Abnormalities of gyration, heterotopias, tuberous sclerosis, focal cortical dysplasia, microdysgenesis, dysembryoplastic neuroepithelial tumour and dysgenesis of the archicortex in epilepsy. Clinical, EEG and neuroimaging features in 100 adult patients

A novel CNS gene required for neuronal migration and involved in X-linked subcortical laminar heterotopia and lissencephaly syndrome

X-SCLH/LIS syndrome is a neuronal migration disorder with disruption of the six-layered neocortex. It consists of subcortical laminar heterotopia (SCLH, band heterotopia, or double cortex) in females and lissencephaly (LIS) in males, leading to epilepsy and cognitive impairment. We report the characterization of a novel CNS gene encoding a 40 kDa predicted protein that we named Doublecortin and the identification of mutations in four unrelated X-SCLH/LIS cases. The predicted protein shares significant homology with the N-terminal segment of a protein containing a protein kinase domain at its C-terminal part. This novel gene is highly expressed during brain development, mainly in fetal neurons including precursors. The complete disorganization observed in lissencephaly and heterotopia thus seems to reflect a failure of early events associated with neuron dispersion.

Doublecortin, a brain-specific gene mutated in human X-linked lissencephaly and double cortex syndrome, encodes a putative signaling protein

X-linked lissencephaly and "double cortex" are allelic human disorders mapping to Xq22.3-Xq23 associated with arrest of migrating cerebral cortical neurons. We identified a novel 10 kb brain-specific cDNA interrupted by a balanced translocation in an XLIS patient that encodes a novel 40 kDa predicted protein named Doublecortin. Four double cortex/X-linked lissencephaly families and three sporadic double cortex patients show independent doublecortin mutations, at least one of them a de novo mutation. Doublecortin contains a consensus Abl phosphorylation site and other sites of potential phosphorylation. Although Doublecortin does not contain a kinase domain, it is homologous to the amino terminus of a predicted kinase protein, indicating a likely role in signal transduction. Doublecortin, along with the newly characterized mDab1, may define an Abl-dependent pathway regulating neuronal migration.

What is the relationship between arachnoid cysts and seizure foci?

PURPOSE

Arachnoid cysts are sometimes encountered in MRIs performed for a variety of reasons. In patients with epilepsy, particularly those with refractory epilepsy, arachnoid cysts are often assumed to be related to their seizure focus. We conducted a study to investigate this putative relationship.

METHODS

A retrospective study on the incidence of arachnoid cysts was performed in patients seen in our Epilepsy Clinic who had CT or MRI scans, interictal EEGs or ictal EEGS. Locations of seizure foci in these patients were defined from clinical and electrophysiologic data.

RESULTS

Seventeen of 867 patients had arachnoid cysts. Twelve patients had temporal lobe cysts and only 3 of them had temporal lobe seizures. Four patients had frontal lobe cysts and only 1 had frontal lobe seizures ipsilateral to the cyst. One patient had a cerebello-pontine angle cyst and frontal lobe seizures. Thus, clinical manifestations of seizures and EEG findings (interictal and/or ictal) indicated that the seizure focus was adjacent to the cysts in only 4 patients (23.5%).

CONCLUSIONS

Our findings suggest that arachnoid cysts are often an incidental finding in patients with epilepsy and do not necessarily reflect the location of the seizure focus.

Neuroimaging in the evaluation of children and adolescents with intractable epilepsy: I. Magnetic resonance imaging and the substrates of epilepsy

Recent advances in neuroimaging, particularly the magnetic resonance imaging (MRI) scan, have greatly enhanced our ability to visualize intraparenchymal anatomy. With the linkage between intracranial pathology and epilepsy now clearly established, the MRI data provides detailed information for the clinician treating patients with epilepsy. This article supplies the reader with an overview of the new techniques in MRI brain imaging that allow us to identify intracranial abnormalities, and a survey of some of the MRI-identified substrates of epilepsy.

Function and organization in dysgenic cortex. Case report

Cerebral dysgenesis is a subject of interest because of its relationship to cerebral development and dysfunction and to epilepsy. The authors present a detailed study of a 16-year-old boy who underwent surgery for a severe seizure disorder. This patient had dysgenesis of the right hemisphere, which was composed of a giant central frontoparietal nodular gray matter heterotopia with overlying large islands of cortical dysplasia around a displaced central fissure. Exceptional insight into the function, biochemistry, electrophysiology, and histological structure of this lesion was obtained from neurological studies that revealed complementary information: magnetic resonance (MR) imaging, [18]fluoro-2-deoxy-D-glucose positron emission tomography (PET), functional PET scanning, proton MR spectroscopic (1H-MRS) imaging, intraoperative cortical mapping and electrocorticography, in vitro electrophysiology, and immunocytochemistry. These studies demonstrated compensatory cortical reorganization and showed that large areas of heterotopia and cortical dysplasia in the central area may retain normal motor and sensory function despite strikingly altered cytoarchitectonic organization and neuronal metabolism. Such lesions necessitate appropriate functional imaging studies prior to surgery and cortical mapping to avoid creating neurological deficits. Integrated studies, such as PET, 1H-MRS imaging, cortical mapping, immunocytochemistry, and electrophysiology may provide information on the function of developmental disorders of cerebral organization.

Periventricular nodular heterotopia and intractable temporal lobe epilepsy: poor outcome after temporal lobe resection

We describe 5 women and 5 men with periventricular nodular heterotopia and electroclinical features suggestive of temporal lobe epilepsy, who were surgically treated for control of medically refractory seizures. Magnetic resonance imaging revealed bilateral periventricular nodular heterotopia in 7 of the 10 patients. Because of the lack of clear localization, 6 patients were studied with intracranial depth electrode recordings. Seizures were of hippocampal onset (3 patients), regional temporal lobe onset (2 patients), or occipital-temporal onset (1 patient). Anterior temporal lobectomy was performed in 6 patients; selective amygdalohippocampectomy, in 1; and anterior temporal lobectomy plus resection of the heterotopic tissue, in 3. None of the 9 patients followed for more than 12 months postoperatively were seizure free. Two patients were initially seizure free for approximately 18 months, but then seizures recurred. One patient had a major reduction in seizure frequency at a 39-month follow-up after most of the unilateral heterotopic tissue was included in the temporal resection. Temporal resection did not lead to a long-term favorable outcome in this group of patients with periventricular nodular heterotopia and epileptogenic discharges involving the temporal lobe. This suggests a more widespread disorder with epileptogenic activity possibly originating in or near the heterotopic tissue. The clinical and electrographic features of periventricular nodular heterotopia pointing to temporal lobe origin are misleading and temporal resection does not result in long-term cessation of seizures.

Repetitive and non-repetitive violent offending behaviour in male patients in a maximum security mental hospital--clinical and neuroimaging findings

OBJECTIVE

To examine if different violent offending behaviours are associated with different clinical and neuroimaging profiles.

METHOD

Thirty-nine schizophrenic and schizoaffective offenders from a maximum security mental hospital-20 repetitive violent offenders (RVOs) and 19 non-repetitive violent offenders (NRVOs)-were selected for clinical and neuroimaging assessments.

RESULTS

Both groups had positive family history of mental illness and violence. Age, diagnosis, duration of illness, victim profiles and use of weapons at the time of the index offence were similar. RVOs had a higher prevalence of early parental separation, juvenile conduct problem, previous convictions of crimes not involving violence, impulsive suicide attempts, delusion of their lives being threatened at the time of the index offence and electroencephalographic (EEG) abnormalities localized to temporal lobes. NRVOs had a higher prevalence of sexual inexperience and command hallucinations to kill at the time of the index offence. Asymmetric gyral patterns at the temporo-parietal region were particularly common in RVOs and absent in NRVOs. Non-specific white matter changes in magnetic resonance imaging (MRI) and generalized cortical hypometabolism in positron emission tomography (PET) were present in both groups.

CONCLUSIONS

Different structural and metabolic changes in the brain were associated with different violent offending behaviours. The complex interaction between violent behaviour, clinical features and neuroimaging findings in schizophrenia requires further studies.

Disinhibited in vitro neocortical slices containing experimentally induced cortical dysplasia demonstrate hyperexcitability

Cortical dysplasia, a disorder of neuronal migration, has a strong association with intractable epilepsy in humans but little is known about the physiologic abnormalities that are present in this condition. Fetal rats were exposed to external irradiation to experimentally produce diffuse cortical dysplasia. In vitro neocortical slices from adult irradiated and control animals were examined in physiologic solution and in the presence of the A-type gamma-amino butyric acid (GABAA) receptor antagonist, bicuculline methiodide. Epileptiform bursts were quantified by counting the number of negative field potentials per epileptiform event. In the presence of bicuculline, neocortical slices with cortical dysplasia demonstrated more robust epileptiform activity in the form of an increased number of negative field potentials per epileptiform event. This demonstrates that areas of experimentally induced cortical dysplasia possess an inherent hyperexcitability when GABAA-mediated inhibition is effectively blocked.

Dominant X linked subcortical laminar heterotopia and lissencephaly syndrome (XSCLH/LIS): evidence for the occurrence of mutation in males and mapping of a potential locus in Xq22

X linked subcortical laminar heterotopia and lissencephaly syndrome (XSCLH/ LIS) is an intriguing disorder of cortical development, which causes classical lissencephaly with severe mental retardation and epilepsy in hemizygous males, and subcortical laminar heterotopia (SCLH) associated with milder mental retardation and epilepsy in heterozygous females. Here we report an exclusion mapping study carried out in three unrelated previously described families in which males are affected with lissencephaly and females with SCLH, using 38 microsatellite markers evenly distributed on the X chromosome. Most of the X chromosome was excluded and potential intervals of assignment in Xq22.3-q23 or in Xq27 are reported. Although the number of informative meioses did not allow a decision between these two loci, it is worth noting that the former interval is compatible with the mapping of a breakpoint involved in a de novo X;autosomal balanced translocation 46,XX,t(X;2)(q22;p25) previously described in a female with classical lissencephaly. In addition, haplotype inheritance in two families showed a grandpaternal origin of the mutation and suggested in one family the presence of mosaicism in germline cells of normal transmitting males.

Somatosensory evoked potentials in adults with cortical dysgenesis and epilepsy

Cortical dysgenesis (CD) is a well-recognised cause of epilepsy, but its functional anatomy is not fully understood. We recorded cortical somatosensory evoked potentials (SEPs) in 13 adult patients with epilepsy and various CDs excluding diffuse gyral malformations as diagnosed by MRI. The CD involved the perirolandic/perisylvian region in 7 patients. Six patients had neurological signs but only 3 had sensory dysfunction (astereognosis). As compared with 12 control subjects, SEPs were considered definitely abnormal in 7 patients (including the 6 with neurological signs) and equivocally abnormal in 2. The abnormalities ranged from defects affecting single components to absence of all potentials of cortical origin in one patient with hemiparesis and astereognosis. In this case it appears that gross sensory function must have been mediated by subcortical structures or through diffuse cortical projections. The initial cortical potentials (N20/P20) were absent in 6 patients, 5 of whom had CD in zones involving or bordering on the primary sensory cortex. Parietal potentials following N20 were absent or attenuated in 4 patients and of abnormally wide distribution, spreading to frontal, midline and ipsilateral electrodes, in 3 frontal components following P20 were absent, attenuated, delayed or distorted by abnormal spread of the parietal activity in 5 patients. Five patients with unilateral CD showed definite or equivocal SEP abnormalities to stimulation of both arms, suggesting there may be more widespread disturbance of cortical organisation and/or synaptogenesis, beyond the resolution of present day neuroimaging.

Neuronal migration disorders and epilepsy: a morphological analysis of three surgically treated patients

Despite the increasing number of patients affected by neuronal migration disorders (NMDs) recently diagnosed in vivo by means of magnetic resonance imaging (MRI), few detailed data on the correlation between the neuroradiological and the anatomical features in the single NMD case are available. The present paper reports a combined cytoarchitectural and immunocytochemical analysis, by means of antisera recognizing specific neuronal and glial markers, of three MRI diagnosed NMD patients surgically treated for the relief of intractable seizures. The first case was a giant subcortical nodular heterotopia of morphologically normal neurons lacking any type of cortical lamination. The second case was a layered polymicrogyria with an abnormal amount of ectopic neurons in the underlying white matter. The third case was a focal cortical dysplasia characterized by a dramatic disruption of the normal cortical layering associated with marked cytological abnormalities. The present data demonstrate that the macroscopical and microscopical brain abnormalities can be markedly different in different NMD subtypes, and suggest that different anatomical substrates can underlie the intrinsic hyperexcitability of these brain malformations. The relevance of further prospective clinico-morphological studies for a better understanding of the mechanisms determining the development of these brain malformations is underlined.

Neuroimaging of focal malformations of cortical development

Neuroimaging is playing an increasingly important role in the evaluation of patients with malformations of cerebral cortical development. In this review, the authors address optimal neuroimaging of cortical malformations using x-ray computed tomography, single-photon-emission computed tomography, positron emission tomography, magnetic resonance imaging, and magnetic resonance spectroscopy. Initially, the authors discuss the strengths and weaknesses of the various imaging techniques. This is followed by a discussion of the clinical and neuroimaging characteristics of several different imaging manifestations of focal malformations of cortical development, including polymicrogyria, focal subcortical heterotopia, schizencephaly, focally thickened gyri, focally irregular gyri, hemimegalencephaly, and transmural dysplasia. The authors intend that, after reading this review, the reader will have a better understanding of the optimal neuroimaging techniques for evaluating these malformations and their many neuroimaging appearances.

EEG features of focal malformations of cortical development

Recent advances in neuroimaging have allowed the detection and characterization of focal malformations of cortical developmental in a significant proportion of patients with epilepsy, many of whom were previously labelled as cryptogenic, allowing a better description of the associated electroencephalogram (EEG) features. Alpha activity is usually preserved, although superficial gyral abnormalities are often associated with overlying localized polymorphic delta activity, and occasionally abnormal fast activity. Most affected patients with epilepsy show interictal spikes. These are often broadly concordant with the structural abnormality but may show a wider anatomic distribution and be multifocal, or occasionally appear only in anatomically distant sites. In many patients the spikes are frequent and sometimes they occur continuously or in long trains. EEG findings are often stable over time, but some patients only show the development of slow wave changes or interictal spikes when followed serially for several years. A small proportion of patients with focal malformations of cortical development have EEG features mimicking idiopathic generalized epilepsy, and occasionally patients exhibit continuous generalized spike and slow wave activity in sleep. Electrocorticography studies confirm the often widespread nature of interictal spiking, but may also show highly epileptogenic patterns recorded directly from dysplastic cortex. The intrinsic epileptogenicity of areas of cortical developmental abnormalities has also been demonstrated by chronic intracranial studies and in vitro recordings of slices obtained from resected human dysplastic cortex. In this regard such developmental abnormalities are fundamentally different from acquired lesions such as tumors/vascular anomalies that usually exert their effects through changes in adjacent cortex.

Epilepsy and brain abnormalities in mice lacking the Otx1 gene

The morphogenesis of the brain and the differentiation of the neural structures are highly complex processes. A series of temporally and spatially regulated morphogenetic events gives rise to smaller areas that are phylogenetically, functionally and often morphogenetically different. Candidate genes for positional information and differentiation during morphogenesis have been isolated. Both in vivo inactivation in mice and impairment in human diseases revealed, that they are required in regional specification and/or correct cell-type induction. We have previously cloned and characterized the murine Otx1 gene, which is related to orthodenticle (otd), a homeobox-containing gene required for Drosophila head development. Expression data during murine embryogenesis and postnatal brain development support the idea that Otx1 could be required for correct brain and sense organs development. To decipher its role in vivo we produced null mice by replacing Otx1 with the lacZ gene. Otx1-/- mice showed spontaneous epileptic behaviour and multiple abnormalities affecting mainly the telencephalic temporal and perirhinal areas, the hippocampus, the mesencephalon and the cerebellum, as well as the acoustic and visual sense organs. Our findings indicate that the Otx1 gene product is required for proper brain functions.

The serotonergic innervation of the cerebral cortex in man and its changes in focal cortical dysplasia

We present the morphology and the laminar distribution of the serotonin (5-hydroxytryptamine, 5-HT) innervation of the cerebral cortex of patients who underwent cortical resection for partial seizures. The limits of the resections were established by stereoelectroencephalography. The 5-HT innervation was mapped by using an antiserum anti-5-HT. Two patients had cryptogenic epilepsies and two others had seizures related to focal cortical dysplasia. 5-HT immunoreactive axons were morphologically heterogeneous and projected diffusely to the cerebral cortex with regional-specific densities. Two types of terminal axon were demonstrated. Type I had large and spherical (intensely immunoreactive) varicosities and was distributed sparsely with a characteristic predominance in the molecular layer. Type II had fine and pleiomorphic varicosities (granular or fusiform) and was distributed through all cortical layers. The distribution of the 5-HT innervation varied according to the different architectonic areas investigated. The granular cortical areas characterized by a highly developed layer IV (primary somatosensory, primary visual and prefrontal cortices) had the highest density of 5-HT-ir fibers distributed from layer I to layer V. The agranular primary motor cortex had the lowest density with fibers preferentially seen in layers I, IIIa and V-VI. The orbital cortex with a poorly defined layer IV had an intermediate density with a laminar repartition predominant in the supragranular layers. In patients with cryptogenic epilepsies, the brain epileptogenic tissue was histologically normal as well as the serotonergic innervation. In contrast, in patients with focal cortical dysplasia, the dysplastic epileptogenic tissue was characterized by a serotonergic hyperinnervation. In agreement with previous data in primates, we give morphological evidence for two morphologically distinct serotonergic subsystems and for regional specific densities in the human cerebral cortex. Moreover, we previously reported an altered pattern of the catecholaminergic innervation in the same dysplasia areas. All these results provide evidence that this development epileptogenic lesion involves several sets of neurons which may contribute to epileptogenic activity.

Magnetic resonance imaging in epilepsy with a fast FLAIR sequence

OBJECTIVE

To assess the diagnostic value of the fast FLAIR sequence in patients with epilepsy.

METHODS

One hundred and twenty eight patients with epilepsy and 10 control subjects were scanned with the fast FLAIR sequence with 5 mm slices, a coronal gradient echo (GRE) T1 weighted sequence with 1.5 mm slices and spin echo (SE) or fast spin echo (FSE) proton density and T2 weighted sequences with 5 mm slices. All images were compared by an unblinded neuroradiologist and neurologist. Fast FLAIR images of patients with hippocampal sclerosis (HS) and normal control subjects were also evaluated by two blinded independent raters.

RESULTS

Fast FLAIR provided a high conspicuity of neocortical damage, hamartomas, dysembryoplastic neuroepithelial tumours, and clear cut hippocampal sclerosis. However, the same information could be obtained from the coronal T1 and T2 weighted images. In three patients fast FLAIR showed a clearly abnormal signal when SE T2 weighted images had not been definitely abnormal. Heterotopia was less conspicuous on fast FLAIR than GRE T1 weighted images. The two blinded raters detected all but one of the patients with clear cut hippocampal sclerosis on fast FLAIR images but missed all borderline cases of hippocampal atrophy and there were two false positives. Clear cut hippocampal sclerosis was more conspicuous on fast FLAIR images than on SE T2 weighted images in most patients, but additional patients were not identified.

CONCLUSION

Fast FLAIR has the advantage of identifying neocortical lesions and definite hippocampal sclerosis with a short scanning time and may also demonstrate lesions when other sequences are normal in a limited number of cases. The technique was not useful, however, for identifying mild hippocampal sclerosis or heterotopia.

Advances in neurology

Images

Startle provoked epileptic seizures:features in 19 patients

OBJECTIVES

To define the clinical characteristics of a group of patients with startle provoked epileptic seizures (SPES).

METHODS

Nineteen patients were identified during the course of a larger study of clinical seizure patterns. A witnessed seizure account was obtained in all patients; interictal EEG in 18, video-EEG-telemetry in eight, CT in 18, and high resolution MRI in eight.

RESULTS

The onset of SPES was in childhood or adolescence in 14 of 19 patients. It was preceded by exclusively spontaneous seizures in nine patients and SPES had been replaced by exclusively spontaneous seizures in two patients. Sudden noise was the main triggering stimulus and somatosensory and visual stimuli were also effective in some patients. The clinical seizure pattern involved asymmetric tonic posturing in 16 of 19 patients. Focal neurological signs were present in nine patients, mental retardation in six, and 10 were clinically normal. Ictal scalp EEG showed a clear seizure discharge in only one patient with a tonic seizure pattern; over the lateral frontal electrodes contralateral to the posturing limbs. Brain CT showed a porencephalic cyst in three patients, focal frontal atrophy in one, and generalised atrophy in one. Brain MRI was undertaken in five normal subjects and three neurologically impaired patients, six with normal CT. It showed a porencephalic cyst in one patient. In six patients, there were dysplastic lesions. They affected the lateral premotor cortex in three patients and the perisylvian cortex in three patients, one with bilateral perisylvian abnormality.

CONCLUSIONS

SPES are more frequent than is generally appreciated. They may be transient and occur relatively commonly without fixed deficit, by contrast with previous reports. The imaging abnormalities identified in those without diffuse cerebral damage suggest that SPES are often due to occult congenital lesions and that the lateral premotor and perisylvian cortices are important in this phenomenon.

Surgery for catastrophic localization-related epilepsy in infants

Cortical resection or hemispherectomy has been reported to result in cessation or dramatic reduction of seizures for small numbers of highly selected infants with severe, intractable epilepsy and developmental delay. However, identification of potential surgical candidates during infancy can be especially challenging because seizure semiology and EEG may sometimes give limited localizing information, e.g., in patients with infantile spasms and hypsarrythmia due to focal cortical dysplasia or a tumor. In infants as well as older patients, the location of a potentially resectable epileptogenic lesion must be defined by convergence of results from video EEG, anatomic and functional neuroimaging, and clinical examination. Reported outcomes after surgery in small series include 78% of 23 infants seizure-free or with at least 90% seizure reduction (1993 University of California at Los Angeles series), and 75% of 12 infants seizure-free or with rare seizures (1995 Cleveland Clinic series). A tendency of "catch-up" developmental progress after surgery was observed in both series. A few reports of smaller groups of infants noted similar results. Prospective studies are in progress to better define the potential risks and benefits of early surgical intervention for infants and catastrophic localization-related epilepsy.

Cortical dysgenesis: serial EEG findings in children and adults

Cortical dysgenesis (CD) is becoming increasingly recognised as a cause of epilepsy in otherwise cryptogenic cases. We describe the serial EEG findings in 22 patients with focal/localised CD. The EEGs covered a minimum period of 5 years in each case (median = 13 years, range: 5-30 years), beginning in childhood. Median age at seizure onset was 3 years (range: 3 weeks-10 years, n = 21). The EEG was normal in the one patient, a 6 year old, who did not have epilepsy. Background rhythms appropriate for age were preserved in the majority of patients (18/22). Slow activity localised to the area of CD was seen in 11 patients; in 3 patients, this did not appear until the second decade of life. Epileptiform discharges were seen in at least one EEG in 20 patients: these were continuous or near-continuous (6 patients) or occurred recurrently in short runs (6 patients). In 6 patients, these discharges appeared only after the second decade of life and in 11 patients, they became more widespread over time. In the remaining patients, the EEG changes did not evolve. Sleep failed to produce new abnormalities (n = 15). None of the patients showed EEG features characteristic of lissencephaly or evolution to the Lennox-Gastaut syndrome. Even in this selected cohort of patients who had undergone serial clinical EEGs, the EEG abnormalities in focal/localised CD appeared relatively stable and showed only moderate changes over time. CD must be included in the differential diagnosis of any patient who presents with localised slow activity on EEG.

Clinical correlations: MRI and EEG

Main structural correlates of epileptogenesis include hippocampal sclerosis, cortical dysgenesis, foreign tissue lesions, gliosis, and dual pathology (a combination of any two). These structural abnormalities are now increasingly defined with MRI, enabling systematic EEG correlative analyses. Hippocampal atrophy (HA) and increased T2 signal in medial temporal structures predict the presence of mesial temporal sclerosis with a high degree of sensitivity and specificity. In 50 patients with clinical evidence of temporal lobe epilepsy and isolated HA, ictal scalp EEG was concordant to the atrophic temporal lobe in 33, nonlateralizing in 12, obscured in 3, and bilateral in 2, but it was discordant in none. Earlier reports of higher levels of discordance may be ascribed to the presence of dual pathology or to differing MRI and EEG criteria for localization. In a more inclusive group of 101 patients with unilateral HA, ictal scalp EEG was obtained in 99. It was unlocalized in 53, localized elsewhere in 9, and localized to the atrophic temporal lobe in 38. Of those, 51 patients had intracranial EEG: 12 were unlocalized, 29 were localized to the atrophic hippocampus, and 9 were localized elsewhere. There is thus a rare but definite subgroup of patients with unilateral HA who have EEG localization elsewhere than the atrophy. The successful cure of seizures in half these patients after removal of the EEG focus confirms the importance of this observation and emphasizes the search for more dual pathology that has remained undetected on MRI. About 10% of the patients with HA have significant atrophy bilaterally, and several series have confirmed that surgical success is predicted by removal of the EEG identified seizure onset area, not the more or less atrophic hippocampus. In patients with other kinds of dual pathology, including HA and foreign tissue lesions or cortical dysgenesis, EEG is also paramount in predicting the site of epileptogenesis for surgical intervention. EEG correlates of cortical dysgenesis are heterogeneous, but EEG has potential to provide accurate localization of the site of epileptogenesis in foreign tissue lesions also. In a study of 59 lesional patients, a small number of patients with low grade astrocytomas and oligodendrogliomas consistently localized by EEG to an area elsewhere than the lesion, and failed seizure control when the lesion was removed. Although MRI can demonstrate the structural correlate of the epilepsy in many situations, rare patients, particularly with certain tumors, cortical dysgenesis, and dual pathology, require EEG for accurate localization.