Characteristics of medial temporal lobe epilepsy: II. Interictal and ictal scalp electroencephalography, neuropsychological testing, neuroimaging, surgical results, and pathology

Rolandic epilepsy and cortical dysplasia: MRI correlation of epileptiform discharges

An 8 year-old girl presented with simple facial motor seizures. Although the electroencephalogram (EEG) demonstrated left hemisphere centrotemporal spikes with features consistent with benign rolandic epilepsy, magnetic resonance imaging (MRI) indicated a left hemisphere focal cortical dysplasia. MRI-assisted EEG dipole analysis of the spikes suggested that the rolandic fissure rather than the focal cortical dysplasia was the origin of the epileptic spike discharge. This noninvasive method may be a useful adjunct in evaluation of some patients with epilepsy and focal superficial cerebral lesions.

Comparative study of interictal PET and ictal SPECT in complex partial seizures

OBJECTIVE

To compare the sensitivity of ictal 99mTc-HMPAO single photon emission computed tomography (SPECT) with interictal 18F-fluoro-deoxyglucose positron emission tomography (PET) in localization of the epileptogenic focus in patients with medically intractable complex partial seizures (MI-CPS).

MATERIAL AND METHODS

Retrospective analysis was performed on patients with MI-CPS who underwent anterior temporal lobectomy from January 1993 onwards when PET became available to us for clinical studies at the Indiana University Medical Center. There were 38 female and 29 male patients (total = 67) with MI-CPS, 10 to 55.5 years of age (mean 31) and duration of their epilepsy from 1-46 years (mean 21). Interictal PET was evaluated for evidence of focal hypometabolism and ictal SPECT for focal perfusion abnormality (hyperperfusion or hypoperfusion) by visual analysis.

RESULTS

Both ictal SPECT and interictal FDG-PET studies were obtained in 36 patients with MI-CPS. PET showed definite hypometabolism in 30 and questionable hypometabolism in an additional two patients. Ictal SPECT correctly localized the seizure focus in 27 patients by demonstrating ictal hyperperfusion whereas in one the hyperperfusion was falsely localized. In an additional seven patients the ictal SPECT provided probable localization by demonstrating ictal hypoperfusion in the appropriate temporal lobe. The sensitivity of ictal SPECT and interictal PET was 34/36 and 32/36, respectively, the difference was not statistically significant (chi 2y = 0.18, DF = 1, P = 0.67). In six of the 36 patients the two tests were complementary to each other in providing localizing information.

CONCLUSION

Ictal SPECT and interictal PET are equally sensitive and reliable techniques in localizing the epileptogenic focus in patients with MI-CPS. They play a critical role in providing localization in MRI negative patients allowing surgical resection to be undertaken in many without additional invasive electrographic monitoring.

The pathogenic and progressive features of chronic human hippocampal epilepsy

To design useful experimental models of epilepsy, it is necessary to clearly understand the known clinical-pathologic features of the disease process. Studies of mesial temporal lobe epilepsy (MTLE) patients have identified several distinctive clinical and pathophysiologic characteristics and many of these can be analyzed in experimental models. For example, patients with typical MTLE have medical histories that often contain an initial precipitating injury (IPI), are likely to have hippocampal sclerosis in the surgical specimen, and have better seizure outcomes than patients with typical idiopathic temporal seizures (i.e. cryptogenic). Hippocampal from children as young as age 1 year with IPI histories also demonstrate neuron damage similar to adults with hippocampal sclerosis. Compared to IPI patients without seizures (i.e. trauma, hypoxia, etc.), IPI cases with severe seizures showed younger ages at the IPI, shorter latent periods, and longer durations of habitual MTLE. Hippocampal damage is often bilateral, however, the epileptogenic side shows hippocampal sclerosis and the opposite side usually shows only mild neuron losses. Moreover, MTLE patients show declines in hippocampal neuron densities with very long histories of habitual seizures (15 to 20 years), however, the additional neuron loss adds to the template of hippocampal sclerosis and occurs in limited subfields (granule cells, CA1 and prosubiculum). Hippocampal axon and synaptic reorganization is another pathologic feature of MTLE, and involves granule cell mossy fibers and axons immunoreactive for neuropeptide upsilon, somatostatin, and glutamate decarboxylase (which synthesizes GABA). Finally, MTLE patients with hippocampal sclerosis show increased granule cell mRNA levels for brain derived neurotropic factor, nerve growth factor, and neurotrophin-3 that correlate with mossy fiber sprouting or with declines in Ammon's horn neuron densities. Taken together, our data support the following concepts: (1) The pathogenesis of MTLE is associated with IPI histories that probably injure the hippocampus at some time prior to habitual seizure onsets, (2) most of the damage seems to occur with the IPI, (3) there can be additional neuron loss associated with long histories, (4) another pathologic feature of MTLE is axon reorganization of surviving fascia dentata and hippocampal neurons, and (5) reorganized axon circuits probably contribute to seizure or propagation.

Clinical features of neocortical temporal lobe epilepsy

Few studies have examined the clinical features of neocortical temporal lobe epilepsy (NTLE) in carefully selected patients. We reviewed records from 21 patients with NTLE, defined by intracranial electroencephalogram (EEG), who have been seizure free for 1 year or more following temporal lobectomy. The mean age of onset at the time of first seizure was 14 years (range, 1-41 years). Febrile seizures were reported in only 2 patients (9.5%). In contrast to prior mesial temporal lobe epilepsy (MTLE) studies, seizure-free intervals between the initial cerebral insult or first seizure and habitual seizures were uncommon. Possible or known risk factors for epilepsy were reported in 13 of 21 patients (62%). Fifteen (71%) patients reported auras, with experiential phenomena being the most common type. Magnetic resonance imaging was normal or nonspecific in 15 patients, revealed mild hippocampal atrophy in 2, tumors in 2, and heterotopic gray matter and hippocampal atrophy in 1, and cortical dysgenesis in 1. Neuropsychological testing showed deficits consistent with the seizure focus in 13 patients (62%), and Wada test showed ipsilateral memory deficits in 10 (48%). The most common behavioral manifestation was a motionless stare at ictal onset (48%). In contrast to prior studies of MTLE, only 1 NTLE patient had frequent independent, contralateral temporal lobe epileptiform spikes on scalp EEG.

Preoperative clinical, EEG, and imaging findings do not predict seizure outcome following temporal lobectomy in childhood

Although certain clinical, electroencephalographic (EEG), magnetic resonance imaging (MRI), and pathologic findings in adults with intractable temporal lobe epilepsy predict seizure outcome following temporal lobectomy, predictors of seizure outcome have not been studied systematically in pediatric temporal lobectomy series. We retrospectively analyzed preoperative clinical, EEG, and neuroimaging findings with reference to seizure outcome (seizure free or non-seizure free) in 33 children (mean age, 9.3 years) who underwent tailored temporal lobe resections for intractable temporal lobe epilepsy. Trends were apparent with (1) younger age at seizure onset, younger age at surgery, shorter duration of epilepsy, localized unilateral temporal lesions on MRI, and right-sided surgery more frequently associated with a seizure-free outcome, and (2) significant prior history, daily preoperative seizures, generalized motor seizures, mental retardation, and localized unilateral temporal epileptiform EEG activity more frequently associated with a non-seizure-free outcome. However, none of these findings, alone or in combination, correlated with postoperative seizure status at a statistically significant level. Submitting the four variables generally considered to be most predictive of favorable outcome (ie, normal intelligence, unilateral ictal and interictal EEG discharges, and focal temporal MRI lesion) to a multiple-cutoff procedure did not predict seizure freedom. Our data indicate that predictors of outcome of temporal lobectomy in adults may not apply in children, perhaps due to inherent neurobiologic differences in the etiology and expression of temporal lobe epilepsy, and should therefore not be used as sole determinants of surgical candidacy in children.

Neuroimaging in epilepsy surgery: a review

In recent years there has been a shift away from invasive monitoring, with more emphasis on the role of neuroimaging, in the selection of patients for epilepsy surgery. Although video-EEG is essential to confirm the diagnosis, and to determine the ictal onset, neuroimaging, in particular magnetic resonance imaging (MRI), forms the basis for selection of most surgical candidates. MRI, using visual analysis, is able to detect hippocampal sclerosis, the most common cause of temporal lobe epilepsy, in the majority of patients with this condition, with quantitative MRI increasing the sensitivity of this imaging technique. Other lesions readily detected on MRI include dysplasia, neuronal migration disorders and cavernomas. Studies have shown that the best postoperative results are achieved in patients with a lesion visible on MRI. Functional imaging, both single photon emission computed tomography (SPECT), in particular ictal SPECT, and photon emission tomography (PET), are important ancillary investigations providing valuable corroborative evidence of a seizure focus.

Familial temporal lobe epilepsy: a common disorder identified in twins

We describe a new syndrome of familial temporal lobe epilepsy in 38 individuals from 13 unrelated white families. The disorder was first identified in 5 concordant monozygotic twin pairs as part of a large-scale twin study of epilepsy. When idiopathic partial epilepsy syndromes were excluded, the 5 pairs accounted for 23% of monozygotic pairs with partial epilepsies, and 38% of monozygotic pairs with partial epilepsy and no known etiology. Seizure onset for twin and nontwin subjects usually occurred during adolescence or early adult life. Seizure types were simple partial seizures with psychic or autonomic symptoms, infrequent complex partial seizures, and rare secondarily generalized seizures. Electroencephalograms revealed sparse focal temporal interictal epileptiform discharges in 22% of subjects. Magnetic resonance images appeared normal. Nine affected family members (24%) had not been diagnosed prior to the study. Pedigree analysis suggested autosomal dominant inheritance with age-dependent penetrance. The estimated segregation ratio was 0.3, indicating an overall penetrance of 60% assuming autosomal dominant inheritance. The mild and often subtle nature of the symptoms in some family members may account for lack of prior recognition of this common familial partial epilepsy. This disorder has similarities to the El mouse, a genetic model of temporal lobe epilepsy with a major gene on mouse chromosome 9, which is homologous with a region on human chromosome 3.

Variability of presentation in medial temporal lobe epilepsy: a study of 30 operated cases

INTRODUCTION

To evaluate the homogeneity of patients operated on for MTLE.

MATERIAL AND METHODS

30 consecutive patients prospectively investigated with clinical, EEG, neuropsychology, MRI, FDG-PET criteria and eventually intracranial EEG, who underwent antero-medial temporal resection (follow-up: 24 months).

RESULTS

Clear and well-lateralised MTLE was non-invasively ascertained in 22 patients (73%). Eight patients (27%) had a less pure presentation due to divergent scalp EEG features (bitemporal, widespread or extratemporal), unusual auras or absence of MRI-based hippocampal sclerosis or FDG-PET hypometabolism. They were explored by invasive monitoring which confirmed medial temporal origin of seizures. Outcome was excellent in 95% of the pure cases (Engel's class I) and less favorable in the more difficult ones (65% of class I).

CONCLUSIONS

MTLE criteria, including the strong contribution of brain imaging, permit to select accurately a large percentage of patients. However a consistant number of patients present a less pure presentation suggesting more complex epileptogenic networks.

Relationship of hippocampal sclerosis to duration and age of onset of epilepsy, and childhood febrile seizures in temporal lobectomy patients

Controversy exists as to whether hippocampal sclerosis (HS) is a preexisting cause or a consequence of seizures. We investigated 122 consecutive patients who underwent anterior temporal lobectomy for intractable epilepsy between 1989 and 1992. MRI scans were normal apart from evidence of HS in 5 cases. The degree of HS was graded from 0 to 4. There was a significant inverse correlation between age of seizure onset and grade of HS (P < 0.0001), and a positive correlation between duration of epilepsy and grade of HS (P < 0.001). Using a dichotomous grouping of HS (HPSC - for grades 0 and 1 [no/mild HS], and HPSC + for grades 3 and 4 [moderate/marked HS]), there was a positive correlation between HPSC + and a history of childhood febrile seizures (CFS) (P = 0.003), earlier age of onset of epilepsy (P < 0.001) and longer duration of epilepsy (P < 0.001). There was no correlation with history of particularly prolonged individual seizures. Partial correlations after controlling for age at onset of epilepsy showed that there was no longer a significant relationship between HPSC + and duration of epilepsy. After controlling for duration of epilepsy, the relationship between HPSC + and age of onset remained significant (P < 0.001). The correlation between HPSC + and CFS, controlling for age at onset, was not significant. A series of logistic regression analyses showed age at onset to be the only predictor of HPSC +. It is concluded that this is supportive evidence for preexisting HS being a cause of temporal lobe epilepsy and not a consequence of seizures.

A clinical approach to the classification of seizures and epileptic syndromes

Seizures and epilepsies are heterogeneous. Their classifications are essential for clinicians to achieve a common understanding of the disorders. The diagnosis, treatment, and prognosis of seizure disorders depend on the correct identification of the types of seizures and epilepsy. The two currently accepted classifications are the International Classification of Epileptic Seizures and the International Classification of Epilepsies and Epileptic Syndromes. Both are based on clinical and electrophysiologic data, and both maintain a basic dichotomy between partial (focal, localization-related) and generalized epileptic disorders. Partial seizures are further classified into those that are simple partial, complex partial, and partial with secondary generalization. Generalized seizures are classified predominantly on the basis of their motor manifestations. Epilepsies are divided into idiopathic, cryptogenic, and symptomatic types. The utility and the limitations of these two classifications are discussed. A simplified system that encompasses neuroradiologic advances is offered to enhance the clinical usefulness of classifying epileptic disorders.

The development of psychosis in epilepsy: a re-examination of the kindling hypothesis

It is generally acknowledged that psychosis occurs with increased frequency within epileptic populations. There are several possible explanations for this over-representation: (1) psychosis may develop as a result of anti-epileptic drug or surgical treatment, or as a result of the psychosocial effects of living with epilepsy; (2) epilepsy and psychosis may, in some cases, have a common cause: and (3) chronic seizure activity may sometimes cause psychosis. The objective of this review is to evaluate the hypothesis that focal seizure activity may lead to the development of psychosis through a kindling process. There is some evidence to suggest that secondary epileptogenesis may develop following the spread of seizure activity from a primary focus, possible via a kindling mechanism. Although it has been suggested that long-term potentiation (LTP) may result in the development of secondary epileptic foci; LTP is not necessarily implicated. The kindling hypothesis of the development of psychosis in epilepsy must address the neural mechanism by which the spread of seizures might result in psychosis. At present, the neurochemical mechanisms by which psychosis could result from epilepsy are unclear.

Is interictal temporal hypometabolism related to mesial temporal sclerosis? A positron emission tomography/magnetic resonance imaging confrontation

The mechanism of interictal glucose hypometabolism remains unclear, but this abnormality occurs more frequently in temporal lobe epilepsy (TLE) than in other types of partial epilepsy. Therefore temporal hypometabolism has been suggested to reflect mesial temporal sclerosis (MTS). To investigate this, we selected 22 patients with refractory partial epilepsy of mesial temporal lobe origin (MTLE) who had hippocampal atrophy based on magnetic resonance imaging (MRI) volumetric analysis. We then analyzed the metabolic correlates of unilateral hippocampal atrophy. Thirteen temporal regions of interest (ROI) were defined on MRI scans for each individual and then applied to high-resolution FDG-positron emission tomography (PET) images obtained parallel to the long axis of the hippocampus. The most hypometabolic regions were the temporal pole and the hippocampal region. When we analyzed ensembles of temporal regions grouped into related networks, the temporolimbic network, which included the hippocampal region and the temporal pole, was abnormal in 95% of the patients at a 3-SD threshold. PET hypometabolism was highly correlated with the degree of hippocampal atrophy in this network, but not in other parts of the temporal lobe, which were less frequently hypometabolic. These data indicate that hypometabolism is a consequence of MTS in the temporolimbic region but not necessarily in the other parts of the temporal lobe. Our results also suggest that the combination of PET and MRI may facilitate the noninvasive diagnosis of MTLE.

Successful epilepsy surgery without intracranial EEG recording: criteria for patient selection

Twenty-two patients with intractable complex partial seizures (CPS) were treated with temporal lobectomy. Eighteen of 22 (82%) are seizure-free while receiving medication, with a mean follow-up time of 4 years. In each case, the clinical seizure pattern, interictal and ictal scalp EEG, magnetic resonance imaging (MRI), neuropsychological testing, and results of the intracarotid amobarbital procedure (IAP) converged to indicate a localized abnormality. None of the patients in this series had mass lesions, vascular malformations, or cortical scars, but 18 of 22 had hippocampal atrophy on MRI and 20 had hippocampal sclerosis (HS) on pathologic examination. We believe it is possible, on the basis of the preoperative evaluation described, to identify a population of epileptic patients who will do very well postoperatively. Such patients do not require invasive EEG monitoring, and they represent approximately 20% of the patients treated surgically in our epilepsy unit in the past several years.

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.

Mesial temporal sclerosis: pathogenesis and significance

Mesial temporal sclerosis (MTS) is a common pathologic finding in patients with temporal lobe epilepsy. Rarely MTS can be detected in children during the first decade of life, but is not commonly found until adolescence. Although the etiology of MTS remains controversial, there is now a considerable amount of evidence demonstrating that MTS is both a result and a cause of seizures. Clinical studies suggest that prolonged seizures or complicated febrile seizures may result in MTS. A variety of epileptogenic agents administered to adult animals have resulted in MTS and spontaneous recurrent seizures. The mechanism of the lesions is due to excessive excitability secondary to release of excitatory amino acids, primarily glutamate. Glutamate, acting at a number of subreceptors on the postsynaptic membrane, leads to prolonged depolarization of neurons and results in the entry of cytotoxic amounts of calcium. Interestingly, the same agents that produce MTS in adult animals do not produce MTS in immature animals. Clinical and experimental evidence suggests that although prolonged seizures or complicated febrile seizures can place a child at risk for MTS, a period of time is required for the lesions to develop fully.