Electrocorticography in patients with medically intractable temporal lobe seizures. II. Quantification of epileptiform discharges following successive stages of resective surgery

Neurophysiology during epilepsy surgery

Intraoperative neuromonitoring (IONM) complements modern presurgical investigations by providing information about the epileptic focus as well as real-time identification of critical functional tissue and assessment of ongoing neural integrity during resective epilepsy surgery. This chapter summarizes current IONM methods for mapping the epileptic focus and for mapping and monitoring functionally important structures with direct brain stimulation and evoked potentials. These techniques include electrocorticography, computerized high-frequency oscillation mapping, single-pulse electric stimulation, cortical and subcortical motor evoked potentials, somatosensory evoked potentials, visual evoked potentials, and cortico-cortical evoked potentials. They may help to maximize epileptic tissue resection while avoiding permanent postoperative neurologic deficits.

Electrographic waveform structure predicts laminar focus location in a model of temporal lobe seizures in vitro

Temporal lobe epilepsy is the most common form of partial-onset epilepsy and accounts for the majority of adult epilepsy cases in most countries. A critical role for the hippocampus (and to some extent amygdala) in the pathology of these epilepsies is clear, with selective removal of these regions almost as effective as temporal lobectomy in reducing subsequent seizure risk. However, there is debate about whether hippocampus is ‘victim’ or ‘perpetrator’: The structure is ideally placed to ‘broadcast’ epileptiform activity to a great many other brain regions, but removal often leaves epileptiform events still occurring in cortex, particularly in adjacent areas, and recruitment of the hippocampus into seizure-like activity has been shown to be difficult in clinically-relevant models. Using a very simple model of acute epileptiform activity with known, single primary pathology (GABA_A Receptor partial blockade), we track the onset and propagation of epileptiform events in hippocampus, parahippocampal areas and neocortex. In this model the hippocampus acts as a potential seizure focus for the majority of observed events. Events with hippocampal focus were far more readily propagated throughout parahippocampal areas and into neocortex than vice versa. The electrographic signature of events of hippocampal origin was significantly different to those of primary neocortical origin – a consequence of differential laminar activation. These data confirm the critical role of the hippocampus in epileptiform activity generation in the temporal lobe and suggest the morphology of non-invasive electrical recording of neocortical interictal events may be useful in confirming this role.

EEG and MEG in mesial temporal lobe epilepsy: where do the spikes really come from?

OBJECTIVE

There is persistent debate as to whether or not EEG and MEG recordings in patients with mesial temporal lobe epilepsy (MTLE) can detect mesial temporal interictal epileptiform discharges (spikes), and this issue is particularly relevant for source localization studies. With the aim of providing direct evidence pertinent to this debate we present detailed examples of the intracranial sources of spikes recorded with EEG and MEG in MTLE.

METHODS

Spikes recorded in five different patients with MTLE during intracranial EEG (n=2), intraoperative electrocorticography (ECOG; n=1), combined scalp-intracranial EEG (n=2) and combined EEG-MEG (n=1) were analyzed and the intracranial sources of the spike foci were matched with their corresponding extracranial EEG and/or MEG fields. EEG and MEG dipole source localization was performed on six independent spike foci identified in one representative patient with bilateral MTLE.

RESULTS

Spikes with an electrical field maximal at F7/8, F9/10≥T3/4 were generated in the anterolateral temporal neocortex. The absence of coincident spiking at mesial locations indicated that these were not propagated from or to the hippocampus. Spikes with an electrical field maximal at T3/4≥T9/10 were generated in the lateral temporal neocortex and likewise did not involve the hippocampus. Individual spikes generated in the mesiobasal temporal neocortex, including the fusiform gyrus, were difficult to detect with EEG (low amplitude diphasic waves most apparent after spike averaging at T3/4, T9/10≥T5/6, P9/10) and only slightly more identifiable with MEG. Spikes generated within and confined to the mesial temporal structures, as confirmed by intracranial recordings, could not be detected with EEG or MEG. Notably, such spikes could not be detected even at intracranial recording sites on the lateral surface of the temporal lobe.

CONCLUSIONS

We present detailed evidence in a small case series showing that typical anterior temporal spikes recorded with EEG and MEG in MTLE arose from the anterolateral temporal neocortex and were neither propagated from nor to the hippocampus. Mid temporal EEG spikes were localized to the lateral temporal neocortex. Intracranially detected mesial temporal spikes were not detected with EEG or MEG.

SIGNIFICANCE

The spikes recorded with EEG and MEG in MTLE are localized to neocortical foci, and not to the mesial temporal structures. Current noninvasive EEG and MEG source localization studies cannot accurately identify true mesial temporal spikes.

Intra-operative electrocorticography in lesional epilepsy

Intra-operative electrocorticography (ECoG) is useful in epilepsy surgery to delineate margins of epileptogenic zone, guide resection and evaluate completeness of resection in surgically remediable intractable epilepsies. The study evaluated 157 cases (2000-2008). The preoperative evaluation also included ictal SPECT (122) and PET in 32 cases. All were lesional cases, 51% (81) of patients had >1 seizure/day and another 1/3rd (51) had >1/week. Pre and post resection ECoG was performed in all cases. A total of 372 recordings were performed in 157 cases. Second post-operative recordings (42) and third post-operative recordings (16) were also performed. Site of recordings included lateral temporal (61), frontal (39), parietal (37), hippocampal (16) and occipital (4). 129/157 cases (82%) showing improvement on ECoG, 30/42 cases showed improvement in 2nd post resection, 8/16 showed improvement in the 3rd post-operative ECoG. 116/157 (73%) patients had good outcome (Engel I and II) at follow up (12-94 months, mean 18.2 months). Of these, 104 patients (80%) showed improvement on post-operative ECoG. 12 had good outcome despite no improvement on ECoG. The improvement in ECoG correlated significantly with clinical improvement [Sensitivity: 100% (95% CI; 96-100%); specificity: 68.3% (95% CI; 51.8-81.4%); positive predictive value: 89.9% (95% CI, 83.1-94.3%); negative predictive value: 100% (95% CI, 85-100%)]. The level of agreement was 91.72% (kappa: 0.76). Concluding, pre and post resection ECoG correlated with its grade of severity and clinical outcome.

Interictal triple ECoG characteristics of temporal lobe epilepsies: An intraoperative ECoG analysis correlated with surgical outcome

OBJECTIVE

Recent reports showed that intraoperative ECoG activities can be analysed with respect to more complex spike patterns. We have systematically investigated different characteristic epileptiform activities in intraoperative ECoG and correlated them to postoperative outcome.

METHODS

Intraoperative ECoG findings of patients with non-tumorous epilepsies (20 patients with Engel outcome 1a, 20 patients with Engel outcome 2-4) were analysed in order to differentiate ECoG characteristics in temporal lobe epilepsies (TLE).

RESULTS

In addition to focal spiking with or without propagation, focal slowing in the theta or delta range and so-called ictaform ECoG patterns were found. These ictaform patterns occurred in 40% of the patients with TLE.

CONCLUSIONS

Leading spikes in combination with focal slowing and ictaform patterns can contribute to a better delineation of mesial temporal epileptic activity in the anterior-posterior alignment. They provide an additional information which can be used for the extent of resection.

SIGNIFICANCE

If the resected area included the anterior mesial regions, where interictal spikes, ictaform activity and slowing were localized, the postoperative outcome was good.

Cortical activation mapping of epileptiform activity derived from interictal ECoG spikes

PURPOSE

To develop and evaluate a new cortical activation mapping (CAM) method to obtain the neuronal activation sequences from the cortical potential distributions.

METHODS

Interictal electrocorticogram (ECoG) recordings were analyzed for eight pediatric epilepsy patients to find the cortical activation maps, which were compared with the patients' seizure-onset zones identified from ictal ECoG recordings. Various relations between the local activation time and cortical potential were assumed. The most effective relation was determined by accessing their capability to predict the seizure-onset zone. Computer simulations using a moving dipole source model were also conducted to test the present approach in imaging the propagated cortical activity.

RESULTS

In both clinical data analysis and computer simulations, the maximal amplitude proved to be the most effective criterion with which to determine the local cortical activation time. The present method successfully predicted the seizure-onset zone in seven of eight patients by the CAM analysis of ECoG-recorded interictal spikes (IISs). For patients with multiple seizure foci, each focus can be revealed by analyzing IISs with different spatial patterns.

CONCLUSIONS

The time difference between spike peaks of the interictal events in the leading channel and other channels can be effectively defined as the local cortical activation time. The cortical activation mapping method based on this time latency can be used to predict the seizure-onset zones, suggesting that the present CAM method is useful to assist the presurgical evaluation for the epilepsy patients.

Facial Emotion Recognition after Curative Nondominant Temporal Lobectomy in Patients with Mesial Temporal Sclerosis

PURPOSE

The right (nondominant) amygdala is crucial for processing facial emotion recognition (FER). Patients with temporal lobe epilepsy (TLE) associated with mesial temporal sclerosis (MTS) often incur right amygdalar damage, resulting in impaired FER if TLE onset occurred before age 6 years. Consequently, early right mesiotemporal insult has been hypothesized to impair plasticity, resulting in FER deficits, whereas damage after age 5 years results in no deficit. The authors performed this study to test this hypothesis in a uniformly seizure-free postsurgical population.

METHODS

Controls (n=10), early-onset patients (n=7), and late-onset patients (n=5) were recruited. All patients had nondominant anteromedial temporal lobectomy (AMTL), Wada-confirmed left-hemisphere language dominance and memory support, MTS on both preoperative MRI and biopsy, and were Engel class I 5 years postoperatively. By using a standardized (Ekman and Friesen) human face series, subjects were asked to match the affect of one of two faces to that of a simultaneously presented target face. Target faces expressed fear, anger, or happiness.

RESULTS

Statistical analysis revealed that the early-onset group had significantly impaired FER (measured by percentage of faces correct) for fear (p=0.036), whereas the FER of the late-onset group for fear was comparable to that of controls. FER for anger and happiness was comparable across all three groups.

CONCLUSIONS

Despite seizure control/freedom after AMTL, early TLE onset continues to impair FER for frightened expressions (but not for angry or happy expression), whereas late TLE onset does not impair FER, with no indication that AMTL resulted in FER impairment. These results indicate that proper development of the right amygdala is necessary for optimal fear recognition, with other neural processes unable to compensate for early amygdalar damage.

Presurgical evaluation and surgical treatment of medically refractory epilepsy

Thanks to today's modern imaging examination techniques and especially to the common use of intracranial electrodes for localizing seizure foci, more and more patients with partial epilepsy can be treated microsurgically. The results of such neurosurgical therapies are very good, particularly in mesial temporal lobe epilepsy. In recent years, good results (60-70% seizure freedom) have also been achieved in extratemporal epilepsy surgery, so that such procedures can now be recommended for carefully selected patients. In this review, presurgical evaluations and the different surgical approaches are presented.

Stereotactic amygdalohippocampotomy for the treatment of medial temporal lobe epilepsy

PURPOSE

This study was carried out to assess the safety and efficacy of stereotactic ablation of the amygdala and hippocampus for the treatment of medial temporal lobe epilepsy.

METHODS

Twenty-two stereotactic amygdalohippocampotomies were performed in 19 patients with unilateral temporal lobe seizures by using magnetic resonance imaging (MRI) localization for target planning and radiofrequency techniques for lesion production. Seizure frequency was assessed at 3-monthly follow-up visits. Two lesion groups were defined. In group I, four to 11 (mean, 6.4) discrete lesions were made, encompassing the amygdala and anterior 13-21 mm (mean, 16.8 mm) of the hippocampus. In group II, a large number of confluent lesions were made (mean, 26.0; range, 12-54) encompassing the amygdala and anterior 15-34 mm (mean, 21.5 mm) of the hippocampus. MRI scanning was carried out 24 h and 6-9 months after surgery.

RESULTS

In five group I patients, one (20%) experienced a favorable seizure outcome. Of 15 group II patients, one of whom had previously undergone limited lesioning and was also analyzed as part of group I, nine (60%) experienced a favorable seizure outcome, with two seizure free. MRI scans at 6- to 9-months' follow-up disclosed discrete areas of atrophy in the amygdala and hippocampus, interspersed with preserved brain in the group I patients. More uniform and complete destruction of amygdala and hippocampus was evident in group II patients. All lesions were confined to the amygdala and hippocampus, sparing the parahippocampal gyrus (PHG).

CONCLUSIONS

The extensive amygdalohippocampal ablation in group II patients improved seizure outcome compared with more limited ablation in group I, but these results were not so good as those from temporal lobectomy in a similar patient group. When considered together with the results of selective amygdalohippocampectomy, and temporal resections that spare hippocampus or amygdala (all producing similar outcomes, and all involving resection of the entorhinal cortex), this study suggests a pivotal role of the entorhinal cortex in temporal epileptogenesis.

Neuromonitoring in the operating room: why, when, and how to monitor?

This review considers the main principles and indications of EEG and evoked potential (EP) neuromonitoring in the operating room. Neuromonitoring has a threefold purpose: to warn the surgeon that he has to adjust his strategy, to confirm his decision, and to help him improve subsequent procedures. The pathophysiology of intraoperative events liable to alter the EEG or the EPs is first considered. The usefulness of neuromonitoring in preventing neurological complication relies on its ability to detect neurological dysfunction at a reversible stage. This applies especially to ischemia and compressive damage. The anesthetic influences on EEG and EPs are then considered. Knowledge of them is essential to disentangle these neurophysiological alterations due to intraoperative events from those merely due to anesthesia and to use neurophysiological parameters to evaluate the depth of anesthesia. Third, the main indications and limitations of neuromonitoring are considered: prevention of ischemic brain or spinal cord damage, prevention of mechanical injuries of the brain, spinal cord or peripheral nerve, and localization of the motor cortex in cortical neurosurgery or of cranial nerves in posterior fossa surgery. Finally, the 3 levels of neuromonitoring (neurophysiological feature extraction, neurophysiological pattern recognition, clinical integration of the neurophysiological patterns) are discussed together with the rules that should guide the dialogue between the surgeon, the anesthesiologist, and the neurophysiologist.

Stereotactic amygdalohippocampotomy and mesial temporal spikes

PURPOSE

To investigate the mechanism of mesial temporal spike generation, we sought to determine whether amygdalohippocampotomy by radiofrequency lesions altered spike rates in patients with medically intractable temporal lobe epilepsy.

METHODS

The subjects were 14 patients whose ictal semiology, ictal and interictal EEGs, and neuropsychological profile were compatible with seizure origin from MRI-demonstrated unilateral mesial temporal sclerosis. Mesial temporal spikes were recorded by a multicontact electrode line stereotactically placed in the temporal horn of the lateral ventricle. A recording and lesioning electrode was also stereotactically advanced to multiple amygdala and hippocampal targets. Several confluent (9 patients) or discrete (5 patients) radiofrequency lesions were made in the amygdala and anterior hippocampus. Scalp and invasive recordings were performed before lesioning, between each lesioning process, and for 48 postoperative h.

RESULTS

As compared to prelesion recordings, no consistent change in anterior and total mesial temporal spike rates occurred intraoperatively or postoperatively. Postlesion: prelesion spike ratios failed to correlate with effectiveness of lesionectomy for seizure control. In contrast, complex partial seizures improved in 13 of 14 patients.

CONCLUSIONS

Amygdala and hippocampal destructive lesions had no consistent effect on meisal temporal spike quantity, but reduced temporal lobe CPS in 13 of 14 patients. This dichotomy suggests that CPS may result from concerted epileptogenesis of the hippocampus, entorhinal cortex, and possibly the amygdala, while the entorhinal cortex alone can produce interictal spikes.

Mesial temporal versus lateral temporal interictal epileptiform activity: comparison of chronic and acute intracranial recordings

Intracranial interictal epileptiform activity (EA) was recorded by chronic stereotactic depth electroencephalography (SDEEG) and acute electrocorticography (ECOG) in 22 patients with complex partial seizures of temporal lobe origin. Chronic SDEEG recordings defined two groups of patients with respect to the presence or absence of lateral temporal EA; 13 patients showed independent lateral temporal EA during chronic recordings and 9 patients did not. All patients had EA recorded from mesial temporal structures during SDEEG. The presence of lateral temporal EA was correlated with a higher pre-operative seizure frequency but not with ictal onset zones, structural pathology, age at onset of epilepsy, or duration of epilepsy. Results of acute ECOG recordings performed on the same patients 1-24 months after SDEEG accurately reproduced the mesial versus lateral distribution of EA within patients (P < 0.0003). Though ECOG was less sensitive than SDEEG in demonstrating EA confined to mesial structures, positive findings at ECOG were 100% specific with respect to SDEEG. These results suggest that, at least with respect to mesial temporal versus lateral temporal structures, there is a constancy within patients in the distribution of interictal EA recorded with chronic intracranial electrodes. In addition, acute ECOG provides an accurate representation of individual patients' interictal EA.

The predictive value of intraoperative electrocorticography in resections for limbic epilepsy associated with mesial temporal sclerosis

OBJECTIVE

Prior studies on the predictive value of intraoperative electrocorticography (ECoG) have been performed on heterogeneous groups of patients with both temporal and extratemporal interictal spikes, lesional and nonlesional pathological findings, and variably extensive resections by different surgeons.

METHODS

We performed both pre- and postresection intraoperative ECoG on 29 consecutive patients with medial temporal lobe epilepsy (17 left-sided) who underwent standard nontailored resections by one surgeon (RRG). All patients had only temporal interictal spikes (six bitemporal) and mesial temporal sclerosis diagnosed by preoperative magnetic resonance imaging and confirmed by pathological examination of resected tissue.

RESULTS

After a mean follow-up of 24.8 months, there were 15 (52%) patients who were seizure-free, 6 (21%) who were seizure-free except for auras, and 8 (28%) who had any seizure after the 1st postoperative month. Fourteen patients (48%) had active interictal discharges outside the area of planned resection revealed by preresection ECoG. Neither the presence of these spikes nor their mean frequency correlated with seizure outcome. Eleven patients (38%) had residual spike discharges after resection, and 18 patients (62%) had new spikes revealed by the postresection ECoG. Neither of these findings nor the mean spike frequency of residual or new spikes related to seizure outcome. Persistent spikes increased in frequency after resection in all outcome groups.

CONCLUSIONS

Electrocorticographic monitoring of interictal epileptiform activity intraoperatively is not useful in the surgical treatment of patients undergoing standard resection for medial temporal lobe epilepsy with magnetic resonance imaging evidence of mesial temporal sclerosis.

Bilateral hippocampal interictal spiking and kindling expression in rabbits

The temporal/spatial dissemination of interictal spikes among different brain structures was studied during the course of kindling to determine if the long-term dissemination pattern reflects the rate and expression of kindling. The experiments were conducted on adult rabbits with chronically implanted electrodes (dorsal hippocampus, amygdala, caudate, all bilateral, sensory motor and occipital cortices). Rabbits (n = 13) were subjected to once daily electrical stimulation in the left hippocampus. Kindling resulted in the development of two different epileptic phenomena: 7 animals quickly (in 2-3 weeks) achieved a fully kindled state, characterized by generalized seizures, whereas the remaining 6 rabbits did not reliably progress beyond partial seizures even after more prolonged stimulation. Animals were accordingly divided into two groups referred to as generalized seizure and partial seizure. The temporal/spatial dissemination pattern, particularly in the two hippocampi, was very different between the groups. In both groups interictal spiking originated in one of the hippocampi independent of site stimulated and represented formation of the primary hippocampal epileptic focus. The generalized seizure group demonstrated stability of the primary hippocampal epileptic focus with permanent predominance of spiking in it over the course of kindling, and a high level of bilateral synchronous hippocampal interictal spiking. In the partial seizure group the primary hippocampal epileptic focus was established during the first 2-3 weeks of stimulation. This was later suppressed upon the development of an independent secondary focus in the opposite hippocampus. These animals also displayed very low levels of synchronous bilateral hippocampal interictal spiking. We suggest that an antagonistic relationship can develop between mirror hippocampal epileptic foci. This can be associated with a low level of bilateral hippocampal synchronous spiking, kindling retardation, and manifestation of partial seizures.

Significance of spikes at temporal lobe electrocorticography

Among 87 consecutive patients operated on under local anesthesia, few aspects of pre- and posttemporal lobe resection electrocorticograms (ECoG) yielded prognostic data. Preresection spikes were most common in the hippocampus, followed in order of frequency by the anterior temporal convexity and the inferior temporal surface. Moderately frequent (>10 spikes/100 s) preresection spikes appeared beyond the subsequent resection line in the posterior temporal region in 16 of 87 (18%) and in orbital frontal cortex in 12 of 87 (14%). Although many hippocampus spikes portended a favorable outcome and rare spikes an unfavorable one, preresection spike quantity otherwise failed to distinguish outcome groups. Absolute quantity of postresection spikes and change from preresectrion quantity in any region did not correlate with outcome except for the insula, where relatively abundant spikes portended favorable outcomes. Postresection electrographic seizures were rare but occurred equally in all outcome groups. No significant change in spike incidence occurred between the first and last 10-min epoch of the 30-min postresection recording.

Significance of spikes recorded on electrocorticography in nonlesional medial temporal lobe epilepsy

Whether spikes recorded by intraoperative electrocorticography imply active epileptogenicity has not been adequately addressed. We performed preresection and postresection electrocorticography on 47 patients with nonlesional medial temporal lobe epilepsy who were undergoing surgery for the treatment of medically refractory epilepsy. A standard anteromedial temporal lobectomy was performed on all patients, with no additional resection, regardless of electrocorticographic findings. Patients were divided into two groups: Group I (no seizures or rare seizures) and Group II (recurrent seizures). Recorded spikes were analyzed for distribution and spike discharge rate. On preresection electrocorticography, 83% of Group I and 82% of Group II had spikes in the anterior temporal lobe. The spike discharge rate was equally distributed between high frequency and low frequency for both groups (not significant). Although spikes localized to the posterior temporal neocortex were seen more in Group II (64%) than Group I (39%), this was not a significant difference (p > 0.1). Most of these patients had a low-frequency spike discharge rate. On postresection electrocorticography, 80% of Group I and 75% of Group II had residual spikes. The majority of these had a low-frequency spike discharge rate and were localized to the margin of resection. We found no correlation between residual spikes on preresection and postresection electrocorticography and outcome. These findings do not support the role of intraoperative electrocorticography in guiding mesial temporal lobe resection.

Electrocorticography and temporal lobe epilepsy: relationship to quantitative MRI and operative outcome

We investigated the relationship between electrocorticography (ECoG), quantitative magnetic resonance imaging (MRI), and surgical outcome in 165 patients with intractable nonlesional temporal lobe epilepsy (NLTLE). A standard mesial temporal resection was performed in all patients. Patients with an operative follow-up < 1 year were excluded from the study. The extent of the lateral temporal neocortex resection (LCR) was guided by ECoG and the side of surgery. The extent of the LCR was not predictive of seizure outcome in patients with or without hippocampal formation atrophy (p > 0.5). Patients undergoing a right anterior temporal lobectomy had a larger LCR (p < 0.0001), but the side of surgery was not of predictive value in determining seizure outcome (p > 0.1). The topography of the acute intracranial spikes did not correlate with operative outcome (p > 0.5) and was independent of hippocampal volumetric studies (p > 0.5). The postexcision ECoG was also shown not to be of prognostic importance (p > 0.5). Our results indicates that the extent of the lateral temporal cortical resection and the ECoG findings are not important determinants of surgical outcome in patients with NLTLE.

Postoperative EEG and electrocorticography: relation to clinical outcome in patients with temporal lobe surgery

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.

Electrocorticography in patients with medically intractable temporal lobe seizures. I. Quantification of epileptiform discharges prior to resective surgery

We studied retrospectively the intraoperative preresection electrocorticograms (ECoGs) of 72 patients undergoing surgery for medically intractable, mostly complex partial, temporal lobe seizures (TLS). Quantification of interictal epileptiform discharges (EDs) detected visually at each electrode location in 2 min recording epochs included computations of ED rates (EDs/min) and cumulative voltages (CuVs) (microV/min). Of 6388 EDs, 81% involved the infratemporal surface, 18% the lateral temporal surface and 1% the orbital frontal area. Forty-eight patients (67%) demonstrated multiple (up to 5 or more), temporally independent foci. Dominant foci in medial and lateral infratemporal locations were about equally common and were significantly more frequent than in lateral temporal locations. Rankings of ED CuVs and rates at individual cortical locations defined 4 areas of "relative interictal cortical epileptogenicity." These were arranged in an orderly pattern with the anterior parahippocampal gyrus and the inferomedial surface of the temporal tip displaying the highest and the lateral temporal and posterior infratemporal cortices showing the lowest propensity to the interictal epileptiform discharge. Individual areas were not characterized by distinct clinical seizure manifestations. Preresection ECoGs provide information on the epileptogenic dysfunction that involves most of the temporal lobe of patients with medically intractable TLS.