Subdural Grid Research Articles

Epilepsy surgery in children with focal cortical dysplasia (FCD): results of long-term seizure outcome.

The purpose of this study was to assess the effect of epilepsy surgery on seizure outcome in children and adolescents under 18 years with intractable epilepsy due to focal cortical dysplasia. We analysed clinical data, such as age at seizure onset, epilepsy course, localisation of focus from presurgical evaluation, MRI, tissue pathology and seizure outcome in 68 patients 6 months to 9 years after epilepsy surgery. Seizure outcome was classified according to the Engel classification. Mean age at seizure onset was 7 months, ranging from the first days of life to 7 years. All patients had medically intractable epilepsy. Localisation of the lesion was predominantly extratemporal: posterior (uni- or multilobar) 43 %, frontal without central region 26 %, multilobar involving central area 19 % and temporal in 12 %. MRI signs typically seen in cortical dysplasia (FCD) such as localised blurring of gray-white matter junction was found in 68 %, dysgyria in 62 %, thickening of the cortical ribbon in 46 % and T2 signal elongation of the subcortical white matter in 40 % of the patients' MRI. Age at surgery ranged from 5 months to 16 years; 14 patients were under 2 years when operated on. In 34 patients (6 patients under 3 years) subdural grid electrode evaluation was performed prior to surgery. Pathology revealed focal cortical dysplasia without balloon cells (type I) in 60 %, FCD of the balloon cell subtype (type II) in 40 % of the specimens. Postoperative complications were subdural hygroma in 5 and an increased motor deficit in 2 patients. Up to two years after epilepsy surgery 50 % of the children were seizure free (Engel class I), 10 % Engel class II, 33 % Engel class III and 7 % unchanged (Engel class IV). Long-term seizure outcome (> 3 years post surgery) in 32 patients showed similar results (class I 50 %, class II 19 %, class III 28 %, class IV 3 %). Complete resection of the dysplastic lesion was significantly correlated with favorable seizure outcome, whereas seizure outcome was not significantly different in patients with mild (type I) or balloon cell (type II) FCD. Children operated after 6 years of age had no better outcome than children operated in infancy or at preschool age. Epilepsy surgery resulted in good (class I and II) seizure control in 60 % of children with intractable epilepsy due to focal cortical dysplasia.

Botulinum toxin-induced paralysis of frontotemporal muscles improves seizure focus localization.

Scalp EEG localization of epileptic foci may be obscured by electromyographic (EMG) artifact produced by ictal contraction of cranial muscles. Injection of botulinum toxin type A (BTX-A) into frontotemporal scalp muscles reduces EMG activity. Initial scalp video-EEG monitoring in three patients suggested partial seizures, but definitive lateralization or localization was precluded by EMG artifact. EMG-guided BTX-A injection to bilateral frontotemporal muscles was performed. When artifact persisted, BTX-A administration was selectively repeated. Patients subsequently underwent scalp video-EEG monitoring 1 week later. All patients had reduction of EMG artifact during subsequent scalp video-EEG monitoring. No patient had adverse effects after BTX-A administration. All three patients had localization to either frontal or temporal lobes and definitive lateralization. Two of the three patients were able to proceed to invasive placement of frontotemporal subdural grid electrodes based on the BTX-A scalp video-EEG localization, and the third patient was determined to have a multifocal seizure disorder. Paralysis of frontotemporal scalp muscle after BTX-A administration reduces EMG artifact and may improve localization and lateralization of a seizure focus, providing a noninvasive technique for advancement toward epilepsy surgery.

Complications of invasive video-EEG monitoring with subdural grid electrodes.

To evaluate the risk factors, type, and frequency of complications during video-EEG monitoring with subdural grid electrodes. The authors retrospectively reviewed the records of all patients who underwent invasive monitoring with subdural grid electrodes (n = 198 monitoring sessions on 187 patients; median age: 24 years; range: 1 to 50 years) at the Cleveland Clinic Foundation from 1980 to 1997. From 1980 to 1997, the complication rate decreased (p = 0.003). In the last 5 years, 19/99 patients (19%) had complications, including two patients (2%) with permanent sequelae. In the last 3 years, the complication rate was 13.5% (n = 5/37) without permanent deficits. Overall, complications occurred during 52 monitoring sessions (26.3%): infection (n = 24; 12.1%), transient neurologic deficit (n = 22; 11.1%), epidural hematoma (n = 5; 2.5%), increased intracranial pressure (n = 5; 2.5%), and infarction (n = 3; 1.5%). One patient (0.5%) died during grid insertion. Complication occurrence was associated with greater number of grids/electrodes (p = 0.021/p = 0.052; especially >60 electrodes), longer duration of monitoring (p = 0.004; especially >10 days), older age of the patient (p = 0.005), left-sided grid insertion (p = 0.01), and burr holes in addition to the craniotomy (p = 0.022). No association with complications was found for number of seizures, IQ, anticonvulsants, or grid localization. Invasive monitoring with grid electrodes was associated with significant complications. Most of them were transient. Increased complication rates were related to left-sided grid insertion and longer monitoring with a greater number of electrodes (especially more than 60 electrodes). Improvements in grid technology, surgical technique, and postoperative care resulted in significant reductions in the complication rate.

Insertion of depth electrodes with or without subdural grids using frameless stereotactic guidance systems -- technique and outcome

Over recent years frameless stereotactic systems have begun replacing framed systems for many neurosurgical procedures. However, little has been published regarding the use of these systems to guide intracranial electrode implantation for epilepsy surgery patients. Here we report our experience utilising such a system to insert depth electrodes and subdural grid electrodes. The SteathStation Image-Guided System TM (SSIGS) (Sofamor Danek, Memphis TN.) was used to insert bilateral depth electrodes in 13 patients, of whom 5 also underwent the insertion of subdural grids or strip electrodes. Initially, a surgical plan based on an entry and target point on axial and sagittal images was performed for the insertion of electrodes. Navigational views, using three-planar images, were then performed to determine which structures the electrodes would pass through to be correctly placed in the amygdala and hippocampus. The correct site of electrode implantation was confirmed post-operatively by spiral CT scans in 4 patients (which were then co-registered to the preimplantation MRI using a surface matching technique) and the other 9 patients by post-implantation MRI. The SSIGS was found to have a mean registration error of 2.0 mm (range 1.8-2.5) in 10 cases; in the 3 cases where the error was greater than 2.5 mm a surfacemerge technique was used with a mean error 0.9 (0.8-1.00). The post-implantation MRI or CT-MRI co-registration confirmed an accurate electrode placement in the mesial temporal region in all cases. Seizure onset lateralisation was achieved in 11 patients, all of whom went onto formal resections based on these results. The only long-term complication was a case of osteomyelitis which required removal of the bone flap. 73% of patients had an excellent seizure outcome. Frameless stereotactic systems can be safely used to intracranial electrodes, avoid the disadvantages of the framed system and have the added advantage of the surgeon being able to visualise the trajectory and to adjust this to avoid vital structures. As well they eliminate surgical obstruction to the insertion of subdural grids at the same operation, which may be caused by a framed system.

The use of subdural grids in the management of focal malformations due to abnormal cortical development

Because of the spatial limitations of scalp electroencephalographic recordings and the lack of reproducible results using functional MR imaging techniques to map various eloquent areas of the cortex, the accurate mapping of the extent of epileptogenicity and function currently are best accomplished through invasive electrocorticographic recordings and direct cortical stimulation, respectively. The purpose of this article is to give an overview of the presurgical diagnostic tools used in patients with suspected malformations caused by abnormalities of cortical development with special emphasis on subdural electrodes. The indications and the role of invasive subdural electrode evaluation are discussed.

Temporal lobe epilepsy associated with hippocampal sclerosis and a contralateral middle fossa arachnoid cyst

We report on a 13-year-old boy with temporal lobe epilepsy associated with left hippocampal sclerosis and a contralateral arachnoid cyst in the middle cranial fossa (ACMCF). Chronic intracranial recording from subdural grid electrodes showed the left medial temporal lobe to be the ictal onset zone. After left anterior temporal lobectomy with hippocampectomy, seizure control was improved. ACMCF was not considered the direct cause of epilepsy; instead the seizures were attributed to hippocampal sclerosis.

Multimodality image-guided epilepsy surgery

Co-registration of imaging modalities in the diagnosis of epilepsy is now commonly used in major epilepsy centres. However, the incorporation of these registrations into an image-guided system has only recently been developed. Using techniques of segment ation, surface matching and/or voxel-based matching, we have been able to co-register flair MRI, SPECT, PET, and subdural grids into the 3-D space of the volumertic MRI and use these images on the StealthStation Image-Guided System™ to resect these foci. This greatly aids the surgeon in performing a more precise tailoring of the cortical resection so as to maximize the excision of the epileptogenic zone while minimizing the impact on eloquent cortex an epileptogenic foci.

Changes in power and coherence of brain activity in human sensorimotor cortex during performance of visuomotor tasks

Electrocorticograms (ECoG) were recorded using subdural grid electrodes in forearm sensorimotor cortex of six human subjects. The subjects performed three visuomotor tasks, tracking a moving visual target with a joystick-controlled cursor; threading pieces of tubing; and pinching the fingers sequentially against the thumb. Control conditions were resting and active wrist extension. ECoGs were recorded at 14 sites in hand- and arm-sensorimotor area, functionally identified with electrical stimulation. For each behavior we computed spectral power of ECoG in each site and coherence in all pair-wise sites. In three out of six subjects, γ-oscillations were observed when the subjects started the tasks. All subjects showed widespread power decrease in the range of 11–20 Hz and power increase in the 31–60 Hz ranges during performance of the visuomotor tasks. The changes in γ-range power were more vigorous during the tracking and threading tasks compared with the wrist extension. Coherence analysis also showed similar task-related changes in coherence estimates. In contrast to the power changes, coherence estimates increased not only in γ-range but also at lower frequencies during the manipulative visuomotor tasks. Paired sites with significant increases in coherence estimates were located within and between sensory and motor areas. These results support the hypothesis that coherent cortical activity may play a role in sensorimotor integration or attention.

Results of surgery in patients with refractory extratemporal epilepsy with normal or nonlocalizing magnetic resonance findings investigated with subdural grids.

To study the efficacy of extensive coverage of the brain surface with subdural grids in defining extratemporal cortical areas amenable for resection in patients with refractory extratemporal epilepy (R-ExTE) and normal or nonlocalizing magnetic resonance imaging (MRI) scans. Sixteen patients with R-ExTE were studied. Eleven patients had simple partial, eight had complex partial, and three had supplementary motor area seizures. Seizure frequency ranged from three per month to daily episodes. Interictal EEG showed large focal spiking areas in 11 patients, secondary bilateral synchrony in four, and was normal in one patient. Surface ictal recordings were nonlocalizing in six patients, and in 10, they disclosed large ictal focal spiking areas. MRI was normal in 10 patients, and in six patients, focal nonlocalizing potentially epileptogenic lesions were found. All patients were given an extensive coverage of the cortical convexity with subdural electrodes through large unilateral (n = 13) or bilateral (n = 3) craniotomies. Bipolar cortical stimulation was carried out through the implanted electrodes. Interictal invasive recording findings showed widespread spiking areas in 13 patients and secondary bilateral synchrony in three. Ictal invasive recordings showed focal seizure onset in all patients. There were six frontal, two parietal, one temporooccipital, four rolandic, and three posterior quadrant resections. Thirteen patients had been rendered seizure free after surgery, and three had > or =90% of seizure-frequency reduction. Pathologic findings included gliosis (n = 10), cortical dysplasia (n = 5), or no abnormalities (n = 1). Six patients had transient postoperative neurologic morbidity. Extensive subdural electrodes coverage seems to be an effective way to investigate patients with R-ExTE and normal or nonlocalizing MRI findings.

Medically intractable, localization-related epilepsy with normal MRI: presurgical evaluation and surgical outcome in 43 patients.

High-resolution magnetic resonance imaging (MRI) plays a crucial role in the presurgical evaluation of patients with medically refractory partial epilepsy. Although MRI detects a morphologic abnormality as the cause of the epilepsy in the majority of patients, some patients have a normal MRI. This study was undertaken to explore the hypothesis that in patients with normal MRI, invasive monitoring can lead to localization of the seizure-onset zone and successful epilepsy surgery. A series of 115 patients with partial epilepsy who had undergone intracranial electrode evaluation (subdural strip, subdural grid, and/or depth electrodes) between February 1992 and February 1999 was analyzed retrospectively. Of these, 43 patients (37%) had a normal MRI. Invasive monitoring detected a focal seizure onset in 25 (58%) patients, multifocal seizure origin in 12 (28%) patients, and in six patients, no focal seizure origin was found. Of the 25 patients with a focal seizure origin, cortical resection was performed in 24, of whom 20 (83%) had a good surgical outcome with respect to seizure control. Six of the 12 patients with multifocal seizure origin underwent other forms of epilepsy surgery (palliative cortical resection in two, anterior callosotomy in two, and vagal nerve stimulator placement in two). Successful epilepsy surgery is possible in patients with normal MRIs, but appropriate presurgical evaluations are necessary. In patients with evidence of multifocal seizure origin during noninvasive evaluation, invasive monitoring should generally be avoided.

Computerized brain-surface voltage topographic mapping for localization of intracranial spikes from electrocorticography. Technical note.

The purpose of this paper is to describe the use of computerized brain-surface voltage topographic mapping to localize and identify epileptic discharges recorded on electrocorticographic (ECoG) studies in which a subdural grid was used during intracranial video electroencephalographic (IVEEG) monitoring. The authors studied 12 children who underwent surgery for intractable extrahippocampal epilepsy. Cortical surfaces and subdural grid electrodes were photographed during the initial surgery to create an electrode map that could be superimposed onto a picture of the brain surface. Spikes were selected from ictal discharges recorded at the beginning of clinically confirmed seizures and from interictal discharges seen on ECoG studies during IVEEG recording. A computer program was used to calculate the sequential amplitude of the spikes by using squared interpolation, and they were then superimposed onto the electrode map. Interictal discharges and high-amplitude spike complexes at seizure onset were plotted on the map. This mapping procedure depicted the ictal zone in nine patients and the interictal zone in 12, and proved to be an accurate and useful source of information for planning corrective surgery.

Extraoperative cortical stimulation of motor function in children

The purpose of the study was to investigate factors altering the amperage threshold needed to provoke functional responses in children with epilepsy. Twenty patients (4–18 years of age) who underwent epilepsy surgery at our institution from 1996–2000 after insertion of subdural grid electrodes were reviewed retrospectively. Extraoperative electrical cortical stimulation was performed with 50-Hz biphasic pulses of 0.2 ms in duration using a “distance reference” technique. Amperage thresholds of primary motor responses and afterdischarges were evaluated. The patients were grouped according to underlying pathology: eight with neuronal migration disorders (group A) and 12 with other disorders (group B). The motor cortex was defined successfully in all children because the afterdischarges threshold was higher than the motor cortical threshold. Amperage thresholds ranged from 2–20 mA (mean = 7.7) for primary motor function. An inverse relationship was found between amperage threshold and age: the younger the patient, the higher the threshold ( P = 0.0005). Patients in group A required a higher amperage (2–20 mA, mean = 8.6) for motor cortical mapping than those in group B (2–14 mA, mean = 6.4). Younger children with neuronal migration disorders require a higher amperage threshold to achieve adequate motor functional mapping with careful observation of afterdischarges.

Mapping of Expressive Language Cortex using Magnetic Source Imaging

The accurate localization of primary language cortex is one of the goals in the evaluation of brain surgery candidates. In this paper we describe the localization of expressive language cortex using magnetic source imaging (MSI) in a patient with refractory epilepsy caused by a tumor affecting the left inferior frontal gyrus. The magnetoencephalographic recordings during an expressive language task were co-registered with a magnetic resonance imaging (MRI) scan obtained after subdural grid placement and provided a cluster of sources of brain activation in the vicinity of the lesion. This map of expressive language provided by MSI was verified with electrocortical stimulation before the operation. No speech problems were reported in our patient after the resection of the lesion, suggesting that MSI is an accurate non-invasive method for the pre-surgical mapping of expressive language in cases where there is clear functional risk during tumor resection.

Complications and results of subdural grid electrode implantation in epilepsy surgery

BACKGROUND We assessed the risk of delayed subdural hematoma and other complications associated with subdural grid implantation. METHODS Forty-nine patients underwent subdural grid implantation with/without subdural strips or depth electrodes from January 1994 to August 1998. To identify the risk associated with subdural grid implantation, a retrospective review of all patients’ medical records and radiological studies was performed. RESULTS The major complications of 50 subdural grid electrode implantations were as follows: four cases (7.8%) of delayed subdural hematoma at the site of the subdural grid, requiring emergency operation; two cases (3.9%) of infection; one case (2.0%) of epidural hematoma; and one case (2.0%) of brain swelling. After subdural hematoma removal, the electrodes were left in place. CCTV monitoring and cortical stimulation studies were continued thereafter. No delayed subdural hematoma has occurred since routine placement of subdural drains was begun. CONCLUSIONS In our experience the worst complication of subdural grid implantation has been delayed subdural hematoma. Placement of subdural drains and close observation may be helpful to prevent this serious complication.

Different activation of presupplementary motor area, supplementary motor area proper, and primary sensorimotor area, depending on the movement repetition rate in humans.

In order to clarify the functional role of the supplementary motor area (SMA) and its rostral part (pre-SMA) in relation to the rate of repetitive finger movements, we recorded movement-related cortical potentials (MRCPs) directly from the surface of the mesial frontal lobe by using subdural electrode grids implanted in four patients with intractable partial epilepsy. Two subregions in the SMA were identified based on the anatomical location and the different response to cortical stimulation. In three of the four subjects, we also recorded MRCPs from the surface of the lateral convexity covering the primary sensorimotor areas (SI-MI), which were defined by cortical stimulation and SEP recording. The subjects extended the middle finger or opposed the thumb against other fingers of the same hand at a self-paced rate of 0.2 Hz (slow) and 2 Hz (rapid), each in separate sessions. As a result, pre-and postmovement potentials were clearly seen at the SI-MI in both slow- and rapid-rate movements. By contrast, in the SMA, especially in the pre-SMA, premovement potentials were not seen and postmovement potentials were seldom seen in the rapid rate movement. In the slow-rate condition, pre- and postmovement potentials were clearly seen in both the pre-SMA and the SMA proper. In conclusion, the SMA, especially the pre-SMA, is less activated electrophysiologically in the rapid-rate movements, while the SI-MI remains active regardless of the movement rate.

Paradigms for subdural grids' implantation in patients with refractory epilepsy.

The need for invasive monitoring in patients with refractory epilepsy has been greatly reduced by the introduction of new technologies such as PET, SPECT and MRI in the clinical practice. On the other hand, 10 to 30% of the patients with refractory epilepsy have non-localizatory non-invasive preoperative work-up results. This paper reports on the paradigms for subdural electrodes implantation in patients with different refractory epileptic syndromes. Twenty-nine adult refractory epileptic patients were studied. Patients were divided into five different epileptic syndromes that represented the majority of the patients who needed invasive recordings: bitemporal (Group I; n=16 ), bi-frontal-mesial (Group II, n=5), hemispheric (Group III; n=2), anterior quadrant (Group IV; n=3) and posterior quadrant (Group V; n=3). All of them were submitted to extensive subdural electrodes' implantation (from 64 to 160 contacts) covering all the cortical surface potentially involved in epileptogenesis under general anesthesia. All patients tolerated well the procedure. There was no sign or symptom of intracranial hypertension except for headache in 22 patients. In all except one Group II patient, prolonged electrocorticographic monitoring using the described subdural cortical coverage patterns was able to define a focal area amenable for resection. In all Groups II-V patients cortical stimulation was able to adequately map the rolandic and speach areas as necessary. Despite recent technological advances invasive neurophysiological studies are still necessary in some patients with refractory epilepsy. The standardization of the paradigms for subdural implantation coupled to the study of homogeneous patients' populations as defined by MRI will certainly lead to a better understanding of the pathophysiology involved in such cases and an improved surgical outcome.

Epileptogenicity correlated with increased N-methyl-D-aspartate receptor subunit NR2A/B in human focal cortical dysplasia.

Human cortical dysplasia (CD) is a frequent cause of medically intractable focal epilepsy. The neurotransmitter mechanisms of epileptogenicity in these lesions have been attributed to changes in various glutamate receptor subtypes. Increased N-methyl-D-aspartate (NMDA) receptor (NR) 2A/B coassembled with NR1 subunits has been shown in focal epileptic CD. The purpose of this study is to correlate in situ CD epileptogenicity and the expression of various glutamate receptor subtypes. The histopathological, morphological, and immunocytochemical findings in cortical tissue resected from five patients with medically intractable epilepsy and CD were correlated with electroencephalographic data recorded from subdural grids. The NMDA antibodies identified subunits NR1 (splicing variants 1a, 1b, 2a, and 2b) and NR2A/B. Epileptogenic specimens displayed the following common features: (a) widespread histological abnormalities of horizontal and columnar dyslamination, neurons with inverted polarity, and more extensive dendritic changes; (b) significantly higher NR2A/B immunoreactivity in both the dysplastic somata and all their dendritic processes; and (c) no statistically significant change in NR1 subunit expression but a more pronounced staining of the apical dendrites in highly epileptogenic cortex. These abnormalities were either absent or minimal in resected specimens that did not show evidence of severe in vivo epileptogenicity. These studies provide direct evidence for a major contribution of the NR2A/B subunit in CD-induced epileptogenicity.

Role of primary sensorimotor cortices in generating inhibitory motor response in humans.

To clarify the mechanism by which inhibitory motor responses such as cortical negative myoclonus are generated in humans, three patients with medically intractable partial epilepsy (two with frontal lobe epilepsy and one with parietal lobe epilepsy) were studied by means of direct cortical stimulation with a single electric pulse through subdural electrodes. All underwent chronic long-term video/EEG monitoring, cortical mapping by 50 Hz electric cortical stimulation and recording of cortical somatosensory evoked potentials with chronically implanted subdural grid electrodes (3 mm in diameter and centre-to-centre distance of 1 cm) to map both epileptogenic and functional zones. After these clinical evaluations, cortical stimulation by single electric pulse (0.3 ms duration, 1 Hz) was carried out through pairs of subdural electrodes located at the primary sensorimotor area (MI-SI), pre-supplementary motor area (pre-SMA) and lateral negative motor area (lateral NMA), while surface EMG was recorded from the muscles of the contralateral hand. The results showed that (i) in all subjects, single pulse stimulation of MI-SI elicited a motor evoked potential (MEP) followed by a silent period (SP) in the contralateral distal hand muscles, the latter lasting 300 ms after the stimulus. The duration of SP was proportional to the size of the preceding MEP. In one subject, SP without any preceding MEP was elicited, and, in another subject, there was a short SP immediately before MEP in the contralateral thenar muscle. (ii) Following the stimulation of either pre-SMA or lateral NMA, no SP was observed. It is concluded that the inhibitory mechanism within the MI-SI, but probably not in the non-primary motor areas, either closely linked to or completely independent of excitation, most likely plays an important role in eliciting brief negative motor phenomena such as cortical negative myoclonus or SP.

784 Multiple Subpial Transections in the Surgical Management of Pediatric Epilepsy

Introduction: Multiple subpial transection (MST) is a technique that is being increasingly used in the surgical management of epilepsy. It allows the interruption of intracortical horizontally oriented fibers and spares the vertically or radially oriented fibers. As such it impairs the generation of epileptiform spikes but preserves the tissue's capability for normal neurologic function. As such it allows treatment of epileptiform activity in eloquent cortex. There are currently no published reports of the use of this technique in pediatric patients. Methods: Retrospective medical record review. Results: Between 1996 and 2000 28 pediatric patients (age 18 or less) (19 female, 9 male) underwent multiple subpial transections at the Hospital for Sick Children. At least 6 months follow up is available for all. Preoperative seizure patterns and frequencies varied widely. All were refractile to medical management. All underwent placement of subdural grid array and mapping of epileptogenic foci. Twenty five patients underwent...

Spatial distribution of neuronal complexity loss in neocortical lesional epilepsies.

Nonlinear EEG analysis is valuable in characterizing the spatiotemporal dynamics of the epileptogenic process in mesial temporal lobe epilepsy. We examined the ability of the measure neuronal complexity loss (L*) to characterize the primary epileptogenic area of neocortical lesional epilepsies during the interictal state. Spatial distribution of L* (L* map) was extracted from electrocorticograms (n = 52) recorded during presurgical assessment via subdural 64-contact grid electrodes covering lesions in either frontal, parietal, or temporal neocortex in 15 patients. The exact location of recording contacts on the brain surface was identified by matching a postimplant lateral x-ray of the skull with a postoperatively obtained sagittal MRI scan. Reprojecting L* maps onto the subject's brain surface allowed us to compare the spatial distribution of L* with the resection range of the extended lesionectomy. In each of the six patients who became seizure-free, maximum values of L* were restricted to recording sites coinciding with the area of resection. In contrast, L* maps of most patients who had no benefit from the resection indicated a more widespread extent or the existence of additional, probably autonomous, foci. The mean of L* values obtained from recording sites outside the area of resection correctly distinguished 13 patients (86.7 %) with respect to seizure outcome. Relevant information obtained from long-lasting interictal electrocorticographic recordings can be compressed to a single L* map that contributes to a spatial characterization of the primary epileptogenic area. In neocortical lesional epilepsies, L* allows for identification and characterization of epileptogenic activity and thus provides an additional diagnostic tool for presurgical assessment.