Faculty Bibliography

Rationale: Partial seizures in temporal lobe epilepsy (TLE) are classified as complex-partial, resulting in a loss of consciousness, or simple-partial, associated with preserved consciousness. The mechanistic underpinnings of impaired consciousness in partial seizures are poorly understood. Investigators have previously suggested that unconsciousness during partial seizures may be related to bilateral temporal lobe involvement, seizure onset in the language-dominant hemisphere, or increased cortico-thalamic synchrony. Earlier work has indeed shown that temporal lobe seizures are often associated with bilateral slow rhythms and decreased cerebral blood flow in the frontoparietal neocortex. Ictal neocortical slow rhythms resemble cortical activity observed during sleep or deep anesthesia. However, no prior investigations have directly examined the relationship between ictal neocortical slow activity and behavioral unresponsiveness. Methods: We analyzed intracranial electroencephalographic (EEG) recordings during 63 partial seizures in 26 TLE patients. Blinded reviewers analyzed behavioral responsiveness based on video recordings of seizures and classified consciousness as impaired (complex-partial) or unimpaired (simple-partial). Results: We found significantly elevated delta-range 1-2 Hz slow activity in the frontal and parietal neocortices during complex-partial compared to simple-partial seizures. Also, fast beta-range EEG activity in the contralateral temporal lobe, indicating seizure propagation, was significantly correlated with slow delta activity in the frontoparietal neocortex. Furthermore, we observed that seizure onset in the languagedominant hemisphere and bilateral temporal lobe involvement were more common during complex- than simple-partial seizures. Conclusions: We have proposed a 'network inhibition hypothesis' based on prior human and animal studies, in which subcortical arousal systems are disrupted by partial seizures, producing a depressed cortical state of slow activity and impaired consciousness. Our present findings illustrate that impaired consciousness is associated with ictal neocortical slow and bilateral temporal fast rhythms, raising the possibility that spread of seizure activity to bilateral temporal lobes may exert a powerful inhibitory effect on subcortical arousal networks. Further investigations are necessary to fully determine the role of cortical-subcortical networks in ictal neocortical dysfunction, and may ultimately lead to specific treatments targeted at preventing this negative consequence of TLE

Impaired consciousness requires altered cortical function. This can occur either directly from disorders that impair widespread bilateral regions of the cortex or indirectly through effects on subcortical arousal systems. It has therefore long been puzzling why focal temporal lobe seizures so often impair consciousness. Early work suggested that altered consciousness may occur with bilateral or dominant temporal lobe seizure involvement. However, other bilateral temporal lobe disorders do not impair consciousness. More recent work supports a 'network inhibition hypothesis' in which temporal lobe seizures disrupt brainstem-diencephalic arousal systems, leading indirectly to depressed cortical function and impaired consciousness. Indeed, prior studies show subcortical involvement in temporal lobe seizures and bilateral frontoparietal slow wave activity on intracranial electroencephalography. However, the relationships between frontoparietal slow waves and impaired consciousness and between cortical slowing and fast seizure activity have not been directly investigated. We analysed intracranial electroencephalography recordings during 63 partial seizures in 26 patients with surgically confirmed mesial temporal lobe epilepsy. Behavioural responsiveness was determined based on blinded review of video during seizures and classified as impaired (complex-partial seizures) or unimpaired (simple-partial seizures). We observed significantly increased delta-range 1-2 Hz slow wave activity in the bilateral frontal and parietal neocortices during complex-partial compared with simple-partial seizures. In addition, we confirmed prior work suggesting that propagation of unilateral mesial temporal fast seizure activity to the bilateral temporal lobes was significantly greater in complex-partial than in simple-partial seizures. Interestingly, we found that the signal power of frontoparietal slow wave activity was significantly correlated with the temporal lobe fast seizure activity in each hemisphere. Finally, we observed that complex-partial seizures were somewhat more common with onset in the language-dominant temporal lobe. These findings provide direct evidence for cortical dysfunction in the form of bilateral frontoparietal slow waves associated with impaired consciousness in temporal lobe seizures. We hypothesize that bilateral temporal lobe seizures may exert a powerful inhibitory effect on subcortical arousal systems. Further investigations will be needed to fully determine the role of cortical-subcortical networks in ictal neocortical dysfunction and may reveal treatments to prevent this important negative consequence of temporal lobe epilepsy

Although many patients with medically refractory focal epilepsy are candidates for resective surgery, patients with multifocal epilepsy and symptomatic generalized epilepsy remain difficult to treat medically and surgically. Corpus callosotomy has been utilized since 1940 for the treatment of seizures, with reports of efficacy in multiple seizure types. Previous studies have demonstrated subsequent lateralization of bilateral/bisynchronous epileptiform activity following callosotomy. To investigate the efficacy of bilateral intracranial electroencephalographic studies immediately following corpus callosotomy, we retrospectively identified 26 patients who underwent corpus callosotomy at our center, 18 of whom had intracranial monitoring following corpus callosotomy. Five of the 18 had focal resections following intracranial electroencephalography (EEG). No patients were seizure free following callosotomy or resection. No differences in postoperative outcomes were seen between patients with intracranial EEG versus those without

Epileptic cortex is characterized by paroxysmal electrical discharges. Analysis of these interictal discharges typically manifests as spike-wave complexes on electroencephalography, and plays a critical role in diagnosing and treating epilepsy. Despite their fundamental importance, little is known about the neurophysiological mechanisms generating these events in human focal epilepsy. Using three different systems of microelectrodes, we recorded local field potentials and single-unit action potentials during interictal discharges in patients with medically intractable focal epilepsy undergoing diagnostic workup for localization of seizure foci. We studied 336 single units in 20 patients. Ten different cortical areas and the hippocampus, including regions both inside and outside the seizure focus, were sampled. In three of these patients, high density microelectrode arrays simultaneously recorded between 43 and 166 single units from a small (4 mm x 4 mm) patch of cortex. We examined how the firing rates of individual neurons changed during interictal discharges by determining whether the firing rate during the event was the same, above or below a median baseline firing rate estimated from interictal discharge-free periods (Kruskal-Wallis one-way analysis, P<0.05). Only 48% of the recorded units showed such a modulation in firing rate within 500 ms of the discharge. Units modulated during the discharge exhibited significantly higher baseline firing and bursting rates than unmodulated units. As expected, many units (27% of the modulated population) showed an increase in firing rate during the fast segment of the discharge (+ or - 35 ms from the peak of the discharge), while 50% showed a decrease during the slow wave. Notably, in direct contrast to predictions based on models of a pure paroxysmal depolarizing shift, 7.7% of modulated units recorded in or near the seizure focus showed a decrease in activity well ahead (0-300 ms) of the discharge onset, while 12.2% of units increased in activity in this period. No such pre-discharge changes were seen in regions well outside the seizure focus. In many recordings there was also a decrease in broadband field potential activity during this same pre-discharge period. The different patterns of interictal discharge-modulated firing were classified into more than 15 different categories. This heterogeneity in single unit activity was present within small cortical regions as well as inside and outside the seizure onset zone, suggesting that interictal epileptiform activity in patients with epilepsy is not a simple paroxysm of hypersynchronous excitatory activity, but rather represents an interplay of multiple distinct neuronal types within complex neuronal networks

OBJECTIVE: The goal of the work described here was to assess the efficacy and safety of vagus nerve stimulation in a cohort of patients with tuberous sclerosis complex with refractory epilepsy. Furthermore, we examined the impact of vagus nerve stimulation failure on the ultimate outcome following subsequent intracranial epilepsy surgery. METHODS: A retrospective review was performed on 19 patients with refractory epilepsy and TSC who underwent vagus nerve stimulator (VNS) implantation. There were 11 (58%) females and 8 (42%) males aged 2 to 44 years when the VNS was implanted (mean: 14.7+/-12 years). Twelve patients underwent primary VNS implantation after having failed a mean of 7.1 antiepileptic drugs. Two patients (17%) had generalized epilepsy, one had a single seizure focus, three (25%) had multifocal epilepsy, and six (50%) had multifocal and generalized epilepsy. Seven patients were referred for device removal and evaluation for intracranial procedures. One patient in the primary implantation group was lost to follow-up and excluded from outcome analysis. RESULTS: All implantations and removals were performed without permanent complications. The duration of treatment for primary VNS implants varied from 8.5 months to 9.6 years (mean: 4.9 years). Mean seizure frequency significantly improved following VNS implantation (mean reduction: 72%, P<0.002). Two patients had Engel Class I (18%), one had Class II (9%), seven had Class III (64%), and one had Class IV (9%) outcome. Three patients with poor response to vagus nerve stimulation therapy at our center underwent resection of one or more seizure foci (Engel Class I, two patients; Engel Class III, one patient). Seven patients referred to our center for VNS removal and craniotomy underwent seizure focus resection (6) or corpus callosotomy (1) (Engel Class II: 2, Engel III: 2; Engel IV: 3). In total, 8 of 10 (80%) patients experienced improved seizure control following intracranial surgery (mean reduction: 65%, range: 0-100%, P<0.05). CONCLUSIONS: VNS is a safe and effective treatment option for medically refractory epilepsy in patients with tuberous sclerosis complex. Nine of 11 patients (82%) experienced at least a 67% reduction in seizure burden. Lack of response to vagus nerve stimulation does not preclude subsequent improvement in seizure burden with intracranial epilepsy surgery

The electroencephalogram (EEG) is a mainstay of clinical neurology and is tightly correlated with brain function, but the specific currents generating human EEG elements remain poorly specified because of a lack of microphysiological recordings. The largest event in healthy human EEGs is the K-complex (KC), which occurs in slow-wave sleep. Here, we show that KCs are generated in widespread cortical areas by outward dendritic currents in the middle and upper cortical layers, accompanied by decreased broadband EEG power and decreased neuronal firing, which demonstrate a steep decline in network activity. Thus, KCs are isolated 'down-states,' a fundamental cortico-thalamic processing mode already characterized in animals. This correspondence is compatible with proposed contributions of the KC to sleep preservation and memory consolidation