Intracranial EEG in temporal lobe epilepsy

Intracranial EEG monitoring before epilepsy surgery, while becoming less commonly performed in patients with unilateral mesial temporal lobe epilepsy, is still widely used when bilateral independent temporal lobe seizures are suspected or when extratemporal foci cannot be ruled out by noninvasive means. Additionally, many epilepsy centers are reporting excellent surgical outcome in patients with neocortical temporal lobe epilepsy, when resections are guided by intracranial EEG studies. This article reviews the indications, technical aspects, risks, and interpretation of intracranial EEG in patients with temporal lobe seizures. It also considers intracranial EEG features predictive of surgical outcome.

Long-term inpatient audiovisual scalp EEG monitoring

Long-term audiovisual scalp EEG monitoring is an essential diagnostic tool for the evaluation of paroxysmal disorders. The definitive classification of both nonepileptic and epileptic events is often possible only with the use of this technique. Assessment of response to treatment and the noninvasive presurgical localization of seizure foci are other important uses. The optimization of both clinical semiology and electrophysiologic data obtained from such studies is the subject of significant research efforts. Outcomes studies and advanced EEG analysis research should ultimately serve to minimize the cost of this valuable technique as well as maximizing its utility.

Visual and quantitative ictal EEG predictors of outcome after temporal lobectomy

PURPOSE

We investigated whether visual and quantitative ictal EEG analysis could predict surgical outcome after anteromesial temporal lobectomy (AMTL) in which mesial structures, basal, and temporal tip cortex were resected.

METHODS

We retrospectively reviewed 282 presurgical scalp-recorded ictal EEGs (21- to 27-channel) from 75 patients who underwent AMTL. We examined the pattern of seizure onset (frequency, distribution, and evolution) and estimated the principal underlying cerebral generators by using a multiple fixed dipole model that decomposes temporal lobe activity into four sublobar sources (Focus 1.1). We correlated findings with a 2-year postoperative outcome.

RESULTS

Sixteen patients had seizures with a well-lateralized, regular 5 to 9-Hz rhythm at onset, that most often had a temporal or subtemporal distribution. All patients became seizure free after surgery. In 51 patients, seizure onset was remarkable for lateralized slow rhythms (<5 Hz), which sometimes appeared as periodic discharges, were often irregular and stable only for short periods (<5 s), and had a widespread lateral temporal distribution. Among these a favorable surgical outcome was encountered in patients with seizures having prominent anterior-tip sources ( 16 of 17 seizure free), whereas those with dominant lateral or oblique sources had a less favorable outcome (three of 14 and 13 of 18, respectively). Irregular, nonlateralized slowing characterized seizure onsets in eight patients. Three patients became seizure free after surgery.

CONCLUSIONS

Both visual and quantitative sublobar source analysis of scalp ictal EEG can predict surgical outcome in most cases after AMTL and complement non-invasive presurgical evaluation.

Defining epileptogenic foci: past, present, future

There is a direct relationship between the geometry (location, area, and orientation) of cortex-generating epileptiform discharges and resultant spike or seizure voltage fields at the scalp. Epileptogenic foci have been localized traditionally with EEG by identifying the negative field maximum (e.g., a phase reversal between adjacent bipolar channels). However, it is the shape of the entire voltage field over the head, including both negative and positive maxima, which provides information necessary to characterize the focus properly. Source location and orientation can be inferred from spike or seizure voltage topography, however, three-dimensional visualization can be obtained from mathematical source models, such as an equivalent dipole. Recent investigations have shown that dipole models can identify the location of epileptogenic foci with sub-lobar precision. Accuracy is enhanced by using additional electrodes, particularly on the lower half of the head, and by measuring their location. Realistic head models obtained from three-dimensional reconstructions of MR images can overcome errors introduced by simple spherical models of the cranium. Co-registering EEG voltage topography and source models with a patient's own cerebral anatomy will make EEG an unparalleled functional imaging technique for defining epileptogenic foci.

Continuous source imaging of scalp ictal rhythms in temporal lobe epilepsy

PURPOSE

We wished to determine whether continuous EEG source imaging can predict the location of seizure onset with sublobar accuracy in temporal lobe epilepsy (TLE).

METHODS

We retrospectively analyzed the earliest scalp ictal rhythms, recorded with 23- to 27-channel EEG, in 40 patients with intractable TLE. A continuous source analysis technique with multiple fixed dipoles (Focus 1.1) decomposed the EEG into source components representing the activity of major cortical sublobar surfaces. For the temporal lobe, these were basal, anterior tip, anterolateral, and posterolateral cortex. Ictal EEG onset was categorized according to its most prominent and leading source component. All patients underwent intracranial EEG studies before epilepsy surgery, and all had a successful surgical outcome (follow-up >1 year).

RESULTS

Most patients with ictal rhythms having a predominant basal source component had hippocampal-onset seizures, whereas those with seizures with prominent lateral source activity had predominantly temporal neocortical seizure origins. Seizures with a prominent anterior temporal tip source component mostly had onset in entorhinal cortex. Seizures in some patients had several equally large and nearly synchronous source components. These seizures, which could be modeled equally well by a single oblique dipole, had onset predominantly in either entorhinal or lateral temporal cortex.

CONCLUSIONS

Multiple fixed dipole analysis of scalp EEG can provide information about the origin of temporal lobe seizures that is useful in presurgical planning. In particular, it can reliably distinguish seizures of mesial temporal origin from those of lateral temporal origin.

New applications of EEG/MEG in epilepsy evaluation

Newly developed digital EEG and MEG recording techniques have provided the ability to ask and at least partially answer questions that were previously beyond our capability. These include the location and character of cerebral sources for epileptiform spike and seizure rhythms and the prediction of anti-epileptic drug efficacy by electrophysiologic means. The techniques of EEG voltage topography and equivalent dipole modeling have now given clinicians a 2-D and 3-D view, respectively, of epilepsy-related brain activity. Quantitative EEG spike morphology measurements have, in addition, shown changes that correlate with and even predict anti-convulsant drug usefulness in a given individual. MEG devices can now measure brain magnetic fields from the entire head and provide localization of epileptic spike sources that are probably more accurate than that achieved by EEG.

Intracranial EEG substrates of scalp ictal patterns from temporal lobe foci

PURPOSE

To determine the intracranial EEG features responsible for producing the various ictal scalp rhythms, which we previously identified in a new EEG classification for temporal lobe seizures.

METHODS

In 24 patients, we analyzed simultaneous intracranial and surface ictal EEG recordings (64 total channels) obtained from a combination of intracerebral depth, subdural strip, and scalp electrodes.

RESULTS

Four of four patients with Type 1 scalp seizure patterns had mesial temporal seizure onsets. However, discharges confined to the hippocampus produced no scalp EEG rhythms. The regular 5- to 9-Hz subtemporal and temporal EEG pattern of Type 1a seizures required the synchronous recruitment of adjacent inferolateral temporal neocortex. Seizure discharges confined to the mesiobasal temporal cortex produced a vertex dominant rhythm (Type 1c) due to the net vertical orientation of dipolar sources located there. Ten of 13 patients with Type 2 seizures had inferolateral or lateral, temporal neocortical seizure onsets. Initial cerebral ictal activity was typically a focal or regional, low voltage, fast rhythm (20-40 Hz) that was often associated with widespread background flattening. Only an attenuation of normal rhythms was reflected in scalp electrodes. Irregular 2- to 4-Hz cortical ictal rhythms that commonly followed resulted in a comparably slow and irregular scalp EEG pattern (Type 2a). Type 2C seizures showed regional, periodic, 1- to 4-Hz sharp waves following intracranial seizure onset. Seven patients had Type 3 scalp seizures, which were characterized by diffuse slowing or attenuation of background scalp EEG activity. This resulted when seizure activity was confined to the hippocampus, when there was rapid seizure propagation to the contralateral temporal lobe, or when cortical ictal activity failed to achieve widespread synchrony.

CONCLUSIONS

Type 1, 2, and 3 scalp EEG patterns of temporal lobe seizures are not a reflection of cortical activity at seizure onset. Differences in the subsequent development, propagation, and synchrony of cortical ictal discharges produce the characteristic scalp EEG rhythms.

Magnetoencephalography/magnetic source imaging in the assessment of patients with epilepsy

Magnetoencephalographic (MEG) dipole source localization is a particularly promising new tool for noninvasive presurgical evaluation of epileptogenic foci. It is potentially more accurate than EEG localization techniques because magnetic fields are not attenuated or distorted by the skull and scalp, which allows cerebral sources to be modeled more simply. MEC spike and seizure sources are routinely co-registered with the patient's brain MRI for clinical interpretation. This has been called magnetic source imaging. Numerous studies have shown that MEG localization of foci agreed with lesion position, depth electrode and ECoG data, PET and MRI findings, and surgical success. The recent development of whole head sensor arrays has greatly enhanced the case with which epileptiform magnetic fields can be recorded and analyzed.

Localization of temporal lobe foci by ictal EEG patterns

Identifying patients whose complex partial seizures originate in temporal neocortex rather than in hippocampus is important because such patients have less favorable outcomes with standard anteromesial temporal resections. We reviewed scalp-recorded ictal EEGs of 93 epilepsy surgery candidates who either underwent intracranial EEG monitoring (n = 58) or who were referred directly for temporal lobectomy (n = 35). We definded seven patterns of early seizure discharges, grouped patients according to their seizure pattern, and correlated these with the site of seizure onset determined by intracranial EEG. Categorization by seizure pattern was also compared with brain magnetic resonance imaging (MRI) findings intracarotid amobarbital (Wada) testing. An initial, regular 5- to 9- Hz inferotemporal rhythm (type 1A) was most specific for hippocampal-onset seizures. Less commonly, a similar vertex/parasagittal positive rhythm (type 1B) or a combination of types 1B and 1A rhythms (type 1C) was recorded. Seizures originating in temporal neocortex were most often associated with irregular, polymorphic, 2- to 5-Hz lateralized activity (type 2A). This pattern was commonly followed by a type 1A theta rhythm (type 2B) or was preceded by repetitive, sometimes periodic, sharp waves (type 2C). Seizures without a clear lateralized EEG discharge (type 3) were most commonly of temporal neocortical origin. These associations between type of seizure pattern and probable site of cerebral origin were statistically significant. MRI and Wada testing did not have as much specificity as ictal patterns in differentiating among seizure origins. We conclude that the initial pattern of ictal discharge on scalp EEG can assist in distinguishing seizures of temporal neocortical onset from those of hippocampal onset. This information can be used to identify patients for invasive monitoring.

Applications of magnetic source imaging in evaluation of candidates for epilepsy surgery

Epilepsy was the first clinical entity to which magnetic source imaging was applied; however, the technology available severely limited early studies. The introduction of large-array magnetic source imaging systems has enabled striking advances in the application of magnetic source imaging in epilepsy surgery programs. The superimposition of functional localization of epileptic sources on anatomic images from MR imaging or computed tomography allows precise definition of the area and extent of epileptogenic tissue for possible resection. Concordance of magnetic source imaging indications with other noninvasive studies may reduce that need for invasive studies in certain patient populations.

Non-invasive localization of the epileptogenic focus by EEG dipole modeling

Localization of epileptogenic foci by traditional visual inspection of EEG traces is simplistic. Voltage topography and subsequent spatio-temporal multiple dipole modeling are new techniques to assess the character of cerebral generators of EEG spikes and seizure rhythms. These predictions have been validated by intracranial monitoring. Patients with mesial temporal seizures have ipsilateral spikes and early ictal rhythms with a strong tangential (vertical) dipole component that often leads any radial source activity. This suggests propagation from baso-mesial to lateral cortex. Those with infero-lateral temporal cortical seizures have similar findings, but tangential sources are synchronous with or lag radial sources. Patients with lateral temporal cortical seizures have spikes and ictal activity that are modeled principally by radial dipoles.

Brain source imaging of focal and multifocal epileptiform EEG activity

Brain electric source analysis (BESA) of the scalp EEG has been used to identify multiple equivalent current sources in the brain during during interictal spikes and seizure onset. To obviate the need for fitting dipole sources to every EEG segment, a new method has been developed on the basis of multiple fixed dipoles, each designed to emphasize functional imaging of particular cortical areas. "FOCUS" can quickly display EEG in various montages including new "sources montages" which provide a high sensitivity for source currents near each dipole while largely suppressing contributions from other brain areas. By comparing this "source EEG" to routine digital EEG in patients with complex partial epilepsy, we have observed that "FOCUS" can more readily determine whether an epileptiform discharge is consistent with a discrete or multifocal generator, characterize likely cerebral source(s), differentiate between spikes and seizures of mesio-basal versus lateral temporal or frontal origin, and estimate the presence and direction of propagation from source potential timing differences. Improved non-invasive EEG evaluations of partial epilepsy will undoubtedly result from this advance.

Aspartame has no effect on seizures or epileptiform discharges in epileptic children

The effects of aspartame (L-aspartyl-L-phenylalanine methyl ester; APM) on the neurological status of children with well-documented seizures were examined in a randomized, double-blind, placebo-controlled, crossover study. We report on 10 children (5 boys, 5 girls, ages 5-13 yr) who were tested for 2 weeks each on APM and placebo (single morning dose, 34 mg/kg). Seven children had generalized convulsions with 4 also having absence episodes. One child had absence seizures and 2 had complex partial seizures only. On each arm of the study, children were admitted to the hospital for a standard 21-lead electroencephalogram (EEG), continuous 24-hour cassette EEG, and determination of biochemical variables in plasma and urine. Subjects completed the Subjects Treatment Emergent Symptoms Scale (STESS) and parents the Conners Behavior Rating Scale. There were no significant differences between APM and placebo in the standard EEG or 24-hour EEG. No differences were noted for the STESS or the Conners ratings, and no differences were noted for any of the biochemical measures (except for expected increases in phenylalanine and tyrosine after APM). Our findings indicate that, in this group of vulnerable children, APM does not provoke seizures.

Models of brain sources

Two categories of models are available for the functional imaging of scalp recorded electric brain activity: single-time-point and spatio-temporal. Instantaneous models require strict assumptions that do not conform with the underlying physiology, because they rely on the few voltage differences measured at only one sampling point. Spatio-temporal models create a spatial image of discrete multiple sources and a temporal image of source current wave forms which reflect the time course of the local activity in circumscribed brain areas at a macroscopic level. The spatial image may be limited in accuracy because it depends both on model and data, but it can be validated by scanning the brain with regional dipole sources. In many cases of temporal lobe epilepsy, for example, interictal spikes can be described adequately by as few as two equivalent dipoles, which image the vertical source current arising from the medio-basal aspect of the temporal lobe and the horizontal source current from its lateral surface.

Spike voltage topography identifies two types of frontotemporal epileptic foci

We characterized voltage topography of frontotemporal EEG spikes in 24 patients with complex partial seizures and identified two distinct patterns. "Type 1" spikes possessed a "dipolar" field with a negative region over the inferolateral temporal scalp and a positive region over the contralateral, centroparietal scalp. "Type 2" spikes showed only a broad, frontotemporal negative field. One or the other spike type predominated in all but two patients. Correlations with clinical data and intracranial EEG suggest that type 2 spikes arise from temporal or frontal neocortex, while type 1 spikes involve mesial temporal structures as well as lateral cortex.

EEG dipole modeling in complex partial epilepsy

Visual inspection and qualitative impressions of clinical EEG abnormalities are being replaced by quantitative characterization of scalp voltage fields and dipole modeling of underlying cerebral sources. Three approaches have been used in the analysis of focal spikes of complex partial epilepsy. 1) Instantaneous, single dipole, inverse solutions for the voltage topography of the spike peak have revealed two distinct equivalent dipole configurations in the brain lobe beneath the negative extreme-radial and oblique (mixed radial and tangential). Only radial dipoles have been found for frontal and fronto-central spikes, while either type have been found for temporal and occipital spike foci. 2) Dipole stability can be assessed by an inspection of sequential instantaneous solutions encompassing the spike complex or by calculating the standard deviation of dipole location (x,y,z) and orientation (elevation, azimuth) parameters during this period. Two-thirds of spike dipoles of the radial type and essentially all of the oblique equivalent dipoles were found to be stable, whereas one-third of the radial dipoles were unstable in position or orientation. 3) Spatio-temporal analysis can identify multiple underlying sources and their potentials. Modeling separate radial and tangential dipoles over the course of the spike has revealed a composite character for spike fields with oblique dipoles and often has defined leads or lags in activity that suggested propagation between infero-mesial and lateral temporal cortex. Correlations with clinical and intracranial EEG data suggest that patients with mesial temporal sclerosis have spikes with oblique and stable equivalent dipoles; patients with discrete cortical lesions have spikes with radial and stable dipoles; patients with extensive or multi-focal cortical insults have spikes with radial and unstable dipoles.

Spike voltage topography and equivalent dipole localization in complex partial epilepsy

The EEG of 45 patients with complex partial epilepsy was recorded from standard and supplementary inferior temporal electrode sites for 2 or more days via cable telemetry onto video (VHS) tape (22-25 channels, common reference). Epochs with "temporal spikes" were read into a topographic EEG device where individual spikes were visually identified and averaged in sums of 8-32. Analysis of spike voltage topography revealed two distinct patterns - dipolar, Type 1 and non-dipolar, Type 2. One or the other spike type predominated in all but two patients. Application of source modeling techniques (3 shells, single dipole, 6 parameters) to the spike topography data revealed that both spike types had similar equivalent dipoles in terms of location and orientation, except for vector elevation. However, calculated dipoles for Type 1 spikes were more stable over the course of the spike peak. Correlations with clinical data and intracranial EEG suggest that Type 1 spikes originate in mesial temporal structures, while Type 2 spikes arise from temporal or frontal neocortex. Spike voltage topography and equivalent dipole localization appear to be useful in the presurgical evaluation of patients with focal epilepsy.

Estimating the importance of epileptiform abnormalities discovered on cassette electroencephalographic monitoring

Assessment of the importance of interictal epileptiform abnormalities discovered with cassette electroencephalographic (EEG) monitoring requires some appreciation of the frequency with which such abnormalities may be encountered in individuals without epilepsy. From a clinical experience involving more than 2500 patients, we have defined a group of 184 patients referred because of headache, with no additional referral information to suggest seizures. Only one (0.5%) of these patients had epileptiform abnormalities on cassette EEG, yielding 95% and 99% confidence limits for the incidence of epileptiform abnormalities in the unselected nonepileptic headache population of 1.5% and 1.8%. Presuming that the incidence of cassette EEG epileptiform abnormalities in the healthy population would be no higher than in this patient group, we suggest that epileptiform abnormalities are no more likely to be incidental findings on cassette EEG monitoring than on routine EEG. Consequently, the detection of such abnormalities seems a worthwhile aspect of cassette EEG interpretation when the goal of monitoring is the detection of evidence to support a diagnosis of epilepsy.

The convulsant effects of kainic acid microinjections into cerebral cortex are concentration but not laminar dependent

The direct excitatory glutamate analog, kainic acid, when injected into different layers of the cat visual cortex, has two effects on this tissue that are fundamentally different from those exerted by the disinhibitory agents penicillin, bicuculline, and strychnine. First, both concentration-dependent suppression and convulsant activity was seen. Second, these concentration-dependent influences were produced effectively in each cortical layer challenged despite a reported preferential localization of kainic acid receptors to layer 6. In contrast, the disinhibitory agents generally produce only convulsant effects, do so in a laminar-specific manner, and at significantly lower concentrations in intact brain (i.e., 50 microM vs 15 mM). These results suggest that kainic acid, in the concentrations employed here, has a generalized excitatory effect on all cortical circuitry and that it's convulsant influences are nonspecific and less potent than those of the inhibitory blockers. Further, a common neural trait shared by these different modes of epileptogenesis is discussed.

Comparisons between strychnine and penicillin epileptogenesis suggest that propagating epileptiform abnormalities require the potentiation of thalamocortical circuitry in neocortical layer 4

Simultaneous recordings from three laminae within the cat visual cortex following differential intralaminar injections of strychnine (i) confirmed that low strychnine concentrations (5 mM) induce interictal-like epileptiform abnormalities (late responses) only when injected into superficial layers 2 and 3, (ii) revealed that these abnormalities are generated locally within these layers, and (iii) showed that they remain local phenomena by not spreading vertically into other cortical layers. Higher strychnine concentrations (20 mM), however, (iv) obscured these laminar differences by increasing layer 4 sensitivity to this agent in addition to the maximally sensitive superficial layers, and further (v) revealed nonlocal, vertically propagating, interictal-like abnormalities (late responses) following layer 4 injections which are preceded by an increase in thalamocortically mediated activity within this layer (enhanced physiologic responses). When penicillin was used as the convulsant, propagated interictal-like responses (late responses) induced in any layer were always preceded by a thalamocortically mediated response from layer 4 (enhanced physiologic responses); a condition clearly unlike the 5 mM but similar to the 20 mM strychnine foci observed in this study. These results suggest that convulsant action upon the thalamocortical circuitry of layer 4 is essential for the development of propagating as opposed to local epileptiform activity. Further, these results may also help explain why some cortical seizure disorders remain localized (focal) whereas others secondarily generalize to distal brain sites (i.e., complex partial seizures of extratemporal origin).

EEG outside the hairline: detection of epileptiform abnormalities

We have assessed the reliability in 25 epileptic patients of EEG recording using disposable self-adhesive electrodes and a seven-channel extra-hairline montage, through comparison to simultaneous standard cable telemetry. Epileptiform abnormalities were detected in all patients on blind interpretation of data acquired using the novel technique. On comparison of specific epileptiform abnormalities identified blindly, a false positive rate of 10% and a false negative rate of 8% were encountered. Posterior temporal complexes accounted for most false negatives, and most false positives did not represent failings of the recording technique. We conclude that EEG recording outside the hairline with disposable electrodes represents a reliable means to acquire, quickly and simply, EEG evidence of epilepsy. As such, it may prove useful in the acute evaluation of patients presenting with apparent seizures.

Supervision of ambulatory cassette EEG screening: a strategy based on the temporal distribution of epileptiform abnormalities

The interposition of screening personnel in the analysis of extended cassette EEG recordings requires a method for supervision by the responsible electroencephalographer. To this end, analysis was undertaken of the temporal distribution of epileptiform abnormalities and seizures on cassette EEG in 40 epilepsy unit inpatients for whom these abnormalities were confirmed by simultaneous cable telemetry. Epileptiform abnormalities were noted in the hour following sleep onset in 38 patients (92%). Seizures occurred primarily in wakefulness. One hundred cassette EEG tapes obtained for clinical purposes and previously found to contain evidence of epilepsy were then subjected to an abbreviated review. Eighty-seven contained epileptiform abnormalities in the first hour of sleep, and 7 additional tapes contained only EEG seizure activity coincident with reported clinical events. Considering that fewer than 20% of cassette recordings obtained in unrestricted clinical use are likely to harbor epileptiform abnormalities or seizures, correct identification of about 99% of tapes as normal or abnormal can be anticipated with an abbreviated review consisting of analysis of the first hour of sleep; and analysis of reported clinical events if no abnormalities are noted in the first hour of sleep. Such a review is sufficiently accurate to serve as a supervisory analysis of cassette EEG tapes that have been previously screened and reported to lack evidence of epilepsy.

Ambulatory cassette EEG in epilepsy diagnosis

The electroencephalographic evaluation of patients with possible or proven epilepsy is no longer limited to routine laboratory EEGs or intensive inpatient monitoring. Expanded temporal sampling of the EEG, which increases the probability of documenting, characterizing, and quantitating the electrographic manifestations of these illnesses, is now available on a portable, outpatient, and less cumbersome inpatient basis by means of ambulatory cassette recordings. The technological advances which have made this technique feasible include small multi-channel tape recorders, miniature preamplifiers, and rapid video/audio playback units. New designs in montages and analysis techniques have made the procedure practical. Clinical series and controlled trials have confirmed the usefulness of cassette EEG monitoring in the evaluation of epilepsy and a wide range of other paroxysmal neurologic disorders. Ambulatory EEG diagnostic yields have been shown to be superior to routine laboratory studies and nearly as good as inpatient telemetry evaluations. The role of cassette recordings in clinical electroencephalography continues to be defined as new applications are established.

Cassette electroencephalography in the evaluation of neonatal seizures

Three-channel cassette electroencephalographic (EEG) recording for up to 24 hours was obtained from 37 neonates with clinically diagnosed or suspected seizures but no seizure activity on routine EEG. EEG seizures were recorded in seven patients, five of whom had experienced clinical seizures in the 24 hours prior to cassette EEG recording. EEG seizures were detected in only one of nine neonates with recurring clinical episodes believed unlikely to be seizures and in only one of 18 without recent clinical events. Cassette EEG can enhance the detection and differentiation of seizures in neonates with persistent clinical episodes but is of low yield otherwise.

Ambulatory cassette EEG in clinical practice

We reviewed ambulatory cassette EEG (A/EEG) records of 500 patients. Epileptiform abnormalities, seizures, or both were detected in 87 patients (17.4%), including 22 who were not taking anticonvulsant drugs. Epileptiform abnormalities were found in 1.5% of patients with syncope and in none without a clear history of episodic complaints. Abnormalities were found in 5.1% of patients referred by nonneurologic physicians. Some clinical seizures were not accompanied by A/EEG change and some episodes were not seizures, despite detection of epileptiform abnormalities.

Ambulatory cassette EEG

The neurophysiologic evaluation of patients with possible or proven paroxysmal disorders is no longer limited to routine laboratory EEGs or intensive inpatient monitoring. Expanded temporal sampling of the EEG, which increases the probability of documenting, characterizing, and quantitating the electrographic manifestations of these illnesses, is now available on a portable, outpatient, and less cumbersome inpatient basis by means of ambulatory cassette recordings. Ambulatory EEG has evolved from clinical need and from new technology that has provided small multichannel tape recorders, miniature preamplifiers, and rapid video/audio playback units. Clinical series and controlled trials have confirmed the usefulness of cassette EEG monitoring in a wide range of neurologic disorders, particularly epilepsy. Ambulatory EEG diagnostic yields have been shown to be superior to routine laboratory studies and nearly as good as inpatient telemetry evaluations. The role of cassette recordings in clinical EEG is being redefined as new applications are established.

Differences between strychnine and penicillin epileptogenesis suggest a laminar organization of neocortical inhibition

Transient foci of epileptiform alteration in neuronal population activity were induced by microinjection of strychnine sulfate into different layers of cat striate cortex. Potentials evoked by visual field-specific photic stimulation were recorded from microelectrodes at the injection site and in adjacent laminae. Epileptogenesis, characterized by an enhancement of the normal primary response followed by the development of a large late potential, occurred only with strychnine injections into superficial pyramidal layers 2 and 3. By contrast, stellate layer 4 has been shown to be most susceptible to epileptogenic effects of penicillin and bicuculline. Since disinhibitory convulsants should be most effective where their actions antagonize the prevalent type of inhibition, these findings suggest that there may be a laminar segregation of neocortical inhibition, possibly glycine-mediated in layers 2-3 and probably gamma aminobutyric acid (GABA)-mediated in layer 4.

Cassette electroencephalographic recording of neonates with apneic episodes

To evaluate the practical importance of seizures as a cause of neonatal apnea, we obtained extended cassette electroencephalographic recording for periods as long as 24 hours from 50 neonates experiencing apneic episodes unassociated with clinical seizure activity. Electroencephalographic recording through definite apneic episodes was obtained in 37 neonates; a total of 153 episodes were detected. None was associated with electroencephalographic seizure activity, although one term neonate without documented apneic episodes had seizure activity detected by cassette electroencephalography and may have had apneic seizures. The episodic apnea and bradycardia commonly encountered in preterm neonates is unlikely to be a manifestation of seizure activity, and extended electroencephlographic monitoring of such patients is of low yield.

Direct comparison of 3- and 8-channel ambulatory cassette EEG with intensive inpatient monitoring

During the intensive monitoring of 30 patients by means of cable telemetry EEG, simultaneous 3- and 8-channel ambulatory EEG recordings were also obtained. Blinded interpretations of the cassette tapes were compared with those of the cable telemetry records. Both 3- and 8-channel ambulatory EEG reviews correctly identified 93% of the records as either normal or epileptiform. Lateralization of abnormalities was equally good with either cassette system, but more detailed characterization was achieved with 8-channel ambulatory EEG. Although 100% of seizures were detected on both systems, there were more false-positive errors when only three data channels were available. Better ability to differentiate renal abnormalities from artifacts was the most significant advantage of 8-channel over 3- to 4-channel ambulatory EEG.

The clinical utility of ambulatory cassette EEG

We obtained ambulatory EEG monitoring (A/EEG) before intensive monitoring in 33 patients newly referred to an epilepsy center. The A/EEG yield of evidence to support a diagnosis of epilepsy was 83% of that of intensive monitoring and 2.5 times that of routine EEG. Accuracy of A/EEG analysis was confirmed by two blind reviews for each tape, with only three misreadings among 99 interpretations rendered. Among 25 patients with specifically diagnostic referrals, A/EEG served the purpose of admission as well as intensive monitoring in 60%. Episodes not accompanied by A/EEG change required behavioral observation for correct identification.

Laminar interactions during neocortical epileptogenesis

Interactions among laminar subpopulations of cat striate cortical neurons were assessed during the evolution of discrete and temporary epileptic foci, which were induced by selective microinjection of penicillin into different cortical layers. Field potentials and multiunit cellular discharges, evoked by selective visual field stimulation, were recorded simultaneously from 3 layers by multibarreled glass microelectrodes. Laminar response profiles at distinct stages of epileptogenesis were characterized for foci induced in superficial pyramidal, middle stellate, and deep pyramidal layers. Layer 4 was verified to be the most susceptible to epileptogenesis. Penicillin's action within this stellate layer appeared to be sufficient for epileptogenesis and was supportative of, if not necessary for, the development of foci originating in pyramidal cell layers. These findings could not be fully appreciated by monitoring only spontaneous interictal spike potentials. Of the two types of neuronal discharge routinely observed, early latency bursting was principally a characteristic of layer 4 stellate populations, whereas longer-latency bursts comparable to paroxysmal depolarization shifts were recorded equally well from both stellate and pyramidal layers. Epileptiform alterations in both field potential and unit responses were quickly evident in cortical laminae having known anatomic connections with the layer where the focus was induced: e.g. in layers 2-3 with layer 4 foci, in layers 5-6 with layers 2-3 foci, and in layer 4 with layers 5-6 foci. The spread of epileptogenesis was slower between laminae where pathways are purported to be less well developed, and appeared to be principally dependent upon the diffusion of penicillin.