On the ictogenic properties of the piriform cortex in vitro

Purpose: The piriform cortex (PC) is known to be epileptic‐prone and it may be involved in the manifestation of limbic seizures. Herein, we have characterized some electrophysiologic and pharmacologic properties of the spontaneous epileptiform activity generated by PC networks maintained in vitro. Methods: We performed field potential recordings from the PC in coronal or sagittal rat brain slices along with pharmacologic manipulations of γ‐aminobutyric acid (GABA)ergic and glutamatergic signaling during application of the convulsant drug 4‐aminopyridine (4AP, 50 μm ). Key Findings: Coronal and sagittal preparations generated interictal‐like and ictal‐like epileptiform discharges with similar duration and frequency. Ictal‐like discharges in sagittal slices were initiated mostly in the PC anterior subregion, whereas interictal activity did not have any preferential site of origin. In sagittal slices, high frequency oscillations (HFOs) at 80–200 Hz were detected mainly at the beginning of the ictal discharge in both posterior and anterior subregions. N‐Methyl‐d ‐aspartate (NMDA) receptor antagonism abolished ictal discharges, but failed to influence interictal activity. In the absence of ionotropic glutamatergic transmission, PC networks generated slow, GABA receptor–dependent events. Finally, GABA_A receptor antagonism during application of 4AP only, abolished ictal discharges and disclosed recurrent interictal activity. Significance: Our findings demonstrate that PC networks can sustain in vitro epileptiform activity induced by 4AP. HFOs, which emerge at the onset of ictal activity, may be involved in PC ictogenesis. As reported in several cortical structures, ionotropic glutamatergic neurotransmission is necessary but not sufficient for ictal discharge generation, a process that also requires operative GABA_A receptor–mediated signaling.     

High-frequency oscillations recorded with surface EEG in neonates with seizures

ObjectiveNeonatal seizures are often the first symptom of perinatal brain injury. High-frequency oscillations (HFOs) are promising new biomarkers for epileptogenic tissue and can be found in intracranial and surface EEG. To date, we cannot reliably predict which neonates with seizures will develop childhood epilepsy. We questioned whether epileptic HFOs can be generated by the neonatal brain and potentially predict epilepsy.MethodsWe selected 24 surface EEGs sampled at 2048 Hz with 175 seizures from 16 neonates and visually reviewed them for HFOs. Interictal epochs were also reviewed.ResultsWe found HFOs in thirteen seizures (7%) from four neonates (25%). 5025 ictal ripples (rate 10 to 1311/min; mean frequency 135 Hz; mean duration 66 ms) and 1427 fast ripples (rate 8 to 356/min; mean frequency 298 Hz; mean duration 25 ms) were marked. Two neonates (13%) showed interictal HFOs (285 ripples and 25 fast ripples). Almost all HFOs co-occurred with sharp transients. We could not find a relationship between neonatal HFOs and outcome yet.ConclusionsNeonatal HFOs co-occur with ictal and interictal sharp transients.SignificanceThe neonatal brain can generate epileptic ripples and fast ripples, particularly during seizures, though their occurrence is not common and potential clinical value not evident yet.     

Effects of the GABA(A) receptor antagonists bicuculline and gabazine on stimulus-induced sharp wave-ripple complexes in adult rat hippocampus in vitro

Hippocampal sharp wave-ripple-complexes (SPW-Rs) are characterized by slow field potential transients superimposed by ripples with a frequency of ∼200 Hz. In epileptic humans and rats frequencies of such transient network oscillations can reach up to 500 Hz potentially due to loss of functional inhibition. Therefore, we investigated whether GABA_A receptor antagonists increased ripple frequency during SPW-Rs. Within area CA3, SPW-Rs were induced by repeated stimulation of stratum radiatum in area CA1 of adult Wistar rat hippocampal slices. Intracellular recordings showed that in approximately 50% of recorded CA3 pyramidal cells SPW-Rs were accompanied by compound excitory postsynaptic potentials (EPSPs) of ∼10 mV superimposed by up to four action potentials (APs). The remaining cells responded with a compound inhibitory postsynaptic potential (IPSP) during SPW-Rs. The GABA_A receptor antagonists bicuculline (BMI) or gabazine (SR-95531) led to a transition of SPW-Rs into prolonged bursts with a significant increase in amplitude and duration reminiscent of recurrent epileptiform discharges (REDs). Ripple frequencies increased from ∼190 Hz to ∼300 Hz. In naïve slices SR-95531 and BMI also evoked REDs with similar incidence and high frequency ripple frequencies of ∼240 Hz. Elevations in extracellular potassium concentration during REDs were approximately 20-fold higher than those observed during SPW-Rs. Intracellular recordings revealed bursts that were characterized by a large (> 25 mV) prolonged depolarization superimposed by up to 40 APs in close synchrony with extracellularly recorded ripples. Our results suggest that the generation of high frequency ripples, which are also observed in epileptic humans and rats, could indicate a loss of functional inhibition.     

Temporal lobe epileptiform activity following systemic administration of 4‐aminopyridine in rats

Purpose: The K⁺ channel blocker 4‐aminopyridine (4AP) induces epileptiform synchronization in brain slices maintained in vitro without interfering with γ‐aminobutyric acid (GABA)_A receptor–mediated inhibition and, actually, even enhancing it. The hypothesis that similar electrographic epileptiform patterns occur in vivo following systemic 4AP injection was tested here. Methods: Sprague‐Dawley rats (n = 13) were implanted with bipolar electrodes aimed at the hippocampal CA3 region, entorhinal cortex, subiculum, dentate gyrus, and amygdala. They were then injected with a single dose of 4AP (4–5 mg/kg, i.p.), and video‐monitoring/electroencephalography (EEG) recordings were performed. Key Findings: 4AP induced convulsive or nonconvulsive seizures in 12 of 13 rats, along with generalized fascicular twitching, wet‐dog shakes, and myoclonic jerks. On EEG, we observed in 7 (58.3%) of 12 animals long‐lasting interictal spikes from the subiculum before the occurrence of the first seizure. Once seizures had started, interictal spikes occurred in all areas with no fixed site of origin. Most seizures (41/60, 68.3%) were characterized by a low‐voltage fast‐activity onset pattern and were convulsive (48/60, 80%). 4AP also induced highly rhythmic theta (6–11 Hz) oscillations in CA3 and entorhinal cortex before seizure occurrence. Significance: Our study shows that systemic 4AP administration in vivo can enhance theta oscillations and induce slow interictal spikes and low‐voltage fast‐onset seizures similar to those reported in brain slices. We propose that these effects may reflect, at least in part, enhanced GABA_A receptor–mediated inhibition as reported in in vitro studies.     

L-Type calcium channel blockade reduces network activity in human epileptic hypothalamic hamartoma tissue

Purpose: Human hypothalamic hamartomas (HHs) are associated with gelastic seizures, intrinsically epileptogenic, and notoriously refractory to medical therapy. We previously reported that the L‐type calcium channel antagonist nifedipine blocks spontaneous firing and γ‐aminobutyric acid (GABA)_A–induced depolarization of single cells in HH tissue slices. In this study, we examined whether blocking L‐type calcium channels attenuates emergent activity of HH neuronal networks. Methods: A high‐density multielectrode array was used to record extracellular signals from surgically resected HH tissue slices. High‐frequency oscillations (HFOs, ripples and fast ripples), field potentials, and multiunit activity (MUA) were studied (1) under normal and provoked [4‐aminopyridine (4‐AP)] conditions; and (2) following nifedipine treatment. Key Findings: Spontaneous activity occurred during normal artificial cerebrospinal fluid (aCSF) conditions. Nifedipine reduced the total number and duration of HFOs, abolished the association of HFOs with field potentials, and increased the inter‐HFO burst intervals. Notably, the number of active regions was decreased by 45 ± 9% (mean ± SEM) after nifedipine treatment. When considering electrodes that detected activity, nifedipine increased MUA in 58% of electrodes and reduced the number of field potentials in 67% of electrodes. Provocation with 4‐AP increased the number of events and, as the number of electrodes that detected activity increased 248 ± 62%, promoted tissue‐wide propagation of activity. During provocation with 4‐AP, nifedipine effectively reduced HFOs, the association of HFOs with field potentials, field potentials, MUA, and the number of active regions, and limited propagation. Significance: This is the first study to report (1) the presence of HFOs in human subcortical epileptic brain tissue in vitro; (2) the modulation of “pathologic” high‐frequency oscillations (i.e., fast ripples) in human epileptic tissue by L‐type calcium channel blockers; and (3) the modulation of network physiology and synchrony of emergent activity in human epileptic tissue following blockade of L‐type calcium channels. Attenuation of activity in HH tissue during normal and provoked conditions supports a potential therapeutic usefulness of L‐type calcium channel blockers in epileptic patients with HH.     

High-frequency (80-500 Hz) oscillations and epileptogenesis in temporal lobe epilepsy

High-frequency oscillations (HFOs), termed ripples (80-200 Hz) and fast ripples (250-600 Hz), are recorded in the EEG of epileptic patients and in animal epilepsy models; HFOs are thought to reflect pathological activity and seizure onset zones. Here, we analyzed the temporal and spatial evolution of interictal spikes with and without HFOs in the rat pilocarpine model of temporal lobe epilepsy. Depth electrode recordings from dentate gyrus (DG), CA3 region, subiculum and entorhinal cortex (EC), were obtained from rats between the 4th and 15th day after a status epilepticus (SE) induced by i.p. injection of pilocarpine. The first seizure occurred 6.1 ± 2.5 days after SE (n = 7 rats). Five of 7 animals exhibited interictal spikes that co-occurred with fast ripples accounting for 4.9 ± 4.6% of all analyzed interictal spikes (n = 12,886) while all rats showed interictal spikes co-occurring with ripples, accounting for 14.3 ± 3.4% of all events. Increased rates of interictal spikes without HFOs in the EC predicted upcoming seizures on the following day, while rates of interictal spikes with fast ripples in CA3 reflected periods of high seizure occurrence. Finally, interictal spikes co-occurring with ripples did not show any specific relation to seizure occurrence. Our findings identify different temporal and spatial developmental patterns for the rates of interictal spikes with or without HFOs in relation with seizure occurrence. These distinct categories of interictal spikes point at dynamic processes that should bring neuronal networks close to seizure generation.     

Pitfalls of high-pass filtering for detecting epileptic oscillations: A technical note on “false” ripples

ObjectivesTo analyze interictal High frequency oscillations (HFOs) as observed in the medial temporal lobe of epileptic patients and animals (ripples, 80–200 Hz and fast ripples, 250–600 Hz). To show that the identification of interictal HFOs raises some methodological issues, as the filtering of sharp transients (e.g., epileptic spikes or artefacts) or signals with harmonics can result in “false” ripples. To illustrate and quantify the occurrence of false ripples on filtered EEG traces.MethodsWe have performed high-pass filtering on both simulated and real data. We have also used two alternate methods: time-frequency analysis and matching pursuit.ResultsTwo types of events were shown to produce oscillations after filtering that could be confounded with actual oscillatory activity: sharp transients and harmonics of non-sinusoidal signals.ConclusionsHigh-pass filtering of EEG traces for detection of oscillatory activity should be performed with great care. Filtered traces should be compared to original traces for verification of presence of transients. Additional techniques such as time-frequency transforms or sparse decompositions are highly beneficial.SignificanceOur study draws the attention on an issue of great importance in the marking of HFOs on EEG traces. We illustrate complementary methods that can help both researchers and clinicians.     

Focal resection of fast ripples on extraoperative intracranial EEG improves seizure outcome in pediatric epilepsy

Purpose: High‐frequency oscillations (HFOs), termed ripples at 80–200 Hz and fast ripples (FRs) at >200/250 Hz, recorded by intracranial electroencephalography (EEG), may be a valuable surrogate marker for the localization of the epileptogenic zone. We evaluated the relationship of the resection of focal brain regions containing high‐rate interictal HFOs and the seizure‐onset zone (SOZ) determined by visual EEG analysis with the postsurgical seizure outcome, using extraoperative intracranial EEG monitoring in pediatric patients and automated HFO detection. Methods: We retrospectively analyzed 28 pediatric epilepsy patients who underwent extraoperative intracranial video‐EEG monitoring prior to focal resection. Utilizing the automated analysis, we identified interictal HFOs during 20 min of sleep EEG and determined the brain regions containing high‐rate HFOs. We investigated spatial relationships between regions with high‐rate HFOs and SOZs. We compared the size of these regions, the surgical resection, and the amount of the regions with high‐rate HFOs/SOZs within the resection area with seizure outcome. Key Findings: Ten patients were completely seizure‐free and 18 were not at 2 years after surgery. The brain regions with high‐rate ripples were larger than those with high‐rate FRs (p = 0.0011) with partial overlap. More complete resection of the regions with high‐rate FRs significantly correlated with a better seizure outcome (p = 0.046). More complete resection of the regions with high‐rate ripples tended to improve seizure outcome (p = 0.091); however, the resection of SOZ did not influence seizure outcome (p = 0.18). The size of surgical resection was not associated with seizure outcome (p = 0.22–0.39). Significance: The interictal high‐rate FRs are a possible surrogate marker of the epileptogenic zone. Interictal ripples are not as specific a marker of the epileptogenic zone as interictal FRs. Resection of the brain regions with high‐rate interictal FRs in addition to the SOZ may achieve a better seizure outcome.     

Antiepileptic drugs abolish ictal but not interictal epileptiform discharges in vitro

Purpose: We established the effects of the antiepileptic drugs (AEDs) carbamazepine (CBZ), topiramate (TPM), and valproic acid (VPA) on the epileptiform activity induced by 4‐aminopyridine (4AP) in the rat entorhinal cortex (EC) in an in vitro brain slice preparation. Methods: Brain slices were obtained from Sprague‐Dawley rats (200–250 g). Field and intracellular recordings were made from the EC during bath application of 4AP (50 μm ). AEDs, and in some experiments, picrotoxin were bath applied concomitantly. Results: Prolonged (>3 s), ictal‐like epileptiform events were abolished by CBZ (50 μm ), TPM (50 μm ), and VPA (1 mm ), whereas shorter (<3 s) interictal‐like discharges continued to occur, even at concentrations up to 4‐fold as high. γ‐Aminobutyric acid (GABA)_A–receptor antagonism changed the 4AP‐induced activity into recurrent interictal‐like events that were not affected by CBZ or TPM, even at the highest concentrations. To establish whether these findings reflected the temporal features of the epileptiform discharges, we tested CBZ and TPM on 4AP‐induced epileptiform activity driven by stimuli delivered at 100‐, 10‐, and 5‐s intervals; these AEDs reduced ictal‐like responses to stimuli at 100‐s intervals at nearly therapeutic concentrations, but did not influence shorter interictal‐like events elicited by stimuli delivered every 10 or 5 s. Conclusions: We conclude that the AED ability to control epileptiform synchronization in vitro depends mainly on activity‐dependent characteristics such as discharge duration. Our data are in keeping with clinical evidence indicating that interictal activity is unaffected by AED levels that are effective to stop seizures.     

GABAA receptor-mediated mechanisms contribute to frequency-dependent depression of IPSPs in the hippocampus

Intracellular recordings from in vitro guinea-pig hippocampal slices were obtained to investigate the mechanisms underlying activity-dependent depression of inhibitory postsynaptic potentials (IPSPs) in Ca1 pyramidal neurons. IPSPs were studied following blockade of glutamatergic transmission. Attenuation of both fast and slow IPSP components was apparent at stimulus rates of >0.2 Hz, but the late IPSP showed depression at even lower rates of stimulation. Overlap of these events made resolving response components during depression difficult. Fast IPSPs were isolated using CsCl-filled microelectrodes, blocking slow potassium-dependent IPSPs and reversing the chloride gradient. Under this condition repetitive stimulation did not induce IPSP depression. Other experiments showed slow IPSP depression at stimulus rates of <0.5 Hz was relieved in the presence of picrotoxin which antagonizes GABA_A events. These results suggest that activity-dependent depression of the fast IPSP results largely from chloride accumulation as a consequence of repetitive activation. Depression of the slow IPSP appears to arise from at least two components: (1) a masking of the slow hyperpolarization resulting from an apparent increase in the depolarizing GABA response due to chloride accumulation; and (2) a novel process involving GABA_A receptors, perhaps mediated through presynaptic inhibition.     

Epileptic high‐frequency oscillations in intraoperative electrocorticography: The effect of propofol

Purpose: Epileptic high‐frequency oscillations (HFOs; 80–500 Hz) may be used to guide neurosurgeons during epilepsy surgery to identify epileptogenic tissue. We studied the effect of the anesthetic agent propofol on the occurrence of HFOs in intraoperative electrocorticography (ECoG). Methods: We selected patients who were undergoing surgery for temporal lobe epilepsy with a standardized electrode grid placement. Intraoperative ECoG was recorded at 2,048 Hz following cessation of propofol. The number and distribution of interictal spikes, ripples (R [80–250 Hz]), and fast ripples (FRs; 250–500 Hz) were analyzed. The amount of events on mesiotemporal channels and lateral neocortical channels were compared between patients with a suspected mesiotemporal and lateral epileptogenic area (Student’s t‐test), and HFOs were compared with the irritative zone, using correlation between amounts of events per channel, to provide evidence for the epileptic nature of the HFOs. Next, the amount of events within the first minute and the last minute were compared to each other and the change in events over the entire epochs was analyzed using correlation analyses of 10 epochs during the emergence periods (Spearman rank test). We studied whether the duration of HFOs changed over time. The change in events within presumed epileptogenic area was compared to the change outside this area (Student’s t‐test). Periods of burst suppression and continuous background activity were compared between and within patients (t‐test). Key Findings: Twelve patients were included: five with suspected mesiotemporal epileptogenic area and three with suspected lateral epileptogenic area (and four were “other”). Spikes, ripples, and FRs were related to the suspected epileptogenic areas, and HFO zones were related to the irritative zones. Ripples and FRs increased during emergence from propofol anesthesia (mean number of ripples from first minute–last minute: 61.5–73.0, R = 0.46, p < 0.01; FRs: 3.1–5.7, R = 0.30, p < 0.01) and spikes remained unchanged (80.1–79.9, R = ?0.05, p = 0.59). There was a decrease in number of channels with spikes (R = ?0.18, p = 0.05), but no change in ripples (R = ?0.13, p = 0.16) or FRs (R = 0.11, p = 0.45). There was no change in the durations of HFOs. The amount of HFOs in the presumed epileptogenic areas did not change more than the amount outside the presumed epileptogenic area, whereas spikes paradoxically decreased more within the suspected epileptogenic area. Six patients showing burst‐suppression had lower rates of ripples than six other patients with continuous background activity (p = 0.02). No significant difference was found between burst suppression and continuous background activity in four patients, but there was a trend toward showing more ripples during continuous background activity (p = 0.16). Significance: Propofol, known for its antiepileptic effects, reduces the number of epileptic HFOs, but has no effect on spikes. This enforces the hypothesis that, in epilepsy, HFOs mirror the disease activity and HFOs might be useful for monitoring antiepileptic drug treatment. It is feasible to record HFOs during surgery, but propofol infusion should be interrupted for some minutes to improve detection.     

Interictal high-frequency oscillations (80-500 Hz) are an indicator of seizure onset areas independent of spikes in the human epileptic brain

Purpose: High‐frequency oscillations (HFOs) known as ripples (80–250 Hz) and fast ripples (250–500 Hz) can be recorded from macroelectrodes inserted in patients with intractable focal epilepsy. They are most likely linked to epileptogenesis and have been found in the seizure onset zone (SOZ) of human ictal and interictal recordings. HFOs occur frequently at the time of interictal spikes, but were also found independently. This study analyses the relationship between spikes and HFOs and the occurrence of HFOs in nonspiking channels. Methods: Intracerebral EEGs of 10 patients with intractable focal epilepsy were studied using macroelectrodes. Rates of HFOs within and outside spikes, the overlap between events, event durations, and the percentage of spikes carrying HFOs were calculated and compared according to anatomical localization, spiking activity, and relationship to the SOZ. Results: HFOs were found in all patients, significantly more within mesial temporal lobe structures than in neocortex. HFOs could be seen in spiking as well as nonspiking channels in all structures. Rates and durations of HFOs were significantly higher in the SOZ than outside. It was possible to establish a rate of HFOs to identify the SOZ with better sensitivity and specificity than with the rate of spikes. Discussion: HFOs occurred to a large extent independently of spikes. They are most frequent in mesial temporal structures. They are prominent in the SOZ and provide additional information on epileptogenicity independently of spikes. It was possible to identify the SOZ with a high specificity by looking at only 10 min of HFO activity.     

Bicuculline‐ and neurosteroid‐sensitive tonic chloride current in rat hypoglossal motoneurons and atypical dual effect of SR95531

Hypoglossal motoneurons (HMs) are known to be under ‘permanent’ bicuculline‐sensitive inhibition and to show ‘transient’ synaptic γ‐aminobutyric acid (GABA)_A and glycine inhibitory responses. The present paper describes a permanent bicuculline‐sensitive current that should contribute to their tonic inhibition. This current was recorded in brainstem slices superfused without any exogenous agonist and remained detectable with tetrodotoxin. It could also be blocked by the other GABA_A antagonists picrotoxin (PTX) and 2‐(3‐carboxypropyl)‐3‐amino‐6‐(4 methoxyphenyl)pyridazinium bromide) (SR95531; gabazine), but persisted in the presence of a specific blocker of α5‐containing GABA_A receptors. Addition of 2 μm 4,5,6,7‐tetrahydroisoxazolo[5,4‐c]pyridin‐3‐ol hydrochloride (THIP), known to preferentially activate GABA_A receptors devoid of a γ‐subunit, induced a sustained anionic current that could be further enhanced by neurosteroids such as allopregnanolone (100 nm ). Thus, HMs show a tonic inhibitory current carried by extrasynaptic γ‐free GABA_A receptors, highly sensitive to neurosteroids. A second result was obtained by using SR95531 at concentrations sufficiently high to rapidly block the tonic current above the chloride equilibrium potential (E_Cl). Surprisingly, below E_Cl, SR95531 (10–40 μm ) activated a sustained inward current, associated with a conductance increase, and resistant to bicuculline or PTX (100 μm ). Similarly, after blockade of the bicuculline‐sensitive current, SR95531 activated an outward current above E_Cl. The bicuculline‐resistant anionic current activated by SR95531 could be blocked by a GABA_C receptor antagonist. Thus, two types of inhibitory GABA receptors, belonging to the GABA_A and GABA_C families, are able to show a sustained activity in HMs and provide promising targets for neuroprotection under overexcitatory situations known to easily damage these particularly fragile neurons.     

High frequency oscillations (80–500 Hz) in the preictal period in patients with focal seizures

Purpose:   Intracranial depth macroelectrode recordings from patients with focal seizures demonstrate interictal and ictal high frequency oscillations (HFOs, 80–500 Hz). These HFOs are more frequent in the seizure-onset zone (SOZ) and reported to be linked to seizure genesis. We evaluated whether HFO activity changes in a systematic way during the preictal period.
Methods:   Fifteen minutes of preictal intracranial electroencephalography (EEG) recordings were evaluated in seven consecutive patients with well-defined SOZ. EEG was filtered at 500 Hz and sampled at 2,000 Hz. Ripples (80–250 Hz) and fast ripples (250–500 Hz) were visually marked, and spectral analysis was performed in seizure-onset as well as nonseizure-onset channels. Linear regressions fitted to the power trends corresponding to intervals of 1, 5, and 15 min before the seizure onset was calculated.
Results:   Total rates of HFOs were significantly higher in the SOZ than outside. Preictal increases and decreases in HFO rates and band power could be detected in all patients, and they were not limited to the SOZs. These measures were very variable, and no systematic trends were observed when comparing patients or seizures in the same patient.
Discussion:   High frequencies in the range of 80–500 Hz are present during the preictal period and are more prominent in the SOZ. They do not change in a systematic way before seizure onset for the horizons we tested. The 80–500 Hz band may be used for the localization of seizure-onset areas but may be more difficult to use for seizure prediction purposes.     

Prospective observational study: Fast ripple localization delineates the epileptogenic zone

ObjectiveTo investigate spatial correlation between interictal HFOs and neuroimaging abnormalities, and to determine if complete removal of prospectively identified interictal HFOs correlates with post-surgical seizure-freedom.MethodsInterictal fast ripples (FRs: 250–500 Hz) in 19 consecutive children with pharmacoresistant focal epilepsy who underwent extra-operative electrocorticography (ECoG) recording were prospectively analyzed. The interictal FRs were sampled at 2000 Hz and were visually identified during 10 min of slow wave sleep. Interictal FRs, MRI and FDG-PET were delineated on patient-specific reconstructed three-dimensional brain MRI.ResultsInterictal FRs were observed in all patients except one. Thirteen out of 18 patients (72%) exhibited FRs beyond the extent of neuroimaging abnormalities. Fifteen of 19 children underwent resective surgery, and survival analysis with log-rank test demonstrated that complete resection of cortical sites showing interictal FRs correlated with longer post-operative seizure-freedom (p < 0.01). Complete resection of seizure onset zones (SOZ) also correlated with longer post-operative seizure-freedom (p = 0.01), yet complete resection of neuroimaging abnormalities did not (p = 0.43).ConclusionsProspective visual analysis of interictal FRs was feasible, and it seemed to accurately localize epileptogenic zones.SignificanceTopological extent of epileptogenic region may exceed what is discernible by multimodal neuroimaging.     

Value of electrical stimulation and high frequency oscillations (80–500 Hz) in identifying epileptogenic areas during intracranial EEG recordings

Purpose: Electrical stimulation (ES) is used during intracranial electroencephalography (EEG) investigations to delineate epileptogenic areas and seizure‐onset zones (SOZs) by provoking afterdischarges (ADs) or patients’ typical seizure. High frequency oscillations (HFOs—ripples, 80–250 Hz; fast ripples, 250–500 Hz) are linked to seizure onset. This study investigates whether interictal HFOs are more frequent in areas with a low threshold to provoke ADs or seizures. Methods: Intracranial EEG studies were filtered at 500 Hz and sampled at 2,000 Hz. HFOs were visually identified. Twenty patients underwent ES, with gradually increasing currents. Results were interpreted as agreeing or disagreeing with the intracranial study (clinical‐EEG seizure onset defined the SOZ). Current thresholds provoking an AD or seizure were correlated with the rate of HFOs of each channel. Results: ES provoked a seizure in 12 and ADs in 19 patients. Sixteen patients showed an ES response inside the SOZ, and 10 had additional areas with ADs. The response was more specific for mesiotemporal than for neocortical channels. HFO rates were negatively correlated with thresholds for ES responses; especially in neocortical regions; areas with low threshold and high HFO rate were colocalized even outside the SOZ. Discussion: Areas showing epileptic HFOs colocalize with those reacting to ES. HFOs may represent a pathologic correlate of regions showing an ES response; both phenomena suggest a more widespread epileptogenicity.     

High frequency oscillations in intra-operative electrocorticography before and after epilepsy surgery

ObjectiveRemoval of brain tissue showing high frequency oscillations (HFOs; ripples: 80–250 Hz and fast ripples: 250–500 Hz) in preresection electrocorticography (preECoG) in epilepsy patients seems a predictor of good surgical outcome. We analyzed occurrence and localization of HFOs in intra-operative preECoG and postresection electrocorticography (postECoG).MethodsHFOs were automatically detected in one-minute epochs of intra-operative ECoG sampled at 2048 Hz of fourteen patients. Ripple, fast ripple, spike, ripples on a spike (RoS) and not on a spike (RnoS) rates were analyzed in pre- and postECoG for resected and nonresected electrodes.ResultsRipple, spike and fast ripple rates decreased after resection. RnoS decreased less than RoS (74% vs. 83%; p = 0.01). Most fast ripples in preECoG were located in resected tissue. PostECoG fast ripples occurred in one patient with poor outcome. Patients with good outcome had relatively high postECoG RnoS rates, specifically in the sensorimotor cortex.ConclusionsOur observations show that fast ripples in intra-operative ECoG, compared to ripples, may be a better biomarker for epileptogenicity. Further studies have to determine the relation between resection of epileptogenic tissue and physiological ripples generated by the sensorimotor cortex.SignificanceFast ripples in intra-operative ECoG can help identify the epileptogenic zone, while ripples might also be physiological.     

Continuous high-frequency activity in mesial temporal lobe structures

Purpose: Many recent studies have reported the importance of high‐frequency oscillations (HFOs) in the intracerebral electroencephalography (EEG) of patients with epilepsy. These HFOs have been defined as events that stand out from the background. We have noticed that this background often consists itself of high‐frequency rhythmic activity. The purpose of this study is to perform a first evaluation of the characteristics of high‐frequency continuous or semicontinuous background activity. Methods: Because the continuous high‐frequency pattern was noted mainly in mesial temporal structures, we reviewed the EEG studies from these structures in 24 unselected patients with electrodes implanted in these regions. Sections of background away from interictal spikes were marked visually during periods of slow‐wave sleep and wakefulness. They were then high‐passed filtered at 80 Hz and categorized as having high‐frequency rhythmic activity in one of three patterns: continuous/semicontinuous, irregular, sporadic. Wavelet entropy, which measures the degree of rhythmicity of a signal, was calculated for the marked background sections. Key Findings: Ninety‐six bipolar channels were analyzed. The continuous/semicontinuous pattern was found frequently (29/96 channels during wake and 34/96 during sleep). The different patterns were consistent between sleep and wakefulness. The continuous/semicontinuous pattern was found significantly more often in the hippocampus than in the parahippocampal gyrus and was rarely found in the amygdala. The types of pattern were not influenced by whether a channel was within the seizure‐onset zone, or whether it was a lesional channel. The continuous/semicontinuous pattern was associated with a higher frequency of spikes and with high rates of ripples and fast ripples. Significance: It appears that high‐frequency activity (above 80 Hz) does not appear only in the form of brief paroxysmal events but also in the form of continuous rhythmic activity or very long bursts. In this study limited to mesial temporal structures, we found a clear anatomic preference for the hippocampus. Although associated with spikes and with distinct HFOs, this pattern was not clearly associated with the seizure‐onset zone. Future studies will need to evaluate systematically the presence of this pattern, as it may have a pathophysiologic significance and it will also have an important influence on the very definition of HFOs.     

Increased fast ripple to ripple ratios correlate with reduced hippocampal volumes and neuron loss in temporal lobe epilepsy patients

PURPOSE: To determine whether hippocampal sclerosis might form an anatomical substrate for pathological high-frequency oscillations in patients with temporal lobe epilepsy (TLE). METHODS: Intracerebral wide bandwidth electroencephalogram was recorded in patients with medically intractable complex partial seizures. A computer-automated program detected interictal normal ripples (80-150 Hz) and pathologic fast ripples (FR, 151-500 Hz) from microelectrodes within hippocampus, entorhinal, and subicular cortices. Hippocampal MRI volumetric analysis and cell density measurements were correlated with rates of FR and ripple discharge. RESULTS: In all 13 patients, higher rates of FR (p = 0.03) and ratios of FR to ripple discharges (p = 0.02) were observed in sites ipsilateral to seizure onset compared with rates within contralateral non-ictal sites. Higher ratios of FR to ripple discharge were associated with smaller ipsilateral hippocampal volumes (p = 0.02) and lower fascia dentata (FD; p = 0.02) and Ammon's horn (p = 0.0005) neuron densities. While reduced FD and Ammon's horn neuron densities correlated with higher ratios of discharges, stepwise multiple regression analysis revealed that decreased neuron densities within CA1 and prosubiculum regions most strongly predicted ratios of FR to ripples (r(2)= 0.78, p = 0.008). CONCLUSIONS: In surgical patients with TLE, higher ratios of FR to ripple discharges are associated with histopathologic changes found in hippocampal sclerosis. These findings support the hypothesis that pathological alterations linked with hippocampal cell loss and synaptic reorganization promote FR and reduce ripple generation.     

Scalp‐recorded high‐frequency oscillations in childhood sleep‐induced electrical status epilepticus

Because high‐frequency oscillations (HFOs) may affect normal brain functions, we examined them using electroencephalography (EEG) in epilepsy with continuous spike‐waves during slow‐wave sleep (CSWS), a condition that can cause neuropsychological regression. In 10 children between 6 and 9 years of age with epilepsy with CSWS or related disorders, we investigated HFOs in scalp EEG spikes during slow‐wave sleep through temporal expansion of the EEG traces with a low‐cut frequency filter at 70 Hz as well as through time‐frequency power spectral analysis. HFOs (ripples) concurrent with spikes were detected in the temporally expanded traces, and the frequency of the high‐frequency peak with the greatest power in each patient’s spectra ranged from 97.7 to 140.6 Hz. This is the first report on the detection of HFOs from scalp EEG recordings in epileptic patients. We speculate that epileptic HFOs may interfere with higher brain functions in epilepsy with CSWS.