Consequences of cortical dysplasia during development in rats

Enhanced Burst-Suppression and Disruption of Local Field Potential Synchrony in a Mouse Model of Focal Cortical Dysplasia Exhibiting Spike-Wave Seizures

Focal cortical dysplasias (FCDs) are a common cause of brain seizures and are often associated with intractable epilepsy. Here we evaluated aberrant brain neurophysiology in an in vivo mouse model of FCD induced by neonatal freeze lesions (FLs) to the right cortical hemisphere (near S1). Linear multi-electrode arrays were used to record extracellular potentials from cortical and subcortical brain regions near the FL in anesthetized mice (5–13 months old) followed by 24 h cortical electroencephalogram (EEG) recordings. Results indicated that FL animals exhibit a high prevalence of spontaneous spike-wave discharges (SWDs), predominately during sleep (EEG), and an increase in the incidence of hyper-excitable burst/suppression activity under general anesthesia (extracellular recordings, 0.5%–3.0% isoflurane). Brief periods of burst activity in the local field potential (LFP) typically presented as an arrhythmic pattern of increased theta-alpha spectral peaks (4–12 Hz) on a background of low-amplitude delta activity (1–4 Hz), were associated with an increase in spontaneous spiking of cortical neurons, and were highly synchronized in control animals across recording sites in both cortical and subcortical layers (average cross-correlation values ranging from +0.73 to +1.0) with minimal phase shift between electrodes. However, in FL animals, cortical vs. subcortical burst activity was strongly out of phase with significantly lower cross-correlation values compared to controls (average values of −0.1 to +0.5, P < 0.05 between groups). In particular, a marked reduction in the level of synchronous burst activity was observed, the closer the recording electrodes were to the malformation (Pearson’s Correlation = 0.525, P < 0.05). In a subset of FL animals (3/9), burst activity also included a spike or spike-wave pattern similar to the SWDs observed in unanesthetized animals. In summary, neonatal FLs increased the hyperexcitable pattern of burst activity induced by anesthesia and disrupted field potential synchrony between cortical and subcortical brain regions near the site of the cortical malformation. Monitoring the altered electrophysiology of burst activity under general anesthesia with multi-dimensional micro-electrode arrays may serve to define distinct neurophysiological biomarkers of epileptogenesis in human brain and improve techniques for surgical resection of epileptogenic malformed brain tissue.

Simultaneous EEG-fMRI in Human Epilepsy

Electroencephalography (EEG) has been used to study and characterize epilepsy for decades, but has a limited ability to localize epileptiform activity to a specific brain region. With recent technological advances, high-quality EEG can now be recorded during functional magnetic resonance imaging (fMRI), which characterizes brain activity through local changes in blood oxygenation. By combining these techniques, the specific timing of interictal events can be identified on the EEG at millisecond resolution and spatially localized with fMRI at millimeter resolution. As a result, simultaneous EEG-fMRI provides the opportunity to better investigate the spatiotemporal mechanisms of the generation of epileptiform activity in the brain. This article discusses the technical considerations and their solutions for recording simultaneous EEG-fMRI and the results of studies to date. It also addresses the application of EEG-fMRI to epilepsy in humans, including clinical applications and ongoing challenges.

Does the region of epileptogenicity influence the pattern of change in cortical excitability?

OBJECTIVE

To investigate whether cortical excitability measures on transcranial magnetic stimulation (TMS) differed between groups of patients with different focal epilepsy syndromes.

METHODS

85 Patients with focal epilepsy syndromes divided into temporal and extra-temporal lobe epilepsy were studied. The cohorts were further divided into drug naïve-new onset, refractory and seizure free groups. Motor threshold (MT) and paired pulse TMS at short (2, 5, 10, 15 ms) and long (100-300 ms) interstimulus intervals (ISIs) were measured. Results were compared to those of 20 controls.

RESULTS

Cortical excitability was higher at 2 & 5 ms and 250, 300 ms ISIs (p<0.01) in focal epilepsy syndromes compared to controls however significant inter-hemispheric differences in MT and the same ISIs were only seen in the drug naïve state early at onset and were much more prominent in temporal lobe epilepsy.

CONCLUSION

Disturbances in cortical excitability are more confined to the affected hemisphere in temporal lobe epilepsy but only early at onset in the drug naïve state.

SIGNIFICANCE

Group TMS studies show that cortical excitability measures are different in temporal lobe epilepsy and can be distinguished from other focal epilepsies early at onset in the drug naïve state. Further studies are needed to determine whether these results can be applied clinically as the utility of TMS in distinguishing between epilepsy syndromes at an individual level remains to be determined.

Pathophysiological mechanisms of focal cortical dysplasia: a critical review of human tissue studies and animal models

Cortical dysplasia (CD, also known as malformations of cortical development) are the pathological substrates in a large percentage of patients with pharmacoresistant epilepsy who may be amenable to surgical treatment. Therefore, research on the mechanisms of dysplastic lesion formation and epileptogenicity is of paramount importance for the prevention, detection, and treatment of CD-induced epilepsy. The purpose of this review is to discuss and critically evaluate the current state and results of human tissue experimentation (focusing on reported results of studies done on neocortical dysplastic tissue resected from patients with pharmacoresistant epilepsy), and to discuss some of the concerns related to research that uses surgically resected epileptic human tissue. The use of better animal models of CD as a tool toward the better understanding of the mechanisms of pathogenesis, epileptogenesis, and epileptogenicity of dysplastic lesions will be reviewed from the perspective of their usefulness in a model of translational research that should ultimately result in better diagnostic and therapeutic techniques of CD.

A randomized clinical trial of repetitive transcranial magnetic stimulation in patients with refractory epilepsy

OBJECTIVE

To study the antiepileptic effects of rTMS in patients with refractory epilepsy and malformations of cortical development in a randomized, double-blind, sham-controlled trial.

METHODS

Twenty-one patients with malformations of cortical development and refractory epilepsy underwent five consecutive sessions of low-frequency rTMS, either sham or active (1Hz, 1,200 pulses), focally targeting the malformations of cortical development. The number of epileptiform discharges in the electroencephalogram and the number of clinical seizures were measured before (baseline), immediately after, as well as 30 and 60 days after rTMS treatment.

RESULTS

rTMS significantly decreased the number of seizures in the active compared with sham rTMS group (p < 0.0001), and this effect lasted for at least 2 months. Furthermore, there was a significant decrease in the number of epileptiform discharges immediately after (p = 0.01) and at week 4 (p = 0.03) in the active rTMS group only. There were few mild adverse effects equally distributed in both groups. The preliminary cognitive evaluation suggests improvement in some aspects of cognition in the active rTMS group only.

INTERPRETATION

Noninvasive brain stimulation for epilepsy may be an alternative treatment for pharmaco-resistant patients with clearly identifiable seizure foci in the cortical convexity and who are not eligible for surgical treatment.

Febrile seizures in the predisposed brain: A new model of temporal lobe epilepsy

The atypical febrile seizure has important clinical implications because of its association with the mesial temporal lobe epilepsy syndrome, which is the most common of the intractable epilepsies. However, whether a causal relation exists between these conditions is currently unknown. We have previously shown that a focal cortical lesion induced in the neonatal rat predisposes to the development of atypical hyperthermic seizures. We show here that 86% of the lesion plus hyperthermia group experience development of spontaneous recurrent seizures recorded from the amygdala ipsilateral to the lesion. Control rats did not have spontaneous recurrent behavioral or electrographic seizures. Lesioned rats with hyperthermic seizures also showed an impaired performance on the Morris water maze when compared with naive control rats, suggesting mild deficits in learning and memory. These findings support a link between the atypical febrile seizure and mesial temporal lobe epilepsy, and at the same time establish a new model for this condition through which new preventative and therapeutic strategies can be tested.

Neurophysiologic findings of neuronal migration disorders: intrinsic epileptogenicity of focal cortical dysplasia on electroencephalography, electrocorticography, and magnetoencephalography

We define specific neurophysiologic characteristics for focal cortical dysplasia, a neuronal migration disorder. We reviewed data from published reports and our patients with focal cortical dysplasia. Our patients underwent preoperative scalp video-electroencephalography (EEG), magnetic resonance imaging (MRI), magnetoencephalography, and intraoperative or extraoperative electrocorticography monitoring. Scalp EEG showed trains of rhythmic epileptiform spike or sharp waves. Positive spikes correlated with early seizure onset, MRI lesion around the rolandic fissure, hemiparesis, and a less favorable outcome. Interictal electrocorticography showed continuous epileptogenic discharges: repetitive electrographic seizures and bursting discharges or continuous or quasicontinuous rhythmic spiking. Ictal electrocorticography showed paroxysmal fast and/or repetitive spiking. Magnetoencephalography showed clustered spike sources within and extending from the lesion. Cortical stimulation gave more frequent, lower-threshold afterdischarges and higher-threshold primary motor function. Focal cortical dysplasias are highly and intrinsically epileptogenic. For surgical seizure control, EEG, electrocorticography, and magnetoencephalography must delineate the intrinsic epileptogenic zone within and extending from the focal cortical dysplasia identified by MRI.

Characteristics of prolonged afterdischarges in children with malformations of cortical development

We investigated aberrant cortical excitability in malformations of cortical development From subdural electrodes, we recorded afterdischarges lasting > or = 6 seconds in 12 of 13 patients with malformations of cortical development and 6 of 10 pediatric patients with nonmalformations of cortical development and reviewed amperage thresholds, distribution of afterdischarges, and motor responses. In patients with malformation of cortical development, motor response thresholds were high; afterdischarge and motor response thresholds, which essentially overlapped, inversely correlated with age (P < .01); afterdischarge thresholds declined with age; and 8 patients showed afterdischarges in remote sites. In nonmalformation of cortical development, afterdischarge thresholds did not significantly correlate with age; motor response thresholds tended to decline with age; and 2 patients had remote afterdischarges. Adolescent patients with malformations of cortical development had lower afterdischarge thresholds than adolescents with nonmalformation of cortical development (P < .05). From their high afterdischarge (and motor response) thresholds, we concluded that preadolescent patients with malformation of cortical development have less excitable, immature cortices, whereas adolescent patients with malformation of cortical development with low afterdischarge thresholds have hyperexcitable cortices. Remote afterdischarges over focal dysplastic cortex suggest aberrant cortical excitability and neural circuits.

Classification issues in malformations caused by abnormalities of cortical development

Malformations caused by abnormalities of cortical development (MCDs) as a group are now widely recognized as a key cause of medically refractory epilepsies, often leading to a consideration of surgical treatment. A practical classification scheme including histopathologic, imaging, and, if possible, clinical-electrographic features of the various different types of MCDs, will be important to the delineation of surgical strategies and anticipation of medical and surgical prognoses. A proposal of such a scheme with emphasis on the focal cortical dysplasias is given in the hopes that it will reopen the debate on the best way to classify these disorders.

Electroencephalographic characterization of an adult rat model of radiation-induced cortical dysplasia

PURPOSE

Cortical dysplasia (CD) is a frequent cause of medically intractable focal epilepsy. The mechanisms of CD-induced epileptogenicity remain unknown. The difficulty in obtaining and testing human tissue warrants the identification and characterization of animal model(s) of CD that share most of the clinical, electroencephalographic (EEG), and histopathologic characteristics of human CD. In this study, we report on the in vivo EEG characterization of the radiation-induced model of CD.

METHODS

Timed-pregnant Sprague-Dawley rats were irradiated on E17 using a single dose of 145 cGy or left untreated. Their litters were identified and implanted with bifrontal epidural and hippocampal depth electrodes for prolonged continuous EEG recordings. After prolonged EEG monitoring, animals were killed and their brains sectioned and stained for histologic studies.

RESULTS

In utero-irradiated rats showed frequent spontaneous interictal epileptiform spikes and spontaneous seizures arising independently from the hippocampal or the frontal neocortical structures. No epileptiform or seizure activities were recorded from age-matched control rats. Histologic studies showed the presence of multiple cortical areas of neuronal clustering and disorganization. Moreover, pyramidal cell dispersion was seen in the CA1>CA3 areas of the hippocampal formations.

CONCLUSIONS

Our results further characterize the in vivo EEG characteristics of the in utero radiation model of CD using long-term EEG monitoring. This model may be used to study the molecular and cellular changes in epileptogenic CD and to test the efficacy of newer antiepileptic medications.

Brain development and generation of brain pathologies

The timing of brain development is genetically programmed and can be profoundly altered by a number of abnormal genes that control proliferation, differentiation, migration, synapse formation, and elimination of cells and synapses. With advances in genetic techniques both the gene and gene product responsible for brain malformation are being identified which will permit a better understanding of the pathophysiology of brain malformations. In addition to genetic abnormalities, brain development can be influenced by a variety of acquired disorders. The type of brain abnormality is highly dependent on the stage of brain development during which the insult occurs. While our understanding of experience- or activity-dependent processes on brain development is limited, it is now clear that this activity can significantly alter the timing of a number of maturational events including receptor maturation and neurogenesis. The future challenge will be both to understand the biological basis of these processes and to use this information to enhance brain development.

Disorders of cortical development

It is only a decade since the realization (facilitated by magnetic resonance imaging) in early 1990s that disorders of cortical development occupy an important place in the aetiologic categorization of epilepsy. Since then research has demonstrated the intrinsic epileptogenicity of disorders of cortical development, their genetic bases and their functional properties. Some of the key points of this most exciting medical and scientific enterprise are reviewed here, with an emphasis in the advances seen within the past 2 years.

Cortical malformations and epilepsy: new insights from animal models

In the last decade, the recognition of the high frequency of cortical malformations among patients with epilepsy especially children, has led to a renewed interest in the study of the pathophysiology of cortical development. This field has also been spurred by the recent development of several experimental genetic and non-genetic, primarily rodent, models of cortical malformations. Epileptiform activity in these animals can appear as spontaneous seizure activity in vivo, in vitro hyperexcitability, or reduced seizure susceptibility in vitro and in vivo. In the neonatal freeze lesion model, that mimics human microgyria, hyperexcitability is caused by a reorganization of the network in the borders of the malformation. In the prenatal methylazoxymethanol model, that causes a diffuse cortical malformation, hyperexcitability is associated with alteration of firing properties of discrete neuronal subpopulations together with the formation of bridges between normally unconnected structures. In agreement with clinical evidence, these experimental data suggest that cortical malformations can both form epileptogenic foci and alter brain development in a manner that causes a diffuse hyperexcitability of the cortical network.