Sleep and epilepsy

Chronobiology of epilepsy and sudden unexpected death in epilepsy

Epilepsy is a neurological disease characterized by spontaneous, unprovoked seizures. Various insults render the brain hyperexcitable and susceptible to seizure. Despite there being dozens of preventative anti-seizure medications available, these drugs fail to control seizures in nearly 1 in 3 patients with epilepsy. Over the last century, a large body of evidence has demonstrated that internal and external rhythms can modify seizure phenotypes. Physiologically relevant rhythms with shorter periodic rhythms, such as endogenous circadian rhythms and sleep-state, as well as rhythms with longer periodicity, including multidien rhythms and menses, influence the timing of seizures through poorly understood mechanisms. The purpose of this review is to discuss the findings from both human and animal studies that consider the effect of such biologically relevant rhythms on epilepsy and seizure-associated death. Patients with medically refractory epilepsy are at increased risk of sudden unexpected death in epilepsy (SUDEP). The role that some of these rhythms play in the nocturnal susceptibility to SUDEP will also be discussed. While the involvement of some of these rhythms in epilepsy has been known for over a century, applying the rhythmic nature of such phenomenon to epilepsy management, particularly in mitigating the risk of SUDEP, has been underutilized. As our understanding of the physiological influence on such rhythmic phenomenon improves, and as technology for chronic intracranial epileptiform monitoring becomes more widespread, smaller and less invasive, novel seizure-prediction technologies and time-dependent chronotherapeutic seizure management strategies can be realized.

Automated detection of ripple oscillations in long-term scalp EEG from patients with infantile spasms

Objective.Scalp high-frequency oscillations (HFOs) are a promising biomarker of epileptogenicity in infantile spasms (IS) and many other epilepsy syndromes, but prior studies have relied on visual analysis of short segments of data due to the prevalence of artifacts in EEG. Here we set out to robustly characterize the rate and spatial distribution of HFOs in large datasets from IS subjects using fully automated HFO detection techniques.Approach.We prospectively collected long-term scalp EEG data from 12 subjects with IS and 18 healthy controls. For patients with IS, recording began prior to diagnosis and continued through initiation of treatment with adrenocorticotropic hormone (ACTH). The median analyzable EEG duration was 18.2 h for controls and 84.5 h for IS subjects (∼1300 h total). Ripples (80-250 Hz) were detected in all EEG data using an automated algorithm.Main results.HFO rates were substantially higher in patients with IS compared to controls. In IS patients, HFO rates were higher during sleep compared to wakefulness (median 5.5 min^-1and 2.9 min^-1, respectively;p = 0.002); controls did not exhibit a difference in HFO rate between sleep and wakefulness (median 0.98 min^-1and 0.82 min^-1, respectively). Spatially, IS patients exhibited significantly higher rates of HFOs in the posterior parasaggital region and significantly lower HFO rates in frontal channels, and this difference was more pronounced during sleep. In IS subjects, ACTH therapy significantly decreased the rate of HFOs.Significance.Here we provide a detailed characterization of the spatial distribution and rates of HFOs associated with IS, which may have relevance for diagnosis and assessment of treatment response. We also demonstrate that our fully automated algorithm can be used to detect HFOs in long-term scalp EEG with sufficient accuracy to clearly discriminate healthy subjects from those with IS.

Temporal patterns in the first unprovoked seizure

OBJECTIVE

Cyclic phenomena in epilepsy are well recognized. We investigated a multicenter cohort of unprovoked first seizure presentations to determine whether seizures have a preponderance to occur in: a particular time of the day, a particular day of the week, a particular month of the year, day time versus night time, and wakefulness versus sleep.

METHODS

We retrospectively studied adults who presented with a first-ever unprovoked seizure to the First Seizure Clinic at two tertiary centers in Australia. Seizure onset time was obtained from the emergency department and ambulance documentations. Electro-clinical and neuroimaging findings were reviewed. We used histograms and Poisson regression modeling to determine whether seizures have a preponderance to occur at a particular time and calculated incidence rate ratios (IRR). We performed further analysis on patients with "first seizure epilepsy" and "first seizure not epilepsy" based on the ILAE criteria for a diagnosis of epilepsy after a single unprovoked seizure, as well as comparing patients that could be categorized as having a generalized-onset seizure versus those with focal-onset seizures.

RESULTS

We analyzed 1724 patients (38% females; age range 14-97 yr, median 39 yr), of whom 18% had epileptiform abnormalities on EEG and potentially epileptogenic lesions were detected on neuroimaging in 28%. Whole cohort analysis shows the incidence rate ratios (IRR) of seizures varied significantly across the 24-hour clock-time of the day (p < 0.001), peaking at hour 12 (IRR 3.18). The first unprovoked seizure was significantly less likely to be reported during the night (IRR 0.61, p < 0.001) and during sleep (IRR 0.29, p < 0.001). Both the "first seizure epilepsy" and "first seizure not epilepsy" subgroups' analysis demonstrated similar patterns. An infraradian pattern was also noted with seizures most likely to occur in May (IRR 1.29, p = 0.02). Both "first seizure epilepsy - generalized" and "first seizure epilepsy - focal" groups had a preponderance for seizures to occur during the day versus night and wakefulness as opposed to sleep, but the association was more robust for generalized seizures.

CONCLUSIONS

Our results suggest that temporal patterns are seen in patients with first-ever unprovoked seizures, including those that meet contemporary criteria for epilepsy. These results raise the possibility that first unprovoked seizures have intrinsic rhythmicity similar to epileptic seizures.

Decoding Circadian Rhythm and Epileptic Activities: Clues From Animal Studies

The relationship between circadian rhythm and epilepsy has been recognized for decades. Yet many questions underlying the complex mechanisms of their interaction remain elusive. A better understanding on this topic allows the development of accurate seizure-detection algorithm and alternative precise therapeutic strategies. Preclinical laboratory studies based on epileptic animal models, with controllable epileptogenic pathology and an array of intervention strategies, shed light on the bidirectional effects between circadian rhythm and epileptic seizures as well as their underlying mechanisms. Here, we reviewed findings on the interaction between circadian rhythm and epileptic seizures in the preclinical setting. We present the possible mechanisms at molecular, cellular and circuitry levels. We propose that future experimental designs should take into account the relationship between circadian rhythm and epilepsy as well as the underlying mechanisms in different types of animal models, which may have a translational significance as stepping stones for clinical benefits.

Circuits generating secondarily generalized seizures

Mapping the circuits underlying the generation and propagation of seizures is critically important for understanding their pathophysiology. We review evidence to suggest that circuits engaged in secondarily generalized seizures are likely to be more complex than those currently proposed. Focal seizures have been proposed to engage canonical thalamocortical circuits that mediate primarily generalized absence seizures, leading to secondarily generalized tonic-clonic seizures. In addition to traveling through the canonical thalamocortical circuits, secondarily generalized seizures could also travel through the striatum, globus pallidus, substantia nigra reticulata, and corpus callosum to the contralateral hemisphere. Recruitment of principal neurons in superficial layers 2/3 of the cortex can play a critical role in corticocortical seizure spread. Understanding the neuronal structures engaged in generating secondarily generalized seizures could provide novel targets for neuromodulation for the treatment of seizures. Furthermore, these sites may be loci of neuronal plasticity facilitating epileptogenesis. This article is part of the Special Issue "Proceedings of the 7th London-Innsbruck Colloquium on Status Epilepticus and Acute Seizures".

Strength and stability of EEG functional connectivity predict treatment response in infants with epileptic spasms

OBJECTIVE

Epileptic spasms (ES) are associated with pathological neuronal networks, which may underlie characteristic EEG patterns such as hypsarrhythmia. Here we evaluate EEG functional connectivity as a quantitative marker of treatment response, in comparison to classic visual EEG features.

METHODS

We retrospectively identified 21 ES patients and 21 healthy controls. EEG data recorded before treatment and after ≥10 days of treatment underwent blinded visual assessment, and functional connectivity was measured using cross-correlation techniques. Short-term treatment response and long-term outcome data were collected.

RESULTS

Subjects with ES had stronger, more stable functional networks than controls. After treatment initiation, all responders (defined by cessation of spasms) exhibited decreases in functional connectivity strength, while an increase in connectivity strength occurred only in non-responders. There were six subjects with unusually strong pre-treatment functional connectivity, and all were responders. Visually assessed EEG features were not predictive of treatment response.

CONCLUSIONS

Changes in network connectivity and stability correlate to treatment response for ES, and high pre-treatment connectivity may predict favorable short-term treatment response. Quantitative measures outperform visual analysis of the EEG.

SIGNIFICANCE

Functional networks may have value as objective markers of treatment response in ES, with potential to facilitate rapid identification of personalized, effective treatments.

Heightened Background Cortical Synchrony in Patients With Epilepsy: EEG Phase Synchrony Analysis During Awake and Sleep Stages Using Novel Ensemble Measure

INTRODUCTION

Excessive cortical synchrony within neural ensembles has been implicated as an important mechanism driving epileptiform activity. The current study measures and compares background electroencephalographic (EEG) phase synchronization in patients having various types of epilepsies and healthy controls during awake and sleep stages.

METHODS

A total of 120 patients with epilepsy (PWE) subdivided into 3 groups (juvenile myoclonic epilepsy [JME], temporal lobe epilepsy [TLE], and extra-temporal lobe epilepsy [Ex-TLE]; n = 40 in each group) and 40 healthy controls were subjected to overnight polysomnography. EEG phase synchronization (SI) between the 8 EEG channels was assessed for delta, theta, alpha, sigma, and high beta frequency bands using ensemble measure on 10-second representative time windows and compared between patients and controls and also between awake and sleep stages. Mean ± SD of SI was compared using 2-way analysis of variance followed by pairwise comparison ( P ≤ .05).

RESULTS

In both delta and theta bands, the SI was significantly higher in patients with JME, TLE, and Ex-TLE compared with controls, whereas in alpha, sigma, and high beta bands, SI was comparable between the groups. On comparison of SI between sleep stages, delta band: progressive increase in SI from wake ⇒ N1 ⇒ N2 ⇒ N3, whereas REM (rapid eye movement) was comparable to wake; theta band: decreased SI during N2 and increase during N3; alpha band: SI was highest in wake and lower in N1, N2, N3, and REM; and sigma and high beta bands: progressive increase in SI from wake ⇒ N1 ⇒ N2 ⇒ N3; however, sigma band showed lower SI during REM.

CONCLUSION

This study found an increased background cortical synchronization in PWE compared with healthy controls in delta and theta bands during wake and sleep. This background hypersynchrony may be an important property of epileptogenic brain circuitry in PWE, which enables them to effortlessly generate a paroxysmal EEG depolarization shift.

An unaware agenda: interictal consciousness impairments in epileptic patients

Consciousness impairments have been described as a cornerstone of epilepsy. Generalized seizures are usually characterized by a complete loss of consciousness, whereas focal seizures have more variable degrees of responsiveness. In addition to these impairments that occur during ictal episodes, alterations of consciousness have also been repeatedly observed between seizures (i.e. during interictal periods). In this opinion article, we review evidence supporting the novel hypothesis that epilepsy produces consciousness impairments which remain present interictally. Then, we discuss therapies aimed to reduce seizure frequency, which may modulate consciousness between epileptic seizures. We conclude with a consideration of relevant pathophysiological mechanisms. In particular, the thalamocortical network seems to be involved in both seizure generation and interictal consciousness impairments, which could inaugurate a promising translational agenda for epilepsy studies.

Reactivation of seizure-related changes to interictal spike shape and synchrony during postseizure sleep in patients

OBJECTIVE

Local field potentials (LFPs) arise from synchronous activation of millions of neurons, producing seemingly consistent waveform shapes and relative synchrony across electrodes. Interictal spikes (IISs) are LFPs associated with epilepsy that are commonly used to guide surgical resection. Recently, changes in neuronal firing patterns observed in the minutes preceding seizure onset were found to be reactivated during postseizure sleep, a process called seizure-related consolidation (SRC), due to similarities with learning-related consolidation. Because IISs arise from summed neural activity, we hypothesized that changes in IIS shape and relative synchrony would be observed in the minutes preceding seizure onset and would be reactivated preferentially during postseizure slow-wave sleep (SWS).

METHODS

Scalp and intracranial recordings were obtained continuously across multiple days from clinical macroelectrodes implanted in patients undergoing treatment for intractable epilepsy. Data from scalp electrodes were used to stage sleep. Data from intracranial electrodes were used to detect IISs using a previously established algorithm. Partial correlations were computed for sleep and wake periods before and after seizures as a function of correlations observed in the minutes preceding seizures. Magnetic resonance imaging (MRI) and computed tomography (CT) scans were co-registered with electroencephalography (EEG) to determine the location of the seizure-onset zone (SOZ).

RESULTS

Changes in IIS shape and relative synchrony were observed on a subset of macroelectrodes minutes before seizure onset, and these changes were reactivated preferentially during postseizure SWS. Changes in synchrony were greatest for pairs of electrodes where at least one electrode was located in the SOZ.

SIGNIFICANCE

These data suggest preseizure changes in neural activity and their subsequent reactivation occur across a broad spatiotemporal scale: from single neurons to LFPs, both within and outside the SOZ. The preferential reactivation of seizure-related changes in IISs during postseizure SWS adds to a growing body of literature suggesting that pathologic neural processes may utilize physiologic mechanisms of synaptic plasticity.

Temporal patterns of epileptiform discharges in genetic generalized epilepsies

OBJECTIVE

We sought to investigate the temporal patterns and sleep-wake cycle-related epileptiform discharges (EDs) in genetic generalized epilepsies (GGEs).

METHODS

We studied 24-hour ambulatory electroencephalography (EEG) recordings of patients with GGE, diagnosed and classified according to the International League against Epilepsy criteria. We manually coded the type of discharge, time of occurrence, duration, and arousal state of each ED. We employed mixed effects Poisson regression modeling to study the temporal distribution of epileptiform discharges. Additionally, we used multinomial regression analysis to explore the significance of the relationship between different states of arousal and types of epileptiform discharges.

RESULTS

We analyzed 6923 EDs from 105 abnormal 24-hour EEGs. Mixed effects Poisson regression analysis demonstrated significant changes in ED counts across time blocks. This distribution was largely influenced by the state of arousal. Generalized fragments (duration<2s) and focal discharges were more frequent during non-REM sleep while paroxysms (duration≥2s) were more frequent in wakefulness. Overall, 67% of epileptiform discharges occurred in non-REM sleep and only 33% occurred in wakefulness. Twenty-four patients (23%) had ED exclusively in sleep. Epileptiform discharges peaked from 23:00 through 07:00h.

SIGNIFICANCE

There is a time-of-day dependency of ED with a significant influence exerted by the state of arousal. Our observations suggest that the generation of epileptiform discharges is not a random process but is the result of complex interactions among biological rhythms such as the sleep-wake cycle and the intrinsic circadian pacemaker. High density of ED in sleep suggests that 24-hour EEG recording with the capture of natural sleep may be more useful than routine EEG to diagnose GGE.

A thalamo-cortical neural mass model for the simulation of brain rhythms during sleep

Cortico-thalamic interactions are known to play a pivotal role in many brain phenomena, including sleep, attention, memory consolidation and rhythm generation. Hence, simple mathematical models that can simulate the dialogue between the cortex and the thalamus, at a mesoscopic level, have a great cognitive value. In the present work we describe a neural mass model of a cortico-thalamic module, based on neurophysiological mechanisms. The model includes two thalamic populations (a thalamo-cortical relay cell population, TCR, and its related thalamic reticular nucleus, TRN), and a cortical column consisting of four connected populations (pyramidal neurons, excitatory interneurons, inhibitory interneurons with slow and fast kinetics). Moreover, thalamic neurons exhibit two firing modes: bursting and tonic. Finally, cortical synapses among pyramidal neurons incorporate a disfacilitation mechanism following prolonged activity. Simulations show that the model is able to mimic the different patterns of rhythmic activity in cortical and thalamic neurons (beta and alpha waves, spindles, delta waves, K-complexes, slow sleep waves) and their progressive changes from wakefulness to deep sleep, by just acting on modulatory inputs. Moreover, simulations performed by providing short sensory inputs to the TCR show that brain rhythms during sleep preserve the cortex from external perturbations, still allowing a high cortical activity necessary to drive synaptic plasticity and memory consolidation. In perspective, the present model may be used within larger cortico-thalamic networks, to gain a deeper understanding of mechanisms beneath synaptic changes during sleep, to investigate the specific role of brain rhythms, and to explore cortical synchronization achieved via thalamic influences.

On-off intermittency of thalamo-cortical oscillations in the electroencephalogram of rats with genetic predisposition to absence epilepsy

Spike-wave discharges (SWD) are electroencephalographic hallmarks of absence epilepsy. SWD are known to originate from thalamo-cortical neuronal network that normally produces sleep spindle oscillations. Although sleep spindles and SWD are considered as thalamo-cortical oscillations, functional relationship between them is not obvious. The present study describes temporal dynamics of SWD and sleep spindles as determined in 24h EEG recorded in WAG/Rij rat model of absence epilepsy. SWD, sleep spindles (10-15 Hz) and 5-9 Hz oscillations were automatically detected in EEG using wavelet-based algorithm. It was found that non-linear dynamics of SWD fitted well to the law of 'on-off intermittency'. Sleep spindles also demonstrated 'on-off intermittency', in contrast to 5-9 Hz oscillations, whose dynamics could not be classified as having any known type of non-linear behavior. Intermittency in sleep spindles and SWD implies that (1) temporal dynamics of these oscillations are deterministic in nature, and (2) it might be controlled by a system-level mechanism responsible for circadian modulation of neuronal network activity.

From sleep spindles of natural sleep to spike and wave discharges of typical absence seizures: is the hypothesis still valid?

The temporal coincidence of sleep spindles and spike-and-wave discharges (SWDs) in patients with idiopathic generalized epilepsies, together with the transformation of spindles into SWDs following intramuscular injection of the weak GABAA receptor (GABAAR) antagonist, penicillin, in an experimental model, brought about the view that SWDs may represent ‘perverted’ sleep spindles. Over the last 20 years, this hypothesis has received considerable support, in particular by in vitro studies of thalamic oscillations following pharmacological/genetic manipulations of GABAARs. However, from a critical appraisal of the evidence in absence epilepsy patients and well-established models of absence epilepsy it emerges that SWDs can occur as frequently during wakefulness as during sleep, with their preferential occurrence in either one of these behavioural states often being patient dependent. Moreover, whereas the EEG expression of both SWDs and sleep spindles requires the integrity of the entire cortico-thalamo-cortical network, SWDs initiates in cortex while sleep spindles in thalamus. Furthermore, the hypothesis of a reduction in GABAAR function across the entire cortico-thalamo-cortical network as the basis for the transformation of sleep spindles into SWDs is no longer tenable. In fact, while a decreased GABAAR function may be present in some cortical layers and in the reticular thalamic nucleus, both phasic and tonic GABAAR inhibitions of thalamo-cortical neurons are either unchanged or increased in this epileptic phenotype. In summary, these differences between SWDs and sleep spindles question the view that the EEG hallmark of absence seizures results from a transformation of this EEG oscillation of natural sleep.

Continuous spike and waves during sleep and electrical status epilepticus in sleep

Continuous spike and waves during sleep is an age-related epileptic encephalopathy that presents with neurocognitive regression, seizures, and an EEG pattern of electrical status epilepticus during sleep. Patients usually present around 5 years of age with infrequent nocturnal unilateral motor seizures that progress within 1 to 2 years to a severe epileptic encephalopathy with frequent seizures of different types, marked neurocognitive regression, and an almost continuous spike-wave EEG pattern during slow-wave sleep. The pathophysiology of continuous spike and waves during sleep is not completely understood, but the corticothalamic neuronal network involved in physiologic oscillating patterns of sleep is thought to be switched into a pathologic discharging mode. Early developmental injury and/or genetic predisposition may play a role in the potentiation of age-related hyperexcitability in the immature brain. A better understanding of the mechanisms leading to electrical status epilepticus during sleep may provide additional therapeutic targets that can improve the outcome of seizures, EEG pattern, and cognitive development in patients with continuous spike and waves during sleep.

Short-Duration Epileptic Discharges Show a Distinct Phase Preference During Ongoing Hippocampal Slow Oscillations

Non-REM (slow-wave) sleep has been shown to facilitate temporal lobe epileptiform events, whereas REM sleep seems more restrictive. This state-dependent modulation may be the result of the enhancement of excitatory synaptic transmission and/or the degree of network synchronization expressed within the hippocampus of the temporal lobe. The slow oscillation (SO), a ∼1 Hz oscillatory pattern expressed during non-REM sleep and urethane anesthesia, has been recently shown to facilitate the generation, maintenance, and propagation of stimulus-evoked epileptiform activity in the hippocampus. To further address the state-dependent modulation of epileptic activity during the SO, we studied the properties of short-duration interictal-like activity generated by focal application of penicillin in the hippocampus of urethane-anesthetized rats. Epileptiform spikes were larger but only slightly more prevalent during the SO as opposed to the theta (REM-like) state. More notably, however, epileptic spikes had a significant tendency to occur just following the peak negativity of ongoing SO cycles. Because of the known phase-dependent changes in 1) synaptic excitability (just following the positive peak of the SO) and 2) network synchronization (during the negative peak of the SO), these results suggest that it is the synchrony and not the changes in synaptic excitability that lead to the facilitation of epileptiform activity during sleep-like slow wave states.

Bimodal ultradian seizure periodicity in human mesial temporal lobe epilepsy

BACKGROUND

Mesial temporal lobe epilepsy (mTLE) has been suggested to follow a circadian rhythm. Previous research found an afternoon peak in mTLE seizure occurrence. We evaluated the pattern of seizure occurrence in patients with well-localized mTLE and hypothesized that peak seizure frequency would occur in the afternoon, and that this pattern would not be altered by age, gender, or seizure focus.

METHODS

We retrospectively identified consecutive mTLE patients with a seizure-free outcome following anterior temporal lobectomy from 1993 to 2004 with video-EEG captured seizures. We recorded and plotted the 24-h clock time for each seizure and performed cosinor analysis. SAS Proc GLIMMIX was used to fit the linearized transform of the cosinor model. Negative binomial regression fitted by the generalized estimating equations (GEE) method was also performed to estimate and compare the mean seizure rates over a 24-h day.

RESULTS

Sixty mTLE patients monitored between 2 and 16 days were analyzed. Mean (standard deviation), median number of seizures per subject were 10.47(7.86), 9.00. Cosinor plots indicated that the function had two modes: 7-8 a.m. and 4-5 p.m. GEE analysis was consistent with peak seizure frequency occurrence at 6-8 a.m. (p<0.0001) and 3-5 p.m. (p<0.01).

CONCLUSIONS

We found a bimodal pattern of seizure occurrence in human mTLE, with peak seizure frequencies occurring between 6-8 a.m. and 3-5 p.m. confirming an afternoon peak, as well as a previously unsuspected morning peak in seizure occurrence that provides rationale for future investigations of antiepileptic drug chronopharmacology and informs patient counseling regarding patterns of seizure occurrence.

Reduced NREM sleep instability in benign childhood epilepsy with centro-temporal spikes

OBJECTIVE

To analyze sleep architecture and NREM sleep instability by means of the cyclic alternating pattern (CAP) in children with benign epilepsy with rolandic spikes (BERS).

METHODS

Ten children with BERS, drug free at the time of the study and 10 age-matched normal controls were included in this study. Sleep was visually scored for sleep architecture and CAP using standard criteria.

RESULTS

Sleep architecture in BERS showed only few significant differences vs. controls with a reduction of total sleep time, sleep efficiency, and REM sleep percentage. CAP analysis revealed several significant differences: reduced total CAP rate, mainly in sleep stage 2, and reduced EEG slow oscillations and arousals during stages N1 and N2.

CONCLUSIONS

Sleep architecture is not importantly affected in BERS but CAP analysis reveals a decrease of NREM instability, mainly in sleep stage 2. Since there is a spindle-related spike activation in BERS, we speculate that the decrease of CAP and of EEG slow oscillations and arousals might be linked with the inhibitory action of spindling activity and spikes on arousals.

SIGNIFICANCE

CAP analysis discloses sleep structure abnormalities in children with BERS not shown by the classical sleep scoring. Spike activity and CAP A1 subtypes seem to be mutually exclusive probably because centro-temporal spikes disturb the physiological synchronization mechanisms needed for the generation of slow-wave components of CAP.

Absence semiologies

Factors Influencing Clinical Features of Absence Seizures. Sadleir LG, Scheffer IE, Smith S, Carstensen B, Carlin J, Connolly MB, Farrell K. Epilepsia 2008;49(12):2100–2107. PURPOSE: The clinical features of absence seizures in idiopathic generalized epilepsy have been held to be syndrome-specific. This hypothesis is central to many aspects of epilepsy research yet has not been critically assessed. We examined whether specific factors such as epilepsy syndrome, age, and state determine the features of absence seizures. METHODS: Children with newly presenting absence seizures were studied using video electroencephalography (EEG) recording. We analyzed whether a child's epilepsy syndrome, age, state of arousal, and provocation influenced specific clinical features of their absence seizures: duration, eyelid movements, eye opening, and level of awareness during the seizure. RESULTS: Seizures (509) were evaluated in 70 children with the following syndromes: Childhood absence epilepsy (CAE), 37; CAE plus photoparoxysmal response (PPR), 10; juvenile absence epilepsy (JAE), 8; juvenile myoclonic epilepsy (JME), 6; unclassified, 9. Seizure duration was associated with epilepsy syndrome as children with JME had shorter seizures than in other syndromes, independent of age. Age independently influences level of awareness and eye opening. Arousal or provocation affected all features except level of awareness. Specific factors unique to the child independently influenced all features; the nature of these factors has not been identified. DISCUSSION: The view that the clinical features of absence seizures have syndrome-specific patterns is not supported by critical analysis. We show that confounding variables profoundly affect clinical features and that syndromes also show marked variation. Variation in clinical features of absence seizures results from a complex interaction of many factors that are likely to be genetically and environmentally determined.

Childhood Absence Epilepsy: Behavioral, Cognitive, and Linguistic Comorbidities. Caplan R, Siddarth P, Stahl L, Lanphier E, Vona P, Gurbani S, Koh S, Sankar R, Shields WD. Epilepsia 2008 Nov;49(11):1838–1846. PURPOSE: Evidence for a poor psychiatric, social, and vocational adult outcome in childhood absence epilepsy (CAE) suggests long-term unmet mental health, social, and vocational needs. This cross-sectional study examined behavioral/emotional, cognitive, and linguistic comorbidities as well as their correlates in children with CAE. METHODS: Sixty-nine CAE children aged 9.6 (SD = 2.49) years and 103 age- and gender-matched normal children had semistructured psychiatric interviews, as well as cognitive and linguistic testing. Parents provided demographic, seizure-related, and behavioral information on their children through a semi-structured psychiatric interview and the child behavior checklist (CBCL). RESULTS: Compared to the normal group, 25% of the CAE children had subtle cognitive deficits, 43% linguistic difficulties, 61% a psychiatric diagnosis, particularly attention deficit hyperactivity disorder (ADHD) and anxiety disorders, and 30% clinically relevant CBCL broad band scores. The most frequent CBCL narrow band factor scores in the clinical/borderline range were attention and somatic complaints, followed by social and thought problems. Duration of illness, seizure frequency, and antiepileptic drug (AED) treatment were related to the severity of the cognitive, linguistic, and psychiatric comorbidities. Only 23% of the CAE subjects had intervention for these problems. CONCLUSIONS: The high rate of impaired behavior, emotions, cognition, and language and low intervention rate should alert clinicians to the need for early identification and treatment of children with CAE, particularly those with longer duration of illness, uncontrolled seizures, and AED treatment.

Slow oscillation state facilitates epileptiform events in the hippocampus

In mesial temporal lobe (MTL) epilepsy, which typically involves the hippocampus (HPC), epileptiform events are enhanced during slow wave sleep (SWS). It remains unclear how and why the electroencephalographic (EEG) states that constitute SWS might predispose the HPC to this type of pathological activity. Recently our laboratory has described a novel state of deactivated hippocampal EEG activity that occurs during both SWS and urethan anesthesia: the slow oscillation (SO). This activity is characterized by a high-amplitude approximately 1-Hz signal, high synchrony within the hippocampus, and a dynamic coordination with neocortical SO. To assess how this activity state might influence epileptiform discharges, we studied the properties of stimulation-evoked and spontaneous epileptiform events elicited in the HPC of urethan-anesthetized rats. We compared those elicited during the SO to those occurring during the theta rhythm. The average duration but not the amplitude of evoked afterdischarges (ADs) was consistently larger during the SO. In addition, spontaneous epileptiform events were more frequent and of higher amplitude during the SO. Last, the bilateral propagation of both ADs and spontaneous events in the hippocampus was enhanced during the SO. These results imply that the threshold for the generation and propagation of epileptiform activity in the hippocampus is lowered during the SO and that this state may be a seed for the initiation, maintenance, and generalization of MTL epilepsy. Further examination of the pathophysiology of sleep-epilepsy interactions in the HPC will be of benefit for an understanding of the mechanisms, prognosis, and therapy for this form of epilepsy.

Determination of a sedative protocol for use in California sea lions (Zalophus californianus) with neurologic abnormalities undergoing electroencephalographic examination

Sedation in sea lions exhibiting abnormal neurologic signs may require modification of established sedatior protocols because of the likely interaction between effects of the sedative and physiologic changes in diseased animals The effects of two sedative combinations, 0.07 mg/kg medetomidine and 0.07 mg/kg medetomidine plus 0.2 mg/kg butorphanol, were compared between California sea lions (Zalophus californianus) with signs of neurologic dysfunctior (n=33) and without neurologic signs (n=8). Sedation depth was scored on a scale of 0 (no effect) to 4 (profound sedation) assessed by response to auditory, tactile, and visual stimuli at the time of perceived maximal sedative effect In the medetomidine-alone group, sea lions with neurologic signs attained a median sedation score of 4 compared to a median sedation score of 1 in the clinically normal sea lions. Sea lions with and without neurologic signs giver medetomidine-butorphanol attained a median sedation score of 4. No statistically significant difference in time to induction and respiratory rate was found between the two sedation protocols in all sea lions. In the sea lions with neurologic signs, the recovery time from medetomidine-butorphanol sedation was prolonged (P < 0.01) and minimum recorded heart rates, although remaining within normal physiologic limits, were lower (P = 0.02) when compared to the sea lions administered medetomidine alone. Muscle jerks were observed in many animals given medetomidine-butorphanol and were detrimental to the diagnostic quality of the electroencephalogram (EEG) recording. Medetomidine alone at a dose rate of 0.07 mg/kg thus provides adequate and safe sedation in sea lions with neurologic signs undergoing EEG evaluation.

Midline spikes

Midline spikes are characterized by spike foci recorded at Cz, Fz, or Pz with amplitude ranging from 20 to 350 microvolts. Out of 7,929 EEGs performed at the Neurodiagnostics Laboratory, Kaiser Permanente Medical Center, Anaheim, California, between 1996 and 2006, 17 EEGs (0.21%) were identified as having interictal midline spikes with or without other epileptiform discharges. Eight EEGs showed midline spikes at Cz, 2 at Fz, 2 at Cz and Fz, 1 at Cz and C3, 1 at Cz, C3, and P3, 1 at Cz and F8, 1 at Cz and T4, and 1 at Cz with 2 Hz generalized spike and slow wave complex. Midline spikes were recorded in 10 males and 7 females. The age ranged from 4 days to 38-years-old with a mean age of 10.8 years. Twelve patients (70.6%) were children. Twelve patients (70.6%) had generalized tonic-clonic seizures and 5 had partial motor seizures. Of the 17 patients, 14 had no known causes, 1 had an agenesis of corpus callosum, 1 had a left frontal arteriovenous malformation, and 1 had a left frontal area stroke. We postulate that the mechanism for the genesis of midline spikes may be heterogeneous. Midline spikes may be triggered by thalamocortical network in a generalized tonic-clonic seizure, or may originate in the parasagittal cortex in a partial motor seizure.

High risk of reading disability and speech sound disorder in rolandic epilepsy families: case-control study

PURPOSE

Associations between rolandic epilepsy (RE) with reading disability (RD) and speech sound disorder (SSD) have not been tested in a controlled study. We conducted a case-control study to determine whether (1) RD and SSD odds are higher in RE probands than controls and (2) an RE proband predicts a family member with RD or SSD, hence suggesting a shared genetic etiology for RE, RD, and SSD.

METHODS

Unmatched case-control study with 55 stringently defined RE cases, 150 controls in the same age range lacking a primary brain disorder diagnosis, and their siblings and parents. Odds ratios (OR) were calculated by multiple logistic regression, adjusted for sex and age, and for relatives, also adjusted for comorbidity of RD and SSD in the proband.

RESULTS

RD was strongly associated with RE after adjustment for sex and age: OR 5.78 (95% CI: 2.86-11.69). An RE proband predicts RD in family members: OR 2.84 (95% CI: 1.38-5.84), but not independently of the RE proband's RD status: OR 1.30 (95% CI: 0.55-12.79). SSD was also comorbid with RE: adjusted OR 2.47 (95%CI: 1.22-4.97). An RE proband predicts SSD in relatives, even after controlling for sex, age and proband SSD comorbidity: OR 4.44 (95% CI: 1.93-10.22).

CONCLUSIONS

RE is strongly comorbid with RD and SSD. Both RD and SSD are likely to be genetically influenced and may contribute to the complex genetic etiology of the RE syndrome. Siblings of RE patients are at high risk of RD and SSD and both RE patients and their younger siblings should be screened early.

Autosomal dominant inheritance of centrotemporal sharp waves in rolandic epilepsy families

PURPOSE

Centrotemporal sharp (CTS) waves, the electroencephalogram (EEG) hallmark of rolandic epilepsy, are found in approximately 4% of the childhood population. The inheritance of CTS is presumed autosomal dominant but this is controversial. Previous studies have varied considerably in methodology, especially in the control of bias and confounding. We aimed to test the hypothesis of autosomal dominant inheritance of CTS in a well-designed family segregation analysis study.

METHODS

Probands with rolandic epilepsy were collected through unambiguous single ascertainment. Siblings in the age range 4-16 years underwent sleep-deprived EEG; observations from those who remained awake were omitted. CTS were rated as present or absent by two independent observers blinded to the study hypothesis and subject identities. We computed the segregation ratio of CTS, corrected for ascertainment. We tested the segregation ratio estimate for consistency with dominant and recessive modes of inheritance, and compared the observed sex ratio of those affected with CTS for consistency with sex linkage.

RESULTS

Thirty siblings from 23 families underwent EEG examination. Twenty-three showed evidence of sleep in their EEG recordings. Eleven of 23 recordings demonstrated CTS, yielding a corrected segregation ratio of 0.48 (95% CI: 0.27-0.69). The male to female ratio of CTS affectedness was approximately equal.

CONCLUSIONS

The segregation ratio of CTS in rolandic epilepsy families is consistent with a highly penetrant autosomal dominant inheritance, with equal sex ratio. Autosomal recessive and X-linked inheritance are rejected. The CTS locus might act in combination with one or more loci to produce the phenotype of rolandic epilepsy.

Midfrequency cortico-thalamic oscillations and the sleep cycle: Genetic, time of day and age effects

WAG/Rij rats have various types of mid frequency cortico-thalamic oscillations, such as anterior and posterior sleep spindles and two types of spike-wave discharges (SWD). The generalized SWD (type I) preferentially occur at transitions from wake to sleep, type II can be found at the occipital cortex during quite wakefulness. In the present experiment sleep spindles, SWD and sleep cycle characteristics of 6-month-old WAG/Rij rats were studied and compared with those of younger WAG/Rij rats with much less SWD and age-matched control (ACI) rats. EEG recordings were made during the beginning (morning) and end (afternoon) of the light period in these four groups of rats. Quantitative characteristics of SWD, sleep spindles and the sleep cycle were determined. There were strain-related and age-dependent effects in the various cortico-thalamic oscillations, older WAG/Rij had more SWDs than younger WAG/Rij rats (both types I and II) and there were more type I SWDs at the end of the light period compared to the beginning. Large strain, age and time of day effects on the sleep cycle were found. The duration of non-REM sleep and the sleep cycle was shorter in WAG/Rij rats but only at the end of the light period and only in older WAG/Rij rats. It can be concluded that the various phasic events and the length of the sleep cycle are under genetic control, and that the sleep cycle length is also controlled by time of day, age and genetic factors. Non-REM sleep and the sleep cycle are disrupted by absence seizures but only in fragile periods when drowsiness and light slow wave sleep dominate.

Sleep spindle abnormalities in children with generalized spike-wave discharges

This study investigated sleep and sleep spindle parameters in children with primary generalized spike-and-wave discharges (untreated primary generalized group, nine patients; treated primary generalized group, six patients) and compared these with an age- and sex-matched nonepileptic control group (n = 47). In the untreated primary generalized group, stage 2 onset was significantly shorter, with less spindles in stage 2. In the last stage 2 period of the night, significantly less fast frequency spindles were observed, indicating abnormal dynamics of sleep architecture. In the treated group, sleep patterns were comparable to that of the control group. The data indicate sleep architecture dysfunctions in children with generalized spike-and-wave discharges. These dysfunctions could account for the frequently encountered sleep problems in children with primary generalized epilepsy.

Cellular and network mechanisms of genetically-determined absence seizures

The absence epilepsies are characterized by recurrent episodes of loss of consciousness associated with generalized spike-and-wave discharges, with an abrupt onset and offset, in the thalamocortical system. In the absence of detailed neurophysiological studies in humans, many of the concepts regarding the pathophysiological basis of absence seizures are based on studies in animal models. Each of these models has its particular strengths and limitations, and the validity of findings from these models for the human condition cannot be assumed. Consequently, studies in different models have produced some conflicting findings and conclusions. A long-standing concept, based primarily from studies in vivo in cats and in vitro brain slices, is that these paroxysmal electrical events develop suddenly from sleep-related spindle oscillations. More specifically, it is proposed that the initial mechanisms that underlie absence-related spike-and-wave discharges are located in the thalamus, involving especially the thalamic reticular nucleus. By contrast, more recent studies in well-established, genetic models of absence epilepsy in rats demonstrate that spike-and-wave discharges originate in a cortical focus and develop from a wake-related natural corticothalamic sensorimotor rhythm. In this review we integrate recent findings showing that, in both the thalamus and the neocortex, genetically-determined, absence-related spike-and-wave discharges are the manifestation of hypersynchronized, cellular, rhythmic excitations and inhibitions that result from a combination of complex, intrinsic, synaptic mechanisms. Arguments are put forward supporting the hypothesis that layer VI corticothalamic neurons act as 'drivers' in the generation of spike-and-wave discharges in the somatosensory thalamocortical system that result in corticothalamic resonances particularly initially involving the thalamic reticular nucleus. However an important unresolved question is: what are the cellular and network mechanisms responsible for the switch from physiological, wake-related, natural oscillations into pathological spike-and-wave discharges? We speculate on possible answers to this, building particularly on recent findings from genetic models in rats.

Diurnal gene expression patterns of T-type calcium channels and their modulation by ethanol

The transient (T-type) calcium channel participates in the generation of normal brain rhythms as well as abnormal rhythms associated with a range of neurological disorders. There are three different isoforms of T-type channels and all are particularly enriched in the thalamus, which is involved in generating many of these rhythms. We report a novel means of T-type channel regulation in the thalamus that involves diurnal regulation of gene expression. Using real time polymerase chain reaction we detected a diurnal pattern of gene expression for all T-type channel transcripts. The peak of gene expression for the CaV3.1 transcript occurred close to the transition from active to inactive (sleep) states, while expression for both CaV3.2 and CaV3.3 peaked near the transition of inactive to active phase. We assessed the effect of chronic consumption of ethanol on these gene expression patterns by examining thalamic tissues of ethanol-consuming cohorts that were housed with the controls, but which received ethanol in the form of a liquid diet. Ethanol consumption resulted in a significant shift of peak gene expression of approximately 5 h for CaV3.2 toward the normally active phase of the mice, as well as increasing the overall gene expression levels by approximately 1.7-fold. Peak gene expression was significantly increased for both CaV3.2 and CaV3.3. Measurements of CaV3.3 protein expression reflected increases in gene expression due to ethanol. Our results illustrate a novel regulatory mechanism for T-type calcium channels that is consistent with their important role in generating thalamocortical sleep rhythms, and suggests that alterations in the pattern of gene expression of these channels could contribute to the disruption of normal sleep by ethanol.

Unique presynaptic and postsynaptic roles of Group II metabotropic glutamate receptors in the modulation of thalamic network activity

The thalamic reticular nucleus (TRN) is a sheet of GABAergic neurons that project to other TRN neurons and to associated thalamocortical relay nuclei. The TRN receives glutamatergic synaptic inputs from cortex as well as reciprocal inputs from the collaterals of thalamocortical neurons. In addition to ionotropic glutamate receptors, metabotropic glutamate receptors (mGluRs) are present in the TRN circuitry. Using whole cell voltage clamp recordings, we pharmacologically characterized unique pre- and postsynaptic functions for Group II mGluRs (mGluR 2 and mGluR 3) within the TRN circuitry in ferrets. mGluR 2 was found on presynaptic cortical axon terminals in the TRN, where it reduced glutamate release, while mGluR 3 acted postsynaptically on TRN cells to increase membrane conductance. Using miniature inhibitory postsynaptic current analysis, we also found that picrotoxin-sensitive intra-TRN GABA-mediated neurotransmission was not affected by administration of a Group II mGluR agonist, indicating that neither mGluR 2 nor 3 acts on presynaptic GABA-containing terminals within the TRN. Because strong corticothalamic activation is implicated in abnormal thalamic rhythms, we used extracellular recordings in the lateral geniculate nucleus to study the effect of Group II mGluR agonists upon these slow oscillations. We induced approximately 3 Hz spike-and-wave discharge activity through corticothalamic stimulation, and found that such activity was reduced in the presence of the Group II mGluR agonist, (-)-2-oxa-4-aminobicyclo[3.1.0]hexane-4,6-dicarboxylate (LY379268). These data indicate that Group II mGluR reduce the impact of corticothalamic excitation, and that they may be a useful target in the reduction of absence-like rhythms.

Grouping of brain rhythms in corticothalamic systems

Different brain rhythms, with both low-frequency and fast-frequency, are grouped within complex wave-sequences. Instead of dissecting various frequency bands of the major oscillations that characterize the brain electrical activity during states of vigilance, it is conceptually more rewarding to analyze their coalescence, which is due to neuronal interactions in corticothalamic systems. This concept of unified brain rhythms does not only include low-frequency sleep oscillations but also fast (beta and gamma) activities that are not exclusively confined to brain-activated states, since they also occur during slow-wave sleep. The major factor behind this coalescence is the cortically generated slow oscillation that, through corticocortical and corticothalamic drives, is effective in grouping other brain rhythms. The experimental evidence for unified oscillations derived from simultaneous intracellular recordings of cortical and thalamic neurons in vivo, while recent studies in humans using global methods provided congruent results of grouping different types of slow and fast oscillatory activities. Far from being epiphenomena, spontaneous brain rhythms have an important role in synaptic plasticity. The role of slow-wave sleep oscillation in consolidating memory traces acquired during wakefulness is being explored in both experimental animals and human subjects. Highly synchronized sleep oscillations may develop into seizures that are generated intracortically and lead to inhibition of thalamocortical neurons, via activation of thalamic reticular neurons, which may explain the obliteration of signals from the external world and unconsciousness during some paroxysmal states.

Cellular and network mechanisms of spike-wave seizures

Spike-wave seizures are often considered a relatively "pure" form of epilepsy, with a uniform defect present in all patients and involvement of the whole brain homogeneously. Here, we present evidence against these common misconceptions. Rather than a uniform disorder, spike-wave rhythms arise from the normal inherent network properties of brain excitatory and inhibitory circuits, where they can be provoked by many different insults in several different brain networks. Here we discuss several different cellular and molecular mechanisms that may contribute to the generation of spike-wave seizures, particularly in idiopathic generalized epilepsy. In addition, we discuss growing evidence that electrical, neuroimaging, and molecular changes in spike-wave seizures do not involve the entire brain homogeneously. Rather, spike-wave discharges occur selectively in some thalamocortical networks, while sparing others. It is hoped that improved understanding of the heterogeneous defects and selective brain regions involved will ultimately lead to more effective treatments for spike-wave seizures.

Early Thalamic Injury Associated with Epilepsy and Continuous Spike-Wave during Slow Sleep

PURPOSE

Mechanisms inducing continuous spike-wave during slow sleep (CSWS) in encephalopathy with electrical status epilepticus during sleep are still unclear. Recently, some sporadic cases with early thalamic injury associated with CSWS have been reported. The aim of the study was to investigate in a population of patients with an early thalamic injury the presence of an activation of paroxysmal activities during sleep, their characteristics, and possible relations to neuroimaging and neuropsychological features.

METHODS

Thirty-two patients with prenatal or perinatal thalamic injuries, mostly due to a vascular mechanisms, were fully examined, including neuroimaging, EEG monitoring, and cognitive follow-up.

RESULTS AND CONCLUSIONS

Twenty-nine of 32 patients showed major sleep EEG activation. Among these 29 patients, two different groups were distinguished: the first included the more or less typical CSWS (12 cases), generally with symmetry of spike and waves (SWs) and often with no spindle at all. The other cases had an usual asymmetry of SWs and presence or reduction of spindles, plus other atypical features concerning synchronism and morphology of SWs. Behavioral disorders were significantly more present in patients with a true CSWS; their improvement (and in one case of the three thoroughly followed the improvement of cognitive competence) paralleled the disappearance of CSWS. The generally predominant injury of the lateral aspect of the thalamus included reticular nucleus and ventral nuclei. An imbalance of gamma-aminobutyric acid (GABA)(B)--versus GABA(A)--mediated receptors may be evoked as a cofactor predisposing to CSWS.

Violência durante o sono

Casos de comportamento violento (CV) durante o sono são relatados na literatura. A incidência de comportamento violento durante o sono não é muito conhecida. Um estudo epidemiológico mostra que cerca de 2% da população geral apresentava comportamento violento dormindo e eram predominantemente homens. Neste artigo, os autores descrevem aspectos clínicos e médico-legais envolvidos na investigação do comportamento violento. O comportamento violento se refere a ferimentos auto-infligidos ou infligidos a um terceiro durante o sono. Ocorre, muito freqüentemente, seguindo um despertar parcial no contexto de um transtorno de despertar (parassonias). Os transtornos do sono predominantes diagnosticados são: transtorno de comportamento REM e sonambulismo. O comportamento violento poderia ser precipitado pelo estresse, uso de álcool e drogas, privação do sono ou febre.

Subjective sleep disturbance in patients with partial epilepsy: a questionnaire-based study on prevalence and impact on quality of life

PURPOSE

This study was designed to assess whether sleep disturbance is more frequent among patients with partial seizures and what impact on quality of life (QoL) sleep disturbance may have on patients with partial seizures.

METHODS

Questionnaire booklets were mailed to 1,183 patients from four Dutch clinics. Each patient was asked to find two age- and gender-matched controls to complete the same set of questionnaires [Sleep Diagnosis List (SDL), Medical Outcomes Study (MOS)-Sleep Scale, Groningen Sleep Questionnaire, Epworth Sleepiness Scale, and the SF-36 Health Survey]. The prevalence of sleep disturbance, based on the SDL, was compared between those with partial epilepsy and controls. Mean scores on sleep and the SF-36 Physical (PCS) and Mental (MCS) Component Summary scales were compared.

RESULTS

Responses from 486 patients and 492 controls were analyzed. Respondents with partial epilepsy had a highly significant, twofold higher prevalence of sleep disturbance compared with controls (38.6 vs. 18.0%; p < 0.0001). Most sleep-disorder subscales showed significant abnormalities in respondents with epilepsy, compared with controls. Mean SF-36 MCS and PCS scores were significantly lower in respondents with epilepsy compared with controls in both the strata with sleep disturbance and without (all p values <0.05). The presence of a sleep disturbance in respondents with epilepsy was associated with the greatest impairment in QoL.

CONCLUSIONS

Sleep disturbance is more than twice as prevalent in persons with partial epilepsy compared with controls, and most domains of sleep are significantly disturbed. Persons with partial epilepsy have significant QoL impairment, and sleep disturbance further compounds this.

Neocortical seizures: initiation, development and cessation

Different forms of electrical paroxysms in experimental animals mimic the patterns of absence seizures associated with spike-wave complexes at approximately 3 Hz and of Lennox-Gastaut seizures with spike-wave or polyspike-wave complexes at approximately 1.5-2.5 Hz, intermingled with fast runs at 10-20 Hz. Both these types of electrical seizures are preferentially generated during slow-wave sleep. Here, we challenge the hypothesis of a subcortical pacemaker that would account for suddenly generalized spike-wave seizures as well as the idea of an exclusive role of synaptic excitation in the generation of paroxysmal depolarizing components, and we focus on three points, based on multiple intracellular and field potential recordings in vivo that are corroborated by some clinical studies: (a) the role of neocortical bursting neurons, especially fast-rhythmic-bursting neurons, and of very fast oscillations (ripples, 80-200 Hz) in seizure initiation; (b) the cortical origin of both these types of electrical paroxysms, the synaptic propagation of seizures from one to other, local and distant, cortical sites, finally reaching the thalamus, where the synchronous cortical firing excites thalamic reticular inhibitory neurons and thus leads to steady hyperpolarization and phasic inhibitory postsynaptic potentials in a majority of thalamocortical neurons, which might explain the obliteration of signals from the external world and the unconsciousness during absence seizures; and (c) the cessation of seizures, whose cellular mechanisms have only begun to be investigated and remain an open avenue for research.

Is spontaneous high-voltage rhythmic spike discharge in Long Evans rats an absence-like seizure activity?

A distinct high-voltage rhythmic spike (HVRS) discharge characterized by a barrage of negative spikes oscillating at 5-12 Hz was observed in chronically implanted Long Evans rats. Spontaneous HVRS discharges were exhibited in 90% of 40 Long Evans rats and occurred during sudden arrest of ongoing behavior (immobility) with occasional facial/whisker twitching. However, the function of HVRS discharges in Long Evans rats remains inconclusive to date and has been associated with alpha tremor/mu rhythm, attentive mu wave, and absence seizure. To elucidate the function of HVRS discharges in Long Evans rats, several experiments were performed. In a 6-h recording session (12:00-18:00), HVRS activities primarily occurred in several specific vigilance states, being particularly abundant in a short-lasting period before vigilance changes. Several characteristics, such as durations, oscillatory frequencies, and interspike intervals (ISIs) of HVRS discharges, were altered during wake-sleep states. Oscillatory frequencies were negatively correlated with durations of HVRS segments. In addition, ISIs of a HVRS episode exhibited a crescendo-decrescendo pattern. These variable ISIs could explain why a negative correlation was found between oscillatory frequencies and durations of HVRS episodes. Moreover, HVRS discharges were demonstrated to have widespread and near-synchronous distribution to bilateral cortical areas. In addition, innocuous electrical stimuli were unable to stop ongoing HVRS discharges. By contrast, noxious stimuli elicited behavioral arousal and immediately terminated most HVRS discharges. Cortical-evoked potentials in response to mild electrical stimulation under HVRS discharges were different from those under waking state but resemble those under slow-wave sleep with a smaller magnitude. Moreover, the temporal and spectral characteristics of spontaneous HVRS activities were analogous to those of seizure activities induced by penicillin and pentylenetetrazol. The incidence of spontaneous HVRS discharges was significantly decreased by ethosuximide administration. Based on these results, HVRS discharge might not be associated with a voluntary mu-rhythm behavior, instead it behaves as an absence-like seizure activity. These results were also collaborated using other genetic absence-seizure rats, such as WAG/Rij and GAERS rats. Possible mechanisms for the generation and termination of paroxysmal HVRS discharges are also discussed.

Sleep Oscillations Developing into Seizures in Corticothalamic Systems

PURPOSE

The aim of this article is to discuss the neuronal substrates of sleep oscillations leading to seizures consisting of spike-wave (SW) complexes at 2-4 Hz, mimicking those seen in absence epilepsy, or SW and polyspike-wave (PSW) complexes at 1.5-2.5 Hz, often associated with fast runs at 10-15 Hz, as in the Lennox-Gastaut syndrome.

METHODS

Extracellular recordings were done in permanently implanted animals during the natural waking-sleep cycle. Single and dual simultaneous recordings from cortical neurons, cortical and thalamic neurons, or cortical neurons and glial cells were performed in cats under ketamine-xylazine anesthesia.

RESULTS

(a) The minimal substrate of SW seizures is the neocortex because such seizures may occur in thalamectomized animals, in which spindles are absent. In intact-brain animals, SW seizures are initiated in neocortex and spread to the thalamus after a few seconds. The majority of thalamocortical (TC) neurons are steadily hyperpolarized throughout the cortical SW seizures. (b) In the Lennox-Gastaut syndrome, the paroxysmal depolarizing shifts (PDSs) associated with the EEG "spike" of SW/PSW complexes contain an important inhibitory component, whereas the hyperpolarization during the EEG "wave" component is not due to gamma-aminobutryic acid (GABA)ergic inhibitory postsynaptic potentials (IPSPs) but is ascribed to a mixture of disfacilitation and K+ currents. As is also the case with seizures consisting of pure SW complexes, the majority of TC neurons are hyperpolarized during the cortical paroxysms and disinhibited after the cessation of cortical seizures.

CONCLUSIONS

Seizures with SW complexes and of the Lennox-Gastaut type preferentially evolve from sleep oscillations. They are initiated in neocortex and spread to the thalamus after a few seconds. The majority of TC neurons are inhibited during these seizures.

Interactions between membrane conductances underlying thalamocortical slow-wave oscillations

Neurons of the central nervous system display a broad spectrum of intrinsic electrophysiological properties that are absent in the traditional "integrate-and-fire" model. A network of neurons with these properties interacting through synaptic receptors with many time scales can produce complex patterns of activity that cannot be intuitively predicted. Computational methods, tightly linked to experimental data, provide insights into the dynamics of neural networks. We review this approach for the case of bursting neurons of the thalamus, with a focus on thalamic and thalamocortical slow-wave oscillations. At the single-cell level, intrinsic bursting or oscillations can be explained by interactions between calcium- and voltage-dependent channels. At the network level, the genesis of oscillations, their initiation, propagation, termination, and large-scale synchrony can be explained by interactions between neurons with a variety of intrinsic cellular properties through different types of synaptic receptors. These interactions can be altered by neuromodulators, which can dramatically shift the large-scale behavior of the network, and can also be disrupted in many ways, resulting in pathological patterns of activity, such as seizures. We suggest a coherent framework that accounts for a large body of experimental data at the ion-channel, single-cell, and network levels. This framework suggests physiological roles for the highly synchronized oscillations of slow-wave sleep.

From molecules to networks: cortical/subcortical interactions in the pathophysiology of idiopathic generalized epilepsy

Generalized epilepsy involves abnormally synchronized activity in large-scale neuronal networks. Burst firing of action potentials is a potent mechanism for increasing neural synchrony and is thought to enhance cortical and thalamic rhythmic network activity. Absence seizures, a form of generalized epilepsy, occur in children as brief 5- to 10-s periods of behavioral arrest associated with massive 3- to 4-Hz spike-wave discharges in cortical and thalamic networks. Prior research has shown that enhanced burst firing may be crucial for the transition from normal to epileptic activity. Can enhanced burst firing in one region of the nervous system, such as the cortex, transform the entire thalamocortical network from normal activity to spike-and-wave seizures? Enhanced burst firing in corticothalamic neurons may increase gamma-aminobutyric acid-B (GABAB) receptor activation in the thalamus, leading to the slower, more synchronous oscillations seen in spike-and-wave seizures. Does "generalized" spike-wave activity homogeneously involve the entire brain, or are there crucial nodes that are more important than others for the generation and behavioral manifestations of generalized seizures? Animal and human data suggest that so-called generalized seizures involve selective thalamocortical networks while sparing others. A greater understanding of these molecular and network mechanisms will ultimately lead to improved targeted therapies for generalized epilepsy.

Neocortical very fast oscillations (ripples, 80-200 Hz) during seizures: intracellular correlates

Multi-site field potential and intracellular recordings from various neocortical areas were used to study very fast oscillations or ripples (80-200 Hz) during electrographic seizures in cats under ketamine-xylazine anesthesia. The animals displayed spontaneously occurring and electrically induced seizures comprising spike-wave complexes (2-3 Hz) and fast runs (10-20 Hz). Neocortical ripples had much higher amplitudes during seizures than during the slow oscillation preceding the onset of seizures. A series of experimental data from the present study supports the hypothesis that ripples are implicated in seizure initiation. Ripples were particularly strong at the onset of seizures and halothane, which antagonizes the occurrence of ripples, also blocked seizures. The firing of electrophysiologically defined cellular types was phase-locked with ripples in simultaneously recorded field potentials. This indicates that ripples during paroxysmal events are associated with a coordination of firing in a majority of neocortical neurons. This was confirmed with dual intracellular recordings. Based on the amplitude that neocortical ripples reach during paroxysmal events, we propose a mechanism by which neocortical ripples during normal network activity could actively participate in the initiation of seizures on reaching a certain threshold amplitude. This mechanism involves a vicious feedback loop in which very fast oscillations in field potentials are a reflection of synchronous action potentials, and in turn these oscillations help generate and synchronize action potentials in adjacent neurons through electrical interactions.

Noradrenergic Control of Thalamic Oscillation: the Role of alpha-2 Receptors

The effects of alpha-adrenergic drugs on neocortical high voltage spike and wave spindles (HVS), reflecting thalamic oscillation, was investigated in freely moving rats. HVS occurred spontaneously in the awake but immobile animal. Peripheral administration of the alpha-1 antagonist, prazosin and alpha-2 agonists, xylazine and clonidine increased the incidence and duration of HVS in a dose-dependent manner. The alpha-2 antagonist, yohimbine and the tricyclic antidepressants, desipramine and amitriptyline, significantly decreased the incidence of the neocortical HVS. Bilateral microinjections of the alpha-2 agonists into the nucleus ventralis lateralis area of the thalamus, but not into the hippocampus or corpus callosum, was as effective as peripheral injection of these drugs. Xylazine was most effective in Fischer 344 rats that display high spontaneous rate of HVS and less effective in the Sprague - Dawley and Buffalo strains. The HVS-promoting effect of clonidine was antagonized by prior intrathalamic injection of the alpha-2 antagonist, yohimbine. The amplitude of the HVS was increased by picomole amounts of unilaterally-injected clonidine. Neurotoxic destruction of the thalamopetal noradrenergic afferents by intracisternal or intrathalamic injection of 6-hydroxydopamine, but not by peripheral administration of DSP-4, increased the incidence of HVS. Importantly, intrathalamic administration of xylazine continued to induce HVS after destroying the thalamic noradrenergic terminals. Following downregulation of the alpha-2 adrenoceptors by chronic administration (3 weeks) of amitriptylene the incidence of HVS decreased and the effectiveness of intrathalamic xylazine on the induction of HVS was significantly reduced. Based on these findings, we suggest that a major action of alpha-2 adrenergic drugs on thalamic oscillation may be mediated by postsynaptic alpha-2 adrenoceptors located on the thalamocortical neurons. We hypothesize that noradrenaline in the thalamus has a dual effect on the relay cells: blocking and promoting thalamic oscillation via alpha-1 and alpha-2 receptors, respectively. The final physiological effect is assumed to be a function of the relative density and affinity of these adrenergic receptor subtypes.

Diurnal variation in the proconvulsant effect of 3-mercaptopropionic acid and the anticonvulsant effect of androsterone in the Syrian hamster

GABA is the principal neurotransmitter of the mammalian circadian system, and its activity is subject to diurnal and circadian variations, with maximal values in hypothalamic turnover, content and binding during the night. In this study we have examined rhythms in the proconvulsant effect of inhibition of glutamate decarboxylase (GAD) in hamsters (Mesocricetus auratus) as well as the anticonvulsant effect of androsterone, a neurosteroid that positively modulates the GABA(A) receptor. Administration of 10-60 mg/Kg of 3-mercaptopropionic acid (3-MPA, a GAD inhibitor) induced convulsions that were analyzed by an ad-hoc severity scale, with a lower sensitivity threshold at 24:00 h. Moreover, the latency for first and maximal convulsive response times was significantly lower at night. A similar temporal profile (maximal effect at midnight) was found for picrotoxin-induced seizures. Androsterone (40 mg/Kg) completely inhibited 3-MPA-induced tonic/clonic seizures at 12:00 h, while it had a partial inhibitory effect at 24:00 h. These results support the importance of temporal regulation of GABAergic modulation in the central nervous system.