups: wakefulness, REM sleep, light sleep and deep sleep. Some disturbance in the periodicity of the sleep cycle was noted in most patients, but the relative proportions of REM sleep, light sleep and deep sleep were close to that reported for normals. Depth spike activity was observed to be most frequent in a majority of sites during deep sleep in 6 patients and during light sleep in 3 patients. In 1 patient equal numbers of sites showed maximal activation during light sleep and deep sleep. In 4 patients, certain sites in the more epileptogenic lobe demonstrated a maximal rate during waking or REM sleep. All patients reported in this study were considered to be suitable for temporal lobectomy. In contrast to the results obtained from a previous study, the side with the site demonstrating maximal mean spike rate did not necessarily correspond to the side chosen for lobectomy. Significant correspondence across patients between the more epileptogenic lobe and maximal spike rate was not found during waking and was further reduced during light sleep and deep sleep. The correspondence was, however, significant during REM sleep and for the side containing the site demonstrating the smallest activation in mean spike rate during light sleep or deep sleep relative to waking. These results indicate that an analysis of sleep induced changes in depth spike activity can be useful in improving predictions concerning epileptogenicity. Quantification of other aspects of the interictal EEG, such as background activity, may further improve such predictions.

Effects of chronic cerebellar stimulation on chronic limbic seizures in monkeys

No influences of chronic cerebellar stimulation were found in 10 different controlled experiments in 5 different monkeys with chronic alumina-induced psychomotor seizures. The stimulation parameters used were comparable to those used in human epileptics, and continuous daily EEG and behavioral monitoring allowed all seizures to be measured for daily frequency and duration over the several weeks of the experiments. Nocturnal seizures were similarly quantified in 3 monkeys to verify that cerebellar stimulation did not affect them. Motor cortex potentials evoked by cerebellar pulses confirmed that the stimulations were activating the cerebellum throughout the experiments, and measures of electrode access resistance and impedance verified that the electrodes remained in contact with the cerebellum. In one monkey given phenobarbital medication, interictal morbid behavior appeared to be improved by chronic stimulation of either cerebellum or dorsolateral frontal cortex, thus indicating an arousal influence of brain stimulation not due to cerebellum per se.

Eating epilepsy associated with a deep forebrain glioma

Seizures associated with deglutition are rare, and previously reported cases of so-called eating epilepsy have not identified specific electrophysiological or anatomical foci. We evaluated a 14-year-old with a deep hemispheric astrocytoma in whom focal seizures were consistently triggered by eating. Electroencephalograms were consistent with a deeply located seizure focus. Eating epilepsy should be grouped with reflex epilepsies that may originate with the diencephalon.

Hippocampal efferents reach widespread areas of cerebral cortex and amygdala in the rhesus monkey

The subiculum of the primate hippocampal formation stands at the end of a polarized sequence of intrinsic hippocampal efferents and is the source of efferents to the medial frontal cortex, the caudal cingulate gyrus, and the parahippocampal area and amygdala in the temporal lobe. In addition, the subiculum sends subcortical efferents to the septum and diencephalon.

Post-EEG seizure depression of human limbic neurons is not determined by their response to probable hypoxia

A subclinical EEG seizure occurred in each of 2 patients during a test that repeatedly evoked probable mild cerebral hypoxia. The firing rate of the neurons was unaffected until the EEG seizure had been ongoing for over a minute. After the EEG seizures, all neurons were profoundly depressed regardless of whether their responses to hypoxia had been an increase, decrease, or no change in firing rate. These observations indicate that the mechanism responsible for post-EEG seizure depression of neuronal firing is probably not cerebral hypoxia.