Characterization of nodular neuronal heterotopia in children

Neurodevelopmental disorders as a cause of seizures: neuropathologic, genetic, and mechanistic considerations

This review will consider patterns of developmental neuropathologic abnormalities-malformations of cortical development (MCD)--encountered in infants (often with infantile spasms), children, and adults with intractable epilepsy. Treatment of epilepsy associated with some MCD, such as focal cortical dysplasia and tubers of tuberous sclerosis, may include cortical resection performed to remove the "dysplastic" region of cortex. In extreme situations (eg, hemimegalencephaly), hemispherectomy may be carried out on selected patients. Neuropathologic (including immunohistochemical) findings within these lesions will be considered. Other conditions that cause intractable epilepsy and often mental retardation, yet are not necessarily amenable to surgical treatment (eg, lissencephaly, periventricular nodular heterotopia, double cortex syndrome) will be discussed. Over the past 10 years there has been an explosion of information on the genetics of MCD. The genes responsible for many MCD (eg, TSC1, TSC2, LIS-1, DCX, FLN1) have been cloned and permit important mechanistic studies to be carried out with the purpose of understanding how mutations within these genes result in abnormal cortical cytoarchitecture and anomalous neuroglial differentiation. Finally, novel techniques allowing for analysis of patterns of gene expression within single cells, including neurons, is likely to provide answers to the most vexing and important question about these lesions: Why are they epileptogenic?

Dysfunction of synaptic inhibition in epilepsy associated with focal cortical dysplasia

Focal cortical dysplasia (FCD) is a common and important cause of medically intractable epilepsy. In patients with temporal lobe epilepsy and in several animal models, compromised neuronal inhibition, mediated by GABA, contributes to seizure genesis. Although reduction in GABAergic interneuron density has been reported in FCD tissue samples, there is little available information on the resulting physiological changes in synaptic inhibition and the potential contribution of these changes to epileptogenesis in the dysplastic human brain. Using visualized whole-cell patch-clamp recordings from identified neurons in tissue slices obtained from patients with FCD, we demonstrate that GABAA-receptor-mediated inhibition is substantially altered in regions of dysplasia. These alterations include a significant reduction in IPSC frequency and a potentially compensatory decrease in transporter-mediated GABA reuptake function; the latter is marked by a significant increase in the decay-time constant for evoked and spontaneous IPSCs and a lack of effect of the GABA transport-inhibitor 1-[2([(diphenylmethylene)imino]oxy)ethyl]-1,2,5,6-tetrahydro-3-pyridinecarboxylic acid hydrochloride on IPSC kinetics. Immunohistochemical staining revealed a scattering of GABAergic interneurons across dysplastic cortex and striking reductions in GABA transporter expression. Together, these results suggest that profound alterations in GABA-mediated synaptic inhibition play an essential role in the process of epileptogenesis in patients with FCD.

Human disorders of cortical development: from past to present

Epilepsy and mental retardation, originally of unknown cause, are now known to result from many defects including cortical malformations, neuronal circuitry disorders and perturbations of neuronal communication and synapse function. Genetic approaches in combination with MRI and related imaging techniques continually allow a re-evaluation and better classification of these disorders. Here we review our current understanding of some of the primary defects involved, with insight from recent molecular biology advances, the study of mouse models and the results of neuropathology analyses. Through these studies the molecular determinants involved in the control of neuron number, neuronal migration, generation of cortical laminations and convolutions, integrity of the basement membrane at the pial surface, and the establishment of neuronal circuitry are being elucidated. We have attempted to integrate these results with the available data concerning, in particular, human brain development, and to emphasize the limitations in some cases of extrapolating from rodent models. Taking such species differences into account is clearly critical for understanding the pathophysiological mechanisms associated with these disorders.

Periventricular Nodular Heterotopia: Classification, Epileptic History, and Genesis of Epileptic Discharges

PURPOSE

Periventricular nodular heterotopia (PNH) is among the most common malformations of cortical development, and affected patients are frequently characterized by focal drug-resistant epilepsy. Here we analyzed clinical, MRI, and electrophysiologic findings in 54 PNH patients to reevaluate the classification of PNH, relate the anatomic features to epileptic outcome, and ascertain the contribution of PNH nodules to the onset of epileptic discharges.

METHODS

The patients were followed up for a prolonged period at the Epilepsy Center of our Institute. In all cases, we related MRI findings to clinical and epileptic outcome and analyzed interictal and ictal EEG abnormalities. In one patient, EEG and stereo-EEG (SEEG) recordings of seizures were compared.

RESULTS

We included cases with periventricular nodules, also extending to white matter and cortex, provided that anatomic continuity was present between nodules and malformed cortex. Based on imaging and clinical data, patients were subdivided into five PNH groups: (a) bilateral and symmetrical; (b) bilateral single-noduled; (c) bilateral and asymmetrical; (d) unilateral; and (e) unilateral with extension to neocortex. The latter three groups were characterized by worse epileptic outcome. No differences in outcome were found between unilateral PNH patients regardless the presence of cortical involvement. Interictal as well as ictal EEG abnormalities were always related to PNH location.

CONCLUSIONS

The distinctive clinical features and epileptic outcomes in each group of patients confirm the reliability of the proposed classification. Ictal EEG and SEEG recordings suggest that seizures are generated by abnormal anatomic circuitries including the heterotopic nodules and adjacent cortical areas.

Grey matter heterotopia: what EEG-fMRI can tell us about epileptogenicity of neuronal migration disorders

Grey matter heterotopia are commonly associated with refractory epilepsy. Depth electrodes recordings have shown that epileptiform activity can be generated within these lesions, and also at a distance in the neocortex. Heterotopia seem to be part of a more complex circuitry involving also the surrounding and distant cerebral cortex. Blood oxygenation level-dependent (BOLD) changes to interictal spikes using continuous EEG and functional MRI (EEG-fMRI) can help to understand non-invasively the mechanisms of epileptogenicity in these patients. We studied 14 patients with epilepsy and heterotopia using simultaneous recording of EEG-fMRI. EEG was continuously acquired from inside the scanner during 2 h sessions. Epileptic spikes were visually identified in the filtered EEG and each type of spike determined one EEG-fMRI study. We looked at positive (activation) and negative (deactivation) changes in the BOLD signal. Eleven patients had nodular heterotopia and three band heterotopia. Four patients had more than one type of spikes, with a total of 26 EEG-fMRI studies. We excluded three with less than three spikes, and therefore a total of 23 studies (12 with nodular and 11 with band heterotopia) were analysed. Nodular heterotopia: Activation was present in nine studies, with involvement of the heterotopia or surrounding cortex in six, three of which had concomitant distant activation. Deactivation was also observed in nine studies, with involvement of the heterotopia and surrounding cortex in four, three of which had concomitant distant deactivation. Band heterotopia: Activation was present in all 11 studies, and always involved the heterotopia and surrounding cortex, 9 of which had concomitant distant activation. Deactivation was also observed in all 11 studies, with involvement of both the heterotopia and surrounding cortex, in addition to distant deactivation in 5 studies. EEG-fMRI studies reveal, non-invasively, metabolic responses in the heterotopia despite the fact that spikes are generated in the neocortex. The responses, activation or deactivation, had different correlation with the lesion and surrounding or distant cortex, activation reflecting intense neuronal activity, or excitation, and deactivation a possible distant (extra-lesional) inhibition. EEG-fMRI may become a useful tool to understand the epileptogenicity of such malformations.

Periventricular heterotopia

Periventricular heterotopia (PH) is clinically diagnosed on the basis of the radiographic characteristics of heterotopic nodules composed of disorganized neurons along the lateral ventricles of the brain. Epilepsy is the main presenting symptom of patients with PH. Behaviorally, patients generally are of normal intelligence, although there have been associated findings of learning disabilities, namely, dyslexia. Two genes responsible for PH have been identified: FilaminA, which encodes for the protein filamin A, and ARFGEF2, which encodes for the vesical transport-regulating protein ARFGEF2. The much more common X-linked dominant form of this disorder is due to filamin A, affects females, and is typically lethal in males. A much rarer autosomal recessive form due to ARFGEF2 mutations leads to microcephaly and developmental delay in addition to PH. Cell motility, adhesion defects, and weakening along the neuroepithelial lining may result from defects in these genes during cortical development and contribute to PH, but the mechanisms are not clear yet. Treatment of PH is largely symptomatic, following basic principles for epilepsy management and genetic counseling.

A distinct asymmetrical pattern of cortical malformation: large unilateral malformation of cortical development with contralateral periventricular nodular heterotopia in three pediatric cases

PURPOSE

To describe a distinct asymmetrical pattern of cortical malformation with large focal malformations of cortical development (MCDs) and contralateral periventricular nodular heterotopia (PNH).

METHODS

We identified three patients with epilepsy and focal EEG abnormalities. Each patient underwent 1.5-Tesla magnetic resonance imaging (MRI) to obtain sagittal T1-weighted, axial fluid-attenuated inversion recovery (FLAIR), fast spin-echo (FSE) T2-weighted, and coronal fast spin-echo inversion recovery (FSEIR) T2-weighted images; coronal spoiled gradient recalled (SPGR) T1-weighted images were obtained in two cases.

RESULTS

Patient 1, an 18-year-old right-handed man, had a 4-year history of intractable seizures. MRI revealed a right frontal subcortical heterotopia (SH) and a single left anterior PNH. Patient 2, a 10-year-old left-handed boy, had a 4-year history of epilepsy. MRI revealed a large region of SH in the left temporal, parietal, and occipital lobes and three right-sided PNH. Patient 3, a 16-month-old girl, had medically refractory infantile spasms. MRI revealed a large MCD in the left parietal lobe with contiguous underlying periventricular heterotopia as well as a small contralateral PNH.

CONCLUSIONS

These cases together illustrate a distinct asymmetrical pattern of a large focal MCD with small contralateral PNH. The asymmetrical involvement of the two hemispheres suggests that the stage of maximal disruption of cortical development may differ between the two hemispheres. Further study into the mechanisms underlying such asymmetrical patterns of cortical malformation should enhance our understanding of cortical development as well as hemispheric lateralization.

Reflex Seizures in Patients with Malformations of Cortical Development and Refractory Epilepsy

PURPOSE

Malformations of cortical development (MCDs) are usually highly epileptogenic, and their hyperexcitability could facilitate the occurrence of reflex seizures. We sought to characterize reflex seizures in patients with MCDs and refractory epilepsy.

METHODS

Clinical, electrographic, and neuroimaging data were reviewed in eight patients with MCDs who had reflex seizures reproduced during presurgical evaluation.

RESULTS

All eight patients had both reflex and spontaneous seizures. In six, however, drop attacks or axial myoclonic seizures occurred only upon specific sensory stimulation. Reflex seizures were induced by more than one type of stimulus in most patients, but anatomofunctional correlations could usually be invoked. Six patients had significant intellectual impairment. Surgical resection controlled seizures in two patients.

CONCLUSIONS

Reflex seizures in patients with MCDs may be medically refractory and may often manifest as drop attacks or axial myoclonus. Surgical resection of focal lesions can bring reflex seizures under control. Putative mechanisms related to the relatively low frequency of reflex seizures in MCDs are discussed.

Periventricular White Matter Flumazenil Binding and Postoperative Outcome in Hippocampal Sclerosis

PURPOSE

In patients with hippocampal sclerosis (HS), anterior temporal lobe resection offers the possibility of a long-lasting suppression of seizures in two thirds of patients. White matter (WM) [11C]flumazenil volume of distribution (FMZ-Vd) reflects the number of neuronal cell bodies in WM. Our objective was to correlate WM FMZ-Vd in patients with unilateral HS and postsurgical outcome.

METHODS

We performed [11C]FMZ-PET in 15 patients with refractory mesial temporal lobe epilepsy (mTLE) and a quantitative MRI diagnosis of unilateral HS subsequently histologically verified in all cases. Median follow-up was 7 years (range, 6-9 years). Metabolite-corrected arterial plasma input functions and spectral analysis were used to generate parametric images of [11C]FMZ-Vd. Statistical parametric mapping (SPM99) with explicit masking was used to investigate the entire brain volume including WM.

RESULTS

Eight patients had Engel class IA outcome (completely seizure free since surgery), and seven were not seizure free. Comparison of seizure-free patients with those who continued to have seizures after surgery revealed areas of increased FMZ binding around the posterior horns of the ipsilateral (z=3.7) and contralateral (z=2.7) ventricles in those with suboptimal outcomes.

CONCLUSIONS

Preoperative [11C]FMZ-PET can detect periventricular increases of WM FMZ binding, implying heterotopic neurons in WM, in patients with mTLE. The presence of such increases correlates with a poorer outcome.

Microdysgenesis in epilepsy

Microdysgenesis is a microscopic malformation of cortical development characterized by heterotopic neurones and abnormal cortical architecture. It has been described in primary generalized and partial epilepsy. Its significance in epileptogenesis is controversial, partly due to lack of consensus of diagnostic criteria. Different terms have also been used for the malformation. Several quantitative studies have been performed of the histopathological aberrations associated with microdysgenesis. A majority of the studies have revealed an increased number of heterotopic neurones in specimens from epilepsy patients. However, the quantitative values given for abnormal numbers of white matter neurones vary greatly between studies and there is no consensus yet on quantitative criteria for microdysgenesis. There have also been conflicting results from studies correlating microdysgenesis with clinical data. Both favourable and worse outcome after epilepsy surgery have been reported in patients with increased numbers of white matter neurones and microdysgenesis. While some studies have shown earlier seizure onset and increased frequency of mental retardation in patients with microdysgenesis, others have not. Differences in inclusion criteria and definition might contribute to the contradictory results. There is some evidence that microdysgenesis could be important in epileptogenesis, but the mechanisms involved remain unknown and difficult to investigate. A consensus on what histopathological criteria to use for the diagnosis of microdysgenesis is needed to explore this further and enable comparisons between centres. There are advantages and disadvantages both with quantitative stereological and with qualitative assessments. It is necessary to evaluate these in the decision on diagnostic criteria, if possible taking both qualitative and quantitative aspects into account.

Electroencephalographic recordings of focal seizures in patients affected by periventricular nodular heterotopia: role of the heterotopic nodules in the genesis of epileptic discharges

Patients affected by periventricular nodular heterotopia are frequently characterized by focal drug-resistant epilepsy. To investigate the role of periventricular nodules in the genesis of seizures, we analyzed the electroencephalographic (EEG) features of focal seizures recorded by means of video-EEG in 10 patients affected by different types of periventricular nodular heterotopia and followed for prolonged periods of time at the epilepsy center of our institute. The ictal EEG recordings with surface electrodes revealed common features in all patients: all seizures originated from the brain regions where the periventricular nodular heterotopia were located; EEG patterns recorded on the leads exploring the periventricular nodular heterotopia were very similar both at the onset and immediately after the seizure's end in all patients. Our data suggest that seizures are generated by abnormal anatomic circuitries, including the heterotopic nodules and adjacent cortical areas. The major role of heterotopic neurons in the genesis and propagation of epileptic discharges must be taken into account when planning surgery for epilepsy in patients with periventricular nodular heterotopia.

Altered Expression of Neurotransmitter-receptor Subunit and Uptake Site mRNAs in Hemimegalencephaly

PURPOSE

Hemimegalencephaly (HMEG) is characterized by unilateral hemispheric enlargement and severe cytoarchitectural abnormalities that are highly associated with intractable epilepsy. No studies have defined alterations in neurotransmitter-receptor subunit gene expression in HMEG. We hypothesize that a differential expression of excitatory amino acid and gamma-aminobutyric acid (GABA)A-receptor subunit messenger RNAs (mRNAs) exists in HMEG.

METHODS

The expression of mRNAs encoding 20 neurotransmitter-receptor subunits, synthetic enzymes, and uptake sites as well as select additional candidate genes was defined in HMEG samples (n=8) compared with homotopic control cortex specimens by using targeted complementary DNA (cDNA) arrays. Expression of GLT-1 (a glial glutamate transporter), EAAC-1 (neuronal glutamate transporter), and NMDA2B was corroborated by immunohistochemical, Western, and ligand-binding assays.

RESULTS

Differential expression of 11 neurotransmitter-related mRNAs was demonstrated in HMEG compared with control cortex. For example, expression of GLT-1 and GluR6 mRNAs was enhanced, whereas diminished expression of the neuronal glutamate transporter EAAC-1, GABAAalpha2, GABAAgamma2, GABAAgamma3, NMDA2B, GluR1, GluR2, GluR4, and GluR5 subunits occurred. Reduced NMDA2B subunit mRNA expression in HMEG was confirmed by receptor ligand-binding assays by using the NMDA2B-receptor antagonist ifenprodil, which revealed barely detectable levels of NMDA2B binding compared with that in control cortex.

CONCLUSIONS

Selective alterations occur in distinct neurotransmitter-receptor and -uptake sites in HMEG. Differential expression of neurotransmitter-receptor and -uptake sites in HMEG may contribute to epileptogenesis in HMEG.

Malformations of cortical development: molecular pathogenesis and experimental strategies

Malformations of cortical development (MCD) are developmental brain lesions characterized by abnormal formation of the cerebral cortex and a high clinical association with epilepsy in infants, children, and adults. Despite multiple anti-epileptic drugs (AEDs), treatment of epilepsy associated with MCD may require cortical resection performed to remove the cytoarchitecturally abnormal region of cortex. Single genes responsible for distinct MCD including lissencephaly, subcortical band heterotopia, and tuberous sclerosis, have been identified and permit important mechanistic insights into how gene mutations result in abnormal cortical cytoarchitecture. The pathogenesis of MCD such as focal cortical dysplasia, hemimegalencephaly, and polymicrogyria, remains unknown. A variety of new techniques including cDNA array analysis now allow for analysis of gene expression within MCD.

Reelin' in Genes for Cortical Dysplasia

Malformations of cortical development are a broad family of disorders that are characterized by abnormal cytoarchitecture of the cerebral cortex and a high association with epilepsy. In recent years positional cloning strategies have been implemented to identify several distinct gene mutations that are responsible for developmental brain malformations. The defined functional roles of proteins encoded by these genes have provided pivotal insights into the cellular mechanisms of brain development. Identification of these genes provides important clinical information used in genetic counseling for patients and their families.

Etiological heterogeneity of familial periventricular heterotopia and hydrocephalus

Periventricular heterotopia (PH) represents a neuronal migration disorder that results in gray matter nodules along the lateral ventricles beneath an otherwise normal appearing cortex. While prior reports have shown that mutations in the filamin A (FLNA) gene can cause X-linked dominant PH, an increasing number of studies suggest the existence of additional PH syndromes. Further classification of these cortical malformation syndromes associated with PH allows for determination of the causal genes. Here we report three familial cases of PH with hydrocephalus. One pedigree has a known FLNA mutation with hydrocephalus occurring in the setting of valproic acid exposure. Another pedigree demonstrated possible linkage to the Xq28 locus including FLNA, although uncharacteristically a male was affected and sequencing of the FLNA gene in this individual revealed no mutation. However, in the third family with an autosomal mode of inheritance, microsatellite analysis ruled out linkage with the FLNA gene. Routine karyotyping and fluorescent in situ hybridization using BAC probes localized to FLNA also showed no evidence of genomic rearrangement. Western blot analysis of one of the affected individuals demonstrated normal expression of the FLNA protein. Lastly, sequencing of greater than 95% of the FLNA gene in an affected member failed to demonstrate a mutation. In conclusion, these findings demonstrate the etiological heterogeneity of PH with hydrocephalus. Furthermore, there likely exists an autosomal PH gene, distinct from the previously described X-linked and autosomal recessive forms. Affected individuals have severe developmental delay and may have radiographic findings of hydrocephalus.

Distribution of Cortical Interneurons in Grey Matter Heterotopia in Patients with Epilepsy

PURPOSE

Grey matter heterotopia are well-defined malformations of the cortex often associated with severe epilepsy. Defects have been identified in genes, including DCX and FLN1, that influence radial migration of postmitotic cells from the ventricular zone to the cortical plate. A proportion of cortical gamma-aminobutyric acid (GABA)-containing interneurons may arise from the ganglionic eminence of the ventral telencephalon. We aimed to identify the subtypes and localisation of interneurons within grey matter heterotopia relative to cortex.

METHODS

By using quantitative immunohistochemistry, we studied the density and distribution of interneurons within six cases of grey matter heterotopia in postmortem tissue from patients with epilepsy.

RESULTS

In many cases, a suggestion of focal rudimentary laminar arrangement and small reelin-positive cells was identified within the heterotopia. Immunohistochemistry for glutamic acid decarboxylase(65/57), parvalbumin, calbindin, and calretinin showed inhibitory neurons of all subtypes represented within the heterotopia, and of normal morphology. The mean densities of interneurons were overall similar to those of the overlying cortex, but the interneurons showed less organisation and were more randomly orientated compared with cortex.

CONCLUSIONS

Interneurons within heterotopia probably arise from the ventricular zone, but their abnormal local organization may influence the epileptogenicity of these lesions.

Malformations of cortical development: burdens and insights from important causes of human epilepsy

Malformations of cortical development (MCD) are important causes of chronic epilepsy in human beings. A blanket term, MCD encompasses many varied developmental disorders with diverse clinical manifestations in patients that neurologists, paediatricians, and learning disability psychiatrists will encounter. Advances in imaging and genetics have led to a significant increase in our understanding of MCD, which has in turn enriched our knowledge of human epileptogenesis and normal brain development and function. In this review, I discuss some of the most common or enlightening MCD: focal cortical dysplasia, periventricular heterotopia, polymicrogyria, band heterotopia and lissencephaly, dysembryoplastic neuroepithelial tumours, and microdysgenesis. Clinical and imaging features, genetic aetiologies, treatments, and the insights that have resulted from MCD study are covered. The burden of epilepsy due to MCD is significant and there is still much to learn about MCD.

Cortical dysplasia and epilepsy: animal models

Cortical dysplasia syndromes--those conditions of abnormal brain structure/organization that arise during aberrant brain development--frequently involve epileptic seizures. Neuropathological and neuroradiological analyses have provided descriptions and categorizations based on gross anatomical and cellular histological features (e.g., lissencephaly, heterotopia, giant cells), as well as on the developmental mechanisms likely to be involved in the abnormality (e.g., cell proliferation, migration). Recently, the genes responsible for several cortical dysplastic conditions have been identified and the underlying molecular processes investigated. However, it is still unclear how the various structural abnormalities associated with cortical dysplasia are related to (i.e., "cause") chronic seizures. To elucidate these relationships, a number of animal models of cortical dysplasia have been developed in rats and mice. Some models are based on laboratory manipulations that injure the brain (e.g., freeze, undercut, irradiation, teratogen exposure) of immature animals; others are based on spontaneous genetic mutations or on gene manipulations (knockouts/transgenics) that give rise to abnormal cortical structures. Such models of cortical dysplasia provide a means by which investigators can not only study the developmental mechanisms that give rise to these brain lesions, but also examine the cause-effect relationships between structural abnormalities and epileptogenesis.

Grey and white matter flumazenil binding in neocortical epilepsy with normal MRI. A PET study of 44 patients

In 20-30% of potential surgical candidates with refractory focal epilepsy, standard MRI does not identify the cause. gamma-Aminobutyric acid (GABA) is the principal inhibitory neurotransmitter in the brain. [(11)C]Flumazenil (FMZ) PET images most subtypes of GABA(A) receptors, present on most neurons. We investigated [(11)C]FMZ binding in grey and white matter in 16 normal controls and in 44 patients with refractory neocortical focal epilepsy and normal optimal MRI. Fourteen patients had unilateral frontal lobe epilepsy, five occipital lobe epilepsy (OLE), six parietal lobe epilepsy (PLE) and 19 neocortical epilepsy that was not clearly lobar. Parametric images of FMZ volume of distribution (FMZ-V(d)) were computed. Statistical parametric mapping (SPM99) with explicit masking, including the white matter, was used to analyse individual patients and groups. Thirty-three of the 44 patients showed focal abnormal FMZ-V(d); increases in 16, decreases in eight, and both increases and decreases in nine. In seven patients, the increases in FMZ binding were periventricular, in locations normally seen in periventricular nodular heterotopia on MRI. There were frontal and parietal increases in FMZ binding in grey and white matter in the PLE group and decreases in the cingulate gyrus in the OLE group. FMZ binding increases, particularly periventricular increases, were a prominent feature of MRI-negative focal epilepsies and may represent neuronal migration disturbances.

The NMDA receptor complex is altered in an animal model of human cerebral heterotopia

Double intraperitoneal injections of methylazoxymethanol (MAM) in pregnant rats induce developmental brain dysgenesis with nodular heterotopia similar to human periventricular nodular heterotopia (PNH) and composed of hyperexcitable neurons. Here we analyzed the NMDA receptor complex and associated proteins in the heterotopic neurons of 2- to 3-month-old MAM-treated rats by means of a combined immunocytochemical/molecular approach. Our data demonstrated a clear reduction of p286-active form of alphaCaMKII and a selective impairment of both the targeting and the CaMKII-dependent phosphorylation of NR2A/B subunits in the postsynaptic membranes of the MAM-induced heterotopia. The reduced NR2A/B immunofluorescence of the cellular membrane was not due to reduced expression since it was decreased only in postsynaptic fractions but not in the homogenate. NMDA-NR1 and AMPA-GluR2/3 subunits, as well as PSD-95 and total alphaCaMKII protein levels, were not affected in MAM-treated rats, thus revealing that the overall composition of the postsynaptic fraction was not altered. These data clearly suggest that the molecular organization of the NMDA/alphaCaMKII complex is selectively altered in the postsynaptic compartment of heterotopic neurons. This alteration can play a role in determining the hyperexcitability of brain heterotopia in MAM rats as well as in human patients affected by PNH.

Experimentally-induced microencephaly: effects on cortical neurons

Genetic and epigenetic factors may alter the normal development of cerebral cortex, producing laminar and cellular abnormalities and heterotopiae, major causes of juvenile, drug-resistant epilepsy. Experimentally-induced migration disorders provide interesting insights in the mechanisms of the determination of neuronal phenotype and connectivity, of congenital cortical dysgenesis and the pathophysiology of associated neurological disorders, such as epilepsy. We investigated the effects of E14 administration of methylazoxymethanol acetate (MAM), which induces microencephaly by ablating dividing cells. Brains from newborn and adult rats were reacted for NADPH-d and CO histochemistry. Moreover, callosally-projecting neurons were retrogradely labeled with DiI at P9 or with BDA in adults. MAM-treated rats displayed a remarkable reduction in cortical thickness, mainly due to reduction in layer IV and in supragranular layers. Heterotopic nodules appeared in the supragranular layers and in the hippocampus. CO-positive barrels in somatosensory cortex were almost absent. The distribution of NADPH-d-positive neurons was regular, but they were rare in heterotopic nodules. Callosally-projecting neurons displayed abnormal orientation of the apical dendrite and increase in the basal dendritic length. Alterations in the dendritic arborization of pyramidal neurons may be one of the substrates for the increased sensitivity to drugs which induce epileptic seizures in these animals.

Reduction of spontaneous inhibitory synaptic activity in experimental heterotopic gray matter

Neuronal heterotopia has a strong association with epilepsy, but the mechanisms that underlie this relationship are largely unknown. We have utilized the in utero irradiated rat model to study circuit abnormalities in experimentally induced subcortical heterotopic gray matter. Spontaneous and miniature inhibitory (IPSCs) and excitatory (EPSCs) postsynaptic currents were recorded from visualized heterotopic pyramidal neurons in in vitro hemispheric slices and compared with control neocortical pyramidal neurons using the whole cell patch-clamp technique. The frequency of spontaneous and miniature IPSCs was significantly reduced in pyramidal neurons from heterotopic cortex. Amplitude and kinetics of IPSCs were not different between the two groups. Spontaneous and miniature EPSCs were not different between the two groups. Short-term synaptic plasticity of stimulus-evoked EPSCs showed depression in heterotopic neurons and facilitation in control pyramidal neurons. This study shows a selective impairment of the GABAergic circuitry in experimental heterotopic gray matter. We have reported similar findings in normotopic dysplastic cortex from this model. Taken together, these studies demonstrate a pervasive defect in inhibition throughout the cortex of irradiated rats with cortical dysplasia and neuronal heterotopia. This may have important implications regarding cortical development and function following in utero injuries.

Abnormalities of grey and white matter [11C]flumazenil binding in temporal lobe epilepsy with normal MRI

In 20% of potential surgical candidates with refractory epilepsy, current optimal MRI does not identify the cause. GABA is the principal inhibitory neurotransmitter in the brain, and GABA(A) receptors are expressed by most neurones. [(11)C]Flumazenil (FMZ) PET images the majority of GABA(A) receptor subtypes. We investigated abnormalities of FMZ binding in grey and white matter in 18 patients with refractory temporal lobe epilepsy (TLE) and normal quantitative MRI. Parametric images of FMZ volume of distribution (FMZ-V(d)) were calculated. Twenty-one healthy controls were scanned for comparison. Statistical parametric mapping (SPM99) was used to localize significant changes in FMZ-V(d) in individual patients and between groups, specifically including the entire white matter in all subjects through explicit masking. Sixteen of 18 patients showed single or multiple abnormalities of FMZ-V(d). Six had hippocampal decreases of FMZ-V(d). Eleven patients showed increased FMZ-V(d) in the temporal lobe white matter (TLWM). Outside the mesial temporal structures, seven showed multiple areas of increase or decrease and only one a single area of decrease. In seven of the 16 patients with abnormalities, findings were concordant with EEG and clinical data, enabling further presurgical evaluation. Group findings were: (i) decreased FMZ-V(d) in the ipsilateral (Z = 3.01) and contralateral (Z = 2.56) hippocampus; (ii) increased FMZ-V(d) in the ipsilateral (Z = 3.71) and contralateral TLWM (two clusters, Z = 3.11 and 2.79); and (iii) increased FMZ-V(d) in the ipsilateral frontal lobe white matter between the superior and medial frontal gyrus (Z = 3.80) with similar changes contralaterally (Z = 4.87). No changes were found in the thalamus and basal ganglia. Region-of-interest analyses indicated an average increase in FMZ binding of 16% in the TLWM ipsilateral to the epileptic focus. PET findings were corroborated by invasive EEG or pathology in five cases. FMZ-PET, analysed by SPM with explicit masking, was sensitive in patients with normal MRI, and hippocampal abnormalities were detected in a third of these patients. Furthermore, increases in FMZ binding in TLWM, indicating microdysgenesis, were detected in the majority of these patients and may represent the structural basis of their epilepsy.

Exploring white matter tracts in band heterotopia using diffusion tractography

Band heterotopia is a malformation of cortical development characterized by bands of gray matter in the white matter parallel to the surface of the neocortex. Histopathological studies have suggested that small white matter tracts pass through the heterotopia, and functional magnetic resonance imaging studies have shown activation in the malformation. We used diffusion tractography to explore the anatomical connectivity of band heterotopia and, in particular, whether in vivo white matter tracts traverse the heterotopic gray matter. Five patients with band heterotopia and five control subjects were scanned with whole brain diffusion tensor imaging. Anisotropy maps were calculated. Using fast marching tractography, we produced maps of connectivity and tract traces from two seed points, in the splenium of the corpus callosum and the right parietal lobe. Eigenvectors were found to pass through the band heterotopia in an aligned fashion. Patterns for maps of connectivity were similar in patients and control subjects. Areas of high connectivity were found in the band heterotopia and in cortical areas on the far side of the malformation from the seed point. The tracts hence appeared to traverse or end within the band heterotopia. The results are in agreement with previous histopathological studies and indicate the structural basis of the functional connectivity and absence of focal deficits in these patients.

Increased excitability and decreased sensitivity to GABA in an animal model of dysplastic cortex

PURPOSE

Cortical dysplasia (CD) is associated with epilepsy in both the pediatric and adult populations. The mechanism underlying seizures with cortical malformations is still poorly understood. To study the physiology of dysplastic cortex, we developed an experimental model of CD.

METHODS

Pregnant rats were given intraperitoneal injections of carmustine (1-3-bis-chloroethyl-nitrosourea; BCNU) on embryonic day 15 (E15). Cortical histology was examined in the resulting pups at P0, P28, and P60. In addition, evoked and spontaneous field potential recordings were obtained in cortical slices from adult control and BCNU-exposed rats. Finally, we used whole-cell recordings to compare physiologic properties of pyramidal neurons and gamma-aminobutyric acid (GABA) responses in control and BCNU-treated animals.

RESULTS

Features characteristic of CD were found in the offspring, including laminar disorganization, cytomegalic neurons, and neuronal heterotopias. Dysplastic cortex also contained abnormal clusters of Cajal-Retzius (CR) cells and disruption of radial glial fibers, as demonstrated with immunohistochemistry. Under conditions of partial GABAA-receptor blockade with 10 microM bicuculline methiodide (BMI), slices of dysplastic cortex demonstrated a significant increase in the number of spontaneous and evoked epileptiform discharges. Individual pyramidal neurons in dysplastic cortex were less sensitive to application of GABA compared with controls.

CONCLUSIONS

BCNU exposure in utero produces histologic alterations suggestive of CD in rat offspring. Dysplastic cortex from this model demonstrates features of hyperexcitability and decreased neuronal sensitivity to GABA. Such physiologic alterations may underlie the increased epileptogenicity of dysplastic cortex.

Effects of antiepileptic drugs on induced epileptiform activity in a rat model of dysplasia

Seizure activity associated with cortical dysplasia (CD) is often resistant to standard pharmacologic treatments. Although several animal models exhibit CD, virtually nothing is known about antiepileptic drug (AED) responses in these animals. Here we have used rats exposed to methylazoxymethanol acetate (MAM) in utero, an animal model featuring nodular heterotopia, to investigate the effects of AEDs in the dysplastic brain. 4-aminopyridine (100 microM), a K(+) channel blocker, was used to induce interictal epileptiform bursting in acute hippocampal slices from MAM-exposed and age-matched vehicle-injected control animals. Extracellular field recordings were used to monitor seizure activity in vitro. Five commonly used AEDs were tested: phenobarbital, 25-400 microM; carbamazepine, 25-200 microM; valproate (VPA), 0.19-4 mM; ethosuximide (ESM), 0.5-8 mM; and lamotrigine (LTG), 49-390 microM. 4-AP-induced bursting occurred with shorter latencies in slices from MAM-exposed rats in comparison with slices from controls, confirming the intrinsic hyperexcitability of dysplastic tissue. Each AED tested demonstrated significant burst suppression in control slices, but interictal epileptiform bursting in MAM-exposed slices was resistant to these treatments. Even at the highest concentrations, VPA, ESM and LTG had no effect on burst amplitude in slices from MAM-exposed rats. Pharmaco-resistance was further tested by measuring seizure latencies in awake, freely-moving rats after kainate administration (15 mg/kg, i.p.) with and without pre-treatment with VPA (400 mg/kg i.p.). Pre-treatment with VPA prolonged seizure latency in control rats, but had no effect in MAM-exposed animals. These results suggest MAM-exposed rats exhibit a dramatically reduced sensitivity to commonly prescribed AEDs.

Pathologic characteristics of the cortical dysplasias

The gross and microscopic features of cortical dysplasia (malformations caused by abnormalities of cortical development) are reviewed and illustrated in this article. The pathologic associations of neurocutaneous disorders, neoplasms, and hippocampal sclerosis with cortical dysplasia also are discussed.

Periventricular heterotopia may result from radial glial fiber disruption

Periventricular heterotopia (PVH) are collections of neurons and glia heterotopically located adjacent to the ventricles. The pathogenesis of periventricular heterotopia is believed to be a failure of cells to migrate from the ventricular zone. Mutations in filamin-1 (FLN1) have recently been identified as a genetic defect that results in an X-linked dominant form of PVH. In addition to this X-linked form, PVH may be found sporadically or occasionally as part of other syndromes. The pathogenesis(es) of PVH has not been entirely elucidated for patients with or without FLN1 mutation. In an attempt to better understand the pathogenesis of PVH, we examined 5 fetuses (gestational ages 21 to 34 wk), 3 females and 2 males, with PVH. Neuropathologic examination of these 5 fetuses revealed several to multiple periventricular nodules. No case showed the extensive periventricular heterotopia most commonly found in females with FLN1 mutations. By immunohistochemistry, neurofilament-positive cells were identified within the PVH in 3 of 5 cases and glial fibrillary acidic protein-positive cells surrounded the nodules in all 5 cases, but positive cells were only found within the nodules of 3 cases. Surprisingly, small collections of CD68-positive macrophages were found at the base of the nodules in 4 of the 5 cases. Moreover, in all cases, the radial glia highlighted with vimentin, showed disorganization specifically around the nodules. These data suggest that at least one pathogenesis for PVH is a disruption of the radial glial organization, resulting in a failure of cells to migrate from the ventricular zone.

Radiologic classification of malformations of cortical development

Malformations of cerebral cortical development are common birth defects that can cause delayed development, epilepsy, focal neurologic deficits, and mental retardation. Rational classification of these disorders is essential for proper prognosis, genetic testing and counseling, and investigation of the underlying molecular causes. A rational approach to this classification is a framework based on whether these disorders are the result of abnormal cell proliferation or apoptosis, abnormal migration of immature neurons, or abnormal horizontal and radial orientation in the cortex. Superimposed on this framework are subclassifications that are based on topology of the malformation, associated central nervous system (CNS) or extra-CNS malformations, and results of molecular genetic testing. Characteristics that correlate with and enforce this system of classification can be identified by modern neuroimaging studies.

Electrophysiological and morphological characterization of neurons within neocortical ectopias

Focal developmental abnormalities in neocortex, including ectopic collections of neurons in layer I (ectopias), have been associated with behavioral and neurological deficits. In this study, we used infrared differential interference contrast microscopy and whole cell patch-clamp to complete the first characterization of neurons within and surrounding neocortical ectopias. Current-clamp recordings revealed that neurons within ectopias display multiple types of action potential firing patterns, and biocytin labeling indicated that approximately 20% of the cells in neocortical ectopias can be classified as nonpyramidal cells and the rest as atypically oriented pyramidal cells. All cells had spontaneous excitatory (glutamatergic) and inhibitory (GABAergic) postsynaptic currents. Exhibitory postsynaptic currents consisted of both N-methyl-D-aspartate (NMDA) receptor-mediated and AMPA/kainate (A/K) receptor-mediated currents. The NMDA receptor-mediated component had decay time constants of 15.35 +/- 2.2 (SE) ms, while the A/K component had faster decay kinetics of 7.6 +/- 1.7 ms at -20 mV. GABA(A) receptor-mediated synaptic currents in ectopic cells reversed at potentials near the Cl- equilibrium potential and had decay kinetics of 16.65 +/- 1.3 ms at 0 mV. Furthermore we show that cells within ectopias receive direct excitatory and inhibitory input from adjacent normatopic cortex and can display a form of epileptiform activity.

Neural tract tracing using Di-I: a review and a new method to make fast Di-I faster in human brain

The use of Di-I in tract-tracing is briefly reviewed and a novel delayed-fixation approach to neural tract-tracing in the postmortem human adult brain is reported. Using the new approach, fast Di-I, a highly lipophilic fluorescent dye was injected into a particular region or nucleus and labelled tracts were followed for distances of some 20-40 mm. The procedure required approximately 36 h, yielding dye penetration rates of 1.0 mm/h or more. This contrasts with previous Di-I, silver impregnation, and horseradish peroxidase protocols, where the tracer penetration rate is typically 0.003 mm/h or less, and the distance traversed amounts to only a few mm even after months of incubation. The new method hinges on the simple consideration that aldehyde fixation, which is normally employed prior to administration of the marker, crosslinks membrane proteins and impedes dye diffusion. The short postmortem samples used in our protocol permit delaying fixation until after the dye has had time to penetrate, dramatically increasing the length and scope of neural circuits that can be traced. Using these methods, for example, we have confirmed the presence of an ipsilateral olivocerebellar climbing fiber projection in the human.

Surgery for malformations of cortical development causing epilepsy

Malformations of cortical development (MCD) are responsible for many cases of refractory epilepsy in adults and children. The results of surgical treatment are difficult to assess from the published literature. Judging from the limited number of adequately reported cases, approximately 40% of all cases of MCD treated surgically may be rendered seizure-free over a minimum 2-year follow-up period. This figure is the same for focal cortical dysplasia (FCD), the most common variety of MCD in surgical reports. In comparison with outcome for epilepsy associated with hippocampal sclerosis, this figure is low. Part of the difference may be artificial and related to limited reporting. Much of the difference is likely to relate to the complex underlying biology of MCD. Analysis of epileptogenesis in MCD has been undertaken. Different types of MCD have different sequelae. Some varieties are intrinsically epileptogenic; these include FCD and heterotopia. Although in most cases, the visualized MCD lies within the region of brain responsible for generating seizures (the epileptogenic zone), it may not constitute the entire epileptogenic zone in all cases. For polymicrogyria and schizencephaly in particular, the visualized abnormalities are probably not the most important component of the epileptogenic zone. There is evidence that the epileptogenic zone is spatially distributed and also, in some cases, temporally distributed. These findings may explain poor surgical outcome and the inadequacy of current presurgical evaluative methods. New preoperative techniques offer the opportunity of improved presurgical planning and selection of cases more likely to be rendered seizure-free by current surgical techniques. Of paramount importance is improved reporting. The establishment of a central registry may facilitate this aim. Specific recommendations are made for surgical strategies based on current experience and understanding.

Development of connections in the human visual system during fetal mid-gestation: a DiI-tracing study

Animal studies have shown that connections between the retina, lateral geniculate nucleus (LGN), and visual cortex begin to develop prenatally. To study the development of these connections in humans, regions of fixed brain from fetuses of 20-22 gestational weeks (GW) were injected with the fluorescent tracer DiI. Placement of DiI in the optic nerve or tract labeled retinogeniculate projections. In the LGN, these projections were already segregated into eye-specific layers by 20 GW. Retinogeniculate segregation thus preceded cellular lamination of the LGN, which did not commence until 22 GW. Thalamocortical axons, labeled from DiI injections into the optic radiations, densely innervated the subplate, but did not significantly innervate the cortical plate. This pattern was consistent with observations of a "waiting period" in animals, when thalamocortical axons synapse in the subplate for days or weeks before entering the cortical plate. Cortical efferent neurons (labeled retrogradely from the optic radiations) were located in the subplate and deep layers of the cortical plate. In summary, human visual connections are partially formed by mid-gestation, and undergo further refinement during and after this period. The program for prenatal development of visual pathways appears remarkably similar between humans and other primates.

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.

Connections between cells of the internal capsule, thalamus, and cerebral cortex in embryonic rat

The aim of our study is to understand the development of the earliest connections in the mammalian pallium by documenting the distribution of cells and fibres labelled from the dorsal and ventral thalamus, internal capsule, perirhinal, and dorsal cortex during the period between embryonic day (E) 14 and 17 by using carbocyanine dye tracing in fixed embryonic rat brains. Dye placed in the thalamus of E14 brains backlabels cells in the thalamic reticular nucleus and within the primitive internal capsule. Both anterograde and retrograde tracing confirmed that the first corticofugal projections reach the internal capsule by E14. At E15-E16, after the first cortical plate cells have migrated into the lateral cortex, some cells of the cortical plate and subplate and marginal zone, are backlabelled from the internal capsule, but still not from the dorsal thalamus, even with very long incubation periods. Crystal placement into the perirhinal cortex at E14-E15 labels numerous cells within the internal capsule, whereas no such cells are revealed from dorsal cerebral cortex until E17, suggesting that internal capsule cells establish early connections with the perirhinal and ventral but not dorsal cortex. We propose that the growth of axons from cortex to dorsal thalamus is delayed in two regions: first from E14-E15 at the lateral entrance of the internal capsule and then, from E16, closer to the thalamus, probably within the thalamic reticular nucleus. Subplate projections reach the proximity of the diencephalon at an early stage, but they might never enter the dorsal thalamus.