The role of the corpus callosum in bilateral interhemispheric synchrony of spike and wave discharge in feline generalized penicillin epilepsy

Functional Dissection of Ipsilateral and Contralateral Neural Activity Propagation Using Voltage-Sensitive Dye Imaging in Mouse Prefrontal Cortex

Prefrontal cortex (PFC) intrahemispheric activity and the interhemispheric connection have a significant impact on neuropsychiatric disorder pathology. This study aimed to generate a functional map of FC intrahemispheric and interhemispheric connections. Functional dissection of mouse PFCs was performed using the voltage-sensitive dye (VSD) imaging method with high speed (1 ms/frame), high resolution (256 × 256 pixels), and a large field of view (∼10 mm). Acute serial 350 μm slices were prepared from the bregma covering the PFC and numbered 1-5 based on their distance from the bregma (i.e., 1.70, 1.34, 0.98, 0.62, and 0.26 mm) with reference to the Mouse Brain Atlas (Paxinos and Franklin, 2008). The neural response to electrical stimulation was measured at nine sites and then averaged, and a functional map of the propagation patterns was created. Intracortical propagation was observed in slices 3-5, encompassing the anterior cingulate cortex (ACC) and corpus callosum (CC). The activity reached area 33 of the ACC. Direct white matter stimulation activated area 33 in both hemispheres. Similar findings were obtained via DiI staining of the CC. Imaging analysis revealed directional biases in neural signals traveling within the ACC, whereby the signal transmission speed and probability varied based on the signal direction. Specifically, the spread of neural signals from cg2 to cg1 was stronger than that from cingulate cortex area 1(cg1) to cingulate cortex area 2(cg2), which has implications for interhemispheric functional connections. These findings highlight the importance of understanding the PFC functional anatomy in evaluating neuromodulators like serotonin and dopamine, as well as other factors related to neuropsychiatric diseases.

Maladaptive myelination promotes generalized epilepsy progression

Activity-dependent myelination can fine-tune neural network dynamics. Conversely, aberrant neuronal activity, as occurs in disorders of recurrent seizures (epilepsy), could promote maladaptive myelination, contributing to pathogenesis. In this study, we tested the hypothesis that activity-dependent myelination resulting from absence seizures, which manifest as frequent behavioral arrests with generalized electroencephalography (EEG) spike-wave discharges, promote thalamocortical network hypersynchrony and contribute to epilepsy progression. We found increased oligodendrogenesis and myelination specifically within the seizure network in two models of generalized epilepsy with absence seizures (Wag/Rij rats and Scn8a^+/mut mice), evident only after epilepsy onset. Aberrant myelination was prevented by pharmacological seizure inhibition in Wag/Rij rats. Blocking activity-dependent myelination decreased seizure burden over time and reduced ictal synchrony as assessed by EEG coherence. These findings indicate that activity-dependent myelination driven by absence seizures contributes to epilepsy progression; maladaptive myelination may be pathogenic in some forms of epilepsy and other neurological diseases.

Distinct roles of rodent thalamus and corpus callosum in seizure generalization

Objective

Bilateral synchronous cortical activity occurs during sleep, attention, and seizures. Canonical models place the thalamus at the center of bilateral cortical synchronization because it generates bilateral sleep spindle oscillations and primarily generalized absence seizures. However, classical studies suggest that the corpus callosum mediates bilateral cortical synchronization.

Methods

We mapped the spread of right frontal lobe‐onset, focal to bilateral seizures in mice and modified it using chemo and optogenetic suppression of motor thalamic nucleus and corpus callosotomy.

Results

Seizures from the right cortex spread faster to the left cortex than to the left thalamus. The 2 thalami have minimal monosynaptic commissural connections compared to the massive commissure corpus callosum. Chemogenetic and closed‐loop optogenetic inhibition of the right ventrolateral thalamic nucleus did not alter inter‐hemispheric seizure spread. However, anterior callosotomy delayed bilateral seizure oscillations.

Interpretation

Thalamocortical oscillations amplify focal onset motor seizures, and corpus callosum spreads them bilaterally. ANN NEUROL 2022;91:682–696

Defective interhemispheric inhibition in drug-treated focal epilepsies

BACKGROUND

Focal epilepsies (FEs) arise from a lateralized network, while in generalized epilepsies (GEs) there is a bilateral involvement from the outset. Intuitively, the corpus callosum is the anatomical substrate for interhemispheric spread.

OBJECTIVE

We used transcranial magnetic stimulation (TMS) to explore whether there are any physiological differences in the corpus callosum of drug-treated patients with FE and those with genetic GE (GGE), compared to healthy subjects (HS).

METHODS

TMS was used to measure the interhemispheric inhibition (IHI) from right-to-left primary motor cortex (M1) and viceversa in 16 patients with FE, 17 patients with GGE and 17 HS. A conditioning stimulus (CS) was given to one M1 10 and 50 ms before a test stimulus delivered to the contralateral M1. Motor evoked potentials (MEPs) were analysed both as a function of the side of stimulation and of the epileptic focus (left-right).

RESULTS

In HS, IHI was reproducible with suppression of MEPs at ISIs of 10 and 50 ms. Similar effects occurred in GGE patients. FE patients behaved differently, since IHI was significantly reduced bilaterally. When FE patients were stratified according to the side of their epileptic focus, the long-ISI IHI (=50 ms) appeared to be defective only when the CS was applied over the "focal" hemisphere.

CONCLUSIONS

FE patients had a defective inhibitory response of contralateral M1 to inputs travelling from the "focal" hemisphere that was residual to the drug action. Whilst IHI changes would not be crucial for the GGE pathophysiology, they may represent one key factor for the contralateral spread of focal discharges, and seizure generalization.

Microstructural Changes in Absence Seizure Children: A Diffusion Tensor Magnetic Resonance Imaging Study

BACKGROUND

Absence seizures are a subtype of epileptic seizures clinically characterized by transient alterations in states of consciousness and by electroencephalography indicating diffuse spike-wave discharges (SWD). Conventional brain magnetic resonance imaging (MRI) is not routinely used to establish the diagnosis, but rather to rule out other diseases. The present study investigated tissue integrity in children with SWD epilepsy using diffusion tensor imaging (DTI).

METHODS

Magnetic resonance imaging (MRI)-DTI was conducted in 18 patients with absence seizures and 10 control participants. Brain areas were evaluated using diffusion maps, and fractional anisotropy (FA), mean diffusivity (MD), parallel diffusivity (λ||), and perpendicular diffusivity (λ⊥) values were extracted and analyzed. Tractography at the regions of abnormal diffusion indices was then reconstructed in each group, and tract symmetry was evaluated by an index of asymmetry (AI). Statistical analyses were performed using nonparametric Mann-Whitney U tests, with p values < 0.05 indicating statistical significance.

RESULTS

Compared to the control group, patients with SWD epilepsy had lower FA values and higher MD values at the genu of the corpus callosum. There was also a stronger negative correlation between MD and FA values at the genu of the corpus callosum in patients than in control participants. The AI for the fiber tracts through the genu of the corpus callosum in the SWD group was significantly higher than that of the control group, indicating that tract distribution was more asymmetric in patients with epilepsy. There were no significant differences between groups in diffusion indices for other brain areas.

CONCLUSION

We observed microstructural changes in the genu of the corpus callosum, as well as reduced FA values, increased λ⊥ values, increased MD values, and asymmetric distribution of fiber tracts, indicating that DTI is more sensitive than conventional MRI to detect brain abnormalities in children with absence seizures.

Heterogeneity of trans-callosal structural connectivity and effects on resting state subnetwork integrity may underlie both wanted and unwanted effects of therapeutic corpus callostomy

BACKGROUND

The corpus callosum (CC) is the primary structure supporting interhemispheric connectivity in the brain. Partial or complete surgical callosotomy may be performed for the palliation of intractable epilepsy. A variety of disconnection syndromes are recognised after injury to or division of the CC however their mechanisms are poorly understood and their occurrence difficult to predict. We use novel high resolution structural connectivity analyses to demonstrate reasons for this poor predictability.

METHODS

Diffusion weighted MRI data from five healthy adult controls was subjected to novel high-resolution structural connectivity analysis. We simulated the effects of CC lesions of varying extents on the integrity of resting state subnetworks (RSNs).

RESULTS

There is substantial between-individual variation in patterns of CC connectivity. However in all individuals termination points of callosal connections mostly involve medial and superior sensory-motor areas. Superior temporal and lateral sensory-motor areas were not involved. Resting state networks showed selective vulnerability to simulated callosotomy of progressively greater anterior to posterior extent. The default mode network was most vulnerable followed by, in decreasing order: frontoparietal, limbic, somatomotor, ventral attention, dorsal attention and visual subnetworks.

CONCLUSION

Consideration of the selective vulnerability of resting state sub-networks, and of between-individual variability in connectivity patterns, sheds new light on the occurrence of both wanted and unwanted effects of callosotomy. We propose that beneficial effects (seizure reduction) relate to disruption of the default mode network, with unwanted "disconnection syndrome" effects due to disruption particularly of the somatomotor and frontoparietal RSNs. Our results may also explain why disconnection syndromes primary reflect lateralised sensory-motor problems (e.g. of limb movement) rather than midline function (e.g. tongue movement). Marked between-subject variation in callosal connectivity may underlie the poor predictability of effects of callosotomy. High resolution structural connectivity studies of this nature may be useful in pre-surgical planning of therapeutic callosotomy for intractable epilepsy.

When spikes are symmetric, ripples are not: Bilateral spike and wave above 80 Hz in focal and generalized epilepsy

OBJECTIVE

To evaluate scalp ripples distribution in secondary bilateral synchrony as a tool to lateralize the epileptic focus and to differentiate focal from generalized epilepsy.

METHODS

Seventeen EEG recordings with bilateral synchronous discharges of focal (focal group-FG: 10) and generalized (generalized group-GG: 7) epilepsy patients were selected for spikes and ripples marking; the spike-normalized ripple rate was calculated in each hemisphere (right/left - anterior/posterior) and a ripple-dominant hemisphere (the one with the highest rate) was identified. Concordance in FG between the ripple dominant hemisphere and the hemisphere of clinical lateralization was evaluated. The ripple-dominant/ripple-nondominant spike-normalized ripple rate ratio was studied to compare groups.

RESULTS

In FG the hemisphere of clinical lateralization and the ripple-dominant hemisphere were 100% concordant. In GG only 3/7 patients showed ripples (vs 10/10 FG), all with anterior dominance. No difference in hemisphere ripple dominance between groups was found.

CONCLUSIONS

Ripples in secondary bilateral synchrony help to lateralize the epileptic focus but do not help to differentiate between focal and generalized epilepsy. This is the first report of visually identified ripples in idiopathic generalized epilepsy.

SIGNIFICANCE

Ripples confirm the clinical lateralization of the epileptic focus in secondary bilateral synchrony but cannot distinguish between focal and generalized epilepsy.

Endoscopic-Assisted (Through a Mini Craniotomy) Corpus Callosotomy Combined With Anterior, Hippocampal, and Posterior Commissurotomy in Lennox-Gastaut Syndrome

BACKGROUND:

Corpus callosotomy is a palliative procedure especially for Lennox-Gastaut semiology without localization with drop attacks.

OBJECTIVE:

To describe endoscopic-assisted complete corpus callosotomy combined with anterior, hippocampal, and posterior commissurotomy.

METHODS:

Patients with drug refractory epilepsy having drop attacks as the predominant seizure type, bilateral abnormalities on imaging, and moderate to severe mental retardation were included. All underwent a complete workup (including magnetic resonance imaging).

RESULTS:

Patients (n = 16, mean age 11.4 ± 6.4 years, range 6-19 years) had a mean seizure frequency of 24.5 ± 19.8/days (range 1-60) and a mean intelligence quotient of 25.23 ± 10.71. All had syndromic diagnosis of Lennox-Gastaut syndrome, with the following etiologies: hypoxic insult (10), lissencephaly (2), bilateral band heterotropia (2), and microgyria and pachygyria (2). Surgery included complete callosotomy and the section of anterior and posterior commissure by microscopic approach through a mini craniotomy (11) and endoscopic-assisted approach (5). Complications included meningitis (1), hyperammonemic encephalopathy (2), and acute transient disconnection (5). There was no mortality or long-term morbidity. Mean follow-up was 18 ± 4.7 months (range 16-27 months). Drop attacks stopped in all. Seizure frequency/duration decreased &gt;90% in 10 patients and &gt;50% in 5 patients, and increased in 1 patient. All patients attained presurgical functional levels in 3 to 6 months. Child behavior checklist scores showed no deterioration. Parental questionnaires reported 90% satisfaction attributed to the control of drop attacks. The series was compared retrospectively with an age/sex-matched cohort (where a callosotomy only was performed), and showed better outcome for drop attacks (P &lt; .003).

CONCLUSION:

This preliminary study demonstrated the efficacy and safety of complete callosotomy with anterior, hippocampal, and posterior commissurotomy in Lennox-Gastaut syndrome (drop attacks) with moderate to severe mental retardation.

Generalized tonic-clonic seizures: aberrant interhemispheric functional and anatomical connectivity

PURPOSE

To characterize interhemispheric functional and anatomic connectivity in patients with idiopathic generalized epilepsy and generalized tonic-clonic seizures (GTCS).

MATERIALS AND METHODS

This retrospective study was approved by the local institutional review board and was HIPAA compliant. All participants provided written informed consent. Resting-state functional and structural magnetic resonance images were acquired in 52 patients with GTCS and 65 healthy control subjects. The functional connectivity between bilateral homotopic voxels was calculated. Homotopic regions showing abnormal functional connectivity in patients were adopted as regions of interest for an analysis of diffusion-tensor imaging tractography. The fractional anisotropy and fiber length were compared between groups. Two-sample t test and nonparametric correlation analysis were used.

RESULTS

Compared with control subjects, patients showed increased interhemispheric functional connectivity between the bilateral cuneus (P = .0008, corrected) and anterior cingulate cortex (P = .0003, corrected) and decreased functional connectivity between the bilateral olfactory cortex (P = .00005, corrected), inferior frontal gyrus (P = .00005, corrected), supramarginal gyrus (P = .0002, corrected), and temporal pole (P = .0003, corrected). Furthermore, the fiber length of the commissural fiber bundles connecting the bilateral anterior cingulate cortex (t = -2.30; P = .03, uncorrected) and the bilateral cuneus was shorter in patients than in control subjects (t = -3.19; P = .002, uncorrected).

CONCLUSION

Our findings show that the bilateral anterior cingulate cortex may be critical to the pathophysiology of patients with GTCS and suggest that the corresponding commissural fiber bundle in the genu of the corpus callosum is a potential target for future surgical treatment in patients with intractable GTCS.

Is there such a thing as "generalized" epilepsy?

The distinction between generalized and partial epilepsies is probably one, if not the most, pregnant assertions in modern epileptology. Both absence and generalized tonic-clonic seizures, the prototypic seizures found in generalized epilepsies, are classically seen as the result of a rapid, synchronous recruitment of neuronal networks resulting in impairment of consciousness and/or convulsive semiology. The term generalized also refers to electroencephalographic presentation, with bilateral, synchronous activity, such as the classical 3 Hz spike and wave discharges of typical absence epilepsy. However, findings obtained from electrophysiological and functional imaging studies over the last few years, contradict this view, showing a rather focal onset for most of the so-called generalized seizure types. Therefore, we ask here the question whether "generalized epilepsy" does indeed exist.

Outcomes After Anterior or Complete Corpus Callosotomy in Children

BACKGROUND:

Corpus callosotomy (CC) is a valuable palliative surgical option for children with medically refractory epilepsy due to generalized or multifocal cortical seizure onset.

OBJECTIVE:

To investigate the extent of CC resulting in optimal seizure control in a pediatric patient population and to evaluate the modification of seizure profile after various CC approaches.

METHODS:

The records of 58 children (3–22 years of age at the time of surgery) with medically refractory epilepsy who underwent CC between 1995 and 2011 were retrospectively reviewed.

RESULTS:

Anterior two thirds callosotomy resulted in resolution of absence (P = .03) and astatic (P = .03) seizures, whereas anterior two thirds callosotomy followed by second-stage completion resulted in resolution of generalized tonic-clonic (GTC) (P = .03), astatic (P = .005), and myoclonic (P = .03) seizures in addition to a trend toward resolution of absence seizures (P = .08). Single-stage upfront complete callosotomy resulted in resolution of absence (P = .002), astatic (P &lt; .0001), myoclonic (P = .007), and complex partial (P = .008) seizures in addition to a trend toward resolution of GTC (P = .06). In comparing a composite of subjects who underwent anterior two thirds callosotomy alone or 2-stage complete callosotomy before the second stage to complete the callosotomy with subjects who underwent upfront complete CC, a more favorable outcome was found in those with the upfront complete CC (P = .02).

CONCLUSION:

Single-stage upfront complete callosotomy is effective in relieving a broader spectrum of seizure types than anterior two thirds callosotomy or 2-stage complete callosotomy in children. The advantages of single-stage complete callosotomy must be weighed against the potentially higher risk of neurological and operative complications.

Increased resting functional connectivity in spike-wave epilepsy in WAG/Rij rats

PURPOSE

Functional magnetic resonance imaging (fMRI)-based resting functional connectivity is well suited for measuring slow correlated activity throughout brain networks. Epilepsy involves chronic changes in normal brain networks, and recent work demonstrated enhanced resting fMRI connectivity between the hemispheres in childhood absence epilepsy. An animal model of this phenomenon would be valuable for investigating fundamental mechanisms and testing therapeutic interventions.

METHODS

We used fMRI-based resting functional connectivity for studying brain networks involved in absence epilepsy. Wistar Albino Glaxo rats from Rijswijk (WAG/Rij) exhibit spontaneous episodes of staring and unresponsiveness accompanied by spike-wave discharges (SWDs) resembling human absence seizures in behavior and electroencephalography (EEG). Simultaneous EEG-fMRI data in epileptic WAG/Rij rats in comparison to nonepileptic Wistar controls were acquired at 9.4 T. Regions showing cortical fMRI increases during SWDs were used to define reference regions for connectivity analysis to investigate whether chronic seizure activity is associated with changes in network resting functional connectivity.

KEY FINDINGS

We observed high degrees of cortical-cortical correlations in all WAG/Rij rats at rest (when no SWDs were present), but not in nonepileptic controls. Strongest connectivity was seen between regions most intensely involved in seizures, mainly in the bilateral somatosensory and adjacent cortices. Group statistics revealed that resting interhemispheric cortical-cortical correlations were significantly higher in WAG/Rij rats compared to nonepileptic controls.

SIGNIFICANCE

These findings suggest that activity-dependent plasticity may lead to long-term changes in epileptic networks even at rest. The results show a marked difference between the epileptic and nonepileptic animals in cortical-cortical connectivity, indicating that this may be a useful interictal biomarker associated with the epileptic state.

Propagation of epileptiform events across the corpus callosum in a cingulate cortical slice preparation

We report on a novel mouse in vitro brain slice preparation that contains intact callosal axons connecting anterior cingulate cortices (ACC). Callosal connections are demonstrated by the ability to regularly record epileptiform events between hemispheres (bilateral events). That the correlation of these events depends on the callosum is demonstrated by the bisection of the callosum in vitro. Epileptiform events are evoked with four different methods: (1) bath application of bicuculline (a GABA-A antagonist); (2) bicuculline+MK801 (an NMDA receptor antagonist), (3) a zero magnesium extracellular solution (0Mg); (4) focal application of bicuculline to a single cortical hemisphere. Significant increases in the number of epileptiform events, as well as increases in the ratio of bilateral events to unilateral events, are observed during bath applications of bicuculline, but not during applications of bicuculline+MK-801. Long ictal-like events (defined as events >20 seconds) are only observed in 0Mg. Whole cell patch clamp recordings of single neurons reveal strong feedforward inhibition during focal epileptiform events in the contralateral hemisphere. Within the ACC, we find differences between the rostral areas of ACC vs. caudal ACC in terms of connectivity between hemispheres, with the caudal regions demonstrating shorter interhemispheric latencies. The morphologies of many patch clamped neurons show callosally-spanning axons, again demonstrating intact callosal circuits in this in vitro preparation.

Resting functional connectivity between the hemispheres in childhood absence epilepsy

OBJECTIVE

The fundamental mechanisms by which childhood absence epilepsy (CAE) changes neural networks even between seizures remain poorly understood. During seizures, cortical and subcortical networks exhibit bihemspheric synchronous activity based on prior EEG-fMRI studies. Our aim was to investigate whether this abnormal bisynchrony may extend to the interictal period, using a blood oxygen level-dependent (BOLD) resting functional connectivity approach.

METHODS

EEG-fMRI data were recorded from 16 patients with CAE and 16 age- and gender-matched controls. Three analyses were performed. 1) Using 16 pairs of seizure-related regions of interest (ROI), we compared the between-hemisphere interictal resting functional connectivity of patients and controls. 2) For regions showing significantly increased interhemispheric connectivity in CAE, we then calculated connectivity to the entire brain. 3) A paired-voxel approach was performed to calculate resting functional connectivity between hemispheres without the constraint of predefined ROIs.

RESULTS

We found significantly increased resting functional connectivity between hemispheres in the lateral orbitofrontal cortex of patients with CAE compared to normal controls. Enhanced between-hemisphere connectivity localized to the lateral orbitofrontal cortex was confirmed by all 3 analysis methods.

CONCLUSIONS

Our results demonstrate abnormal increased connectivity between the hemispheres in patients with CAE in seizure-related regions, even when seizures were not occurring. These findings suggest that the lateral orbitofrontal cortex may play an important role in CAE pathophysiology, warranting further investigation. In addition, resting functional connectivity analysis may provide a promising biomarker to improve our understanding of altered brain function in CAE during the interictal period.

Differences in corpus callosum volume and diffusivity between temporal and frontal lobe epilepsy

We analyzed volume and diffusivity measures of the corpus callosum (CC) in patients with temporal (TLE) and frontal (FLE) lobe epilepsy in comparison with healthy subjects. On high-resolution T1-weighted scans of 18 controls and 44 patients the volumes (cm(3)) of Witelson regions (WRs) and the entire CC were measured. The apparent diffusion coefficients (ADCs, 10(-5)mm(2)s(-1)) for the entire CC and three areas of interest were measured from co-registered ADC maps. The CC of patients with TLE and FLE, corrected for total brain volume, was smaller than that of controls. Patients' ADC values were higher than those of controls. Findings were significant for WR1, WR2, and WR6, the CC regions connecting the frontal and temporal lobes. Patients with FLE had smaller WR1 and higher ADC values; in patients with TLE, the findings were similar for WR6. Atrophy and increased diffusivity in subregions of the CC connecting homotopic contralateral cortical regions indicate anatomical abnormalities extending beyond the epileptogenic zone in FLE and TLE.

Imaging foci of epileptic discharges from simultaneous EEG and fMRI using the canonical HRF

PURPOSE

Simultaneous electroencephalography and functional magnetic resonance imaging (EEG-fMRI) is considered as a powerful and non-invasive method that allows definition of the irritative zone. However, the complex interictal epileptic discharge (IED) may be present in some patients, and sometimes no active foci can be localized using General Linear Model (GLM) which is a widely adopted tool in EEG-fMRI study. The purpose of this study is to develop a new scheme to improve the detectability and localize the canonical HRF localizable foci.

METHOD

Various IEDs are classified using a combination of an independent component analysis (ICA) and a temporal correlation analysis between the independent components and the raw EEG channel; and the classified IEDs are then separately used for foci localization. This scheme is tested by ten patients with variable IEDs, including two patients whose activity could not be identified by common method.

RESULT

Applying this scheme to the two patients, some foci consistent with electroclinical data were localized. When it was applied to the remaining eight patients with positive results using common method, 2-4 types of IEDs were classified, and the activity could be identified from at least one type of IED. The results were similar to that received from common method.

CONCLUSION

These results indicate that the proposed scheme could enhance the imaging of the localizable foci by isolating its IEDs. This scheme is potentially a useful tool for epilepsy clinic.

Centro-median stimulation yields additional seizure frequency and attention improvement in patients previously submitted to callosotomy

RATIONALE

Deep brain stimulation (DBS) has been increasingly used in the treatment of refractory epilepsy over the last decade. We report on the outcome after thalamic centro-median (CM) DBS in patients with generalized epilepsy who had been previously treated with extended callosal section.

METHODS

Four consecutive patients with generalized epilepsy who were previously submitted to callosal section and had at least 1 year of follow-up after deep brain implantation were studied. Age ranged from 19 to 44 years. All patients were submitted to bilateral CM thalamic DBS. Post-operative CT scans documented the electrode position in all patients. All patients had pre- and post-stimulation prolonged interictal scalp EEG recordings, including spike counts. Attention level was evaluated by means of the SNAP-IV questionnaire. The pre-implantation anti-epileptic drug regimen was maintained post-operatively in all patients.

RESULTS

Post-operative CT documented that all electrodes were correctly located. There was no morbidity or mortality. Seizure frequency reduction ranging from 65 to 95% and increased attention level was seen in all patients. Interictal spiking frequency was reduced from 25 to 95%, but their morphology remained the same. There was re-synchronization of interictal discharges during slow-wave sleep in 2 patients.

CONCLUSION

All patients benefit from the procedure. The CM seems to play a role in modulating the epileptic discharges and attention in these patients. On the other hand, it is not the generator of the epileptic abnormality and appeared not to be involved in non-REM sleep-related interictal spiking modulation.

DTI abnormalities in anterior corpus callosum of rats with spike-wave epilepsy

OBJECTIVE

Absence epilepsy is a common seizure disorder in children which can produce chronic psychosocial sequelae. Human patients and rat absence models show bilateral spike-wave discharges (SWD) in cortical regions. We employed diffusion tensor imaging (DTI) in rat absence models to detect abnormalities in white matter pathways connecting regions of seizure activity.

METHODS

We studied Wistar albino Glaxo rats of Rijswijk (WAG/Rij), genetic absence epilepsy rats of Strasbourg (GAERS), and corresponding nonepileptic control strains. Ex vivo DTI was performed at 9.4 T with diffusion gradients applied in 16 orientations. We compared fractional anisotropy (FA), perpendicular (lambda(perpendicular)) and parallel (lambda(||)) diffusivity between groups using t-maps and region of interest (ROI) measurements.

RESULTS

Adult epileptic WAG/Rij rats exhibited a localized decrease in FA in the anterior corpus callosum. This area was confirmed by tractography to interconnect somatosensory cortex regions most intensely involved in seizures. This FA decrease was not present in young WAG/Rij rats before onset of SWD. GAERS, which have more severe SWD than WAG/Rij, exhibited even more pronounced callosal FA decreases. Reduced FA in the epileptic animals originated from an increased lambda(perpendicular) with no significant changes in lambda(||).

INTERPRETATION

Reduced FA with increased lambda(perpendicular) suggests that chronic seizures cause reduction in myelin or decreased axon fiber density in white matter pathways connecting regions of seizure activity. These DTI abnormalities may improve the understanding of chronic neurological difficulties in children suffering with absence epilepsy, and may also serve as a noninvasive biomarker for monitoring beneficial effects of treatment.

Outcome after extended callosal section in patients with primary idiopathic generalized epilepsy

PURPOSE

We report the outcome of patients with refractory idiopathic generalized epilepsy (IGE) who were submitted to extended one-stage callosal section.

METHODS

Eleven patients with IGE who were submitted to extended one-stage callosal section were studied. Preoperative workup included history and neurologic examination, interictal, and ictal electroencephalography (EEG) recording, high resolution 1.5T magnetic resonance imaging (MRI) and intelligence quotient (IQ) testing. All patients were submitted to extended one-stage microsurgical callosal section, leaving only the splenium intact.

RESULTS

Preoperative ictal patterns included repetitive spike and wave or polyspike and wave discharges or fast epileptic recruiting rhythm. MRI showed no focal lesions. Preoperatively, mean general IQ was 85. Postoperatively, at least a 75% reduction in the frequency of generalized tonic-clonic seizures was noted in all patients. In three patients absences disappeared completely, and the others had at least 90% reduction in seizure frequency. Only one patient had myoclonic seizures preoperatively, and these seizures disappeared after callosal section. After surgery, mean general IQ was 89. A very clear increase in attention level was noted in all patients. Postoperative interictal EEG recordings showed rupture of bilateral synchrony in all patients.

DISCUSSION

This article reports on a large and homogeneous series of patients with refractory IGE submitted for callosal section. There was a marked decrease in generalized seizure frequency and increase in the attention level in this patient population. Our results suggest that corticocortical interaction might have a role in IGE pathogenesis. Callosotomy is a safe, effective, and underused palliative procedure in these well-selected patients with refractory IGE.

Influence of levetiracetame on ictal and postictal EEG in patients with partial seizures

To investigate the influence of levetiracetame (LEV) treatment on the interhemispheric seizure pattern propagation and postictal recovery of electroencephalography (EEG) background activity. Twenty-three adult patients (age > 16 years) with pharmacoresistant focal epilepsies presenting at the Epilepsy Center Erlangen for pre-surgical evaluation were enrolled in the study. Those eligible patients receiving only one antiepileptic drugs were recruited to the 48-h baseline phase and, after at least two seizures, were randomized to the 7-day treatment phase with either LEV (n = 11) or placebo (n = 12). All participants were submitted to continuous day-and-night video-EEG monitoring. The daily dose of LEV was 1000 mg (500 mg bid.) on the first treatment day and was increased to 2000 mg (1000 mg bid.) from the second day onward. The EEG changes relating to the time delay of the interhemispheric seizure pattern propagation and to the postictal recovery of the background activity were analysed by computerized video-EEG recording and compared using the non-parameter Mann-Whitney U-exact test (alpha = 0.05). A prolonged latency of the contralateral seizure pattern propagation was observed in the LEV group, whereas a more rapid propagation was observed in the placebo group (P = 0.009). Postictal generalized slowing of the background activity was recorded in 21 patients during the baseline phase. More rapid postictal recovery of the EEG background activity was observed in the LEV, but not in the placebo group (P = 0.03). This study demonstrated that LEV not only prevented the seizure pattern propagation but also helped the speedy recovery of the postictal background activity in the EEG.

Corpus callosotomy in refractory idiopathic generalized epilepsy

RATIONALE

A small percentage of patients with idiopathic generalized epilepsy (IGE) do not respond to medical therapy. Generalized tonic-clonic (GTC) seizures are especially debilitating and can be associated with severe injuries. The benefit, safety and effect of corpus callosotomy (CC) in patients with IGE have not been studied.

METHODS

We reviewed patients with presumed IGE who underwent CC between 1991 and 2000. Criteria for selection included history, examination, brain imagining, interictal and ictal EEG. All patients had refractory and debilitating tonic-clonic seizures (GTCS) and had failed four or more antiepileptic drugs. Seizure frequency was calculated per month over the last year and pre-operative baseline was compared to last follow-up using paired t-tests. IQ, executive function, language and verbal, non-verbal memory and quality of life (QOL) was compared before and after surgery. Serial EEGs after surgery were reviewed.

RESULTS

There were nine patients (seven men), mean age 37.9 (range: 22-49), mean IQ 87.3 (range: 75-107). All had anterior CC. Mean follow-up time was 5.4 years (range: 0.6-10.3 years). One patient died from sudden death in epilepsy 9 months after surgery. There was a significant reduction of GTC seizures from 6.3 to 1.1 (p<0.005). Four patients had more than 80% and eight more than 50% reduction. Of five patients with absence seizures, two became seizure free and one had more than 80% reduction and two worsened slightly, and of three with myoclonic seizures one had more than 90% reduction. One patient had completion of the CC with improvement of myoclonus and absence seizures, but not of GTC seizures and suffered a disconnection syndrome. Another had right frontal focal resection without improvement after new seizures of focal onset. Cognitive testing showed a good outcome (improved or no change) in all cognitive domains. Post-surgical EEG showed new focal slowing and sharp waves. There was no change in QOL.

CONCLUSION

CC can be effective in reducing GTC, absence and myoclonic seizures in patients with refractory IGE. These findings suggest that interhemispheric communication of the cerebral cortices plays an important role in the generation of seizures in IGE. Anterior CC appears safe while complete callosotomy has a risk of disconnection syndrome.

Efficacy and Safety of Radiosurgical Callosotomy: A Retrospective Analysis

PURPOSE

Anterior callosotomy is a surgical option for the treatment of generalized tonic or atonic seizures associated with drop attacks. Besides open surgery, a radiosurgical callosal disconnection using the gamma knife (GK) also can be performed, but reliable data about tolerability and efficacy are sparse.

METHODS

Eight patients (three female, five male age range, 5 to 69 years) with severe generalized epilepsy associated with disabling drop attacks underwent GK callosotomy between 1993 and 2004. In six patients, the anterior third of the corpus callosum was radiosurgically disconnected. In one patient a second procedure with GK treatment of the middle third of the corpus callosum was added 17 months later. In two patients posterior GK callosotomy had followed partial hemispherotomy.

RESULTS

Drop attacks (DAs) were completely abolished in three patients, and two patients had a marked DA seizure reduction of 60%. Two of four patients with additional generalized tonic-clonic seizures showed a reduction of 100%, and the remaining, a 50% and 60% decrease, respectively. Other seizure types responded less well to the radiosurgical treatment. In both patients with posterior GK callosotomy after hemispherotomy, partial seizures decreased. Beside transient headache in two patients, no immediate or long-term postradiosurgical side effects were observed.

CONCLUSIONS

Palliative radiosurgical callosotomy is an efficient and safe noninvasive alternative to the open procedure with comparable results. No signs of postradiosurgical side effects were noted within an up to 12-year posttreatment period.

Callosal bisection and transcallosal secondary antiepileptogenesis

More than 28,000 neuroscientists and 3,000 epileptologists gathered at their respective 2001 meetings of the Society for Neuroscience and the American Epilepsy Society. Yet only six articles, one directly and five indirectly, discussed the corpus callosum (CC). Is not this in itself a remarkable finding? Are there no mysteries left? The reality is that considerable uncertainties exist regarding the rationale for callosal bisection (CCB) that causes contrasting effects (i.e., amelioration of generalized seizure, at times leading to freedom from seizure, and intensification of postoperatively fragmented seizure, at times leading to status epilepticus). Similarly, the clinical relevance of EEG mirror focus formation, an experimentally well-established transcallosal consequence of partial cortical epileptogenesis, continues to be debated. This presentation revisits these unresolved issues (a) to gain insight into the dynamic role played by the CC in medically refractory epilepsy, and (b) to promote the development of antiepileptogenic tools that are currently unavailable.

Independent component analysis of generalized spike-and-wave discharges: primary versus secondary bilateral synchrony

OBJECTIVE

To differentiate between primary and secondary bilateral synchrony (PBS and SBS, respectively) in generalized spike-and-wave discharges (GSWD), we applied independent component analysis (ICA) to GSWD, and analyzed the characteristic patterns of independent components.

METHODS

EEGs from 19 patients with GSWD (9 PBS patients, 10 SBS patients) were studied. Thirty GSWD epochs were selected and concatenated to construct an EEG data matrix that was subjected to ICA. Selected independent components were localized by mapping them on a spherical model of the head by means of brain electrical source analysis (BESA) to define dipole sources.

RESULTS

Epileptic components of GSWD were clearly separated by the ICA algorithm in all patients. Between one and three components per patient were responsible for GSWD. In PBS patients, 70.6% of the independent components had dipole sources within the dorsolateral frontal region, and 56.5% of the independent components in SBS patients were within the medial frontal region (P=0.012). The orientation of all of the independent components in PBS patients was radial, whereas 47.8% of the independent components in SBS patients were tangential; this difference was statistically significant (P=0.001).

CONCLUSIONS

Spatiotemporal decomposition of GSWD by using ICA might be helpful for differentiating SBS from PBS.

SIGNIFICANCE

The localization of the dipole sources of the independent components may provide insight into the pathophysiological origins of GSWD.

Magnetoencephalographic analysis of bilaterally synchronous discharges in benign rolandic epilepsy of childhood

The purpose of this study was to examine the spatial and temporal relationship between bilateral foci of bilaterally synchronous discharges in benign rolandic epilepsy of childhood (BREC) using a whole-scalp neuromagnetometer. We simultaneously recorded interictal magnetoencephalographic (MEG) and electroencephalographic (EEG) signals in six children with BREC. Interictal spikes were classified into three groups: bilaterally synchronous discharges (BSDs), unilateral discharges on right side (UD-R), and unilateral discharges on left side (UD-L). We used equivalent current dipole (ECD) modelling to analyse the cortical sources of interictal spikes. Both BSDs and UDs were found in Patients 1-4, whereas only UDs were identified in Patients 5 and 6. The ECDs of interictal spikes were located in rolandic regions, 10-20mm anterior and lateral to hand somatosensory cortices. Multi-dipole analysis of BSDs showed two ECDs in homotopic motor areas of the hemispheres. During BSDs, the right-sided activation preceded the left-sided activation by 15-21 milliseconds in Patients 1 and 2. In Patients 3 and 4, the activation occurred 17-20 milliseconds earlier in the left than the right hemisphere. Within the same hemisphere, the sources of BSDs and UDs were located in similar areas. In conclusion, our results imply the cortical epileptogenicity in bilateral perirolandic areas in BREC. The sequential activation during BSDs in both hemispheres suggest the existence of synaptic connections, possibly via the corpus callosum, between bilateral irritative foci.

Neurological function-based research

Function of the nervous system has been the main domain of interest of an elite group of philosophers, scientists and physicians. This topic may only be understood within a historical perspective, since the progression of knowledge of the function of the nervous system has always paralleled the expansion of our understanding of the outer world and of biological organisms observed. However, data gained from functional experimental and clinical researches in neurosurgery, neurology and neuroscience constitute not only a traditional and historical accumulation of this knowledge but also serve as one of the most important aspects of experimental and clinical neuroscience in the future. This article summarizes the basic principles and methods used in neurological function-based research. In order to elucidate the topic, the text was practically divided into three main sections: (1) historical perspective regarding functional research models in clinical and experimental neuroscience; (2) neurological function-based clinical studies in humans; and (3) experimental research models on animals.

Is a cortical spike discharge "transferred" to the contralateral cortex via the corpus callosum?: An intraoperative observation of electrocorticogram and callosal compound action potentials

PURPOSE

By means of the intraoperative electrophysiologic observation, we reevaluated the "transfer" theory that a transcallosal volley invoked by a cortical spike discharge in one hemisphere directly causes its contralateral counterpart via the corpus callosum (CC).

METHODS

Twenty-six patients who underwent corpus callosotomy were the subjects of this study. Intraoperatively, electrocorticograms from both hemispheres were simultaneously monitored with callosal compound action potentials (CCAPs) from the CC. Analysis was conducted on (a) the interhemispheric delay of bilaterally synchronous spike-and-wave discharges (BSSWs), and (b) the chronological relation between BSSWs and CCAPs.

RESULTS

The side of prior spike discharges was never fixed but was occasionally reversed. Interhemispheric delays between the BSSWs were not constant, regardless of direction, and fluctuated in all patients. Most of the interhemispheric delays were distributed within 20 ms with a mode of 0 ms. The waveform of the CCAP was characterized by slow-rising negative potential change that attained its peak after a cortical spike discharge. These findings were identical in all the patients regardless of whether the BSSWs were changed or unchanged after callosotomy.

CONCLUSIONS

If the "transfer" role of the CC is true, interhemispheric delays between BSSWs must be longer than interhemispheric axonal conduction time (about 20 ms), and a preceding cortical spike discharge must produce a CCAP and then a contralateral one in order of time. However, this hypothesis was not confirmed in the present study. We propose the interhemispheric recruitment of the epileptogenic state as a different role of the CC on epileptogenesis.

Surgery in Lennox-Gastaut syndrome. Corpus callosum division for children

A tonic or "drop" attacks--often seen in Lennox-Gastaut syndrome--are some of the most feared childhood seizures, since they may lead to head injuries. Corpus callosum division will often control atonic seizures, and other types of generalized seizure.

Neurophysiological study of secondary synchronous occipito-frontopolar spikes in childhood

OBJECTIVES

We conducted this latency study to clarify the neurophysiological mechanism underlying the synchronous appearance of independent occipital and frontopolar spike discharges in childhood epilepsies.

METHODS

The subjects were 13 children with localization-related epilepsies (LRE) who showed apparently synchronous occipital and frontopolar EEG spike discharges. There was idiopathic LRE in 7 children, symptomatic LRE in 4, and cryptogenic LRE and a history of cryptogenic West syndrome in one patient each. Patient ages at the time of the study ranged from 4 years 3 months to 14 years 0 month with a mean of 9 years 4 months. The EEGs were digitized at 1024 samples/s. The latency was measured between the peak of the occipital and frontopolar spike discharges. The conduction velocity was calculated by dividing the distance between the occipital and frontopolar electrodes by the latencies.

RESULTS

We studied 19 EEGs including 6 serial EEGs recorded longitudinally in 5 patients. The number of occipito-frontal spike discharges available for the study ranged from 12 to 70 with an average of 36+/-17 in each EEG record. Occipital spikes always preceded the frontopolar spikes by 11.1-31.6 ms (average 19.3+/-5.4 ms). The estimated conduction velocity ranged from 6.7 to 19.2 m/s with a mean of 12.2+/-3.7 m/s.

CONCLUSIONS

The synchronizing spike phenomenon we showed in this study was in the posterior to anterior direction (intrahemispheric synchrony) in contrast to that of secondary bilateral interhemispheric synchrony through the corpus callosum. It is suggested that the long occipito-frontal association fibers play a role in synchronizing both spike discharges. This secondary occipito-frontopolar synchrony should be one of the developmental EEG phenomena related to the maturation of brain, and contribute to the multiplication or diffusion of the pre-existing localized spike discharges often seen in pediatric LRE regardless of etiology.

Pathophysiological mechanisms of genetic absence epilepsy in the rat

Generalized non-convulsive absence seizures are characterized by the occurrence of synchronous and bilateral spike and wave discharges (SWDs) on the electroencephalogram, that are concomitant with a behavioral arrest. Many similarities between rodent and human absence seizures support the use of genetic rodent models, in which spontaneous SWDs occur. This review summarizes data obtained on the neurophysiological and neurochemical mechanisms of absence seizures with special emphasis on the Genetic Absence Epilepsy Rats from Strasbourg (GAERS). EEG recordings from various brain regions and lesion experiments showed that the cortex, the reticular nucleus and the relay nuclei of the thalamus play a predominant role in the development of SWDs. Neither the cortex, nor the thalamus alone can sustain SWDs, indicating that both structures are intimely involved in the genesis of SWDs. Pharmacological data confirmed that both inhibitory and excitatory neurotransmissions are involved in the genesis and control of absence seizures. Whether the generation of SWDs is the result of an excessive cortical excitability, due to an unbalance between inhibition and excitation, or excessive thalamic oscillations, due to abnormal intrinsic neuronal properties under the control of inhibitory GABAergic mechanisms, remains controversial. The thalamo-cortical activity is regulated by several monoaminergic and cholinergic projections. An alteration of the activity of these different ascending inputs may induce a temporary inadequation of the functional state between the cortex and the thalamus and thus promote SWDs. The experimental data are discussed in view of these possible pathophysiological mechanisms.

Decreased multi-band posterior interhemispheric coherence with a lipoma on the corpus callosum: a case report of a possible association

The corpus callosum plays a role in mediating interhemisphere communication. Coherence may be a quantitative EEG-based measure of this communication. The present report is of a female schizophrenic patient with a marked coherence deficit in the temporal-parietal-occipital region involving multiple frequency bands. An MRI scan of her brain revealed a lipoma involving the splenium of the corpus callosum. It is speculated that this lipoma may have caused a physical impingment on or developmental aberration of adjacent callosal fibers, resulting in the observed coherence deficit. Further studies of coherence measures in patients with collosal lipomas are proposed.

Effects of age upon interhemispheric EEG coherence in normal adults

Age-related differences in quantified electrophysiological measures of interhemispheric EEG coherence were studied in 371 subjects (171 males and 200 females), ages 20-80, all of whom were judged to be optimally healthy. Principal components analysis (PCA) was performed on interhemispheric coherence of Laplacian referenced data from eight homologous left-right electrode pairs, from 0.5 to 32 Hz. Regression procedures, using signals from artifact monitoring channels, were used to minimize effects of eye movement and muscle artifact. Forty-six factors described 80% of the total variance, with electrode location the primary source of communality in factor formation. Within 350 right-handed subjects, results indicated a broad trend for decreased interhemispheric coherence with advancing age. Using canonical correlation, the coherence-based factors also successfully predicted spectral variables, previously found to maximally illustrate age-related EEG desynchronization. We speculate that age-related reduction of interhemispheric coherence may in part explain age-related EEG desynchrony and stems from age-related reduction of cortical connectivity. Gender differences of interhemispheric coherence were also evident. Females demonstrated higher interhemispheric coherence than males. Within a smaller subpopulation of 63 subjects (21 left and 42 right handed), there was a gender-by-handedness interaction, with higher interhemispheric coherence in right-handed females than right-handed males and the reverse in left-handed male and female subjects.

Investigation of bilateral synchronous spike-wave discharge by EEG topography

To investigate the generator mechanism of the generalized discharge in childhood epilepsy, we studied its voltage field distribution using linked ear reference (L + R), common averaged reference (CA), and source derivation methods (SD). Two cases of epilepsy with clinical lapse of consciousness, and diffuse EEG spike-wave burst were divided into primary bilateral synchrony (PBS) and secondary bilateral synchrony (SBS), using coherence-phase analysis. Spike topograph images were shown by three different references. The difference in spike voltage between CA and SD was also investigated (CA-SD). By all three different references, PBS spike topography showed a broad symmetrical voltage gradient. In SBS, a left frontal focal negativity was shown with a steep voltage gradient in the topographic images of spikes by L + R, CA and SD. This asymmetry disappeared gradually at the end of the spike-wave burst. Both patients showed almost identical broad symmetrical voltage gradients on CA-SD topogram. EEG topography using various references suggested that the potential fields produced by deep generators were identical for PBS and SBS, although those generated by surface generators were different. These findings support the hypothesis that SBS may be projected from the subcortical system, and activated by cortical firing.

Callosotomy for severe epilepsies with generalized seizures: outcome and prognostic factors

The purpose of the present study was to verify the effect of callosotomy on generalized seizures, to check the effect on other seizure types and to search for possible prognostic factors. Twenty patients with a minimum follow-up of one year (mean 3.5 years) were available for our analysis. In six of them the callosotomy was performed in two stages (total: 26 surgical procedures). Age ranged from 14 to 40 years (mean 23 years). Different aetiologies were known in 15 patients. Duration of epilepsy ranged from 6 to 23 years (mean 15 years). The frequency of seizures ranged between 19 and 750 per month. The most significant effect of surgery was the complete suppression of the generalized seizures associated with falling in 9/19 and their reduction of more than 80% in 7/19 patients (total "good results": 16/19). The generalized tonic-clonic seizures were less affected. The surgical effect on the partial seizures was very variable, the partial simple seizures being the most affected. A positive statistical association with the outcome of the generalized seizures with fall was found for a presurgical seizure frequency below 90 per month, a prevalent bilateral EEG epileptic activity and, to a less extent, the absence of cerebral structural lesions. The role of age, aetiology, duration of the disease, single or more seizure types, mental impairment and extent of callosotomy remains uncertain. Disconnection syndrome does not appear if the splenium is spared. The present findings confirm that the main indication for callosotomy is the occurrence of generalized seizures with fall. Surgery can be initially limited to the anterior 2/3 of the corpus callosum; further posterior section of the corpus, excluding the splenium, should be regarded as a second step, when necessary.

Effect of anterior callosotomy on bilaterally synchronous spike and wave and other EEG discharges

We analyzed pre- and postoperative interictal and ictal EEG patterns in relation to seizure outcome in 29 patients with intractable epilepsy who had undergone anterior callosotomy. Twenty-two patients had generalized bilaterally synchronous sharp or spike and slow wave discharges (GSW) and 7 did not: Postoperatively, patients with preoperative GSW showed disruption of synchrony, increased amplitude asymmetry, and a decreased amount of GSW. No significant decrease was noted in the amount of total epileptic discharge after operation. There was a significant relation between seizure outcome and decrease in amount of GSW. The degree of disruption of synchrony, total amount of epileptic discharge, and pre- and postoperative EEG patterns were not good indicators of seizure outcome. Postoperative changes in lateralization of epileptic foci consisted either of increased lateralization, less lateralization but increased independent discharges in the previously nonpredominant hemisphere, or appearance of some lateralization. There appears to be a spectrum of GSW: Both secondary bilateral synchrony and secondarily generalized corticoreticular epileptic discharges are distributed along this spectrum.

Electroclinical signs of benign neonatal familial convulsions

Benign neonatal familial convulsions comprise a distinct epileptic syndrome with an autosomal mode of transmission. The electroclinical signs of seizures in this syndrome are not yet well defined. In 3 children from two families presenting with benign neonatal familial convulsions, 14 seizures were recorded during electroencephalographic (EEG)-video sessions. All seizures occurred during sleep, after a short arousal reaction. Seizures started with bilateral, symmetrical flattening of the EEG for 5 to 19 seconds; simultaneously there was apnea and tonic motor activity. The EEG flattening was followed by a long (1-2-minute) bilateral discharge of spikes and sharp waves; simultaneously, there were vocalizations, chewing, and focal or generalized clonic activity. The prominence of EEG and motor abnormalities varied between the left and the right from one seizure to the next in any given child. The seizures stopped without EEG or clinical postictal depression. These electroclinical observations suggest that the convulsions of benign neonatal familial convulsions are a form of generalized tonic-clonic seizure whose expression may be asymmetrical, probably because of the immaturity of the corpus callosum or other structures ensuring seizure synchronization.

Value of intraoperative EEG changes during corpus callosotomy in predicting surgical results

The intraoperative transformation of generalized epileptiform discharges (GED) to lateralized epileptiform activity during the course of corpus callosum sectioning for intractable epilepsy in 37 patients was correlated with percentage of decrease in atonic-tonic seizures with "drops" at mean follow-up of 26 months (range 12-86 months). Twenty-seven (73%) patients had intraoperative interictal discharges, and 21 (78%) showed varying degrees of lateralization of GED during corpus callosum sectioning (two thirds to total). All patients experienced > 80% reduction in atonic-tonic seizures with drops. The group (n = 7) with largest decrease in GED had the greatest decrease in seizures (95.5%). Six patients without change in GED had 88% decrease in seizures, as did 14 patients (85-86%) with mild or moderate decreases in GED, but there was no statistically significant correlation between decrease in GED and seizure frequency after operation. Thus, although lateralization of GED after corpus callosum sectioning was evident in 78% of patients with GED, the degree of lateralization of GED did not correlate with degree of reduction of tonic-atonic seizures. Therefore, intraoperative surface EEG monitoring does not appear to be helpful at this time as a guide to extent of callosotomy.

Primary and secondary bilateral synchrony in epilepsy: differentiation by estimation of interhemispheric small time differences during short spike-wave activity

Estimation of interhemispheric small time differences (TDs) during spike-wave bursts in the EEG by coherence and phase analysis is useful for differentiation between primary bilateral synchrony (PBS) and secondary bilateral synchrony (SBS) in epilepsy. Because the previous method via Fast Fourier Transform needed long bursts for reliable analysis, a method using a 2-dimensional autoregressive model was newly developed to enable estimation of TDs even in 1.2 sec bursts, and applied to 19 epileptic patients with apparently bilaterally synchronous spike-wave bursts. At the onsets of bursts, estimated maximal TDs were 5.8 msec or less and inconsistent in leading hemispheres in 10 patients with a clinical diagnosis of idiopathic, cryptogenic or symptomatic generalized epilepsy indicating PBS, while the maximal TDs were 9.3-41.5 msec and consistent in leading in 7 patients with clinically symptomatic partial epilepsy and also in two with idiopathic and symptomatic generalized epilepsy suggesting SBS. Among 8 patients with bursts which suggested SBS and long enough for evaluation of intra-burst TD variation, TDs tended to disappear in the middle to end parts of the bursts in 5 cases, but not in the other 3, suggesting 2 different pathophysiological mechanisms in SBS.