Invasive Presurgical Evaluation Research Articles

Neuronal substrates of sensory gating within the human brain

Background For the human brain, habituation to irrelevant sensory input is an important function whose failure is associated with behavioral disturbances. Sensory gating can be studied by recording the brain’s electrical responses to repeated clicks: the P50 potential is normally reduced to the second of two paired clicks but not in schizophrenia patients. To identify its neural correlates, we recorded electrical traces of sensory gating directly from the human hippocampus and neocortex. Methods Intracranial evoked potentials were recorded using hippocampal depth electrodes and subdural strip and grid electrodes in 32 epilepsy patients undergoing invasive presurgical evaluation. Results We found evidence of sensory gating only in the hippocampus, the temporo-parietal region (Brodmann’s areas 22 and 2), and the prefrontal cortex (Brodmann’s areas 6 and 24); however, whereas neocortical habituating responses to paired clicks were peaking around 50 msec, responses within the hippocampus proper had a latency of about 250 msec. Conclusions Consistent with data from animal studies, our findings show that the hippocampus proper contributes to sensory gating, albeit during a time window following neocortical habituation processes. Thus, sensory gating may be a multistep process, with an early phase subserved by the temporo-parietal and prefrontal cortex and a later phase mediated by the hippocampus.

Surgical treatment for neocortical temporal lobe epilepsy: clinical and surgical aspects and seizure outcome.

The goal of this paper was to describe the clinical and surgical aspects of a group of patients suffering from drug-resistant neocortical temporal lobe epilepsy (TLE), as well as seizure outcomes and factors affecting seizure outcomes in these patients. This study was based on data prospectively collected and retrospectively evaluated. Sixty-two patients with neocortical TLE constituted the study population. Only patients who underwent corticectomies, lesionectomies, lateral anterior lobe resections, and/or multiple subpial transections were included. The pathological areas resected in these patients could be separated into three groups composed of 35 neoplastic lesions, 23 nonneoplastic lesions, and three nonlesional areas. The mean duration of follow-up review in these patients was 21.9+/-14 months. Outcomes were categorized according to Engel classes. Class I was found in 79% of the patients and Class II in 11%. Invasive presurgical evaluation was performed in 43% of the patients. There were only temporary complications (3.3% surgical and 1.6% neurological) and no deaths. In summary, lesions confirmed on histological examination were rarely found in patients with neocortical TLE. Low-grade tumors were the most commonly found lesions in these patients and the most common tumor was ganglioglioma. Outcome was best for those patients with neoplastic lesions and was independent of the duration of their seizures. Outcome was little influenced by the type of resection performed and was found to be as good as that achieved in patients with mesial TLE. These results demonstrate that the concept of lateral or neocortical TLE as a distinct entity is useful. Surgery for neocortical TLE can be considered a viable treatment option that is associated with a low morbidity rate and good outcomes.

Electrical Cortical Stimulation of the Human Prefrontal Cortex Evokes Complex Visual Hallucinations

Complex visual hallucinations are a well-known feature of electrical stimulation or epileptic discharge in the temporal lobe. It has been proposed that these visual hallucinations result from an electrical interference with the ventral visual processing stream in the lateral temporal lobe and the memory system in medial temporal structures, which explains their frequent visual and mnestic features. Even though recent studies have demonstrated visual and memory functions in the prefrontal cortex, up to now epileptic discharge or electrical stimulation of prefrontal structures has only rarely been reported to induce visual phenomena. We report on two patients undergoing invasive presurgical epilepsy evaluation in which electrical cortical stimulation of the left prefrontal cortex repeatedly induced complex visual hallucinations. Interestingly, the induced visual responses differed with respect to their spatial organization: whereas those evoked on the inferior frontal gyrus were perceived in the whole visual field, complex visual responses on the middle frontal gyrus were restricted to the contralateral hemispace. Based on the spatial organization of the visual experiences in our patients, animal work, and neuroimaging data it might be suggested that specific subregions of the human prefrontal cortex might contain separate visual and mnemonic processing mechanisms.

Presurgical evaluation: current role of invasive EEG.

Our purpose was to review the current role of invasive and semi-invasive EEG in the presurgical evaluation of candidates for epilepsy surgery. The use of stereotactically implanted intracranial depth (stereo-EEG), subdural strip and grid, and foramen ovale electrodes, as well as intraoperative electrocorticography and electrical brain stimulation ("functional mapping") at the Epilepsy Center University Hospital Zurich, from 1984 to 1998, is analyzed. Advantages and disadvantages of the various intracranial EEG techniques are critically discussed. Out of 422 selective amygdalohippocampectomies performed in Zurich, 54% had non-invasive, 32% had semi-invasive, and 14% had invasive presurgical EEG evaluation. Because patients currently referred to our center increasingly present with a complex history of disease, i.e., constitute so-called "difficult cases", there is trend to combine several invasive and semi-invasive, pre- and intraoperative neurophysiological techniques.

Protocolo de evaluación neurofisiológica prequirúrgica en epilepsia resistente a tratamiento

Nowadays approximately twenty per cent of epileptic patients who are on a pharmacological treatment carry on their fits. Their instability can be eliminated with an operation in any center that is specialized in epileptic surgery. The center must have a cross disciplinary team. The ideal operation is one that only eliminates the tissue needed to end up with the fits The main goal of the neurophysiological evaluation is to delimit the cerebral area that generates an epileptogenic activity so as to be able to eliminate it without causing further damage. We present a medical record of pre-surgical neurophysiological evaluation that we would like to put into practice in our department in coordination with the services of neurology, radiodiagnosis, and neurosurgery. This medical record contains the following sections: 1. Selection of patients. 2. Non invasive pre-surgical evaluation. 3. Invasive pre-surgical evaluation. 4. Postsurgical evaluation. The invasive pre-surgical evaluation constitutes the most interesting part of the whole process, it allows thanks to intracranial registers to locate accurately epileptogenic focuses; what is more, it allows to carry out a functional mapping of areas that cannot be explored with surface techniques; and finally this evaluation allows to open new fields of investigation about the way in which the encephalon works. Epilepsy surgery is a subject to develop in Spain in which the neurophysiologic exploration is something essential.

Memory function during low intensity hippocampal electrical stimulation in patients with temporal lobe epilepsy

Hippocampal memory dysfunction is a main symptom of mesiotemporal lobe epilepsy (TLE). It may increase after temporal lobectomy for seizure relief. The aim of this study was to assess the ability of psychometry during hippocampal electrical stimulation to predict post‐operative memory deficits and thereby to contribute to lateralization of the epileptogenic zone. A computerized memory test was performed during low intensity hippocampal stimulation in nine patients with TLE during invasive presurgical evaluation. The following results were obtained: speech dominant hippocampal stimulation induced subtle verbal memory deficits in patients with the epileptogenic zone in the speech dominant hemisphere and normal baseline memory performance. Verbal memory deficits could not be induced in patients with the epileptogenic zone in the speech dominant hemisphere and pre‐existing memory deficits, or if the seizure origin was contralateral to speech dominance. Thus it was possible to lateralize the epileptogenic zone in patients with normal baseline memory performance by revealing hippocampal dysfunction only during electrical stimulation. Post‐operative psychometric testing demonstrated that the individual risk for impairment of verbal memory performance following temporal lobectomy could be predicted correctly. In conclusion, psychometry during hippocampal stimulation may allow prediction of memory deficits following temporal lobectomy and improve determination of the epileptogenic zone.

Specific response of the epileptic focus to anesthesia with propofol

Twenty patients suffering from medically intractable epilepsy were submitted to invasive presurgical evaluation using subdural strip electrodes for chronic electrocorticographic (ECoG) recordings. A short anesthesia of 12–15 min was induced to ensure painless percutaneous removal of the subdural electrode strips. Anesthesia was provided by two consecutive intravenous injections of propofol (50 and 70 mg or 70 and 140 mg) preceded by fentanyl (0.1–0.15 mg) in 12 patients. In order to test whether the epileptic focus showed a specific response following application of propofol, the first 12 min of ECoG recordings, prior to removal of the subdural strip electrodes, were visually analyzed. Specific patterns of response confined to the epileptic focus (foci) were observed in 18 of 20 patients following application of at least one of the two doses of propofol. Patterns observed in the epileptogenic area consisted of (a) a loss or marked reduction of activity induced by propofol (n = 16), (b) a maximum of suppression of brain activity (n = 12), (c) suppression of spontaneous interictal activity (n = 10), (d) induction of epileptiform activity (n = 5), and (e) induction of a habitual seizure (n = 1). In summary, ECoG recordings during anesthesia with propofol provide complementary information about the epileptic focus in the process of presurgical evaluation of epileptics.