Pathogenesis of mesial temporal sclerosis

Reduced cortical thickness in children with new-onset seizures

BACKGROUND AND PURPOSE

Children with new-onset seizures may have antecedent neurobiologic alterations that predispose them to developing seizures. Our aim was to evaluate hippocampal and thalamic volumes and lobar cortical thickness of children with new-onset seizures.

MATERIALS AND METHODS

Twenty-nine children with new-onset seizures and normal MR imaging findings were recruited. Ten patients had generalized seizures, 19 had partial seizures, and 15 were on antiepileptic medications. Twenty-three age-matched healthy controls were also recruited. Hippocampal and thalamic volumes and lobar cortical thickness, including frontal, medial temporal, lateral temporal, parietal, cingulate, and occipital cortical thickness, were assessed by using volumetric T1-weighted imaging and were compared between patients and controls.

RESULTS

There were no significant differences in hippocampal and thalamic volumes of patients with new-onset seizures, including the subgroups with generalized and partial seizures and those on and off antiepileptic medications, compared with controls (P > .01). There was significant reduction in cortical thickness in right cingulate (P = .004), right medial temporal (P = .006), and left frontal (P = .007) cortices in patients with new-onset seizures. Patients with generalized seizures did not demonstrate a significant reduction in cortical thickness (P > .01). Patients with partial seizures demonstrated a significant reduction in cortical thickness in the right frontal (P = .008), right parietal (P = .003), and left frontal (P = .007) cortices. There were no significant differences in cortical thickness among patients on or off antiepileptic medications (P > .01).

CONCLUSIONS

We found reduced cortical thickness in children with new-onset seizures. Further studies are necessary to elucidate the neurobiologic relevance of these structural changes.

Chronic deficit in the expression of voltage-gated potassium channel Kv3.4 subunit in the hippocampus of pilocarpine-treated epileptic rats

Voltage gated K(+) channels (Kv) are a highly diverse group of channels critical in determining neuronal excitability. Deficits of Kv channel subunit expression and function have been implicated in the pathogenesis of epilepsy. In this study, we investigate whether the expression of the specific subunit Kv3.4 is affected during epileptogenesis following pilocarpine-induced status epilepticus. For this purpose, we used immunohistochemistry, Western blotting assays and comparative analysis of gene expression using TaqMan-based probes and delta-delta cycle threshold (ΔΔCT) method of quantitative real-time polymerase chain reaction (qPCR) technique in samples obtained from age-matched control and epileptic rats. A marked down-regulation of Kv3.4 immunoreactivity was detected in the stratum lucidum and hilus of dentate gyrus in areas corresponding to the mossy fiber system of chronically epileptic rats. Correspondingly, a 20% reduction of Kv3.4 protein levels was detected in the hippocampus of chronic epileptic rats. Real-time quantitative PCR analysis of gene expression revealed that a significant 33% reduction of transcripts for Kv3.4 (gene Kcnc4) occurred after 1 month of pilocarpine-induced status epilepticus and persisted during the chronic phase of the model. These data indicate a reduced expression of Kv3.4 channels at protein and transcript levels in the epileptic hippocampus. Down-regulation of Kv3.4 in mossy fibers may contribute to enhanced presynaptic excitability leading to recurrent seizures in the pilocarpine model of temporal lobe epilepsy.

Altered expression and function of small-conductance (SK) Ca(2+)-activated K+ channels in pilocarpine-treated epileptic rats

Small conductance calcium (Ca(2+)) activated SK channels are critical regulators of neuronal excitability in hippocampus. Accordingly, these channels are thought to play a key role in controlling neuronal activity in acute models of epilepsy. In this study, we investigate the expression and function of SK channels in the pilocarpine model of mesial temporal lobe epilepsy. For this purpose, protein expression was assessed using western blotting assays and gene expression was analyzed using TaqMan-based probes and the quantitative real-time polymerase chain reaction (qPCR) comparative method delta-delta cycle threshold ( big up tri, open big up tri, openCT) in samples extracted from control and epileptic rats. In addition, the effect of SK channel antagonist UCL1684 and agonist NS309 on CA1 evoked population spikes was studied in hippocampal slices. Western blotting analysis showed a significant reduction in the expression of SK1 and SK2 channels at 10days following status epilepticus (SE), but levels recovered at 1month and at more than 2months after SE. In contrast, a significant down-regulation of SK3 channels was detected after 10days of SE. Analysis of gene expression by qPCR revealed a significant reduction of transcripts for SK2 (Kcnn1) and SK3 (Kcnn3) channels as early as 10days following pilocarpine-induced SE and during the chronic phase of the pilocarpine model. Moreover, bath application of UCL1684 (100nM for 15min) induced a significant increase of the population spike amplitude and number of spikes in the hippocampal CA1 area of slices obtained control and chronic epileptic rats. This effect was obliterated by co-administration of UCL1684 with SK channel agonist NS309 (1microM). Application of NS309 failed to modify population spikes in the CA1 area of slices taken from control and epileptic rats. These data indicate an abnormal expression of SK channels and a possible dysfunction of these channels in experimental MTLE.

CA3 axonal sprouting in kainate-induced chronic epilepsy

Latency between an early neurological insult and development of spontaneous recurrent seizures suggests aberrant chronological reorganization in patients with mesial temporal sclerosis associated epilepsy. Kainate-induced status similarly results in delayed development of spontaneous recurrent seizures. Mossy fiber sprouting by the dentate granule cells is a well-characterized manifestation of such temporal structural reorganization in both patients and animal models. However, alterations in other components of hippocampal circuitry have not been evaluated. We present results from studies using precise anterograde and retrograde tract tracing methodologies to evaluate the reorganization of outflow of the CA3 pyramidal cells. Although septotemporal relationships of the normal CA3 outflow tract through the Schaffer collaterals are well known, their aberrant reorganization following kainate-induced spontaneous recurrent seizures is not known. We provide the first definitive evidence of widespread CA3 structural reorganization in the form of sprouting of CA3 axons to widespread areas throughout the hippocampus and entorhinal cortex. This includes an apparent increase in the density of projection to areas that normally receive CA3 outflow such as CA1 and subiculum as well as novel projections beyond the confines of the hippocampus to the pre and parasubiculum and medial and lateral entorhinal cortex. We provide the first evidence of novel CA3 Schaffer collateral projection to the entorhinal cortex. The sprouting of CA3 outflow to widespread regions of the hippocampus and the entorhinal cortex may provide insight into how the injured hippocampus propagates unconventional impulse excitation to cortical fields which have a critical role in providing excitatory inputs into the hippocampus possibly setting up reverberating excitatory circuits as well as widespread connections throughout the cortical mantle. Sprouting-related mechanisms may also explain the latency associated with development of spontaneous recurrent seizures, the hallmark of temporal lobe epilepsy.

Cerebral damage in epilepsy: a population-based longitudinal quantitative MRI study

PURPOSE

Whether cerebral damage results from epileptic seizures remains a contentious issue. We report on the first longitudinal community-based quantitative magnetic resonance imaging (MRI) study to investigate the effect of seizures on the hippocampus, cerebellum, and neocortex.

METHODS

One hundred seventy-nine patients with epilepsy (66 temporal lobe epilepsy, 51 extratemporal partial epilepsy, and 62 generalized epilepsy) and 90 control subjects underwent two MRI brain scans 3.5 years apart. Automated and manual measurement techniques identified changes in global and regional brain volumes and hippocampal T2 relaxation times.

RESULTS

Baseline hippocampal volumes were significantly reduced in patients with temporal lobe epilepsy and could be attributed to an antecedent neurologic insult. Rates of hippocampal, cerebral, and cerebellar atrophy were not syndrome specific and were similar in control and patient groups. Global and regional brain atrophy was determined primarily by age. A prior neurologic insult was associated with reduced hippocampal and cerebellar volumes and an increased rate of cerebellar atrophy. Significant atrophy of the hippocampus, neocortex, or cerebellum occurred in 17% of patients compared with 6.7% of control subjects. Patients with and without significant volume reduction were comparable in terms of seizure frequency, antiepileptic drug (AED) use, and epilepsy duration, with no identifiable risk factors for the development of atrophy.

CONCLUSIONS

Overt structural cerebral damage is not an inevitable consequence of epileptic seizures. In general, brain volume reduction in epilepsy is the cumulative effect of an initial precipitating injury and age-related cerebral atrophy. Significant atrophy developed in individual patients, particularly those with temporal lobe and generalized epilepsy. Longer periods of observation may detect more subtle effects of seizures.

Neuropsychological Outcome after Selective Amygdalohippocampectomy with Transsylvian versus Transcortical Approach: A Randomized Prospective Clinical Trial of Surgery for Temporal Lobe Epilepsy

PURPOSE

Selective amygdalohippocampectomy (SAH) is a surgical treatment option for patients with medically intractable mesial temporal lobe epilepsy. In contrast to standard anterior temporal lobectomy, resection of unaffected tissue is limited, although it achieves equal seizure outcomes in selected patients. In SAH, the mesial structures can be approached by different routes, the transsylvian approach and the transcortical approach. Advantages or disadvantages with respect to postoperative cognitive outcome are still a matter of debate.

METHODS

Eighty randomized patients were included in the analyses. In 41 patients, the transsylvian approach, and in 39 patients, the transcortical approach was performed. All patients received comprehensive neuropsychological testing of verbal and nonverbal memory, attention, and executive functions before and 6 months or 1 year after SAH.

RESULTS

Seventy-five percent of patients became completely seizure free with no difference depending on the chosen approach. Repeated measures multivariate analysis of variance (MANOVA) showed that cognitive outcomes after both approaches were essentially the same. The only exception was phonemic fluency, which was significantly improved after transcortical but not after transsylvian SAH.

CONCLUSIONS

The results indicate that either surgical approach can be chosen independent of cognitive outcome criteria. Improvement in phonemic fluency after transcortical SAH may reflect selective normalization of cognitive function after epilepsy surgery, whereas frontal lobe manipulation might have hindered recovery of this function after transsylvian SAH.

Refractory epilepsy: a progressive, intractable but preventable condition?

Intractable seizures are just one manifestation of 'refractory epilepsy', which can be recognized as a distinct condition with multifaceted dimensions, including neurobiochemical plastic changes, cognitive decline and psychosocial dysfunction, leading to dependent behaviour and a restricted lifestyle. The biological basis of 'refractoriness' is likely to be multifactorial, and may include the severity of the syndrome and/or underlying neuropathology, abnormal reorganization of neuronal circuitry, alteration in neurotransmitter receptors, ion channelopathies, reactive autoimmunity, and impaired antiepileptic drug (AED) penetration to the seizure focus. Some of these deleterious changes may be a consequence of recurrent seizures. We hypothesize that 'refractory epilepsy' may be prevented by interrupting this self-perpetuating progression. There is increasing evidence that these patients can be identified early in the clinical course and, thus, be targeted early for effective therapeutic intervention. Failure of two first-line AEDs due to lack of efficacy or poor tolerability should prompt consideration of epilepsy surgery in a patient with a resectable brain abnormality. For the majority not suitable for 'curative' surgery, AEDs should be combined with the aim of achieving 'synergism'. This strategy has the potential to improve outcome by preventing the insidious progression to intractable 'refractoriness' and a downward spiraling quality of life.

A longitudinal quantitative MRI study of community-based patients with chronic epilepsy and newly diagnosed seizures: methodology and preliminary findings

Experimental and human data suggest that progressive cerebral damage may result from the cumulative effect of brief recurrent seizures. Longitudinal studies addressing this fundamental question, however, are lacking. We have addressed this need with a large prospective community-based observational study, which aims to rescan 154 patients with chronic active epilepsy and 90 patients with newly diagnosed seizures, after an interval of 3.5 years. Here, we describe the quantitative magnetic resonance methods used to identify subtle volume changes in hippocampal, cerebellar, and neocortical structures over time and report preliminary findings. Using this methodology, we have previously shown that we can reliably detect individual hippocampal volume (HV) and cerebellar volume (CBV) changes greater than 3.1 and 3.0%, respectively (Lemieux et al, 2000). Analysis of the first 53 subjects (24 patients with chronic active epilepsy, 9 patients with newly diagnosed seizures, and 20 controls) has demonstrated significant HV losses in 4 individuals. Automated and semiautomated calculation has detected significant reductions in CBV, total brain volume, and gray matter volume in 2, 3, and 1 subject, respectively. There were no significant white matter volume losses detected. Data collected from rescanning the entire cohorts will help to provide further information on the relationship between recurrent seizures and secondary brain damage.

Lateralization of temporal lobe foci: depth versus subdural electrodes

OBJECTIVES

Definitive localization of an epileptic focus correlates with a favorable outcome following epilepsy surgery. This study was undertaken to determine the incremental value of data yielded for surgical decision making when using subdural electrodes alone and in addition to depth electrodes for temporal lobe epilepsy.

METHODS

Standardized placement for intracranial electrodes included: (1) longitudinal placement of bilateral temporal lobe depth electrodes; (2) bilateral subtemporal subdural strips; and (3) bilateral orbitofrontal subdural strips. Sixty-three events were randomly reviewed for: (1) subdural electrodes alone; and (2) depth electrodes in conjunction with subdural electrodes.

RESULTS

Of the 63 seizures, 54 (85.7%) demonstrated congruent lateralization to ipsilateral subtemporal subdural strip electrodes (based on depth electrode localization) when subdural strip electrodes were utilized alone. In 3 of 22 patients, 7 seizures demonstrated 'false localization' on subdural electrode analysis alone when compared with depth recording and post-surgical outcome. For these 3 patients, retrospective review of neuroimaging demonstrated suboptimal ipsilateral placement of subtemporal subdural electrodes with the most mesial electrode lateral to the collateral sulcus. Four additional patients had suboptimal placement of subtemporal subdural electrodes. Two of these 4 patients had congruent localization with subdural electrodes to ipsilateral depth electrodes despite suboptimal placement. Subtemporal subdural electrodes accurately localized for all seizures from the mesial temporal lobe when the mesial electrodes of the subtemporal subdural strip recorded mesial to the collateral sulcus from the parahippocampal region.

CONCLUSION

We conclude that although there are high concordance rates between subdural and depth electrodes, localization of seizure onset based on subdural strip electrodes alone may result in inaccurate focus identification with potential for possible suboptimal treatment of temporal lobe epilepsy. When subtemporal subdural electrodes provide recording from the parahippocampal region, there is accurate localization of the seizure focus. If suboptimal placement occurs lateral to the collateral sulcus, the electroencephalographer cannot make a definitive identification of the seizure focus.

Comparison of [18F]FDG-PET, [99mTc]-HMPAO-SPECT, and [123I]-iomazenil-SPECT in localising the epileptogenic cortex

OBJECTIVES

Firstly, to compare the findings of interictal 18F-fluorodeoxyglucose (FDG) positron emission tomography (PET) and of single photon emission computed tomography (SPECT) using 99mTc-hexamethyl propylene-amine-oxime (HMPAO) and 123I-iomazenil in localising the epileptogenic cortex in patients who were candidates for epilepsy surgery, but in whom clinical findings, video EEG monitoring (V-EEG), MRI, and neuropsychological evaluations did not give any definite localisation of the seizure onset. Secondly, to assess the ability of these functional methods to help in the decision about the epilepsy surgery.

METHODS

Eighteen epileptic patients were studied with FDG-PET and iomazenil-SPECT. HMPAO-SPECT was performed in 11 of these 18 patients. Two references for localisation was used--ictal subdural EEG recordings (S-EEG) and the operated region.

RESULTS

Fifteen of 18 patients had localising findings in S-EEG. FDG-PET findings were in accordance with the references in 13 patients and iomazenil-SPECT in nine patients. HMPAO-SPECT visualised the focus less accurately than the two other methods. In three patients S-EEG showed independent bitemporal seizure onset. In these patients FDG-PET showed no lateralisation. However, iomazenil-SPECT showed temporal lobe lateralisation in two of them.

CONCLUSION

FDG-PET seemed to localise the epileptogenic cortex more accurately than interictal iomazenil-SPECT in patients with complicated focal epilepsy.

Frequency of unilateral and bilateral mesial temporal sclerosis in primary and secondary epilepsy: a forensic autopsy study

Controversy exists regarding the pathogenetic relationship of mesial temporal sclerosis (MTS) to epileptogenesis. Some investigators view hippocampal sclerosis as the primary cause of temporal lobe epilepsy, whereas others interpret the changes to be the result of chronic seizure activity. The present autopsy-based study attempts to clarify the etiologic relationship between mesial temporal sclerosis and epilepsy. To investigate the assumption that bilateral MTS is more likely to be the result of chronic seizure activity associated with a seizure focus outside the hippocampus, two subject groups were identified. The first group comprised 43 patients who had no extrahippocampal pathology and were classified as having primary epilepsy. The second group comprised 35 patients who, had identifiable extrahippocampal pathology and were classified as having secondary epilepsy. Fifteen of the 35 cases of secondary epilepsy also had MTS; seven of these were unilateral and eight were bilateral. Of the 43 cases with primary epilepsy, only one had MTS, and it was unilateral. Significantly more cases of primary epilepsy than secondary epilepsy had no MTS (p < 0.001), suggesting that both unilateral and bilateral forms of MTS occur with greater frequency in subjects with seizure foci outside the hippocampus. These results also suggest that unilaterality of MTS does not exclude an extrahippocampal cause.

Functional anatomy of human hippocampal formation and related structures

Data on the internal organization, and neuronal connections of the human hippocampal formation and related structures of the limbic system are briefly reviewed. In the healthy brain, somatosensory, visual, and auditory input proceeds through neocortical core and belt fields to a variety of association areas, and from here the data is transported via long corticocortical pathways to the extended prefrontal association cortex. Tracts generated from this highest organization level of the brain guide the data via the frontal belt (premotor cortex) to the frontal core (primary motor area). The striatal and cerebellar loops provide the major routes for this data transfer. The main components of the limbic system (the hippocampal formation, the entorhinal region, and the amygdala) maintain a strategic position between the sensory and the motor association areas. Part of the stream of data from the sensory association areas to the prefrontal cortex branches off and eventually converges on the entorhinal region and the amygdala, These connections establish the afferent leg of the limbic loop. In addition, the limbic centers receive substantial input from nuclei processing viscerosensory information. The entorhinal region, the hippocampal formation, and the amygdala are densely interconnected. Important among these connections is the perforant path, which originates in the entorhinal cortex and projects to the hippocampal formation (fascia dentata, Ammon's horn, and subiculum). The subiculum projects to the amygdala, entorhinal region, mamillary nuclei, and anterior and midline thalamic nuclei. The hippocampal formation, the entorhinal region, and the amygdala generate the efferent leg of the limbic loop, which is directed toward the prefrontal cortex. Additional projections reach the key nuclei that control endocrine and autonomic functions. Furthermore, the amygdala exerts influence on all nonthalamic nuclei projecting in a nonspecific manner to the cerebral cortex (ie, the cholinergic magnocellular forebrain nuclei, the histaminergic tuberomamillary nucleus, the dopaminergic nuclei of the ventral tegmentum, the serotonergic anterior raphe nuclei, and the noradrenergic locus ceruleus). The limbic loop centers thus are in the unique strategic position to perform integration of exteroceptive sensory data of various sources with interoceptive stimuli from autonomic centers. Their efferent projections exert influence on both the prefrontal association cortex and the key centers controlling endocrine and autonomic functions.