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An epidural performed for obstetric analgesia was initially characteristic of a subdural block but subsequently behaved as a normal epidural. Radiological confirmation of the catheter having been subdural was established by performing a magnetic resonance imaging scan.
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A case is described where an epidural anaesthetic was complicated by dural puncture, following which attempted conversion to a continuous subarachnoid infusion technique resulted in failure of anaesthesia. Accidental subdural catheter placement was confirmed radiologically, subdural spread of solution being demonstrated; however, only minimal anaesthetic effects were produced.
The indications and applications of brain functional mapping with subdurally implanted electrodes are reviewed briefly. The mapping can be performed outside the operating room and is useful for presurgical study of patients who have a complex distribution of epileptiform discharges and for minimizing functional deficits postoperatively. The major disadvantages of the procedure are the inflexibility of the electrode positions and the necessity of a craniotomy to insert and remove the subdural electrodes.
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Eleven patients had seizures with unilateral temporal lobe onset recorded with simultaneous bilateral medial temporal depth electrodes and neocortical (subdural) electrodes at least on the side of seizure onset. Of a total of 55 seizures, four had simultaneous onset in neocortex and hippocampus, and 51 had onset in unilateral hippocampus. None originated solely in temporal neocortex. Three reproducible patterns of seizure spread from hippocampus were defined in which seizures spread initially to ipsilateral neocortex (32 seizures), spread first to contralateral hippocampus (13 seizures), or spread simultaneously to ipsilateral neocortex and contralateral hippocampus. Although the region of hippocampus in which seizures arose was constant, patterns of spread sometimes varied in the same patient. When contralateral neocortical involvement occurred, it was after or with contralateral hippocampus but never before. These results suggest the existence of an operational hippocampal commissure in humans.
We report magnetoencephalographic localization of subdural electrode dipoles placed at the basal and mesial surfaces of the temporal lobe in a patient with temporal lobe epilepsy. The locations of the three dipoles were predicted from their magnetic fields with a computer model of the head as a conducting sphere. The predicted locations were within 1, 3, and 4 mm of the actual locations. These results, obtained in an area of the brain from which epileptiform discharges are frequently recorded, strongly support the capability of magnetoencephalography to accurately localize electrical events in this brain region.
During the course of a seizure, subdural recordings often show secondary areas which develop independent electrographic sequences that may outlast the primary seizure sequence. We reviewed the subdural data on 8 patients with intraictal secondarily activated foci (ISF). In 6 patients, ISFs were documented to be capable of generating independent seizures; ISF seizures occurred during the initial subdural monitoring period in 3, but became ictal generators only after excision of the primary focus in the other 3. Prominent interictal abnormalities were observed at the ISFs in 5 of 8 patients. The ISF in 1 patient correlated with the structural lesion on magnetic resonance imaging (MRI) scan. We believe that ISFs have significant epileptogenicity and should be resected when possible.
A 28-year-old woman with epilepsy developed the new onset of paroxysmal tongue tingling during subdural electrode monitoring. Her symptoms coincided with electrographic seizure activity arising from an area of the perisylvian region that had not previously been involved in her habitual seizures. At electrode removal, a focal 2 x 2 cm hematoma was detected and evacuated from beneath these electrode contacts. Unexpected episodic events may represent nonhabitual seizure activity related to the surgical procedure.
PURPOSE: This study evaluates the localization of stimulation-induced auras (SIA) and tries to determine whether the SIA can help to define the boundaries of resection in epilepsy surgery. METHODS: Using subdural grid electrodes, 31 patients with drug resistant focal epilepsy were examined in a retrospective and prospective study lasting 2 years. RESULTS: On stimulation by subdural electrodes, we elicited habitual auras in 16 patients (52%). The zone of SIA overlapped the epileptogenic lesion in 12 patients (75%), the EEG seizure onset zone in 12 patients (75%), and the irritative zone of interictal spikes in eight patients (50%). Postoperative results showed a significant correlation with the complete removal of the epileptogenic lesion (p < 0.001). Because the number of patients in the study was small, we could not find a significant correlation with the complete removal of the SIA zone, EEG seizure onset zone, and irritative zone. CONCLUSIONS: Our study confirms previous analyses which indicate that complete resection of the epileptogenic lesion is essential to achieve a good outcome. Frequent overlap of the SIA zone with the epileptogenic lesion and the EEG seizure onset zone indicates proximity of the SIA with the epileptogenic zone. Nevertheless, this study does not support the concept that the SIA zone and the EEG seizure onset zone have additional value in defining the boundaries of resection in epilepsy surgery. Three case presentations suggest that SIA result from facilitated pathways between the stimulated cortex, the epileptogenic zone around the lesion, and the symptomatogenic zone. Functional reorganization in the vicinity of the cortical lesion cannot be ruled out but was not seen in our patients. Thus, SIA often do not reflect the normal function of the stimulated underlying cortex.
PURPOSE: To describe regularly recurring ictal phenomena as recorded in a single patient whose seizure disorder related to a left temporal cavernous angioma. METHODS: Seizures were recorded by left temporal subdural strip electrodes which were bilaterally placed because of ambiguous seizure origin and right temporal memory dysfunction at Wada testing. Seizure length, interseizure intervals, and interseizure onset intervals were visually assessed. The coefficient of variation was used as an estimate of variability in seizure periodicity. RESULTS: Mean seizure durations varied from 68 to 95 s; coefficients of variation varied from 0.08 to 0.18, values similar to other periodic biological phenomena. Mean daily seizure intervals varied from 96 to 152 s with coefficients of variation ranging from 0.12 to 0.35. Interseizure onset intervals had smaller coefficients of variation (0.03-0.20). Manifestations of these simple partial seizures included a rising epigastric sensation and nausea. CONCLUSIONS: Previously described periodic EEG phenomena are interictal events or appear with peripheral events of similar cadence. Therefore, we remain unaware of any documentation of periodic ictal electrographic events. The restricted propagation and duration of these recorded seizures may have simplified their pathophysiology allowing a stereotyped involvement of excitatory and inhibitory mechanisms producing the periodicity.
A 26-year-old man with intractable focal motor seizure beginning with tonic contraction of the left orbicularis oculi muscle had prolonged EEG monitoring with subdural grid electrodes placed over the right perirolandic cortex. Electrical stimulation of the cortex with implanted subdural electrodes showed a relatively low threshold for afterdischarges (ADs) but could not disclose the motor area for the left upper face where or near where the epileptogenic area was expected to be present. Bereitschaftspotential recorded from the subdural electrodes in association with self-paced voluntary blink (eyelid closing) disclosed the motor area specifically related to voluntary movements of the left upper face, which was most likely buried in the sulcus. This observation suggests that recording of Bereitschaftspotential from subdural electrodes is useful for mapping the motor cortex, especially in patients with focal motor seizure with low threshold for ADs to electric stimuli.
PURPOSE: We wished to define the localization of cortical generators of visual (pattern) evoked potentials (VEP) and the temporal sequence of activation in the occipital region. METHODS: In 4 candidates for epilepsy surgery, a large array of subdural electrodes was placed over occipital areas. Checkerboard pattern reversal stimuli were generated and the epileptogenic focus was localized and functionally mapped. Magnetic resonance imaging did not show any occipital lesions in any of the 4 patients. RESULTS: The area first activated was the lingual gyrus in the mesial occipital lobe (negative potential peaks at approximately 70 ms), followed by an area superior to the calcarine fissure (negative peaks at approximately 80 ms). Later (starting at approximately 90 ms), there were positive potentials over the occipital pole and lingual gyrus, followed by potentials at the lateral occipital lobe. CONCLUSIONS: These data support the idea that VEP are generated in the mesial and lateral occipital cortex by different circumscribed neuronal generators with different latencies of activation. The scalp-recorded N1 and P1 potential peaks most likely derive from the progressive activation of neuronal masses in different regions of the occipital lobe.
PURPOSE: The yield of subdural versus intracerebral electrodes for ictal localization remains a point of controversy. We assessed the relative sensitivity of these two types of electrodes per case. METHODS: Eighty-three intracranial recordings obtained from 82 patients were retrospectively reviewed to establish which type of electrode performed best in which patients and which seizure types. RESULTS: Sixty (73%) of 82 patients had temporal lobe seizure onsets, eight frontal, nine widespread or multifocal/multilobar or both, whereas in five, seizure onset was not localized. Exclusive use of intracerebral electrodes would have been sufficient for accurate localization of the seizure-onset zone in all 35 patients with strictly mesial temporal seizure onsets. In only 20 (57%) of these 35 patients, the same decision would have been reached with exclusive use of subdural electrodes. In widespread neocortical and mesial temporal seizures (n = 25), yield of both electrode types was at about the same level, but neither was sufficient to identify the zone of ictal onset on its own. In frontal or multilobar seizures (n = 22), yield of subdural electrodes was slightly better then that of the intracerebral electrodes, but was not sufficient in all cases. CONCLUSIONS: This study indicates that, depending on the characteristics of the seizure disorder, exclusive use of either intracerebral or subdural electrodes may easily result in erroneous diagnosis because of insufficient sampling of the brain. These findings are in contrast with other studies emphasizing the high yield of reliable EEG findings in evaluations with a single type of electrode and corroborate the results of one of our previous studies.
PURPOSE: Numerous factors have been analyzed in attempts to predict the outcome of surgical resections in patients with neocortical epilepsy. We examined the correlation between surgical outcome and electrocorticographic features of neocortical ictal patterns. METHODS: Twenty six patients with neocortical epilepsy underwent monitoring with subdural grid electrodes before surgery. Ictal patterns were analyzed retrospectively and correlated with three types of outcome: seizure free, worthwhile improvement (>75% reduction of seizure frequency), and no worthwhile improvement. The duration of follow-up was 2-5 years. RESULTS: Ictal patterns were divided according to the size of epileptogenic zone (focal, regional, multifocal); velocity and type of seizure propagation (fast contiguous, slow contiguous, noncontiguous); pattern of the onset of ictal activity; part of the cortex involved in the origin of the seizure (frontal, frontocentroparietal, etc.). Spread to medial temporal structures (as assessed by subtemporal strips) also was evaluated in selected cases. Statistically significant correlation with surgical outcome (p = 0.026) was shown for only one variable: type of spread. Patients with slow spread (n = 8) demonstrated the best outcomes (five are seizure free), whereas patients with noncontiguous spread (n = 5) demonstrated the worst outcomes (four did not improve significantly). Patients with fast contiguous spread (n = 13) showed intermediate outcomes. CONCLUSIONS: Types of propagation of ictal neocortical activity correlate with surgical outcome. Analysis of ictal pattern during intracranial recordings may help to predict surgical outcome for neocortical epilepsy.
PURPOSE: Performance of automatic spike-detection algorithms and interrater reliability of human EEG reviewers were investigated previously by using scalp EEG recordings. However, it is not known, whether the findings of these studies hold for intracranial recordings. To address this question, we analyzed spike detection in intracranial recordings by two human reviewers and three automatic systems covering major lines in the development of automatic spike-detection systems. METHODS: Intracranial recordings from subdural and intrahippocampal depth electrode contacts in seven patients were analyzed by two reviewers and three spike-detection systems: (a) The rule-based system by Gotman, (b) a two-stage system consisting of a linear predictor and a second rule-based stage, and (c) a system using wavelet coefficients of the intracranial EEG data. RESULTS: Agreement between the two human reviewers with respect to spike identification was <50%. The automatic systems achieved agreements of 24% (Gotman), 26% (wavelet detector), and 32% (two-stage system) with the individual human reviewers. In spite of the small proportion of agreements, the same anatomic regions were identified by human and automatic EEG analysis as generators for the majority of spikes. CONCLUSIONS: The poor agreement between the human EEG reviewers suggests that the definition of spikes and spike-like episodes in intracranial electrodes is far from unequivocal. Nevertheless, localizing information is highly consistent by either visual or automatic spike detection, independent of the algorithm used for automatic spike detection.
PURPOSE: To evaluate combined [18F]fluorodeoxyglucose (18F-FDG) positron emission tomography (PET) and 122-channel whole-scalp magnetoencephalography (MEG) in lateralizing the epileptogenic cortex in patients whose routine presurgical evaluations gave discordant results about the location of the epileptic focus. METHODS: Nine patients (five women, four men) aged 13-40 years were studied. Subdural EEG (SEEG) was recorded from eight patients. Six patients were operated on. RESULTS: In seven of nine patients, PET and MEG agreed in localizing the epileptogenic cortex. When PET and MEG were in congruence, SEEG agreed with the findings. In five of six operated-on patients, PET and MEG results were congruent, and the outcome of the operation was successful. Two patients had discordant PET and MEG results. In one patient, PET showed bitemporal hypometabolism, whereas MEG showed epileptiform activity in the right parietal lobe. The surgical outcome of the palliative temporal lobectomy was poor. Another patient had unilateral temporal hypometabolism in PET and bitemporal activity in MEG. She was not operated on. CONCLUSIONS: In most patients, PET and MEG were congruent in locating the epileptogenic cortex. Thus the combination of these techniques may provide useful support for the localization of the seizure onset and reduce the need for invasive procedures.
PURPOSE: To study the mechanisms of startle-induced supplementary sensorimotor area (SSMA) seizures. METHODS: We present a patient investigated with indwelling subdural grid electrodes covering both the SSMA and the dorsolateral frontal lobe. RESULTS: We found a simultaneous seizure onset in the right dorsolateral premotor cortex and the right SSMA. High-resolution magnetic resonance imaging (MRI) showed a small subcortical lesion adjacent to the right SSMA. The patient became seizure free after resection of the lesion and the ictal-onset zone. CONCLUSIONS: We conclude that an extended region of abnormally excitable tissue within the frontal lobe could facilitate the generation of startle-induced seizures. We speculate that a widespread epileptogenic zone could help to explain why some patients with SSMA or dorsolateral frontal lobe seizures are more likely to have startle-induced seizures.