Postoperative EEG and Electrocorticography: Relation to Clinical Outcome in Patients with Temporal Lobe Surgery

Summary: To evaluate the role of different EEG methods with respect to postoperative clinical follow-up, 32 non-lesionary epilepsy patients who had undergone temporal lobectomy were studied preoperatively and at 2-week, 3-month, and 1-year postoperative follow-up. Routine, sleep, and sphenoidal EEG recordings as well as intraoperative electrocorticography (ECoG) were made for all patients. At 1-year follow-up, the EEGs with sphenoidal electrodes and with sleep deprivation procedure provided important prognostic information; the appearance of seizures was associated with the presence of interictal epileptiform abnormalities in EEG. In the postresection ECoG, however, epileptiform abnormalities were not associated with clinical outcome or with postoperative epileptiform EEG at 1 year. Routine EEG reliably reflects clinical outcome after temporal lobectomy; with sphenoidal electrodes as well as with sleep deprivation procedure, the diagnostic yield can be further improved.     

Visual versus computer evaluation of thiopental-induced EEG changes in temporal lobe epilepsy

Thiopental-induced EEG beta activity was analyzed both visually and by computer in 33 patients with complex partial epilepsy. Studies were done in 16 patients with depth electrodes in limbic structures and 17 patients with scalp and sphenoidal electrodes. The percentage of drug-induced change in beta activity was quantified by computer using spectral analysis. The statistical significance of asymmetries between homologous sites in the amount of change was determined. The spatial distribution of significant asymmetries was used for localization and compared with the results of independent visual analysis of the thiopental EEG. Concordance between computer and visual evaluation occurred in 10 of 17 scalp/sphenoidal and 10 of 16 depth electrode tests. The accuracy of visual and computer localization was determined by comparing them with locus of itcal EEG onset, interictal spikes, and positron emission tomography. In scalp/sphenoidal studies, computer analysis indicating asymmetry appeared more likely to correlate with independent clinical criteria than visual analysis. In depth studies the reverse appeared to be true. Scalp/sphenoidal tests yielded positive results in 25-30% of patients whereas depth electrode tests were positive in 50-70% of patients. The results indicate that computer analysis of surface thiopental tests is an accurate and useful supplement to visual evaluation of these tests.     

Sphenoidal EEG recording in complex partial seizures

A prospective study was performed to evaluate the usefulness of sphenoidal EEG recording during wakefulness, as compared to routine tracings awake and asleep, for recognizing epileptic electroencephalographic foci in patients with complex partial seizures. Fifty patients were investigated. Following sleep deprivation a routine waking EEG, a sleep tracing and an awake recording with sphenoidal needles were obtained. In nine patients temporal epileptiform activity was apparent in all three conditions (wakefulness, sleep and with sphenoidal electrodes). In 21 patients temporal epileptiform activity was seen during sleep only, while the sphenoidal leads were non-contributory. In 20 patients epileptiform activity was not recorded under any of the above conditions. This study indicates that sphenoidal recording during wakefulness does not contribute to the detection of epileptic discharges in patients with complex partial seizures.     

Interictal Epileptiform Discharges in Temporal Lobe Epilepsy Due to Hippocampal Sclerosis Versus Medial Temporal Lobe Tumors

PURPOSE: It remains controversial whether a specific pattern of interictal epileptiform activity exists that may help to differentiate temporal lobe epilepsy (TLE) due to hippocampal sclerosis (HS) from other forms of TLE. In this study, we characterized the distribution of interictal epileptiform discharges in TLE due to HS as compared with those in patients with tumors restricted to the medial temporal lobe structures. METHODS: The study included 21 adult patients with unilateral HS who remained seizure free (>1 year) after anterior temporal lobectomy with amygdalohippocampectomy. Patients with "dual pathology" were excluded. The comparison group consisted of nine patients with tumors restricted to the amygdala and hippocampus. All patients underwent video-EEG monitoring preoperatively by using 39 scalp electrodes (including the 10-10 system over both temporal regions) and bilateral sphenoidal electrodes. RESULTS: The HS patient group showed a significantly higher percentage of ipsilateral epileptiform discharges maximal at anterior temporal electrodes (median, 97.0%; sphenoidal electrode alone, 88.1%), as compared with the tumor group (median, 72.1%; p<0.001; sphenoidal electrode alone, 24.8%; p<0.001). The HS group had significantly fewer extratemporal spikes/sharp waves (median, 0.0), as compared with the tumor group (10.0%; p<0.001). At least 90% of the interictal discharges were located in the anterior temporal region in 20 (95.2%) of 21 HS patients, but in none of the tumor patients (p<0.001). Bilateral temporal discharges were found in nine (42.9%) of 21 patients with HS and in two (22.2%) of nine tumor patients (p = 0.42). CONCLUSIONS: We conclude that ipsilateral interictal epileptiform discharges outside the anterior temporal region are rare (<10%) in adults with intractable TLE due to unilateral HS. Frequent posterior or extratemporal sharp waves may detract from the certainty of this diagnosis in complicated cases. These restricted epileptiform discharges suggest a smaller irritative zone in HS as compared with medial tumors, or a more organized activity associated with intrinsic hippocampal disease. Bilateral epileptiform discharges were not uncommon in both groups.     

Sphenoidal electrodes THEIR USE AND VALUE IN THE ELECTROENCEPHALOGRAPHIC INVESTIGATION OF COMPLEX PARTIAL EPILEPSY

The diagnostic value of sphenoidal electrode EEG recordings in patients with seizures characteristic for epilepsy with complex partial symptomatology was assessed in a study comprising 404 patients; 71.3% of the patients had seizures with automatisms and amnesia, and 28.7% had psychic seizures with subjective phenomena such as hallucinations and illusions. A total of 59.6% of the patients had diagnostic EEG changes in routine waking or sleep EEG. In sphenoidal EEG recording including thiopenthone activation, diagnostic changes were found in 40.5% of the patients without specific changes in waking or sleep EEG, the chance of a positive finding being more than five times higher in patients with automatisms than patients with psychic seizures. Apart from cases where surgical treatment of temporal lobe epilepsy is considered, sphenoidal electrode EEG recording, including intravenous thiopenthone activation, should be performed in patients with seizure phenomena raising suspicion of epilepsy with complex partial symptomatology but where waking and sleep EEGs fail to demonstrate specific abnormalities.     

Significance of Simple Partial Seizures in Temporal Lobe Epilepsy

Summary: We determined how localization of simple partial seizures (SPS) correlated with localization of complex partial seizure (CPS) in scalp/sphenoidal EEG and assessed prognosis after temporal lobe resective surgery in patients with an ictal correlate of SPS in scalp/sphenoidal EEG recordings. EEGs were recorded with the 10–20 system of electrode placement and supplemented with sphenoidal electrodes. Between 1985 and 1992, 183 patients with temporal lobe epilepsy (TLE) reported an aura (SPS) during inpatient monitoring; all were eligible for inclusion in our study. The EEGs during SPS showed ictal changes in 51 patients (28%, 117 SPS). Forty-four patients had unilateral temporal interictal spikes (IIS), and SPS and CPS always arose from the same region. Seven patients had bitemporal interictal spikes; SPS colocalized with CPS in 4 patients (57%), SPS were contralateral to CPS in 2 patients, and 1 patient had bilateral independent CPS but unilateral SPS. SPS accompanied by EEG ictal changes conveyed a favorable prognosis in patients who underwent epilepsy surgery. Scalp/sphenoidal recorded IIS but were less reliable in identifying the location of CPS onset in patients with bitemporal spikes.     

Comparison of sphenoidal, foramen ovale and anterior temporal placements for detecting interictal epileptiform discharges in presurgical assessment for temporal lobe epilepsy.

OBJECTIVES: Some authors have recently stressed that the position of the tip of the sphenoidal electrode plays a crucial role in its efficacy to detect mesio-basal spikes. We have tested this hypothesis by comparing the sensitivity of a contact of a foramen ovale bundle located at the foramen ovale (CFO) with scalp electrodes in detecting interictal epileptiform discharges. We have also compared deep and superficial foramen ovale contacts in the same bundle in order to establish whether deeper contacts can detect epileptiform discharges not seen at the foramen ovale or on the scalp. METHODS: The sensitivity for detecting epileptiform discharges of simultaneous intracranial and scalp EEG recordings from 20 patients under telemetric presurgical assessment for temporal lobe epilepsy were compared. RESULTS: Out of 2280 epileptiform discharges evaluated, about 70% were seen only at the deep foramen ovale contacts. Out of the 722 discharges recorded by CFO and/or scalp electrodes, 698 were seen at the CFO and 690 at the scalp anterior temporal electrode. Only on 29 occasions (4.15%) were discharges recorded at the CFO and not at the anterior temporal electrode. On 21 occasions (3.04%) CFO failed to detect discharges seen at the anterior temporal electrode. CONCLUSIONS: Our findings confirm previous results suggesting that sphenoidal electrodes, however accurately positioned, offer no significant increase in detection sensitivity compared with anterior temporal scalp electrodes. In addition, these results confirm that a large proportion of discharges seen at the deepest foramen ovale contacts are not seen either on the scalp nor at the superficial foramen ovale contacts.     

The necessity for sphenoidal electrodes in the presurgical evaluation of temporal lobe epilepsy: con position.

This article reviews several lines of evidence that efface the requirement for sphenoidal leads in the EEG investigation of temporal lobe epilepsy. Mandibular notch or anterior temporal electrodes, each situated well within the anterior temporal spike field, detect interictal and ictal epileptiform phenomena virtually as well as do sphenoidal leads, provide consistent recording circumstances, do not require physician expertise for their placement, and create no discomfort. This article also cites many studies demonstrating the reliability of ictal semeiology and of MRI in lateralizing and localizing temporal epileptogenesis. Thus, EEG constitutes one element in a matrix of lateralizing data.     

EEG changes,recorded with subdural electrodes,after thiopental injections in patients with temporal lobe seizures

Nineteen patients with temporal lobe epilepsy, examined with intracranial subdural electrodes for seizure monitoring, were subjected to intravenous thiopental injections to the level of deep narcosis. The induced EEG changes were recorded, analyzed, and correlated to the localization of the seizure-generating focus. In 14 patients with unilateral seizure onset, the thiopental injections evoked a focal spike activation on the focus side. The first suppression period in the EEG with a duration exceeding 1 s was recorded from the strip electrode overlying the focus. Only in six patients did asymmetries in the barbiturate-induced beta activity in the background EEG show lateralizing signs in concordance with the side of seizure onset. In five patients with bilateral seizure-generating foci, no significant lateralization in induced beta activity, focal spike activation, or focal suppression pattern could be found. The results are discussed, comparing similar studies performed with surface, sphenoidal, and depth electrodes.     

Techniques and applications of sphenoidal recording

Sphenoidal electrodes were introduced in the late 1940s and early 1950s to record EEG activity from the inferior mesial temporal region. They are inserted percutaneously beneath the zygomatic arch to rest in the vicinity of the foramen ovale. They are safe, easy to insert, and well tolerated for up to 3 weeks. Artifacts are less prominent than with other types of basal electrodes. Complications are rare. Sphenoidal electrodes are superior to scalp electrodes in detecting interictal epileptiform discharges and ictal EEG patterns from the inferior mesial temporal lobe. Clinical indications include: documentation of epileptiform activity in patients with partial complex seizures in whom scalp recording has been inconclusive; diagnosis of episodes of uncertain mechanism; and localization of an epileptogenic focus in patients being evaluated for temporal lobectomy.     

Comparative localization of epileptic foci in partial epilepsy by PCT and EEG

One or more interictal positron computed tomograms of 18F-fluorodeoxyglucose were obtained on 50 patients with partial seizure disorders. Ictal as well as interictal electroencephalographic (EEG) data were available for all 50 patients, with scalp, sphenoidal, and depth electrode recordings done on 27 and scalp and sphenoidal recordings alone on 23. Thirty-five patients demonstrated one or more abnormal interictal zones of hypometabolism, while combined EEG studies were localizing for 36. There were considerable disagreements between the location of metabolic deficits and the epileptic focus revealed by individual scalp and depth EEG recorded ictal and interictal epileptiform activity; however, there was good correlation between the site of focal hypometabolism and the epileptic focus determined by the combined results of all electrophysiological studies. When focal hypometabolism and focal nonepileptiform EEG abnormalities (i.e., slow waves and attenuation of fast rhythms) were both present in the same patient, their localization agreed completely. Metabolic and combined electrophysiological techniques both occasionally produced false positive as well as false negative results. When used together, the EEG can confirm that a hypometabolic zone is epileptogenic, while FDG scans may indicate whether an epileptic EEG focus represents a lesion or propagation from a distant site.     

Anterior "cheek" electrodes are comparable to sphenoidal electrodes for the identification of ictal activity.

Sphenoidal electrodes are used to localize epileptiform activity originating in the temporal lobe during complex partial seizures. Sphenoidal electrodes, however, are semi-invasive and uncomfortable to the patient. We compared skin electrodes placed on the cheek ("cheek electrodes") with sphenoidal electrodes for the detection of the side and site of complex partial seizure onset. In a masked, randomized comparison of single ictal recordings in 22 patients, there were no significant differences between sphenoidal and cheek electrode montages in detecting the side or site of ictal onset (P < 0.01). Signal/noise ratios for interictal spikes were a mean 16.5% greater at sphenoidal sites compared to cheek sites (paired t test, t = 2.4, P < 0.05). This difference, however, did not influence the detection of rhythmical ictal activity in cheek and sphenoidal montages in our study, nor the assignment of side, site or time of seizure onset by unbiased readers. Recordings from cheek electrodes are comparable to those from sphenoidal electrodes and are useful for localizing ictal activity.     

Intracranial volume conduction of cortical spikes and sleep potentials recorded with deep brain stimulating electrodes.

OBJECTIVE: To examine interictal epileptiform and sleep potentials recorded intracranially from deep brain stimulation (DBS) electrodes in patients treated with DBS for epilepsy. Specifically, this study sought to determine whether the DBS-recorded potentials represent: (a) volume conduction from surface neocortical discharges or (b) transsynaptic propagation along cortical-subcortical pathways with local generation of the subcortical potentials near the DBS targets. METHODS: Six patients with intractable epilepsy treated with thalamic DBS of the central median nucleus (CM; one patient) or anterior thalamus (5 patients) who had focal interictal spikes were studied. Sleep potentials were also studied in a 7th patient with Parkinson disease treated with DBS of the subthalamic nucleus (STN). RESULTS: Focal interictal cortical spikes recorded by scalp electroencephalography (EEG) were recorded synchronously, but with opposite polarity, from the DBS electrodes in CM as well as the more superficial anterior thalamic contacts situated in the anterior nucleus (AN) and dorsal medial nucleus (DM). In referential montages, the subcortical potentials were of highest amplitude ipsilateral to the focal cortical spikes, with a small but reproducible amplitude decrement present at each electrode contact more distant from the cortical source, irrespective of the specific DBS target. Subcortical sleep potentials (K-complexes and sleep spindles) were also recorded synchronously and with inverse polarity compared to the corresponding scalp potentials, and appeared in a similar fashion at all subcortical sites sampled by the DBS electrodes. Amplitude attenuation in the thalamus of intracranial volume conducted potentials with increasing distance from their cortical spike sources was measured at approximately 5-10 microV/mm. DISCUSSION: Recent reports on scalp-CM or scalp-STN EEG recordings in patients treated with DBS for epilepsy have interpreted the intracranial waveforms as evidence of transsynaptic cortical-subcortical transmission across neuroanatomical pathways presumed to be involved in the generation of sleep potentials (Clin. Neurophysiol. 113 (2002) 25) and epileptiform activity (Clin. Neurophysiol. 113 (2002) 1391). However, our results show that the intracranial spikes recorded from DBS electrodes in various regions of the thalamus (CM, AN and DM) represent subcortical volume conduction of the synchronous cortical spikes recorded with scalp EEG. The same is true for the intracranial reflections of scalp EEG sleep potentials recorded from DBS electrodes in CM, AN, DM and STN. These interictal DBS waveforms thus cannot be used to support hypotheses of specific cortical-subcortical pathways of neural propagation or subcortical generation of the DBS-recorded potentials associated with scalp EEG interictal spikes and sleep potentials. SIGNIFICANCE: Detailed analysis of the intracranial potentials recorded from DBS electrodes in association with scalp EEG spikes and sleep discharges shows that the intracranial waveforms represent volume conduction from discharges generated in the neocortex and not, as has been suggested, locally generated activity resulting from cortical-subcortical neural propagation.     

Combined use of sphenoidal electrodes and the dipole localization method for the identification of the mesial temporal focus

We attempted to sub-classify four cases who show temporal spikes on standard scalp electroencephalogram (EEG), using sphenoidal electrodes and the dipole localization METHOD: In a case with mesial temporal epilepsy, spikes showed phase reversal in a sphenoidal electrode, and the spike dipoles were estimated to be in the mesial temporal lobe. In a case with lateral temporal epilepsy, spikes showed no phase reversal in a sphenoidal electrode, and the spike dipoles were estimated to be in the lateral temporal lobe. In two cases out of four, spikes showed phase reversal in sphenoidal electrodes, whilst the dipoles were estimated to be in the frontal lobe. Clinical features also suggested a diagnosis of frontal lobe epilepsy. In one of the two cases in which frontal lobe epilepsy was suspected, ictal dipoles as well as interictal spike dipoles indicated participation of the frontal lobe in the genesis of seizures. Nevertheless, only mesial temporal lobectomy was performed based on results obtained by invasive subdural electrodes. As a result, seizures were not controlled. Although sphenoidal electrodes were useful for differentiating between mesial and lateral temporal lobe foci, it is advisable to use them in combination with the dipole localization method to identify frontal lobe foci.     

Detection of epileptiform activity by different noninvasive EEG methods in complex partial epilepsy

The diagnostic utility of different noninvasive electrode placements for deriving the electroencephalogram and detecting interictal epileptiform discharges was compared. Anterior temporal and nasopharyngeal electrodes in combination with routine scalp electrodes detected over 97% of the spikes, whereas recording from only standard electrode placements detected 58%. Minisphenoidal and surface sphenoidal electrodes were generally not helpful. In some circumstances, however, the use of surface sphenoidal electrodes provided important confirmatory information. In no case did the minisphenoidal electrodes provide unique information, and their use seems unjustified, although the inclusion of other nonstandard electrodes in the recording montage is important to increase the yield.     

New sphenoidal electrode assembly to permit long-term monitoring of the patient's ictal or interictal EEG.

A new sphenoidal wire electrode is described which greatly increases the clinical and diagnostic usefulness of sphenoidal electrode recordings. These very fine wire electrodes are easy to insert; they are comfortable and acceptable to the patient. In contrast to sphenoidal needle electrodes they expose the patient to no risk should he have a seizure during recording. These electrodes also allow one to extend the recording time to several days, thus increasing the chances of recording a spontaneous seizure, for instance while the patient's EEG is being recorded with a telemetry system. The extended recording time also allows for continuous automatic sampling of the interictal EEG over a period of several days. The quality and the reliability of the EEG record are also enhanced. The new sphenoidal electrodes have been used on over 100 patients and are now being used routinely on suspected temporal lobe epileptics recorded with conventional techniques, while 50 of the patients have also been recorded with a cable-telemetry seizure monitoring system which has captured 65 spontaneous seizures.     

Controversies in clinical neurophysiology. MEG is superior to EEG in the localization of interictal epileptiform activity: Con.

OBJECTIVE: To assess whether MEG is superior to scalp-EEG in the localization of interictal epileptiform activity and to stress the 'con' part in this controversy. METHODS: Advantages and disadvantages of the two techniques were systematically reviewed. RESULTS: While MEG and EEG complement each other for the detection of interictal epileptiform discharges, EEG offers the advantage of long-term recording significantly increasing its diagnostic yield which is not feasible with MEG. Localization accuracies of EEG and MEG are comparable once inaccuracies for the solution of the forward problem are eliminated. MEG may be more sensitive for the detection of neocortical spike sources. EEG and MEG source localizations show comparable agreement with invasive electrical recordings, can clarify the spatial relationship between the irritative zone and structural lesions, guide the placement of invasive electrodes and attribute epileptic activity to lobar subcompartments in temporal lobe epilepsy and to a lesser extent in extratemporal epilepsy. CONCLUSIONS: A clear superiority of MEG over EEG for the localization of interictal epileptiform activity cannot be derived from the studies presently available. SIGNIFICANCE: The combination of EEG and MEG provides information for the localization of interictal epileptiform activity which cannot be obtained with either technique alone.     

Scalp EEG in temporal lobe epilepsy surgery

Electroencephalography (EEG) with standard scalp and additional noninvasive electrodes plays a major role in the selection of patients for temporal lobe epilepsy surgery. Recent studies have provided data supporting the value of interictal and postictal EEG in assessing the site of ictal onset. Scalp ictal rhythms are morphologically complex but at least one pattern (a five cycles/second rhythm maximum at the sphenoidal or anterior temporal electrode) occurs in >50% of patients and has a high predictive value and interobserver reliability for temporal lobe originating seizures. Thorough interictal and ictal scalp EEG evaluation, in conjunction with modern neuroimaging, is sufficient for proceeding to surgery without invasive recordings in some patients. Further studies are required to define the scalp ictal characteristics of mesial vs. lateral temporal lobe epilepsy.     

Sphenoidal EEG recording using acupuncture needle electrode in complex partial seizure.

Sphenoidal EEG recording using an uninsulated acupuncture needle electrode were performed in 41 patients with or suspected of complex partial seizures of temporal lobe origin. The anterior temporal spikes were detected by the routine EEG in 17 patients (41%) and by the acupuncture sphenoidal needle in 29 patients (70%). The anterior temporal spikes recorded by the acupuncture needle were almost identical in configuration, amplitude and distribution to those recorded by conventional wire or insulated needle sphenoidal electrodes. The sequence in the frequency of spike detection by these 3 types of sphenoidal electrode were SP1-2, T1-2, F7-8 and A1-2 locations. The spikes of maximal amplitude were most frequently recorded by the SP electrode followed by the T1-2 electrode. The placement of the disposable acupuncture needle was simple and safe. Patients experienced minimal discomfort or pain that lasted at most 0.5 h. No complications occurred. The records were generally free of artifacts. It is concluded that the acupuncture needle can be used as sphenoidal electrode in outpatient EEG recording for the diagnosis of complex partial seizures of anterior temporal-origin.     

Use of cavernous sinus EEG in the detection of seizure onset and spread in mesial temporal lobe epilepsy

PURPOSE: The study goal was to evaluate the clinical usefulness of intravenous EEG recording by placing wire electrodes in the cavernous sinus (CS) and the superior petrosal sinus (SPS) in patients with intractable temporal lobe epilepsy (TLE), with special emphasis on the ictal recording. METHODS: We placed Seeker Lite-10 guide wire as electrodes in the bilateral CS, SPS, or both to simultaneously record both ictal and interictal EEGs with the scalp EEG in five patients with TLE. In addition, in one patient, we averaged interictal scalp and intravascular EEG time-locked to the epileptiform discharge recorded from the CS/SPS-EEG to further delineate the relationship of the spikes between scalp and intravenous recording. RESULTS: In four of five patients, clinically useful recording was obtained to determine ictal focus. We recorded habitual seizures in three patients, and the detailed characteristics of ictal epileptiform discharges were shown. The averaged waveform of interictal epileptiform discharges clarified the spike distribution in the scalp EEGs, which was otherwise undetectable in the single trace. All of the patients completed the intravenous EEG monitoring without any neurological or psychological problems. CONCLUSIONS: The CS/SPS-EEG is a relatively noninvasive method that is useful for the detection of ictal focus and its spreading pattern and thus for the selection of surgical candidate among patients with intractable TLE. Although the number of seizures detected during the short monitoring period may be limited, due to the advantages of its safety and simplicity, it is worth trying for potential surgical candidates before more invasive examinations are applied. A further study with a larger number of patients is needed to estimate its practical risk.