Pathophysiology of generalized penicillin epilepsy in the cat: the role of cortical and subcortical structures. I. Systemic application of penicillin

Advancing thalamic neuromodulation in epilepsy: Bridging adult data to pediatric care

Thalamic neuromodulation has emerged as a treatment option for drug-resistant epilepsy (DRE) with widespread and/or undefined epileptogenic networks. While deep brain stimulation (DBS) and responsive neurostimulation (RNS) depth electrodes offer means for electrical stimulation of the thalamus in adult patients with DRE, the application of thalamic neuromodulation in pediatric epilepsy remains limited. To address this gap, the Neuromodulation Expert Collaborative was established within the Pediatric Epilepsy Research Consortium (PERC) Epilepsy Surgery Special Interest Group. In this expert review, existing evidence and recommendations for thalamic neuromodulation modalities using DBS and RNS are summarized, with a focus on the anterior (ANT), centromedian(CMN), and pulvinar nuclei of the thalamus. To-date, only DBS of the ANT is FDA approved for treatment of DRE in adult patients based on the results of the pivotal SANTE (Stimulation of the Anterior Nucleus of Thalamus for Epilepsy) study. Evidence for other thalamic neurmodulation indications and targets is less abundant. Despite the lack of evidence, positive responses to thalamic stimulation in adults with DRE have led to its off-label use in pediatric patients. Although caution is warranted due to differences between pediatric and adult epilepsy, the efficacy and safety of pediatric neuromodulation appear comparable to that in adults. Indeed, CMN stimulation is increasingly accepted for generalized and diffuse onset epilepsies, with recent completion of one randomized trial. There is also growing interest in using pulvinar stimulation for temporal plus and posterior quadrant epilepsies with one ongoing clinical trial in Europe. The future of thalamic neuromodulation holds promise for revolutionizing the treatment landscape of childhood epilepsy. Ongoing research, technological advancements, and collaborative efforts are poised to refine and improve thalamic neuromodulation strategies, ultimately enhancing the quality of life for children with DRE.

Seizure Detection and Network Dynamics of Generalized Convulsive Seizures: Towards Rational Designing of Closed-Loop Neuromodulation

Objective

Studies have demonstrated the utility of closed-loop neuromodulation in treating focal onset seizures. There is an utmost need of neurostimulation therapy for generalized tonic-clonic seizures. The study goals are to map the thalamocortical network dynamics during the generalized convulsive seizures and identify targets for reliable seizure detection.

Methods

Local field potentials were recorded from bilateral cortex, hippocampi, and centromedian thalami in Sprague-Dawley rats. Pentylenetetrazol was used to induce multiple convulsive seizures. The performances of two automated seizure detection methods (line length and P-operators) as a function of different cortical and subcortical structures were estimated. Multiple linear correlations-Granger's Causality was used to determine the effective connectivity.

Results

Of the 29 generalized tonic-clonic seizures analyzed, line length detected 100% of seizures in all the channels while the P-operator detected only 35% of seizures. The detection latencies were shortest in the thalamus in comparison to the cortex. There was a decrease in amplitude correlation within the thalamocortical network during the seizure, and flow of information was decreased from thalamus to hippocampal-parietal nodes.

Significance

The preclinical study confirms thalamus as a superior target for automated detection of generalized seizures and modulation of synchrony to increase coupling may be a strategy to abate seizures.

The relationship between the localization of the generalized spike and wave discharge generators and the response to valproate

PURPOSE

Up to 30% of patients with idiopathic generalized epilepsy (IGE) have seizures that are refractory to medication despite appropriate therapy that commonly includes valproate (VPA). The aim of this study was to compare patients with VPA-refractory and VPA-responsive IGE in order to determine whether there are group differences in generalized spike and wave discharge (GSWD) generators that may be associated with VPA resistance.

METHODS

Of 89 IGE patients who underwent electroencephalography (EEG) combined with functional magnetic resonance imaging (fMRI; EEG/fMRI), 25 with GSWDs identified in EEG/fMRI data were included. Simultaneous acquisition of 64 channels of EEG data at 10 kHz was performed using an MRI-compatible EEG cap and amplifier at 4T. VPA resistance was defined as lack of seizure control despite therapeutic dose of VPA.

KEY FINDINGS

The fMRI blood oxygen-level dependent (BOLD) correlates of GSWD in the entire group involved midline thalamus, frontal regions comprising Brodmann areas 6, 24, and 32, and temporal lobes diffusely. When VPA-responsive and VPA-resistant patients were compared, BOLD signal increases were noted in the VPA-resistant patients in medial frontal cortex, along the paracingulate gyrus (Montreal Neurological Institute; MNI x = 2, y = 13.6, z = 45.9), and anterior insula bilaterally (right MNI x = 37.6, y = 7.8, z = 0.6, left MNI x = -35.3, y = 13.6, z = -5.3).

SIGNIFICANCE

Our findings support the hypothesis that VPA-resistant and VPA-responsive patients may have different GSWD generators. Furthermore, we hypothesize that these differences in GSWD generators may be the reason for different responses to VPA.

Cortical and subcortical contributions to absence seizure onset examined with EEG/fMRI

In patients with idiopathic generalized epilepsies (IGEs), bursts of generalized spike and wave discharges (GSWDs) lasting > or =2 seconds are considered absence seizures. The location of the absence seizures generators in IGEs is thought to involve interplay between various components of thalamocortical circuits; we have recently postulated that medication resistance may, in part, be related to the location of the GSWD generators [Szaflarski JP, Lindsell CJ, Zakaria T, Banks C, Privitera MD. Epilepsy Behav. 2010;17:525-30]. In the present study we hypothesized that patients with medication-refractory IGE (R-IGE) and continued absence seizures may have GSWD generators in locations other than the thalamus, as typically seen in patients with IGE. Hence, the objective of this study was to determine the location of the GSWD generators in patients with R-IGE using EEG/fMRI. Eighty-three patients with IGE received concurrent EEG/fMRI at 4 T. Nine of them (aged 15-55) experienced absence seizures during EEG/fMRI and were included; all were diagnosed with R-IGE. Subjects participated in up to three 20-minute EEG/fMRI sessions (400 volumes, TR=3 seconds) performed at 4 T. After removal of fMRI and ballistocardiographic artifacts, 36 absence seizures were identified. Statistical parametric maps were generated for each of these sessions correlating seizures to BOLD response. Timing differences between brain regions were tested using statistical parametric maps generated by modeling seizures with onset times shifted relative to the GSWD onsets. Although thalamic BOLD responses peaked approximately 6 seconds after the onset of absence seizures, other areas including the prefrontal and dorsolateral cortices showed brief and nonsustained peaks occurring approximately 2 seconds prior to the maximum of the thalamic peak. Temporal lobe peaks occurred at the same time as the thalamic peak, with a cerebellar peak occurring approximately 1 second later. Confirmatory analysis averaging cross-correlation between cortical and thalamic regions of interest across seizures corroborated these findings. Finally, Granger causality analysis showed effective connectivity directed from frontal lobe to thalamus, supporting the notion of earlier frontal than thalamic involvement. The results of this study support our original hypothesis and indicate that in the patients with R-IGE studied, absence seizures may be initiated by widespread cortical (frontal and parietal) areas and sustained in subcortical (thalamic) regions, suggesting that the examined patients have cortical onset epilepsy with propagation to thalamus.

Thalamic lesions in a genetic rat model of absence epilepsy: dissociation between spike-wave discharges and sleep spindles

Recent findings have challenged the traditional view that the thalamus is the primary driving source of generalized spike-wave discharges (SWDs) characteristic for absence seizures, and indicate a leading role for the cortex instead. In light of this we investigated the effects of thalamic lesions on SWDs and sleep spindles in the WAG/Rij rat, a genetic model of absence epilepsy. EEG was recorded from neocortex and thalamus in freely moving rats, both before and after unilateral thalamic ibotenic acid lesions. Complete unilateral destruction of the reticular thalamic nucleus (RTN) combined with extensive destruction of the thalamocortical relay (TCR) nuclei, resulted in the bilateral abolishment of SWDs and ipsilateral abolishment of sleep spindles. A suppression of both types of thalamocortical oscillations was found when complete or extensive damage to the RTN was combined with minor to moderate damage to the TCR nuclei. Lesions that left the rostral pole of the RTN and part of the TCR nuclei intact, resulted in an ipsilateral suppression of sleep spindles, but a large increase of bilateral SWDs. These findings demonstrate that the thalamus in general and the RTN in particular are a prerequisite for both the typical bilateral 7-11 Hz SWDs and natural occurring sleep spindles in the WAG/Rij rat, but suggest that different intrathalamic subcircuits are involved in the two types of thalamocortical oscillations. Whereas the whole RTN appears to be critical for the generation of sleep spindles, the rostral pole of the RTN seems to be the most likely part that generates SWDs.

Vers une théorie unifiée des symptômes positifs

Defined more than one century ago, the concept of positive symptoms has become obsolete, except in the psychiatric domain. However, its relevance remains intact today when considering such pathophysiologies as neuropathic (phantom) pain, movement disorders, tinnitus, epilepsy, and psychiatric disorders. Beside their very different clinical characteristics, all these symptoms arise from a lesion in the nervous system. Furthermore, they are paradoxical in the sense that they correspond to a spontaneous hyperactivity of the injured functional system, concomitant to the usual deficits resulting from the lesion. Could these similarities reflect the existence of some common pathophysiological process? A peculiar electrophysiological property of thalamic cells is likely to be compatible with this hypothesis. A thalamic cell produces action potentials when depolarised by excitatory inputs. Conversely, its ability to produce action potentials is decreased or even completely suppressed when the same cell is hyperpolarized by inhibitory influences. However, depending on its level of hyperpolarization, this cell can also produce rhythmic paradoxical bursts of activity at low frequency (3-4 Hz). In this context, a lesion involving, for example, the somatosensory excitatory fibres gives rise to hyperpolarization of the corresponding thalamic cells, which may produce such rhythmic bursting activity. This causes an increase of low frequency thalamo-cortical activity, which, through reduction of collateral cortico-cortical inhibition, induces high frequency activity in neighbouring thalamo-cortical loops ("edge effect"). This leads to the appearance of the clinical symptoms, in this case, pain. Electrophysiological recordings performed in patients suffering from sensory or motor positive symptoms have shown the presence of such deleterious sequence of events. Furthermore, the efficiency of neurosurgical treatments that are used against some positive symptoms can be explained on the basis of such a dynamic process. Both considerations support the validity of the proposed hypothesis and open avenues for the control of other positive symptoms.

Seizures induced by penicillin microinjections in the mesencephalic tegmentum

The location and extension of a convulsive area in the brain stem in cats was determined through penicillin microinjections (0.5-1.0 microl) of a concentrated sodium penicillin solution (500 IU/microl), stereotactically oriented to multiple structures, in fully awake animals, partially restrained through a rod fixation system that avoided pain, allowed the observation of clinical seizures and simultaneous recording of EEG, EMG and multiple unit activity (MUA) from the injected site and the motor cortex (Cx). Clinical and EEG seizure patterns in relation to the injected sites and penicillin doses were studied in another group of animals using doses from 12.5 IU/0.1 microl to 125 IU/1.0 microl. The time relationship between muscular clonus, EEG spikes and MUA at the injected site and Cx were analyzed. The only area in which penicillin induced seizures was the mesencephalic tegmentum (MT). The amount of penicillin but not the stereotactic coordinates determined the seizure type. MT EEG and MUA paroxysms anticipated clinical seizure and Cx EEG spikes. When Cx EEG appeared, they were accompanied by an increase in MUA beginning in the Cx and EMG, followed by significant increase in MT MUA. The sequence of events suggest that MT seizure activity propagates via alternative pathways not involving direct reticulospinal or pyramidal tract pathways.

Modulation of sleep interictal epileptiform discharges in partial epilepsy of childhood

OBJECTIVE

NREM sleep increases the Interictal Epileptic Discharges (IEDs) in the majority of children affected by partial epilepsy (both symptomatic or cryptogenetic). Experimental data revealed that the normal sleep oscillations, leading to the appearance of spindles and delta waves on the surface EEG during NREM sleep, might develop into paroxysmal synchronization. Spectral analysis enables the quantitative description of the dynamics of delta (slow wave activity, SWA, 0,5-4,5 Hz) and sigma activity (SA, 12.0-16.0 Hz) and can be used to assess the relationship between SA, SWA and IEDs during sleep.

DESIGN AND METHODS

We have performed overnight continuous EEG-polysomnographic studies in 7 patients (mean age 7.2+/-1.3). The temporal series of SWA and SA were obtained from a spike-free derivation lead. The IEDs count was performed on the most active lead. Relationships between sigma and SWA and time series of IEDs were tested by means of correlation techniques after data normalization.

RESULTS

Our results revealed a significant higher correlation between IEDs and SA with respect to SWA in all the subjects, in total sleep time. The same analysis limited to NREM sleep highlights the better correlation between SA and IEDs.

CONCLUSIONS

Our data suggest that the neural mechanisms involved in the generation of sleep spindles facilitate the IEDs production in childhood partial epilepsies at least in those strongly activated by sleep.

Chronic periodic lateralized epileptiform discharges during sleep in a patient with caudate nucleus atrophy: insights into the anatomical circuitry of PLEDs

OBJECTIVE

Periodic lateralized epileptiform discharges have been recognized for 33 years; however, little is known about the underlying mechanism causing periodic discharges. The following case provides an opportunity to study PLEDs in a patient with precisely localized subcortical grey matter lesions.

METHODS

Routine EEGs and overnight polysomnography were performed on the study patient. Standard 10-20 electrode positions were used, as well as EOG and chin EMG for polysomnography.

RESULTS

The study patient was a 39-year-old woman with severe left caudate nucleus atrophy and right hemi-dystonia. She had left ventral-lateral (VL) thalamotomies in 1989 and 1991, pallidotomy in 1992, and centromedian thalamic stimulator implantation in 1997. EEGs prior to surgical intervention demonstrated left hemisphere PLEDs during sleep. Following CM nucleus stimulatory implantation, the patient had overnight polsomnography. EEG during wakefulness and REM sleep was normal. With stages I-IV sleep left hemisphere PLEDs at 1-2 Hz were seen with fronto-temporal predominance. Sleep spindles were present bilaterally. There was no history of seizures, before or after surgery.

CONCLUSIONS

The finding of PLEDs confined to synchronized sleep which were not affected by surgical manipulation of the motor basal ganglia circuit suggests a role of the associative basal ganglia circuit in the generation of periodic phenomenon.

The connection between absence-like seizures and hypothermia induced by penicillin: possible implication on other animal models of petit mal epilepsy

In this study we investigated the relationship between penicillin-induced hypothermia and petit mal epilepsy induced by this proconvulsant antibiotic. In order to find a possible dose-dependent relationship, we used two doses: 1500.000 and 1000.000 U/kg b.wt., both known as being sufficient to induce absence-like attacks with subsequent spike and wave discharges (SWD) in electrocorticogram (ECoG). Because of experimental data suggesting penicillin binding to benzodiazepine receptor recognition site, we also studied penicillin-induced changes in body temperature after diazepam pretreatment. Results of this study clearly show that penicillin in doses known to induce petit mal-like epilepsy concomitantly induces statistically significant dose-dependent decrease in body temperature. Pretreatment with diazepam completely prevents both penicillin-induced hypothermia and SWDs. On the other hand, both the diazepam and mixed diazepam + penicillin treatments did not significantly alter body temperature. These results suggest, however, that at least some of the penicillin effects described could be assigned to its binding to the benzodiazepine receptor recognition site at GABA(A) ionophore. This may have an important clinical implication because the inhibitory action of penicillin at the benzodiazepine receptor recognition site could account for the mechanism of penicillin-induced unspecific encephalopathies in humans. The relationship between petit mal epilepsy and hypothermia sheds new light on the action mechanisms of penicillin-induced absence seizures.

Electrocortical and behavioral responses produced by acute electrical stimulation of the human centromedian thalamic nucleus

Incremental, desynchronizing and spike-wave electrocortical responses and concomitant symptoms to acute electrical stimulation of the centromedian thalamic nucleus (CM) were studied in 12 patients with intractable complex partial and tonic-clonic generalized seizures. Low-frequency (6/s), 320-800 microA stimulation of the caudal-basal and central portions of CM elicited incremental recruiting and augmenting-like responses with a bilateral regional scalp distribution, with emphasis at the ipsilateral frontal (recruiting) and central (augmenting) regions, while ventral-basal CM stimulation elicited primary-like responses with a focal distribution at the ipsilateral parietal region. High-frequency (60/s), 320-800 microA stimulation of caudal-basal and central, but not ventral-basal CM, elicited EEG desynchronization and a slow negative shift of the EEG baseline with scalp distribution similar to that showed by recruiting- and augmenting-like responses. Neither incremental nor desynchronization EEG responses were accompanied by evident patient sensory or motor responses. Low-frequency (3/s), high-intensity (30 V = 2400 microA) combined stimulation of the right CM and left non-specific mesencephalic ascending pathways elicited a response similar to the typical absence attack with all EEG and clinical ingredients: S1, S2, P1 and W components of the individual spike-wave complex and generalized spike-wave discharges followed by sleep spindle EEG afterdischarges, accompanied by motionless stare, 3/s eye blinking, lip smacking and total failure to respond to visual stimuli in patients under a simple responding task.

Expression of epileptiform activity induced by a penicillin focus within the posterior thalamus in the awake rat

Investigations were performed to study the epileptiform activity, induced by a local injection of penicillin (PCN) into the posterior thalamus (pTh) of the awake rat, and to compare it with the epilepsy induced in the same animals 2 weeks later by an injection of PCN into the motor cortex (MC). Using EEG recordings, 1) the distribution of focal cortical activity, and 2) the severity of the epileptiform activity (frequency of focal activity, occurrence and duration of generalized episodes) were analyzed. The focal activity of pTh rats was characterized by two types of potentials: (a) sharp potentials with a spike-like shape that developed during the first hour after PCN injection only in the visual cortex, but in the transition area between the motor and sensory cortex during the last period of epileptiform activity; and (2) large potentials with a wave- or spike-wave-like shape that had their center of focal expression in the transition zone between the motor and sensory cortex. MC rats exhibited only a spike-like potential with or without short-lasting afterdischarges in the homotopic areas of the MC of both hemispheres. During periods with large potentials only, the number of generalized episodes was significantly reduced with respect to those periods with sharp potentials. When the epileptiform activity changed from large to sharp potentials, the interictal frequency increased significantly. It is postulated (a) and a pTh focus activates the lateral and/or the reticular thalamic areas, which, due to their high intrinsic potential for synchronization, cause a self-sustained interictal activity of the large potential type; and (b) that the wave of the large potentials is involved in an anticonvulsive mechanism that reduces the extent of ictal as well as interictal activity.

Triphasic waves during post-ictal stupor

BACKGROUND

The term, "triphasic wave" originally described an EEG pattern believed to be a marker for a specific stage of hepatic coma. For 4 decades, the diagnostic and prognostic specificity of the pattern remains controversial. Its pathophysiology also continues to be elusive.

METHODS

EEG recordings were obtained in three patients known or suspected to have primary generalized epilepsy. In 2 patients, the EEGs were part of long-term monitoring using simultaneous video-EEG telemetry. For the third patient, the EEG was secured only during the post-ictal unconsciousness. These 3 patients were specifically selected because of the presence of triphasic waves in their EEGs.

RESULTS

Triphasic waves were observed in the EEG of the 3 patients only during post-ictal unconsciousness. The pattern was transient, being preceded by generalized suppression and delta slow waves and followed by theta activities. Alpha rhythms supervened when the patients became fully alert.

CONCLUSION

A post-ictal state should be considered in unconscious patients with triphasic EEG waves.

Primary (idiopathic) generalized epilepsy and underlying mechanisms

Primary Generalized Epilepsy (PGE) has been more hotly debated over the past decades than other forms of epileptic seizure disorder. The sudden synchronous appearance of bilateral spikes and spike-waves (mainly with myoclonus resp. absence) used to perplex the earliest generation of electroencephalographers, and the enigmatic genesis of these discharges (and seizures) has not ceased to fascinate the investigators of this phenomenon. A "centrencephalic" concept with paroxysmal discharges arising from thalamic structures and "projecting" to the cortex was championed for many years and eventually laid aside. More recently, the role of the thalamic level has been re-emphasized, mainly on the basis of experimental work. In this article, the bulk of experimental work is critically reviewed: the simian model (Papio papio), the feline, and the rodent models (Wistar rat, tottering mouse). Stress is being laid on fundamental differences between all of these models and human PGE. EEG evidence indicates a superior frontal origin of bilateral-synchronous spikes and spike-waves; depth EEG recordings in patients have failed to demonstrate primary thalamic spike generation. The heart of the matter in PGE appears to be the mechanism underlying paroxysmal discharges; above all the role of arousal. It is not awakening from sleep but the ensuing period that is critical in its epileptogenic thrust caused by alternating periods of return to drowsiness and arousing stimuli. This biphasic process gradually escalates EEG bursts to myoclonus (or absences) and possibly to a generalized tonic-clonic convulsion. Most conducive to this crescendo is the state of tiredness following a night of poor sleep. Bilateral synchrony is not precise and small time differences exist. The line between primary and secondary bilateral synchrony (with a primary cortical focus) can become blurred. Genetic predisposition to generalized paroxysms must always be considered, even in the face of a primary focus with secondary bilateral synchrony. Photosensitivity is a second paroxysm-inducing mechanism in PGE; it is much less common than the abnormal arousal ("dyshormia"); both mechanisms can be present in the same patient. Therapy and prevention of seizures in PGE are finally discussed. The concept of abnormal arousal mechanisms can be put into practice in order to prevent seizures: avoidance of sleepless nights, not always an easy task in adolescents and young adults.

Inter- and intrahemispheric phase changes of diffuse 3 Hz spike-and-wave complex

The inter- and intrahemispheric phase characteristics were investigated on diffuse 3 Hz spike-and-wave complex (D3SW) in eight epileptic patients who were diagnosed with typical absence. The phase of D3SW was analyzed sequentially using cross-power spectral arrays dividing D3SW into two components (spike-and-wave complex, spike component). The phase of spike-and-wave complex preceded most at the midline structure, and delayed symmetrically toward the lateral side of each hemisphere in all cases, while not in all cases when spike component was investigated. In spite of this symmetry, there was sparse correlation in phase change between the homologous hemispheres. There was almost no linear correlation between the phase of spike-and-wave complex and the corresponding spike component. The author concludes that the 'centrencephalic system' hypothesis still plays an important role for the generation of D3SW, and suggest a mechanism other than the cortical recurrent inhibition for the generation of wave component.

Epileptogenic spikes and seizures but not high voltage spindles are induced by local frontal cortical application of gamma-hydroxybutyrate

Combining the methods of microdialysis and EEG recording, we have examined the effect of unilaterally, intracortically applied gamma-hydroxybutyrate (GHB) on frontal cortical EEG activity in freely moving rats. GHB, a natural endogenous GABA metabolite, is known to induce rhythmic spike and wave activity, resembling generalized petit mal epilepsy. Without GHB, spontaneous high voltage spindles (HVS, 6-9 Hz) were observed during awake and immobile behavior in most of the animals (HVS rats), while others never had any HVS. In those both groups of animals intracortical application of GHB induced epileptogenic spikes (< 0.5 Hz) behaviorally accompanied by occasional myoclonic jerks and epileptic discharges (< 2 Hz) with behavioral convulsions and contraversive movements towards the left hindlimb (seizures) but did not induce HVS or spike and waves, as reported after systemic application. In the group of rats with spontaneous occurring HVS the amplitude of the HVS on the side of the microdialysis probe was suppressed by GHB and GHB-induced spikes invading the contralateral cortex frequently triggered and terminated local HVS. The results point to different neural mechanisms for the generation of HVS and spikes and epileptic discharges (seizures) induced after local intracortical application of GHB.

Kainic acid-induced thalamic seizure in cats--a possible model of petit mal seizure

Bipolar depth electrodes were implanted stereotaxically in the thalamus, hippocampus and midbrain reticular formation of cats. Cortical screw electrodes were placed over the bilateral sensorimotor cortex. A guide cannula with an inner injection cannula was inserted unilaterally into the posterolateral ventral nuclei (VPL) of the thalamus. Eight days after the procedures, kainic acid (2.0 micrograms) was injected unilaterally into the VPL via the injection cannula in freely moving animals and electro-clinical observations were made. About 1 h after the kainic acid injection, multiple spikes were observed in the VPL (injection site), which propagated to the subcortical structures. These seizures finally propagated bilaterally to the cortex about 2 h after the injection. About 3-4 h after the injection, small spike and wave complexes repeatedly appeared for a short period of time in cortical leads and cats exhibited behavioral arrest with unresponsiveness during the seizures. About 24 h after the injection, generalized small spike and wave complexes were observed intermittently in cortical and subcortical structures. They persisted for 4-5 s and were associated with behavioral arrest and staring. The results demonstrate that a unilateral microinjection of kainic acid into VPL induced petit mal-like seizure, and suggest that VPL plays an important role in the generation or transfer of spike and wave complexes.

Panoramic imaging of brain pHi and CBF during penicillin and metrazole induced status epilepticus

Using real-time in vivo umbelliferone fluorescent imaging, cortical intracellular brain pH (pHi) and cortical blood flow (CBF) were measured in New Zealand white rabbits during generalized seizures induced by intravenous metrazole or sodium penicillin. In the former, brain pHi declined from 7.04 +/- 0.07 to 6.78 +/- 0.07 within 15 min of generalized seizures and remained at this level for 1 h. In the penicillin group, pHi fell from 7.05 +/- 0.10 to 6.81 +/- 0.07 and also remained at this level over 60 min. This brain acidosis was uniform across the brain's surface. With the onset of status epilepticus there was a hyperemia which occurred in a heterogeneous pattern with blood flow appearing to be greater adjacent to cortical vasculature and slower in border zones between surface blood vessels. In the metrazole group, there was evidence of vasomotor paralysis with loss of autoregulation involving both cortical surface vasculature and penetrating arterioles with their capillary beds.

Genetic and phenotypic heterogeneity of inherited spike-wave epilepsy: two mutant gene loci with independent cerebral excitability defects

Two recessive gene loci controlling cerebral excitability in the mouse (tg, chr 8 and stg, chr 15) share generalized neocortical spike-wave seizures as a common mutant phenotype. Although the primary molecular defects are unknown, homozygous tg mutants display a gene-linked hyperplasia of central noradrenergic axons originating in the locus ceruleus, and early selective lesions of these fibers correct the epileptic phenotype in the adult. In contrast, we find that stg homozygotes, despite a more severe seizure disorder, show no alterations in regional noradrenergic fiber innervation, and seizure frequency is unaffected by neonatal noradrenergic depletion. These mutations demonstrate that excessive synchronous neuronal discharges alone are insufficient to trigger abnormal growth of locus ceruleus fibers, and reveal the existence of two distinct intervening brain neuromodulatory mechanisms, norepinephrine (NE)-dependent and NE-independent, underlying the inheritance of this common pattern of epilepsy.

Ethosuximide alters intrathalamic and thalamocortical synchronizing mechanisms: a possible explanation of its antiabsence effect

Effects of systemic administration of a single dose (50 mg/kg) of ethosuximide (ESM) on extracellularly recorded thalamic (nucleus centralis lateralis, CL; nucleus reticularis, RE) and cortical neurons and on cortical EEG activity of acute cats, have been studied. In intact animals ESM led to: (a) desynchronization of cortical EEG activity; (b) reduction of cortical recruiting responses to 6 Hz stimulation of nucleus centralis medialis (CeM); (c) increased firing rate of CL units; and (d) reduction of incremental responses (IRs) of CL neurons to CeM stimulation. In midbrain reticular formation (MRF)-lesioned animals, ESM induced: (a) reduction of cortical spindle waves; (b) increment of their intraburst frequency; (c) reduction of the IR of CL neurons to 3 and 6 Hz CeM stimulation; (d) shortening of the inhibitory period following each response; and (e) no increment of spontaneous firing rate of CL units. Moreover, ESM led to important changes in the spontaneous activity of RE neurons: spike barrages, typical of these neurons in MRF-lesioned animals, became less frequent and of longer duration, being also constituted by longer interspike intervals. However, responses of RE neurons to low frequency CeM stimulation, when present, did not show any incremental phenomenon and appeared unchanged after ESM. Responses of cortical neurons to paired stimuli, applied with different interstimulus intervals, to nucleus ventralis posterolateralis or in animals with isolated cortex, to subcortical white matter, disclosed a reduction of the cortical inhibitory period following the response to the conditioning stimulus. These data suggest that ESM exerts a moderate diffuse anti-inhibitory action at both cortical and thalamic levels and an activating effect on MRF, which could also be accomplished through disinhibition. The reduction of the inhibitory phases in thalamic nuclei would alter spontaneous intrathalamic synchronizing mechanisms, leading to a decreased effectiveness of thalamocortical volleys, which are believed to be fundamental for the appearance of cortical spike and wave discharges. This hypothesis would therefore explain the specific efficacy of ESM against absence seizures.

Animal models of the epilepsies

The study of mechanisms of the epilepsies requires employment of animal models. Choice of a model system depends upon several factors, including the question to be studied, the type of epilepsy to be modelled, familiarity and convenience. Over 50 models are reviewed. Major categories of models are those for simple partial seizures: topical convulsants, acute electrical stimulation, cortically implanted metals, cryogenic injury; for complex partial seizures: kainic acid, tetanus toxin, injections into area tempesta, kindling, rodent hippocampal slice, isolated cell preparations, human neurosurgical tissue; for generalized tonic-clonic seizures: genetically seizure-prone strains of mouse, rat, gerbil, fruitfly and baboon, maximal electroshock seizures, systemic chemical convulsants, metabolic derangements; and for generalized absence seizures: thalamic stimulation, bilateral cortical foci, systemic penicillin, gamma-hydroxy-butyrate, intraventricular opiates, genetic rat models. The lithium-pilocarpine, homocysteine and rapid repetitive stimulation models are most useful in studies of status epilepticus. Key findings learned from each of the models, the model's strengths and weaknesses are detailed. Interpretation of findings from each of these models can be difficult. Do results pertain to the epilepsies or to the particular model under study? How important are species differences? Which clinical seizure type is really being modelled? In a model are behavior or EEG findings only similar superficially to epilepsy, or are the mechanisms comparable? The wealth of preparations available to model the epilepsies underscores the need for unifying themes, and for better understanding of basic mechanisms of the epilepsies.

Electrical stimulation of the centromedian thalamic nucleus in the treatment of convulsive seizures: a preliminary report

Five patients with clinical and EEG primary generalized or multifocal uncontrollable seizures underwent stereotaxic implantation of electrodes in both centromedian thalamic nuclei (CM). Each electrode consisted of a semiflexible array of three platinum-iridium wires, isolated except at their tips, which were separated by 4 mm. Bipolar, biphasic rectangular pulses were delivered in trains of 1 min every 5 min, alternating right and left side for sessions 2 h/day. Patients were followed for 3 months with charting of clinical seizures, daily 4-h EEG recordings from scalp and depth for 5 days and weekly thereafter. Baseline and 3-month evaluation of psychological performance through selected Beta R, Wechsler memory scale, visual discrimination, MMPI, and Zung's rated depression scale. Tests were evaluated for significant changes by the parametric student's t test and Mann Whitney nonparametric test. Clinical seizures were significantly reduced by electrical stimulation (ES), as were EEG interictal spikes and EEG slow waves. Psychological performance improved beyond that expected by reduction in seizure activity. ESCM induced a local afterdischarge (AD) that progressively developed in time and intensity, and the beneficial effects outlasted ES for periods of weeks to months, suggesting that a state of hyperexcitability of stimulated tissue, similar to "kindling," was created by chronic ES.

Intracellular recordings in pericruciate neurons during spike and wave discharges of feline generalized penicillin epilepsy

Concurrent EEG and intracellular recordings from pericruciate neurons of cats obtained before and after i.m. injection of penicillin inducing the syndrome of feline generalized penicillin epilepsy (FGPE) characterized by spike and wave (SW) discharge in the EEG, display large excitatory postsynaptic potentials (EPSPs) at the time of the EEG 'spike' which alternate with hyperpolarizing potentials occurring in coincidence with the EEG 'wave' component of the SW complex. The large EPSPs trigger discharges of single or multiple high-frequency action potentials which do not show a progressive decrement in amplitude nor an appreciable increase in duration. These bursts thus differ in some respects from typical paroxysmal depolarization shifts. The hyperpolarizing potentials show an early phase which is reversed by intracellular Cl- injection or diffusion and thus behaves like a classical inhibitory postsynaptic potential (IPSP). The late phase is unaffected by Cl-. Hyperpolarizing potentials of pericruciate neurons induced by antidromic activation of the cerebral peduncle (CP) or by direct cortical stimulation are not altered after i.m. injections of penicillin at doses sufficient to induce generalized SW discharge. The early phase of hyperpolarization both before and after i.m. penicillin is reversed by intracellular Cl- injection or diffusion, the late phase remains unchanged. The early phase thus represents a classical IPSP, which does not appear to be affected by the low brain penicillin concentrations sufficient to induce generalized SW discharge. It is concluded that this form of epileptic discharge cannot be attributed to blockage of phasic (presumably somatic) postsynaptic inhibition by penicillin. These results indicate that to regard all forms of epileptic discharge as the consequence of a blockage of gamma-aminobutyric acid-mediated phasic postsynaptic inhibition acting on the soma represents an unduly restrictive view of epileptogenesis.

Anterior thalamic mediation of generalized pentylenetetrazol seizures

The effects of microinjection of various neuroactive compounds into the anterior thalamic nucleus (AN) and other selected subcortical regions of guinea pig brain on the expression of pentylenetetrazol (PTZ)-induced behavioral and electrical seizure activity were examined. Excitatory agents, kainic acid (KA), bicuculline (BIC) or PTZ, injected into the AN or other thalamic nuclei, striatum, but not the mammillary bodies (MB), facilitated the EEG convulsant action of systemically administered PTZ. Injection of muscimol into the AN protected against the expression of PTZ-induced repetitive high-voltage EEG seizure discharges and inhibited the facilitatory effects of subcortically applied KA or BIC. Injection of muscimol into the AN was also able to terminate established ongoing seizure discharges. Unilateral application of muscimol to the AN did not prevent the repetitive hypersynchronous EEG discharges following systemic PTZ but did result in the delay in the onset of cortical hypersynchrony in the ipsilateral hemisphere. Muscimol injections into other thalamic nuclei, MB, cortex, striatum or directly into the CSF space had no anticonvulsant effect, however. Microinjection of gamma-vinyl-gamma-aminobutyric acid, a selective GABA transaminase inhibitor, resulted in protection from the behavioral convulsant action and lethal effects of PTZ when administered into the thalamus, especially the AN, but not when injected into the striatum or CSF. These data demonstrate that the AN is an important subcortical nucleus for the mediation of both cortical EEG synchrony and behavioral seizure expression induced by PTZ. In light of previous results establishing a role for the brainstem and diencephalon in PTZ seizure expression, the AN may serve, in part, as a gating mechanism for the propagation of paroxysmal activity between subcortical areas and the cerebral cortex.

Multiunitary activity analysis of cortical and subcortical structures in paroxysmal discharges and grand mal seizures in photosensitive baboons

Cortical and subcortical multiunitary activities (MUA) and EEG were simultaneously recorded in baboons made photosensitive by a subconvulsant dose of DL-allylglycine. Intermittent light stimulation (ILS) trains induced in these animals fronto-rolandic (FR) paroxysmal discharges (PDs, constituted as spikes and waves) and grand mal seizures. During the induction of FR PDs by ILS trains, the visual structures (occipital cortex, colliculi superioris, pulvinar) showed a significant MUA increase which was not related to the PD spike or wave but correlated with the flashes. The first structure showing bursts of MUA that frequently preceded the PD appearance was the FR cortex. When PDs appeared, the bursts were related to the spikes of PDs and were followed by an inhibition during the slow wave. The pontine and mesencephalic reticular formations and the facial nuclei were activated in bursts after the FR PDs had reached a certain amplitude. The thalamic nuclei ventralis lateralis, centrum medianum and lateralis posterior were activated only later, when the FR PDs had reached an even greater amplitude. It is suggested that the activation of visual structures is necessary for FR PD appearance. The secondary pontine and mesencephalic activation could reinforce that of the FR cortex and then the thalamus, and could determine the myoclonus observed in unparalysed animals. When the ILS is continued, grand mal seizures appear. The onset of the seizures could be linked to the loss of FR cortical control of the subcortical structures. The resulting reticular activation would be responsible for the vasomotor modifications which constitute the first clinical signs of a seizure.

Interictal EEG pattern in rabbit penicillin epilepsy

The effects of Na-penicillin G (1,000,000 I.U./kg i.v.) were studied in 28 rabbits implanted with surface and deep electrodes in the medial (mT) and lateral (lT) nuclei of the thalamus and in the cornu Ammonis dorsalis (CAd). Attention was focused on interictal spike activity, cortical spindle activity and spikes-spindles relationship. A multifocal interictal EEG pattern, represented by cortical and thalamic spikes, was observed in 20 animals. A clear association between cortical spikes and spindles occurred almost constantly. Moreover a statistically significant increase in the average duration of spindles without changes in the average frequency was noticed. Seizure discharges had always a focal start and secondary generalization. The EEG features of parenteral penicillin epilepsy in the rabbit appeared to be more similar to those of the rat than to those of the cat. The spindle activity changes and the paroxysmal spike activity can be interpreted as two independent penicillin induced features appearing during raised cortical excitability periods.

Involvement of cortical, thalamic and midbrain reticular formation neurons in spike and wave discharges: extracellular study in feline generalized penicillin epilepsy

Extracellular activity of single units, simultaneously recorded in cortex, thalamus, and midbrain reticular formation was investigated during feline generalized penicillin epilepsy. The firing activity of neurons recorded in the cortex was invariably and consistently enhanced in coincidence with the positive peak and the positive-negative transient of the "spike" of the spike and wave complex, and it was greatly decreased during the wave. In the nonspecific thalamic nuclei three classes of neurons were identified according to their patterns of activity during the spike and wave complex: (i) neurons behaving like cortical units, (ii) neurons with enhanced firing activity during the wave and a decreased activity during the "spike," and (iii) unmodified neurons. In the nucleus lateralis posterior neurons of the third class were not found. Most midbrain reticular neurons could be classified in the same three classes of the nonspecific thalamic nuclei; however, 11% of those units increased their activity 20 to 30 ms earlier than did the cortical units (class IV). Investigation of the activities of all these neuronal populations immediately prior to a spike and wave discharge showed that the rhythmic cycle of excitation-inhibition commenced earlier in the cortical neurons than in any other subcortical neuron. Moreover, there were some nonspecific thalamic neurons of class II with an inhibitory phase exactly coincident with the activation of class IV midbrain reticular neurons. These data suggest (i) a leading role of cortical neurons in initiating and maintaining a spike and wave burst; (ii) the involvement of a corticothalamocortical circuit in timing the bursts, and (iii) an accessory reticulothalamic loop also involved in regulating the intraburst frequency of the spike and wave complex.

Ventromedial thalamic lesions and seizure susceptibility

Recent data indicate that the substantia nigra is an important site in a circuitry involved in the modification of various experimental seizures with neocortical and limbic involvement. Since there are no direct nigral projections to either area, we assumed that the nigral effects on seizures are relayed by other sites such as the thalamus. To evaluate this hypothesis we produced bilateral high-radiofrequency thermocoagulative lesions of the ventromedial (VM) thalamic nuclei which receive the nigral efferents in the rat. We determined the susceptibility of lesioned and control adult rats to the development of flurothyl seizures 2 and 4 weeks later. The latency to the onset of a generalized seizure was considered as the convulsive threshold. There were no differences in the mean latencies between the groups. The results suggest that bilateral destruction of the VM thalamic nuclei does not modify the susceptibility to the development of flurothyl seizures in the rat.

Sleep and epilepsy

Epileptic mechanisms in the brain are subject to long-duration, time-ordered neuromodulatory processes controlled by endogenous oscillators which are responsible for appropriately phased modulation of various normal physiological processes, including the 24-h sleep/wakefulness cycle and the ultradian 100-min cycle of rapid eye movement/non-rapid eye movement sleep. Both focal and generalized types of epileptiform activity in humans are subject to biorhythmic modulation, and the various modulation patterns observed are in accord with a model which explains these patterns as a consequence of the interaction of two endogenous modulatory processes: one with a period of about 24 h, the other with a period of about 100 min. Differences in the phase angle between the two cyclic processes, determined by time of sleep onset, explain the various modulatory patterns observed. The mechanisms involved in the genesis and elaboration of electrical epileptiform activity in animal models are examined in relation to known processes involved in the physiology of sleep, and compared with data derived from long-term studies of the time distribution of epileptic events in humans. In infantile spasms, clinical seizure activity and the ictal and interictal EEG patterns in relationship to the phases of the sleep cycle, the significant defects in the quality and quantity of sleep in this disorder, and the changes that take place in all of these when seizures are abolished by effective treatment, suggest that pontine mechanisms responsible for the sleep cycle may be involved in the elaboration of infantile spasms and hypsarrhythmia.

Comparative effects of topical perfusions of pentylenetetrazol in the mesencephalon and cerebral cortex of cats

Push-pull perfusions of pentylenetetrazol (PTZ) were carried out in the mesencephalon and cerebral cortex (orbitofrontal, motor, and suprasylvian) of "encéphale isolé" cats, while EEG recordings from motor cortices and EMG of facial muscles were obtained. There were significant differences between perfusions in the mesencephalic reticular formation (MRF) and in the cerebral cortex: (i) EEG spikes not accompanied by muscular contractions occurred during perfusion of cerebral cortex (motor cortex included), but never with perfusions in MRF. (ii) In some instances muscular tonic and clonic contractions occurred in the absence of EEG spikes when perfusing the mesencephalon, but never with cortical perfusions. (iii) Both MRF and cortical perfusions induced EEG spikes accompanied by myoclonic seizures; however, muscular seizures were practically of the same amplitude in both sides after perfusion of the MRF and were predominant in the contralateral side after cortical perfusions. In addition, significantly less perfusion time (total dose) of PTZ was needed to induce such events in the MRF than in the cerebral cortex. (iv) Generalized seizures induced by cortical perfusions showed a clear predominance of contractions in the muscles contralateral to the perfusion site, whereas perfusions in the MRF induced generalized seizures indistinguishable from those produced by i.v. administration of PTZ. Results suggest that PTZ generalized seizures, closely resembling the so called "primary generalized seizures," result from activation of the MRF, whereas PTZ acting in the cerebral cortex produces a model of focal convulsions that may become secondarily generalized.

Local cerebral metabolic rate for glucose during petit mal absences

Four patients with primary generalized or true petit mal epilepsy were studied with positron emission tomography using [18F]fluorodeoxyglucose (FDG). FDG studies were carried out during 10 minutes of hyperventilation before and again after medical control of spontaneous absences. Before seizures were controlled all 4 patients demonstrated frequent bilaterally synchronous three-per-second spike-and-wave discharges associated with altered consciousness. After spontaneous seizures were controlled, hyperventilation produced only electroencephalographic slowing without clinical symptoms in 3; the fourth patient had absences less frequently. Patterns of local cerebral metabolic rate for glucose (CMRGlc) were normal and identical for ictal and interictal scans; there was, however, a 2.5- to 3.5-fold diffuse ictal increase in global CMRGlc evident when ictal studies were compared with hyperventilation control studies in which no seizures occurred. The CMRGlc was similar in the two scans obtained from the patient who had absences during both studies. No anatomical substrate of petit mal epilepsy was identified. The CMRGlc in these patients during petit mal absences was higher than that recorded in other patients during partial or generalized convulsive seizures. This difference may reflect the fact that petit mal absences are not associated with postictal depression.

Interruption of the mammillothalamic tract prevents seizures in guinea pigs

Interruption of the connection between the mammillary bodies and the anterior nucleus of the thalamus in guinea pigs, by discrete bilateral electrolytic lesions of the mammillothalamic tract, resulted in essentially complete protection from the behavioral and electroencephalographic convulsant action and lethal effect of pentylenetetrazol. This result demonstrates that the mammillary bodies and their rostral efferent connections are important for the propagation and perhaps initiation of generalized seizures.

Transition from spindles to generalized spike and wave discharges in the cat: simultaneous single-cell recordings in cortex and thalamus

The relationships between the activity of the cortex and that of a "specific" (n. lateralis posterior, LP) and an intralaminar thalamic nucleus (n. centralis medialis, NCM) were studied in the cat during the transition from spontaneous spindles to generalized spike and wave (SW) discharge following i.m. penicillin injection. The EEG and extracellular single-unit activity were recorded in cortex and thalamus during the spindle stage and at different intervals after penicillin until well developed SW discharges were present. Computer-generated EEG averages and histograms of single-unit activity were triggered by either peaks of cortical or thalamic EEG transients or by cortical or thalamic action potentials. In agreement with previous observations, cortical neurons increasingly fired during the spindle wave as it was transformed into the "spike" of the SW complex, while a period of neuronal silence gradually developed as the "wave" of the SW complex emerged. Similar changes developed in the thalamus, particularly in LP, either concurrently with or more often after the onset of the changes in the cortex. Most neurons in NCM, continued to fire randomly even after well developed SWs and rhythmic neuronal discharges had developed in cortex and LP. Only 4/11 NCM neurons did ultimately exhibit a rhythmic firing pattern similar to that seen in the cortex and LP. The correlation between cortical and thalamic unit activity was low during spindles, but gradually increased during the development of SW discharges. These data confirm that the cortex is the leading element in the transition from spindles to SWs. Increasingly, in the course of this transition, cortical and thalamic neuronal firing becomes more intimately phase-locked. This mutual interrelationship appears to be more pronounced between cortex and "specific" than intralaminar thalamic nuclei.

Differential participation of some 'specific' and 'non-specific' thalamic nuclei in generalized spike and wave discharges of feline generalized penicillin epilepsy

Extracellular single unit and electroencephalographic (EEG) activity during generalized spike and wave discharges (SW) induced by i.m. penicillin was recorded simultaneously in the cortex, in a 'specific' thalamic nucleus (n. lateralis posterior, LP) and in some 'non-specific' thalamic nuclei (n. centralis medialis, NCM; n. centrum medianum, CM; n. centralis lateralis, CL) Computer-generated EEG averages and histograms of single unit activity were triggered by either peaks of EEG transients or action potentials. The time at which cortical neurons (66/66) were most likely to fire was during the 'spike' of the SW complex while absence of firing was the rule during the 'wave'. Most LP neurons (23/26) showed a similar pattern, 3 cells firing preferentially during the 'wave'. In NCM only 17 of 39 neurons fired during the 'spike', 8 of 39 neurons during the 'wave' while the others showed no change in their firing pattern during SWs. Twenty-six of 30 CM and 20 of 24 CL neurons fired during the 'spike' of SW; the other cells in these nuclei did not change their firing pattern during SWs. When present, rhythmic fluctuations in firing linked to SW discharge were less prominent in these 'non-specific' thalamic nuclei than in cortex and LP. Furthermore, participation of NCM, CM and CL neurons in the SW rhythm occurred only after neurons in cortex and LP had become involved in it. Thus, as is the case for cortical neurons, the main firing pattern of thalamic cells during SWs consists of an oscillation between 'excitation' during the 'spike' and 'inhibition' during the 'wave' of the SW complex. However, the coupling between cortical and thalamic neuronal firing is less intimate for cells of the 'non-specific' thalamic nuclei than for a 'specific' nucleus such as LP. Thus, at least some 'specific' thalamic nuclei are more intimately involved in the mechanism of SW discharge than the midline intralaminar nuclei.

Pathophysiology of generalized photosensitive epilepsy in the cat

Feline generalized penicillin-induced epilepsy is a reliable experimental model of epileptic photosensitivity. Binocular photic stimulation at 4-8 flashes/s consistently triggers generalized, bilaterally synchronous spike-and-wave discharges in cats submitted to long-term intramuscular penicillin administration at low dosage (50,000-150,000 IU/kg). The photically induced epileptic activity is first and mainly recorded from the cerebral cortex as compared with the lateral geniculate (GL) body, therefore supporting a cortical onset of epileptic photosensitivity. At this low penicillin dosage, only minimal spontaneous spike-and-wave activity is recorded. Bilateral GL stimulation is more effective than binocular photic stimulation in triggering spike-and-wave discharges, whereas unilateral GL stimulation is grossly ineffective. This observation implies a role of bilateral specific visual thalamocortical volleys in the genesis of epileptic photosensitivity. Five of 12 randomly selected cats receiving long-term penicillin exhibited spontaneous epileptic photosensitivity, a finding that suggests an underlying genetic predisposition of the feline species to this condition.

Familial photosensitive epilepsy: effectiveness of clonazepam

We describe the clinical features of a family with photosensitive epilepsy, followed for 13 years. Generalized paroxysmal discharges induced by photic stimulation appeared in all 9 siblings. Generalized seizures, myoclonic jerks, and absences appeared in variable combination in 7 of them who on clonazepam remained free from seizures for a seven-year follow-up period. On EEG the paroxysmal abnormalities induced by photic stimulation abated during clonazepam medication.

Partial inhibitory seizures: a report on two cases

This paper described two epileptic children with the clinical features of tonic-clonic seizures, brief atonic episodes, drop attacks and myoclonic jerks. Patient 2 also displayed absences and behavioral disturbance with episodes of minor status epilepticus. When the patients held both arms outstretched, polygraphic records revealed the spike-and-wave (sp-w) discharges in association with the sudden dropping of an outstretched arm. The drops were due to a loss of tone recorded in deltoid muscles, and coincided with the slow wave components of bilateral sp-w complexes, predominantly on the contralateral side. In the present paper, the clinical and electrophysiologic features of this particular type of seizure are presented in detail. A cortical origin of these episodes is suggested.

Behavioral alterations associated with generalized spike and wave discharges in the EEG of the cat

Instrumental conditioning procedures demonstrated that in feline generalized penicillin epilepsy (FGPE) the cat's ability to respond to sensory (visual or auditory) stimuli was selectively impaired during penicillin-induced generalized spike and wave (SW) discharge. Responsiveness between SW bursts remained unimpaired. Most often the performance deficit consisted of a total absence of a learned response to stimuli presented during SW bursts or, if such a response occurred, reaction times were on the average significantly longer than to stimuli presented between SW bursts. Stimuli falling in the middle of a SW burst were associated with the highest likelihood of response failure. Spontaneous motor performance which was not contingent on any stimulus was also impaired during SW discharge. Response failure during SW bursts is either attributable to a cognitive defect or to motor impairment associated with temporary amnesia. Impairment of motor performance unassociated with amnesia or a cognitive defect was sometimes present during SW discharge, as evidenced by failure to carry out a motor response or to complete it until the SW burst was over. These deficits are similar to those seen in human absence attacks associated with generalized SW discharge. These observations thus support the validity of FGPE as an acceptable model of human primary generalized epilepsy.

Participation of corticothalamic cells in penicillin-induced generalized spike and wave discharges

Single unit extracellular recordings were performed in the cortex of awake painlessly immobilized unanesthetized cats during generalized spike and wave discharges (SW) induced by i.m. penicillin. Corticothalamic cells were identified in cortical areas 3a and 4 gamma by stimulating n. ventralis lateralis (VL) and in cortical areas 5 and 7 by stimulating n. lateralis posterior (LP). Twelve of 24 neurons antidromically invaded from VL were also pyramidal tract cells. Two of 11 neurons antidromically invaded from LP also displayed orthodromic responses. Corticothalamic cells fired bursts of action potentials in association with the 'spike' whereas a period of inhibition was associated with the 'wave' of the SW complex. The data suggest that in this experimental model the appearance of SW in the thalamus is due to secondary activation of thalamic neurons by volleys arising from the cortex and mediated through corticothalamic connections.

Potentiation and modification of recruiting responses precedes the appearance of spike and wave discharges in feline generalized penicillin epilepsy

Recruiting responses (RR) were evoked by stimulation of nucleus centralis medialis in awake and painlessly immobilized cats. Following the administration of sodium penicillin G (350,000 IU/kg i.m.) and at a time preceding the development of generalized spike-and-wave discharge we observed a strong potentiation of RR in 10 out of 15 experiments (50-200% increase in amplitude). The major feature of wave form modification consisted mainly of a development or increase of positive phases of individual recruiting waves. In between such large amplitude negative-positive recruiting waves a slow negative wave developed. One of every two recruiting waves was often diminished when the preceding recruiting wave had reached considerable amplitude. The changes in the RR were antagonized by barbiturates and by caffeine. In conjunction with previous evidence these results support the hypothesis that spikes of spike-and-wave discharges in FGPE are generated by similar thalamocortical volleys as those creating RR and spindles. They further suggest that the crucial neuronal mechanism underlying this effect of penicillin is a shift in the emphasis from distal apical dendritic thalamocortical synapses on cortical pyramidal neurons to more proximal ones.

Feline generalized epilepsy induced by tranexamic acid (AMCA)

Epileptic activities induced by topical application of tranexamic acid (AMCA) and penicillin to the cortex of 12 cats in acute experiments were compared. Both substances when diffusely applied on a wide cortical area of both hemispheres at very low concentration produced an EEG pattern consisting of spike-wave bursts similar to the electrographic manifestations seen in feline generalized epilepsy induced by large parenteral doses of sodium penicillin. These epileptic bursts could be triggered by repetitive stimulation of nucleus centralis medialis. Increased concentrations of both AMCA and penicillin led to the appearance of bilaterally synchronous spikes and poly-spikes which were not further excited by NCM stimulation. Two factors seem to play an important role in eliciting spike-wave bursts in both models: (1) the area of the cortex exposed to the epileptogenic agent and (2) the concentration of the epileptogenic agent used. The similar effects observed in 5 chronic animals either by intravenous injection of high doses of AMCA or by intramuscular injection of sodium penicillin confirm the results obtained in acute experiments and suggest a new way of inducing feline generalized epilepsy.

EEG monitoring of focal lesions of the blood-brain barrier

This paper presents a modified method for the EEG detection and monitoring of small focal blood-brain barrier (BBB) lesions. The method uses high dose IV penicillin (1.2 X 10(6) mu/kg) to produce a spike focus at BBB lesions, penicillin encephalopathy. The EEG is recorded and the epileptic spikes are counted by an automatic computer program. The computer produces an objective quantitative results. The method is verified empirically on traumatic and radiation BBB lesions.

[Indications and long-term results of stereotactic operations in therapy-resistant epilepsy (author's transl)]

Stereotactic functional surgery was performed on 45 patients with predominantly temporal epilepsy. Results of up to 24 years (= 5.85 +/- 2.25 years) are reported. Depending on the type of seizure and after careful analysis of the EEG focus, the frequency and intensity of the various seizure types, as well as personality and behavior disturbances, can be improved in more than two-thirds of the cases. This improvement is achieved by combining different target points with coagulation of the subcortical motor and senso-motor centers and neuronal structures, thereby cutting off the mostly double-track corticosubcortical connections