Advances in understanding the process of epileptogenesis based on patient material: what can the patient tell us?

Epilepsy as a dynamical system, a most needed paradigm shift in epileptology

The idea of the epileptic brain being highly excitable and facilitated to synchronic activity has guided pharmacological treatment since the early twentieth century. Although tackling epilepsy's seizure-prone feature, by tonically modifying overall circuit excitability and/or connectivity, the last 50 years of drug development has not seen a substantial improvement in seizure suppression of refractory epilepsies. This review presents a new conceptual framework for epilepsy in which the temporal dynamics of the disease plays a more critical role in both its understanding and therapeutic strategies. The repetitive epileptiform pattern (characteristic during ictal activity) and other well-defined electrographic signatures (i.e., present during the interictal period) are discussed in terms of the sequential activation of the circuit motifs. Lessons learned from the physiological activation of neural circuitry are used to further corroborate the argument and explore the transition from proper function to a state of instability. Furthermore, the review explores how interfering in the temporally dependent abnormal connectivity between circuits may work as a therapeutic approach. We also review the use of probing stimulation to access network connectivity and evaluate its power to determine transitional states of the dynamical system as it moves towards regions of instability, especially when conventional electrographic monitoring is proven inefficient. Unorthodox cases, with little or no scalp electrographic correlate, in which ictogenic circuitry and/or seizure spread is temporally restricted to neurovegetative, cognitive, and motivational areas are shown as possible explanations for sudden death in epilepsy (SUDEP) and other psychiatric comorbidities. In short, this review presents a paradigm shift in the way that we address the disease and is aimed to encourage debate rather than narrow the rationale epilepsy is currently engaged in. This article is part of the Special Issue "NEWroscience 2018".

Downregulation of hyperpolarization-activated cyclic nucleotide-gated channels (HCN) in the hippocampus of patients with medial temporal lobe epilepsy and hippocampal sclerosis (MTLE-HS)

Changes in the expression of HCN ion channels leading to changes in I_h function and neuronal excitability are considered to be possible mechanisms involved in epileptogenesis in kinds of human epilepsy. In previous animal studies of febrile seizures and temporal lobe epilepsy, changes in the expression of HCN1 and HCN2 channels at different time points and in different parts of the brain were not consistent, suggesting that transcriptional disorders involving HCNs play a crucial role in the epileptogenic process. Therefore, we aimed to assess the transcriptional regulation of HCN channels in Medial temporal lobe epilepsy with hippocampal sclerosis (MTLE-HS) patients. This study included eight nonhippocampal sclerosis patients and 40 MTLE-HS patients. The mRNA expression of HCN channels was evaluated by qRT-PCR, while the protein expression was quantitatively analyzed by Western blotting. The subcellular localization of HCN channels in the hippocampus was explored by immunofluorescence. We demonstrated that the mRNA and protein expression of HCN1 and HCN2 are downregulated in controls compared to that in MTLE-HS patients. In the hippocampal CA1/CA4 subregion and GCL, in addition to a large decrease in neurons, the expression of HCN1 and HCN2 on neuronal cell membranes was also downregulated in MTLE-HS patients. These findings suggest that the expression of HCN channels are downregulated in MTLE-HS, which indicates that the decline in HCN channels in the hippocampus during chronic epilepsy in MTLE-HS patients leads to the downregulation of I_h current density and function, thereby reducing the inhibitory effect and increasing neuronal excitability and eventually causing disturbances in the electrical activity of neurons.

Assessing Epileptogenicity Using Phase-Locked High Frequency Oscillations: A Systematic Comparison of Methods

High frequency oscillations (HFOs) are traditional biomarkers to identify the epileptogenic tissue during presurgical evaluation in pharmacoresistant epileptic patients. Recently, the resection of brain tissue exhibiting coupling between the amplitude of HFOs and the phase of low frequencies demonstrated a more favorable surgical outcome. Here we compare the predictive value of ictal HFOs and four methods for quantifying the ictal phase-amplitude coupling, namely mean vector length, phase-locked high gamma, phase locking value, and modulation index (MI). We analyzed 32 seizures from 16 patients to identify the channels that exhibit HFOs and phase-locked HFOs during seizures. We compared the resection ratio, defined as the percentage of channels exhibiting coupling located in the resected tissue, with the postsurgical outcome. We found that the MI is the only method to show a significant difference between the resection ratios of patients with good and poor outcomes. We further show that the whole seizure, not only the onset, is critical to assess epileptogenicity using the phase-locked HFOs. We postulate that the superiority of MI stems from its capacity to assess coupling of discrete HFO events and its independence from the HFO power. These results confirm that quantitative analysis of HFOs can boost presurgical evaluation and indicate the paramount importance of algorithm selection for clinical applications.

Automated epileptic seizure detection based on break of excitation/inhibition balance

Physiological models are attractive for seizure detection, as their parameters are related to physiological meanings. We propose an algorithm to early detect epileptic seizures based on automatic estimation of average synaptic gains (excitatory Ae, slow and fast inhibitory B and G) by combining clinical data with a neural mass model. Three indices (Ae/B, Ae/G and Ae/(B + G)), all related to excitation/inhibition balance, were calculated and used as cues to detect seizures. A simple thresholding method was employed. We evaluated the algorithm against the manual scoring of a human expert on intracranial EEG samples from 23 patients suffering from different types of epilepsy. Best performance was achieved using Ae/(B + G) as a cue, i.e. excitation/(slow + fast) inhibition, on temporal lobe epilepsy (TLE) patients. A leave-one-out cross-validation showed that the algorithm achieved 92.98% sensitivity for TLE patients. The median false positive rate was 0.16 per hour, and median detection delay was 14.5 s. Of interest, the threshold values determined by a leave-one-out cross-validation did nearly not vary among TLE patients, suggesting a general excitation/inhibition balance baseline in TLE patients. The same approach could be used with other types of epilepsy by adapting the neural mass model to these types.

Seizure evolution can be characterized as path through synaptic gain space of a neural mass model

Physiologically based models could facilitate better understanding of mechanisms underlying epileptic seizures. In this paper, we attempt to reveal the dynamic evolution of intracranial EEG activity during epileptic seizures based on synaptic gain identification procedure of a neural mass model. The distribution of average excitatory, slow and fast inhibitory synaptic gain in the parameter space and their temporal evolution, i.e., the path through the model parameter space, were analyzed in thirty seizures from ten temporal lobe epileptic patients. Results showed that the synaptic gain values located roughly on a plane before seizure onset, dispersed during seizure and returned to the plane when seizure terminated. Cluster analysis was performed on seizure paths and demonstrated consistency in synaptic gain evolution across different seizures from the individual patient. Furthermore, two patient groups were identified, each one corresponding to a specific synaptic gain evolution in the parameter space during a seizure. Results were validated by a bootstrapping approach based on comparison with random paths. The differences in the path revealed variations in EEG dynamics for patients despite showing identical seizure onset pattern. Our approach may have the potential to classify the epileptic patients into subgroups based on different mechanisms revealed by subtle changes in synaptic gains and further enable more robust decisions regarding treatment strategy.

Dynamics underlying interictal to ictal transition in temporal lobe epilepsy: insights from a neural mass model

We propose an approach that combines a neural mass model and clinical intracranial electroencephalographic (iEEG) recordings to explore the potential pathophysiological mechanisms (at the neuronal population level) of ictogenesis. Thirty iEEG recordings from 10 temporal lobe epilepsy (TLE) patients around seizure onset were investigated. Physiologically meaningful parameters [average excitatory (A_e ), slow (B), and fast (G) inhibitory synaptic gain] were identified during interictal to ictal transition. Four ratios (A_e /G, A_e /B, A_e /(B + G), and B/G) were derived from these parameters, and their evolution over time was analyzed. The excitation/inhibition ratio increased around seizure onset and decreased before seizure offset, indicating the impairment and re-emergence of excitation/inhibition balance around seizure onset and before seizure offset, respectively. Moreover, the slow inhibition may have an earlier effect on excitation/inhibition imbalance. We confirm the decrease in excitation/inhibition ratio upon seizure termination in human temporal lobe epilepsy, as revealed by optogenetic approaches both in vivo in animal models and in vitro. The increase in excitation/inhibition ratio around seizure occurrence could be an indicator to detect seizures.

MicroRNA expression profiles in chronic epilepsy rats and neuroprotection from seizures by targeting miR-344a

MicroRNA (miRNA) is believed to play a crucial role in the cause and treatment of epilepsy by controlling gene expression. However, it is still unclear how miRNA profiles change after multiple prolonged seizures and aggravation of brain injury in chronic epilepsy (CE). To investigate the role of miRNA in epilepsy, we utilized the CE rat models with pentylenetetrazol (PTZ) and miRNA profiles in the hippocampus. miRNA profiles were characterized using miRNA microarray analysis and were compared with the rats in the sham group, which received 0.9% physiological saline treatment at the same dose. Four up-regulated miRNAs (miR-139-3p, -770-5p, -127-5p, -331-3p) and 5 down-regulated miRNAs (miR-802-5p, -380-5p, -183-5p, -547-5p, -344a/-344a-5p) were found in the CE rats (fold change >1.5, P<0.05). Three of the dysregulated miRNAs were validated by quantitative real-time polymerase chain reaction, which revealed an outcome consistent with the initial results of the miRNA microarray analyses. Then, miR-344a agomir was intracerebroventricularly injected and followed by PTZ induction of CE models to investigate the effect of miR-344a in chronic neocortical epileptogenesis. After miRNA-344a agomir and scramble treatment, results showed a restoration of seizure behavior and a reduction in neuron damage in the cortex in miRNA-334a agomir treated rats. These data suggest that miRNA-344a might have a small modulatory effect on seizure-induced apoptosis signaling pathways in the cortex.

High-frequency Oscillations and the Seizure Onset Zones in Neocortical Epilepsy

Background:

To study the characters of high-frequency oscillations (HFOs) in the seizure onset zones (SOZ) and the nonseizure onset zones (NSOZ) in the electrocorticography (ECoG) of patients with neocortical epilepsy.

Methods:

Only patients with neocortical epilepsy who were seizure-free after surgery as determined with ECoG were included. We selected patients with normal magnetic resonance imaging before surgery in order to avoid the influence of HFOs by other lesions. Three minutes preictal and 10 min interictal ECoG as recorded in 39 channels in the SOZ and 256 channels in the NSOZ were analyzed. Ripples and fast ripples (FRs) were analyzed by Advanced Source Analysis software (ASA, The Netherlands). Average duration of HFOs was analyzed in SOZ and NSOZ separately.

Results:

For ripples, the permillage time occupied by HFOs was 0.83 in NSOZ and 1.17 in SOZ during the interictal period. During preictal period, they were 2.02 in NSOZ and 7.93 in SOZ. For FRs, the permillage time occupied by HFOs was 0.02 in NSOZ and 0.42 in SOZ during the interictal period. During preictal period, they were 0.03 in NSOZ and 2 in SOZ.

Conclusions:

High-frequency oscillations are linked to SOZ in neocortical epilepsy. Our study demonstrates the prevalent occurrence of HFOs in SOZ. More and more burst of HFOs, especially FRs, means the onset of seizures.

Curcumin supplementation improves mitochondrial and behavioral deficits in experimental model of chronic epilepsy

The present study was aimed to investigate the potential beneficial effect of curcumin, a polyphenol with pleiotropic properties, on mitochondrial dysfunctions, oxidative stress and cognitive deficits in a kindled model of epilepsy. Kindled epilepsy was induced in rats by administering a sub-convulsive dose of pentylenetetrazole (PTZ, 40 mg/kg body weight) every alternate day for 30 days. PTZ administered rats exhibited marked cognitive deficits assessed using active and passive avoidance tasks. This was accompanied by a significant decrease in NADH:cytochrome-c reductase (complex I) and cytochrome-c oxidase (complex IV) activities along with an increase in ROS, lipid peroxidation and protein carbonyls. The levels of glutathione also decreased in the cortex and hippocampus. Electron micrographs revealed disruption of mitochondrial membrane integrity with distorted cristae in PTZ treated animals. Histopathological examination showed pyknotic nuclei and cell loss in the hippocampus as well as in the cortex of PTZ treated animals. Curcumin administration at a dose of 100 mg/kg, p.o. throughout the treatment paradigm was able to ameliorate cognitive deficits with no significant effect on seizure score. Curcumin was able to restore the activity of mitochondrial complexes. In addition, significant reduction in ROS generation, lipid peroxidation and protein carbonyls was observed in PTZ animals supplemented with curcumin. Moreover, glutathione levels were also restored in PTZ treated rats supplemented with curcumin. Curcumin protected mitochondria from seizure induced structural alterations. Further, the curcumin supplemented PTZ rats had normal cell morphology and reduced cell loss. These results suggest that curcumin supplementation has potential to prevent mitochondrial dysfunctions and oxidative stress with improved cognitive functions in a chronic model of epilepsy.

High frequency oscillations and infraslow activity in epilepsy

In pre-surgical evaluation of epilepsy, there has been an increased interest in the study of electroencephalogram (EEG) activity outside the 1-70 Hz band of conventional frequency activity (CFA). Research over the last couple of decades has shown that EEG activity in the 70-600 Hz range, termed high frequency oscillations (HFOs), can be recorded intracranially from all brain regions both interictally and at seizure onset. In patients with epilepsy, HFOs are now considered as pathologic regardless of their frequency band although it may be difficult to distinguish them from the physiologic HFOs, which occur in a similar frequency range. Interictal HFOs are likely to be confined mostly to the seizure onset zone, thus providing a new measure for localizing it. More importantly, several studies have linked HFOs to underlying epileptogenicity, suggesting that HFOs can serve as potential biomarkers for the illness. Along with HFOs, analysis of ictal baseline shifts (IBS; or direct current shifts) and infraslow activity (ISA) (ISA: <0.1 Hz) has also attracted attention. Studies have shown that: IBSs can be recorded using the routine AC amplifiers with long time constants; IBSs occur at the time of conventional EEG onset, but in a restricted spatial distribution compared with conventional frequencies; and inclusion of IBS contacts in the resection can be associated with favorable seizure outcome. Only a handful of studies have evaluated all the EEG frequencies together in the same patient group. The latter studies suggest that the seizure onset is best localized by the ictal HFOs, the IBSs tend to provide a broader localization and the conventional frequencies could be non-localizing. However, small number of patients included in these studies precludes definitive conclusions regarding post-operative seizure outcome based on selective or combined resection of HFO, IBS and CFA contacts. Large, preferably prospective, studies are needed to further evaluate the implications of different EEG frequencies in epilepsy.

Role of oxidative stress in refractory epilepsy: evidence in patients and experimental models

Oxidative stress, a state of imbalance in the production of reactive oxygen species and nitrogen, is induced by a wide variety of factors. This biochemical state is associated with systemic diseases, and diseases affecting the central nervous system. Epilepsy is a chronic neurological disorder with refractoriness to drug therapy at about 30%. Currently, experimental evidence supports the involvement of oxidative stress in seizures, in the process of their generation, and in the mechanisms associated with refractoriness to drug therapy. Hence, the aim of this review is to present information in order to facilitate the handling of this evidence and determine the therapeutic impact of the biochemical status for this pathology.

High-frequency activity in experimental and clinical epileptic foci

Pathological high-frequency electrographic activity (pHFA, >80Hz) represents one of the major discoveries in epilepsy research over the past few decades. In this review we focus on the high-frequency activity recorded in vivo in chronic models of epilepsy. The presence of HFA particularly of fast ripples (250-600Hz)reflects epileptogenic reorganization of brain tissue, endogenous epileptogenicity and ability to generate spontaneous seizures. The spatial distribution of epileptic HFA can be used to localize epileptic foci. In some regions of brain the localizing value of epileptic HFA is weakened by frequency overlap with physiological HFA. In this situation, only detailed knowledge of the regional physiological activity may provide relevant information which frequencies provide localizing information. In the epileptic hippocampus, the activity from 250Hz to 600Hz frequency band (fast ripples) is always epileptic and can be used as reliable marker of epileptic tissue in all hippocampal subregions. The localizing value of HFA in the identification of the epileptic focus is discussed from an experimental and clinical perspective; as the information provided by HFA can improve presurgical diagnosis and surgical outcome. Finally, research into HFA has contributed to improved understanding and new insights into the cellular and network organization of epileptic foci and the pathophysiology of epilepsy.

Upregulation of adenosine kinase in astrocytes in experimental and human temporal lobe epilepsy

PURPOSE

Adenosine kinase (ADK) represents the key metabolic enzyme for the regulation of extracellular adenosine levels in the brain. In adult brain, ADK is primarily present in astrocytes. Several lines of experimental evidence support a critical role of ADK in different types of brain injury associated with astrogliosis, which is also a prominent morphologic feature of temporal lobe epilepsy (TLE). We hypothesized that dysregulation of ADK is an ubiquitous pathologic hallmark of TLE.

METHODS

Using immunocytochemistry and Western blot analysis, we investigated ADK protein expression in a rat model of TLE during epileptogenesis and the chronic epileptic phase and compared those findings with tissue resected from TLE patients with mesial temporal sclerosis (MTS).

KEY FINDINGS

In rat control hippocampus and cortex, a low baseline expression of ADK was found with mainly nuclear localization. One week after the electrical induction of status epilepticus (SE), prominent up-regulation of ADK became evident in astrocytes with a characteristic cytoplasmic localization. This increase in ADK persisted at least for 3-4 months after SE in rats developing a progressive form of epilepsy. In line with the findings from the rat model, expression of astrocytic ADK was also found to be increased in the hippocampus and temporal cortex of patients with TLE. In addition, in vitro experiments in human astrocyte cultures showed that ADK expression was increased by several proinflammatory molecules (interleukin-1β and lipopolysaccharide).

SIGNIFICANCE

These results suggest that dysregulation of ADK in astrocytes is a common pathologic hallmark of TLE. Moreover, in vitro data suggest the existence of an additional layer of modulatory crosstalk between the astrocyte-based adenosine cycle and inflammation. Whether this interaction also can play a role in vivo needs to be further investigated.

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.

A review of current applications of mass spectrometry for neuroproteomics in epilepsy

The brain is unquestionably the most fascinating organ, and the hippocampus is crucial in memory storage and retrieval and plays an important role in stress response. In temporal lobe epilepsy (TLE), the seizure origin typically involves the hippocampal formation. Despite tremendous progress, current knowledge falls short of being able to explain its function. An emerging approach toward an improved understanding of the complex molecular mechanisms that underlie functions of the brain and hippocampus is neuroproteomics. Mass spectrometry has been widely used to analyze biological samples, and has evolved into an indispensable tool for proteomics research. In this review, we present a general overview of the application of mass spectrometry in proteomics, summarize neuroproteomics and systems biology-based discovery of protein biomarkers for epilepsy, discuss the methodology needed to explore the epileptic hippocampus proteome, and also focus on applications of ingenuity pathway analysis (IPA) in disease research. This neuroproteomics survey presents a framework for large-scale protein research in epilepsy that can be applied for immediate epileptic biomarker discovery and the far-reaching systems biology understanding of the protein regulatory networks. Ultimately, knowledge attained through neuroproteomics could lead to clinical diagnostics and therapeutics to lessen the burden of epilepsy on society.

Decreased neuronal differentiation of newly generated cells underlies reduced hippocampal neurogenesis in chronic temporal lobe epilepsy

Hippocampal neurogenesis declines substantially in chronic temporal lobe epilepsy (TLE). However, it is unclear whether this decline is linked to altered production of new cells and/or diminished survival and neuronal fate-choice decision of newly born cells. We quantified different components of hippocampal neurogenesis in rats exhibiting chronic TLE. Through intraperitoneal administration of 5'-bromodeoxyuridine (BrdU) for 12 days, we measured numbers of newly born cells in the subgranular zone-granule cell layer (SGZ-GCL) at 24 h and 2.5 months post-BrdU administration. Furthermore, the differentiation of newly added cells into neurons and glia was quantified via dual immunofluorescence for BrdU and various markers of neurons and glia. Addition of new cells to the SGZ-GCL over 12 days was comparable between the chronically epileptic hippocampus and the age-matched intact hippocampus. Furthermore, comparison of BrdU+ cells measured at 24 h and 2.5 months post-BrdU administration revealed similar survival of newly born cells between the two groups. However, only 4-5% of newly born cells (i.e., BrdU+ cells) differentiated into neurons in the chronically epileptic hippocampus, in comparison to 73-80% of such cells exhibiting neuronal differentiation in the intact hippocampus. Moreover, differentiation of newly born cells into S-100beta+ astrocytes or NG2+ oligodendrocyte progenitors increased to approximately 79% in the chronically epileptic hippocampus from approximately 25% observed in the intact hippocampus. Interestingly, the extent of proliferation of astrocytes and microglia (identified through Ki-67 and S-100beta and Ki-67 and OX-42 dual immunofluorescence) in the SGZ-GCL was similar between the chronically epileptic hippocampus and the age-matched intact hippocampus, implying that the proliferation of neural stem/progenitor cells in the SGZ-GCL of the chronically epileptic hippocampus was not obscured by an increased division of glia. Thus, severely diminished DG neurogenesis in chronic TLE is not associated with either decreased production of new cells or reduced survival of newly born cells in the SGZ-GCL. Rather, it is linked to a dramatic decline in the neuronal fate-choice decision of newly generated cells. Overall, the differentiation of newly born cells turns mainly into glia with chronic TLE from predominantly neuronal differentiation seen in control conditions.

Synaptic noise and physiological coupling generate high-frequency oscillations in a hippocampal computational model

There is great interest in the role of coherent oscillations in the brain. In some cases, high-frequency oscillations (HFOs) are integral to normal brain function, whereas at other times they are implicated as markers of epileptic tissue. Mechanisms underlying HFO generation, especially in abnormal tissue, are not well understood. Using a physiological computer model of hippocampus, we investigate random synaptic activity (noise) as a potential initiator of HFOs. We explore parameters necessary to produce these oscillations and quantify the response using the tools of stochastic resonance (SR) and coherence resonance (CR). As predicted by SR, when noise was added to the network the model was able to detect a subthreshold periodic signal. Addition of basket cell interneurons produced two novel SR effects: 1) improved signal detection at low noise levels and 2) formation of coherent oscillations at high noise that were entrained to harmonics of the signal frequency. The periodic signal was then removed to study oscillations generated only by noise. The combined effects of network coupling and synaptic noise produced coherent, periodic oscillations within the network, an example of CR. Our results show that, under normal coupling conditions, synaptic noise was able to produce gamma (30-100 Hz) frequency oscillations. Synaptic noise generated HFOs in the ripple range (100-200 Hz) when the network had parameters similar to pathological findings in epilepsy: increased gap junctions or recurrent synaptic connections, loss of inhibitory interneurons such as basket cells, and increased synaptic noise. The model parameters that generated these effects are comparable with published experimental data. We propose that increased synaptic noise and physiological coupling mechanisms are sufficient to generate gamma oscillations and that pathologic changes in noise and coupling similar to those in epilepsy can produce abnormal ripples.

Effect of sleep stage on interictal high-frequency oscillations recorded from depth macroelectrodes in patients with focal epilepsy

PURPOSE

To investigate the effect of sleep stage on the properties of high-frequency oscillations (HFOs) recorded from depth macroelectrodes in patients with focal epilepsy.

METHODS

Ten-minute epochs of wakefulness (W), stage 1-2 non-REM (N1-N2), stage 3 non-REM (N3) and REM sleep (R) were identified from stereo-electroencephalography (SEEG) data recorded at 2 kHz in nine patients. Rates of spikes, ripples (>80 Hz), and fast ripples (>250 Hz) were calculated, as were HFO durations, degree of spike-HFO overlap, HFO rates inside and outside of spikes, and inside and outside of the seizure-onset zone (SOZ).

RESULTS

Ripples were observed in nine patients and fast ripples in eight. Spike rate was highest in N1-N2 in 5 of 9 patients, and in N3 in 4 of 9 patients, whereas ripple rate was highest in N1-N2 in 4 of 9 patients, in N3 in 4 of 9 patients, and in W in 1 of 9 patients. Fast ripple rate was highest in N1-N2 in 4 of 8 patients, and in N3 in 4 of 8 patients. HFO properties changed significantly with sleep stage, although the absolute effects were small. The difference in HFO rates inside and outside of the SOZ was highly significant (p < 0.000001) in all stages except for R and, for fast ripples, only marginally significant (p = 0.018) in W.

CONCLUSIONS

Rates of HFOs recorded from depth macroelectrodes are highest in non-REM sleep. HFO properties were similar in stages N1-N2 and N3, suggesting that accurate sleep staging is not necessary. The spatial specificity of HFO, particularly fast ripples, was affected by sleep stage, suggesting that recordings excluding REM sleep and wakefulness provide a more reliable indicator of the SOZ.

High-frequency oscillations: what is normal and what is not?

High-frequency oscillations (HFOs) in the 80-200 Hz range can be recorded from normal hippocampus and parahippocampal structures of humans and animals. They are believed to reflect inhibitory field potentials, which facilitate information transfer by synchronizing neuronal activity over long distances. HFOs in the range of 250-600 Hz (fast ripples, FRs) are pathologic and are readily recorded from hippocampus and parahippocampal structures of patients with mesial temporal lobe epilepsy, as well as rodent models of this disorder. These oscillations, and similar HFOs recorded from neocortex of patients, appear to identify brain tissue capable of spontaneous ictogenesis and are believed to reflect the neuronal substrates of epileptogenesis and epileptogenicity. The distinction between normal and pathologic HFOs (pHFOs), however, cannot be made on the basis of frequency alone, as oscillations in the FR frequency range can be recorded from some areas of normal neocortex, whereas oscillations in the ripple frequency range are present in epileptic dentate gyrus where normal ripples never occur and, therefore, appear to be pathologic. The suggestion that FRs may be harmonics of normal ripples is unlikely, because of their spatially distinct generators, and evidence that FRs reflect synchronized firing of abnormally bursting neurons rather than inhibitory field potentials. These synchronous population spikes, however, can fire at ripple frequencies, and their harmonics appear to give rise to FRs. Investigations into the fundamental neuronal processes responsible for pHFOs could provide insights into basic mechanisms of epilepsy. The potential for pHFOs to act as biomarkers for epileptogenesis and epileptogenicity is also discussed.

Implications of decreased hippocampal neurogenesis in chronic temporal lobe epilepsy

Temporal lobe epilepsy (TLE), characterized by spontaneous recurrent motor seizures (SRMS), learning and memory impairments, and depression, is associated with neurodegeneration, abnormal reorganization of the circuitry, and loss of functional inhibition in the hippocampal and extrahippocampal regions. Over the last decade, abnormal neurogenesis in the dentate gyrus (DG) has emerged as another hallmark of TLE. Increased DG neurogenesis and recruitment of newly born neurons into the epileptogenic hippocampal circuitry is a characteristic phenomenon occurring during the early phase after the initial precipitating injury such as status epilepticus. However, the chronic phase of the disease displays substantially declined DG neurogenesis, which is associated with SRMS, learning and memory impairments, and depression. This review focuses on DG neurogenesis in the chronic phase of TLE and first confers the extent and mechanisms of declined DG neurogenesis in chronic TLE. The available data on production, survival and neuronal fate choice decision of newly born cells, stability of hippocampal stem cell numbers, and changes in the hippocampal microenvironment in chronic TLE are considered. The next section discusses the possible contribution of declined DG neurogenesis to the pathophysiology of chronic TLE, which includes its potential effects on spontaneous recurrent seizures, cognitive dysfunction, and depression. The subsequent section considers strategies that may be useful for augmenting DG neurogenesis in chronic TLE, which encompass stem cell grafting, administration of distinct neurotrophic factors, physical exercise, exposure to enriched environment, and antidepressant therapy. The final section suggests possible ramifications of increasing the DG neurogenesis in chronic epilepsy.

Neuronal population oscillations of rat hippocampus during epileptic seizures

Neuronal population oscillations in the hippocampus have an important effect in the information processing in the brain and the generation of epileptic seizures. In this paper, we investigate the neuronal population oscillations in the hippocampus of epileptic rats in vivo using an empirical mode decomposition (EMD) method. A neuronal population oscillation can be decomposed into several relaxation oscillations, which possess a recovery and release phase, with the different frequencies that ranges from 0 to 600 Hz. The natures of relaxation oscillations at the pre-ictal, seizure onset and ictal states are distinctly different. The analysis of relaxation oscillations show that the gamma wave is a lead relaxation oscillation at the pre-ictal stage, then it moves to beta oscillation or theta oscillation while the ictal stage starts; the fast relaxation oscillations are associated with the slow relaxation oscillations in the CA1 or CA3, in particular, the fast relaxation oscillations are associated on the recovery phase of the slow relaxation oscillations during the pre-ictal interval, however move to the release phase of the slow relaxation oscillations during the ictal interval. Comparison of the relaxation oscillations in CA1 and CA3 shows that the neurons in the CA1 are more active during the epileptic seizures than during the pre-ictal stage. These findings demonstrate that this method is very helpful to decompose neuronal population for understanding the underlying mechanism of epileptic seizures.

Metabolic changes in temporal lobe structures measured by HR-MAS NMR at early stage of electrogenic rat epilepsy

The purpose of this study was to determine cerebral metabolic profile changes in response to electric stimulation to the right dorsal hippocampus (HPC) for the establishment of an epileptic rat model. Electroencephalogram measurements and behavioral results indicated that the experimental rats were in an early stage of epilepsy. Metabolites were determined by high-resolution magic-angle-spinning nuclear magnetic resonance (HR-MAS NMR) spectroscopy of the following intact brain tissue: bilateral hippocampi, entorhinal cortices (ECs), and temporal lobes (TLs). The NMR data was statistically analyzed using principal component analysis (PCA). Results demonstrated that metabolic profiles were significantly different between the experimental and sham rats in the bilateral hippocampi and the ipsilateral EC. Significant increases in total creatine in the ipsilateral HPC and alanine in the ipsilateral TL were measured (p<0.05). Some metabolite levels were disturbed in the bilateral HPC-EC loops. In the sham group, glutamate and choline concentrations were significantly higher or lower in the ipsilateral EC than bilateral hippocampi, respectively (p<0.01). However, such differences were not observed in the experimental group. In addition, N-acetylaspartate levels in the experimental group were significantly less in the ipsilateral HPC than in bilateral ECs (p<0.05). The level of myo-inositol in the ipsilateral EC significantly increased in the experimental group, compared to the contralateral EC (p<0.05). These results may provide metabolic information about temporal lobe structures to provide more knowledge about epileptic abnormalities at the early stage.

Developing cell transplantation for temporal lobe epilepsy

Mesial temporal lobe epilepsy (MTLE) is presumed to develop progressively as a consequence of synaptic reorganization and neuronal loss, although the exact etiology of seizure development is unknown. Nearly 30% of patients with MTLE have disabling seizures despite pharmacological treatment, and the majority of these patients are recommended for resection. The authors review cell transplantation as an alternative approach to the treatment of epilepsy. Recent work in animal models shows that grafted neuronal precursors that differentiate into inhibitory interneurons can increase the level of local inhibition. Grafts of these inhibitory neurons could help restore equilibrium in MTLE. Developing a sound transplantation strategy involves careful consideration of the etiology of MTLE and the expected functional role of transplanted cells. These issues are reviewed, with a focus on those factors most likely to influence clinically applicable results.

Closed head injury causes hyperexcitability in rat hippocampal CA1 but not in CA3 pyramidal cells

Traumatic brain injury frequently elicits epileptic seizures hours or days after the impact. The mechanisms on cellular level are poorly understood. Because posttraumatic epilepsy appears in many cases as a temporal-lobe epilepsy which originated the hippocampus, we studied trauma-induced hyperexcitability on the cellular level in this brain area. We used the model of closed head injury to analyse the electrophysiological changes in CA1 and CA3 pyramidal cells and in interneurones of the CA1 field, which is extremely sensitive to ischemia. We found that morphologically closed head injury (CHI) led to a gradual progressive, cell type specific time course in neuronal degeneration. To analyse electrophysiological impairment we measured resting membrane potential, recorded spontaneous action potentials and induced action potentials by current pulses at different times after CHI. We found a dramatic increase in the frequency of spontaneous action potentials of CA1 but not of CA3 pyramidal cells after CHI. This hyperexcitability was maximal at 2 h (4.5-fold higher than sham), was also observed at 24 h after CHI and disappeared after 3 days. We found that CA1 interneurones responded by a much weaker increase of AP frequency after CHI. We conclude that the strong hyperexcitability after CHI is cell-type specific and transient. The understanding of the complex neuronal interactions probably offers a promising possibility for pharmacological intervention to prevent posttraumatic epilepsy.

Cellular and network mechanisms of electrographic seizures

Epileptic seizures constitute a complex multiscale phenomenon that is characterized by synchronized hyperexcitation of neurons in neuronal networks. Recent progress in understanding pathological seizure dynamics provides crucial insights into underlying mechanisms and possible new avenues for the development of novel treatment modalities. Here we review some recent work that combines in vivo experiments and computational modeling to unravel the pathophysiology of seizures of cortical origin. We particularly focus on how activity-dependent changes in extracellular potassium concentration affects the intrinsic dynamics of neurons involved in cortical seizures characterized by spike/wave complexes and fast runs.

Studies of stimulus parameters for seizure disruption using neural network simulations

A large scale neural network simulation with realistic cortical architecture has been undertaken to investigate the effects of external electrical stimulation on the propagation and evolution of ongoing seizure activity. This is an effort to explore the parameter space of stimulation variables to uncover promising avenues of research for this therapeutic modality. The model consists of an approximately 800 mum x 800 mum region of simulated cortex, and includes seven neuron classes organized by cortical layer, inhibitory or excitatory properties, and electrophysiological characteristics. The cell dynamics are governed by a modified version of the Hodgkin-Huxley equations in single compartment format. Axonal connections are patterned after histological data and published models of local cortical wiring. Stimulation induced action potentials take place at the axon initial segments, according to threshold requirements on the applied electric field distribution. Stimulation induced action potentials in horizontal axonal branches are also separately simulated. The calculations are performed on a 16 node distributed 32-bit processor system. Clear differences in seizure evolution are presented for stimulated versus the undisturbed rhythmic activity. Data is provided for frequency dependent stimulation effects demonstrating a plateau effect of stimulation efficacy as the applied frequency is increased from 60 to 200 Hz. Timing of the stimulation with respect to the underlying rhythmic activity demonstrates a phase dependent sensitivity. Electrode height and position effects are also presented. Using a dipole stimulation electrode arrangement, clear orientation effects of the dipole with respect to the model connectivity is also demonstrated. A sensitivity analysis of these results as a function of the stimulation threshold is also provided.

Molecular neuropathology of temporal lobe epilepsy: complementary approaches in animal models and human disease tissue

Patients with temporal lobe epilepsies (TLE) frequently develop pharmacoresistance to antiepileptic treatment. In individuals with drug-refractory TLE, neurosurgical removal of the epileptogenic focus provides a therapy option with high potential for seizure control. Biopsy specimens from TLE patients constitute unique tissue resources to gain insights in neuropathological and molecular alterations involved in human TLE. Compared to human tissue specimens in most neurological diseases, where only autopsy material is available, the bioptic tissue samples from pharmacoresistant TLE patients open rather exceptional preconditions for molecular biological, electrophysiological as well as biochemical experimental approaches in human brain tissue, which cannot be carried out in postmortem material. Pathological changes in human TLE tissue are multiple and relate to structural and cellular reorganization of the hippocampal formation, selective neurodegeneration, and acquired changes of expression and distribution of neurotransmitter receptors and ion channels, underlying modified neuronal excitability. Nevertheless, human TLE tissue specimens have some limitations. For obvious reasons, human TLE tissue samples are only available from advanced, drug-resistant stages of the disease. However, in many patients, a transient episode of status epilepticus (SE) or febrile seizures in childhood can induce multiple structural and functional alterations that after a latency period result in a chronic epileptic condition. This latency period, also referred to as epileptogenesis, cannot be studied in human TLE specimens. TLE animal models may be particularly helpful in order to shed characterize new molecular pathomechanisms related to epileptogenesis and open novel therapeutic strategies for TLE. Here, we will discuss experimental approaches to unravel molecular-neuropathological aspects of TLE and highlight characteristics and potential of molecular studies in human and/or experimental TLE.

Interictal high-frequency oscillations (100-500 Hz) in the intracerebral EEG of epileptic patients

Interictal fast oscillations between 100 and 500 Hz have been reported in signals recorded from implanted microelectrodes in epileptic patients and experimental rat models. Oscillations between 250 and 500 Hz, or fast ripples (FR), appeared related to the epileptic focus whereas ripples (80-200 Hz) were not. We report high-frequency oscillations recorded with intracranial macroelectrodes in seven patients with refractory focal epilepsy during slow-wave sleep. We characterize the relation of fast oscillations to the seizure focus and quantify their concordance with epileptiform transients, with which they are strongly associated. The patients were selected because interictal spikes were found within and outside the seizure onset zone. Visual inspection was used to identify and classify the ripples and FRs according to their relation to epileptiform spikes. Continuous-time wavelet analysis was used to compute their power. Ripples were present in all patients while FRs where found in five of the seven patients. Most ripples and FRs occurred at the same time as epileptiform transients. The rate of occurrence of ripples was higher within the seizure onset zone than outside in four of seven patients. The rate of FRs was much higher within the seizure onset zone than outside in four of the five patients with FRs (in these four patients, FRs were almost inexistent outside the seizure onset zone). The power of ripples and FRs tended to be higher in the electrodes where their rate was also higher. These results indicate that FRs were more restricted to the electrodes located within the seizure onset zone, especially to the hippocampus, than ripples. In only one patient, FRs were more frequent outside the seizure onset zone; this patient was the only one with cortical dysplasia and the electrode with a high rate of FRs was inside the lesion. This study demonstrates that interictal ripples and FRs can be recorded with depth macroelectrodes in patients. Most occur at the time of epileptiform spikes but some are isolated. Ripples do not show a clear differentiation between the seizure onset zone and remote areas, whereas FRs have a higher rate and higher power in the seizure onset zone. Our results also suggest a special capacity of the abnormal hippocampus to generate FRs, although they were also recorded in other structures.

Concise review: prospects of stem cell therapy for temporal lobe epilepsy

Certain regions of the adult brain have the ability for partial self-repair after injury through production of new neurons via activation of neural stem/progenitor cells (NSCs). Nonetheless, there is no evidence yet for pervasive spontaneous replacement of dead neurons by newly formed neurons leading to functional recovery in the injured brain. Consequently, there is enormous interest for stimulating endogenous NSCs in the brain to produce new neurons or for grafting of NSCs isolated and expanded from different brain regions or embryonic stem cells into the injured brain. Temporal lobe epilepsy (TLE), characterized by hyperexcitability in the hippocampus and spontaneous seizures, is a possible clinical target for stem cell-based therapies. This is because these approaches have the potential to curb epileptogenesis and prevent chronic epilepsy development and learning and memory dysfunction after hippocampal damage related to status epilepticus or head injury. Grafting of NSCs may also be useful for restraining seizures during chronic epilepsy. The aim of this review is to evaluate current knowledge and outlook pertaining to stem cell-based therapies for TLE. The first section discusses the behavior of endogenous hippocampal NSCs in human TLE and animal models of TLE and evaluates the role of hippocampal neurogenesis in the pathophysiology and treatment of TLE. The second segment considers the prospects for preventing or suppressing seizures in TLE using exogenously applied stem cells. The final part analyzes problems that remain to be resolved before initiating clinical application of stem cell-based therapies for TLE. Disclosure of potential conflicts of interest is found at the end of this article.

High-frequency intracerebral EEG activity (100-500 Hz) following interictal spikes

PURPOSE

High-frequency activity has been recorded with intracerebral microelectrodes in epileptic patients and related to seizure genesis. Our goal was to analyze high-frequency activity recorded with electroencephalograph (EEG) macroelectrodes during the slow wave immediately following interictal spikes, given the potential importance of this presumed hyperpolarization in transforming spikes into seizures.

METHODS

Depth electrode EEG recordings from 10 patients with intractable focal epilepsy were low-pass filtered at 500 Hz and sampled at 2,000 Hz. Spikes were categorized according to localization and morphology. Segments of 256 ms were selected immediately following (postspike), and 2 s before each spike (baseline). Power was estimated in subgamma (0-40 Hz), gamma (40-100 Hz), high frequency (100-200 Hz), and very high frequency (250-500 Hz) bands.

RESULTS

Changes in power above 100 Hz were seen in 22 of 29 spike categories, consisting primarily of a widespread decrease in frequencies above 100 Hz. This decrease became spatially more restricted as frequencies increased, and coincided with the localization of largest spikes for the highest frequencies. High-frequency power decreases were prominent in the hippocampus but less common in amygdala and neocortex. High-frequency power increases were observed in the amygdala.

CONCLUSIONS

Thus high-frequency EEG activity can be recorded with macroelectrodes in humans and may provide insights on neuronal mechanisms related to human epilepsy. This activity undergoes consistent modifications after EEG spikes. We propose that the reduction in high frequencies reflects a postspike depression in neuronal activity that is more pronounced in the region of spike generation. This depression is almost always seen in hippocampus but less in amygdala.

Fast oscillations trigger bursts of action potentials in neocortical neurons in vitro: A quasi-white-noise analysis study

PURPOSE

Recent evidence supports the importance of action potential bursts in physiological neural coding, as well as in pathological epileptogenesis. To better understand the temporal dynamics of neuronal input currents that trigger burst firing, we characterized spectral patterns of stimulation current that generate bursts of action potentials from regularly spiking neocortical neurons in vitro.

METHODS

Sharp microelectrodes were used for intracellular recording and stimulation of cortical neurons in rat brain slices. Quasi-white-noise (0-2 kHz) and "chirp" sine wave currents of decreasing wavelength were applied to represent a broad spectrum of stimulation frequencies. Action potential-related averaging of the stimulation current variations preceding bursting was used to characterize stimulation current patterns more likely to result in a burst rather than a single-spike response.

RESULTS

Bursts of action potentials were most reliably generated by a preceding series of > or = 2 positive current transients at 164+/-37 Hz of the quasi-white-noise, and to sine wave currents with frequencies greater than 90 Hz. The intraburst action potential rate was linearly related to the frequency of the input sine wave current.

CONCLUSIONS

This study demonstrates that regularly spiking cortical neurons in vitro burst in response to fast oscillations of input currents. In the presence of positive cortical feedback loops, encoding input frequency in the intraburst action potential rate may be safer than producing a high-frequency regular output spike train. This leads to the experimentally testable and therapeutically important hypothesis that burst firing could be an antiepileptogenic and/or anti-ictogenic mechanism.

Slow state transitions of sustained neural oscillations by activity-dependent modulation of intrinsic excitability

Little is known about the dynamics and mechanisms of transitions between tonic firing and bursting in cortical networks. Here, we use a computational model of a neocortical circuit with extracellular potassium dynamics to show that activity-dependent modulation of intrinsic excitability can lead to sustained oscillations with slow transitions between two distinct firing modes: fast run (tonic spiking or fast bursts with few spikes) and slow bursting. These transitions are caused by a bistability with hysteresis in a pyramidal cell model. Balanced excitation and inhibition stabilizes a network of pyramidal cells and inhibitory interneurons in the bistable region and causes sustained periodic alternations between distinct oscillatory states. During spike-wave seizures, neocortical paroxysmal activity exhibits qualitatively similar slow transitions between fast run and bursting. We therefore predict that extracellular potassium dynamics can cause alternating episodes of fast and slow oscillatory states in both normal and epileptic neocortical networks.

Prophylactic treatment of bipolar disorder in pregnancy and breastfeeding: focus on emerging mood stabilizers

OBJECTIVES

Bipolar disorders are reported to have a high incidence during childbearing years and the need may arise to start or continue a pharmacological treatment during pregnancy and the postpartum period. In the last few years several investigations have evaluated the efficacy of emerging mood-stabilizing agents in the treatment of bipolar disorders, such as lamotrigine, olanzapine, risperidone, quetiapine, aripiprazole and ziprasidone. A number of studies, which examined the use of oxcarbazepine, point to its potential usefulness in prophylactic treatment. The aim of this review is to compare information from the literature on the safety of lamotrigine, oxcarbazepine, risperidone, olanzapine, and quetiapine to the safety data on classic mood stabilizers during pregnancy and the postpartum period.

METHODS

A computerized search carried out from 1980 to April 5, 2006 led to the summarization of the results. (References were updated after acceptance and prior to publication.)

RESULTS

Emerging mood stabilizers show uncertain safety parameters in pregnancy and lactation. Limited information on lamotrigine and oxcarbazepine does not suggest a clear increase in teratogenicity, while olanzapine appears to be associated with a higher risk of metabolic complications in pregnant women. Data about risperidone and quetiapine are still inconclusive. Finally, the literature on the safety of these compounds in breastfeeding is anecdotal.

CONCLUSIONS

Untreated pregnant bipolar women are at an increased risk of poor obstetrical outcomes and relapse of affective symptoms. On the other hand, classic antiepileptic drugs are well-known human teratogens, whereas data on lithium are partially ambiguous. The safety of emerging mood stabilizers in pregnancy and breastfeeding has not been examined extensively. Therefore, when approaching bipolar disorder, if possible, each episode must be considered separately.

Kainic acid-induced recurrent mossy fiber innervation of dentate gyrus inhibitory interneurons: possible anatomical substrate of granule cell hyper-inhibition in chronically epileptic rats

Kainic acid-induced neuron loss in the hippocampal dentate gyrus may cause epileptogenic hyperexcitability by triggering the formation of recurrent excitatory connections among normally unconnected granule cells. We tested this hypothesis by assessing granule cell excitability repeatedly within the same awake rats at different stages of the synaptic reorganization process initiated by kainate-induced status epilepticus (SE). Granule cells were maximally hyperexcitable to afferent stimulation immediately after SE and became gradually less excitable during the first month post-SE. The chronic epileptic state was characterized by granule cell hyper-inhibition, i.e., abnormally increased paired-pulse suppression and an abnormally high resistance to generating epileptiform discharges in response to afferent stimulation. Focal application of the gamma-aminobutyric acid type A (GABA(A)) receptor antagonist bicuculline methiodide within the dentate gyrus abolished the abnormally increased paired-pulse suppression recorded in chronically hyper-inhibited rats. Combined Timm staining and parvalbumin immunocytochemistry revealed dense innervation of dentate inhibitory interneurons by newly formed, Timm-positive, mossy fiber terminals. Ultrastructural analysis by conventional and postembedding GABA immunocytochemical electron microscopy confirmed that abnormal mossy fiber terminals of the dentate inner molecular layer formed frequent asymmetrical synapses with inhibitory interneurons and with GABA-immunopositive dendrites as well as with GABA-immunonegative dendrites of presumed granule cells. These results in chronically epileptic rats demonstrate that dentate granule cells are maximally hyperexcitable immediately after SE, prior to mossy fiber sprouting, and that synaptic reorganization following kainate-induced injury is temporally associated with GABA(A) receptor-dependent granule cell hyper-inhibition rather than a hypothesized progressive hyperexcitability. The anatomical data provide evidence of a possible anatomical substrate for the chronically hyper-inhibited state.

Full-band EEG (fbEEG): a new standard for clinical electroencephalography

A variety of neuroimaging techniques, such as functional magnetic resonance imaging (fMRI), positron emission tomography (PET) and magnetoencephalography (MEG), have been established during the last few decades, with progressive improvements continuously taking place in the underlying technologies. In contrast to this, the recording bandwidth of the routine clinical EEG (typically around 0.5-50 Hz) that was originally set by trivial technical limitations has remained practically unaltered for over half a decade. An increasing amount of evidence shows that salient EEG signals take place and can be recorded beyond the conventional clinical EEG bandwidth. These physiological and pathological EEG activities range from 0.01 Hz to several hundred Hz, and they have been demonstrated in recordings of spontaneous activity in the preterm human brain, and during epileptic seizures, sleep, as well as in various kinds of cognitive tasks and states in the adult brain. In the present paper, we will describe the practical aspects of recording the full physiological frequency band of the EEG (Full-band EEG; FbEEG), and we review the currently available data on the clinical applications of FbEEG. Recording the FbEEG is readily attained with commercially available direct-current (DC) coupled amplifiers if the recording setup includes electrodes providing a DC-stable electrode-skin interface. FbEEG does not have trade-offs that would favor any frequency band at the expense of another. We present several arguments showing that elimination of the lower (infraslow) or higher (ultrafast) bands of the EEG frequency spectrum in routine EEG has led, and will lead, to situations where salient and physiologically meaningful features of brain activity remain undetected or become seriously attenuated and distorted. With the currently available electrode, amplifier and data acquisition technology, it is to be expected that FbEEG will become the standard approach in both clinical and basic science.

Progressive hippocampal and extrahippocampal atrophy in drug resistant epilepsy

PURPOSE OF REVIEW

This article reviews recent experimental and clinical evidence for seizure-related progressive brain damage and discusses possible mechanisms of ongoing brain atrophy in epilepsy.

RECENT FINDINGS

Experimental data indicate that seizures induce brain plasticity that may result in either damage or protection. Brief seizures or status epilepticus may promote resistance to additional damage but also induce cumulative neuronal loss and increase susceptibility to network synchronization. Some experimental studies indicated that, following the initial damage caused by status epilepticus, further brief seizures may not produce significant continuing neuronal loss and hippocampal atrophy, whereas other studies showed the contrary. There is recent evidence that progressive damage and atrophy may occur after an acute insult but are not directly associated with recurrent seizures. Clinical research data continue to show discrepancies regarding whether ongoing seizures cause progressive atrophy. Some cross-sectional and longitudinal magnetic resonance imaging studies in patients with partial epilepsies have shown progressive hippocampal and extrahippocampal atrophy, the severity of which correlated with the duration of epilepsy, seizure frequency, or lifetime seizure number, whereas others have failed to show a clear association.

SUMMARY

Experimental data indicate that epileptogenesis in developing brain may not require significant neuronal loss, which is in keeping with clinical observations that progressive cognitive and behavioural impairment may occur in patients with no detectable brain atrophy. A better understanding of why, when and how progressive brain atrophy occurs will lead to better clinical management, earlier surgical intervention when necessary and, ultimately, prevention of epileptogenesis.

Full-band EEG (FbEEG): an emerging standard in electroencephalography

While enormous resources have been recently invested into the development of a variety of neuroimaging techniques, the bandwidth of the clinical EEG, originally set by trivial technical limitations, has remained practically unaltered for over 50 years. An increasing amount of evidence shows that salient EEG signals are observed beyond the bandwidth of the routine clinical EEG, which is typically around 0.5-50 Hz. Physiological and pathological EEG activity ranges at least from 0.01 Hz to several hundred Hz, as demonstrated in recordings of spontaneous activity in the immature human brain, as well as during epileptic seizures, or various kinds of cognitive tasks and states in the adult brain. In the present paper, we will review several arguments leading to the conclusion that elimination of the lower (infraslow) or higher (ultrafast) bands of the EEG frequency spectrum in routine EEG leads to situations where salient and physiologically meaningful features of brain activity are ignored. Recording the full, physiologically relevant range of frequencies is readily attained with commercially available direct-current (DC) coupled amplifiers, which have a wide dynamic range and a high sampling rate. Such amplifiers, combined with appropriate DC-stable electrode-skin interface, provide a genuine full-band EEG (FbEEG). FbEEG is mandatory for a faithful, non-distorted and non-attenuated recording, and it does not have trade-offs that would favor any frequency band at the expense of another. With the currently available electrode, amplifier and data acquisition technology, FbEEG is likely to become the standard approach for a wide range of applications in both basic science and in the clinic.

High-frequency Oscillations after Status Epilepticus: Epileptogenesis and Seizure Genesis

PURPOSE

To investigate the temporal relation between high-frequency oscillations (HFOs) in the dentate gyrus and recurrent spontaneous seizures after intrahippocampal kainite-induced status epilepticus.

METHODS

Recording microelectrodes were implanted bilaterally in different regions of hippocampus and entorhinal cortex. A guide cannula for microinjection of kainic acid (KA) was implanted above the right posterior CA3 area of hippocampus. After recording baseline electrical activity, KA (0.4 microg/0.2 microl) was injected. Beginning on the next day, electrographic activity was recorded with video monitoring for seizures every day for 8 h/day for > or = 30 days.

RESULTS

Of the 26 rats studied, 19 revealed the appearance of sharp-wave activity and HFOs in the frequency range of 80 to 500 Hz in the dentate gyrus ipsilateral to the KA injection. In the remaining seven rats, no appreciable activity was noted in this frequency range. In some rats with recurrent seizures, HFOs were in the ripple frequency range (100-200 Hz); in others, HFOs were in the fast ripple frequency range (200-500 Hz), or a mixture of both oscillation frequencies was found. The time of detection of the first HFOs after status epilepticus varied between 1 and 30 days, with a mean of 6.3 +/- 2.0 (SEM). Of the 19 rats in which HFO activity appeared, all later developed recurrent spontaneous seizures, whereas none of the rats without HFOs developed seizures. The sooner HFO activity was detected after status epilepticus, the sooner the first spontaneous seizure occurred. A significant inverse relation was found between the time to the first HFO detection and the subsequent rate of spontaneous seizures.

CONCLUSIONS

A strong correlation was found between a decreased time to detection of HFOs and an increased rate of spontaneous seizures, as well as with a decrease in the duration of the latent period between KA injection and the detection of spontaneous seizures. Two types of HFOs were found after KA injection, one in the frequency range of 100 to 200 Hz, and the other, in the frequency range of 200 to 500 Hz, and both should be considered pathological, suggesting that both are epileptogenic.

Pregabalin As Adjunctive Therapy for Partial Seizures

The efficacy and safety of pregabalin as adjunctive therapy for patients with partial epilepsy with or without secondary generalization has been studied in three randomized, double-blind, placebo-controlled trials involving 1,052 patients. Patients (> or =12 years of age) participating in the trials were highly refractory to treatment, experiencing at least six seizures and no 4-week seizure-free period during the 8-week baseline phase, even though 73% received at least two antiepileptic drugs and 23% received three. Each fixed-dose study was 12 weeks in duration. In Study 1, patients received pregabalin 50, 150, 300, or 600 mg/day given two times daily with no titration period. Studies 2 and 3 had a 1-week titration to dose levels of 150 and 600 mg/day given three times daily (Study 2), 600 mg/day given two times daily, and 600 mg/day given three times daily (Study 3). Pregabalin, at 150, 300, and 600 mg/day, was significantly superior to placebo in reducing seizure frequency with a clear dose-response relationship. Responder rates, defined as the percentage of patients with > or =50% reduction in seizure frequency from baseline, approached 50% at 600 mg/day. In the effective dose range (150-600 mg/day), seizure freedom in the last 28 days of treatment was 3-17%. There was no difference between two times daily and three times daily dosing regimens. Efficacy was apparent as early as week one. The most commonly reported adverse events were CNS related, and either mild or moderate in intensity and generally self limiting. Few patients (< or =5% in any treatment group) discontinued due to lack of efficacy. These results indicate that pregabalin is highly effective as adjunctive therapy in the treatment of patients with partial seizures with or without secondary generalization.