SPECT perfusion changes during complex partial seizures in patients with hippocampal sclerosis

Application of dynamic susceptibility contrast-enhanced perfusion in temporal lobe epilepsy

BACKGROUND

Accurately locating the epileptogenic focus in temporal lobe epilepsy (TLE) is important in clinical practice. Single-photon emission computed tomography (SPECT) and positron-emission tomography (PET) have been widely used in the lateralization of TLE, but both have limitations. Magnetic resonance perfusion imaging can accurately and reliably reflect differences in cerebral blood flow and volume.

PURPOSE

To investigate the diagnostic value of dynamic susceptibility contrast-enhanced (DSC) perfusion magnetic resonance imaging (MRI) in the lateralization of the epileptogenic focus in TLE.

MATERIAL AND METHODS

Conventional MRI and DSC-MRI scanning was performed in 20 interictal cases of TLE and 20 healthy volunteers. The relative cerebral blood volume (rCBV) and relative cerebral blood flow (rCBF) of the bilateral mesial temporal lobes of the TLE cases and healthy control groups were calculated. The differences in the perfusion asymmetry indices (AIs), derived from the rCBV and rCBF of the bilateral mesial temporal lobes, were compared between the two groups.

RESULTS

In the control group, there were no statistically significant differences between the left and right sides in terms of rCBV (left 1.55 ± 0.32, right 1.57 ± 0.28) or rCBF (left 99.00 ± 24.61, right 100.38 ± 23.46) of the bilateral mesial temporal lobes. However, in the case group the ipsilateral rCBV and rCBF values (1.75 ± 0.64 and 96.35 ± 22.63, respectively) were markedly lower than those of the contralateral side (2.01 ± 0.79 and 108.56 ± 26.92; P < 0.05). Both the AI of the rCBV (AI(rCBV); 13.03 ± 10.33) and the AI of the rCBF (AI(rCBF); 11.24 ± 8.70) of the case group were significantly higher than that of the control group (AI(rCBV) 5.55 ± 3.74, AI(rCBF) 5.12 ± 3.48; P < 0.05). The epileptogenic foci of nine patients were correctly lateralized using the 95th percentile of the AI(rCBV) and AI(rCBF) of the control group as the normal upper limits.

CONCLUSION

In patients with TLE interictal, both rCBV and rCBF of the ipsilateral mesial temporal lobe were markedly lower than that of healthy control subjects. DSC-MRI can provide lateralization for TLE.

Localizing value of ictal SPECT is comparable to MRI and EEG in children with focal cortical dysplasia

PURPOSE

To assess the predictive value of ictal single-photon emission computed tomography (SPECT) for outcome after excisional epilepsy surgery in a large population of children with focal cortical dysplasia (FCD).

METHODS

One hundred seventy-three ictal SPECT studies in 106 children with histologically proven FCD were retrospectively analyzed. The extent and location of ictal hyperperfusion and completeness of surgical removal were assessed. Completeness of resection of epileptogenic regions defined by ictal SPECT, electroencephalography (EEG), and magnetic resonance imaging (MRI) were compared and correlated with postoperative seizure outcome. In addition, subcortical activation of the cerebellum, basal ganglia, and thalamus were analyzed.

KEY FINDINGS

The extent of hyperperfusion was focal or lobar in 58%, whereas multilobar activations occurred in only 32%; hemispheric or bilateral findings were rare. Favorable postsurgical seizure outcome was achieved in 67% patients with nonlocalized SPECT findings, 45% with nonresected ictal hyperperfusion, 36% with partially resected ictal hyperperfusion, and 86% when the zone of ictal hyperperfusion was completely resected (p = 0.000198). The favorable postsurgical outcome after complete removal of the SPECT hyperperfusion zone surpassed the 75% rate of seizure freedom in patients with removal of MRI/EEG-defined epileptogenic region. A similar predictive value of ictal SPECT for seizure outcome was found in nonoperated patients and subjects who were undergoing reoperation. Subcortical activation conferred no predictive value.

SIGNIFICANCE

Ictal SPECT helps to define the epileptogenic zone in a high proportion of children with FCD undergoing surgical evaluation. Complete removal of both SPECT and MRI/EEG-defined regions is a strong predictor of surgical success and has important implications for surgical planning.

Alterations in regional homogeneity of resting-state brain activity in mesial temporal lobe epilepsy

PURPOSE

The purpose of the present study was to identify abnormal areas of regional synchronization in patients with mesial temporal lobe epilepsy and hippocampus sclerosis (mTLE-HS) compared to healthy controls, by applying a relatively novel method, the Regional Homogeneity (ReHo) method to resting state fMRI (RS-fMRI) data.

METHODS

Eyes closed RS-fMRI data were acquired from 10 mTLE-HS patients (four right-side, six left-side) and 15 age- and gender-matched healthy subjects, and were analyzed by using ReHo. For group analysis, four right-side MTLE-HS patients' functional images were flipped, in order to make a homogeneous left MTLE-HS group (10 cases) and increase the sample size.

KEY FINDINGS

Compared to the healthy control group, patients showed significantly increased ReHo in ipsilateral parahippocampal gyrus, midbrain, insula, corpus callosum, bilateral sensorimotor cortex, and frontoparietal subcortical structures, whereas decreased ReHo was observed mainly in default mode network (DMN) (including precuneus and posterior cingulate gyrus, bilateral inferior lateral parietal, and mesial prefrontal cortex) and cerebellum in patients relative to the control group.

SIGNIFICANCE

This study identified that ReHo pattern in mTLE-HS patients was altered compared to healthy controls. We consider decreased ReHo in DMN to be responsible for wide functional impairments in cognitive processes. We propose that the increased ReHo in specific regions may form a network that might be responsible for seizure genesis and propagation.

Impaired consciousness in epilepsy

Consciousness is essential to normal human life. In epileptic seizures consciousness is often transiently lost, which makes it impossible for the individual to experience or respond. These effects have huge consequences for safety, productivity, emotional health, and quality of life. To prevent impaired consciousness in epilepsy, it is necessary to understand the mechanisms that lead to brain dysfunction during seizures. Normally the consciousness system-a specialised set of cortical-subcortical structures-maintains alertness, attention, and awareness. Advances in neuroimaging, electrophysiology, and prospective behavioural testing have shed light on how epileptic seizures disrupt the consciousness system. Diverse seizure types, including absence, generalised tonic-clonic, and complex partial seizures, converge on the same set of anatomical structures through different mechanisms to disrupt consciousness. Understanding of these mechanisms could lead to improved treatment strategies to prevent impairment of consciousness and improve the quality of life of people with epilepsy.

Epilepsy and the consciousness system: transient vegetative state?

Recent advances have shown much in common between epilepsy and other disorders of consciousness. Behavior in epileptic seizures often resembles a transient vegetative or minimally conscious state. These disorders all converge on the "consciousness system" -the bilateral medial and lateral fronto-parietal association cortex and subcortical arousal systems. Epileptic unconsciousness has enormous clinical significance leading to accidental injuries, decreased work and school productivity, and social stigmatization. Ongoing research to better understand the mechanisms of impaired consciousness in epilepsy, including neuroimaging studies and fundamental animal models, will hopefully soon enable treatment trails to reduce epileptic unconsciousness and improve patient quality of life.

The default mode network and altered consciousness in epilepsy

The default mode network has been hypothesized based on the observation that specific regions of the brain are consistently activated during the resting state and deactivated during engagement with task. The primary nodes of this network, which typically include the precuneus/posterior cingulate, the medial frontal and lateral parietal cortices, are thought to be involved in introspective and social cognitive functions. Interestingly, this same network has been shown to be selectively impaired during epileptic seizures associated with loss of consciousness. Using a wide range of neuroimaging and electrophysiological modalities, decreased activity in the default mode network has been confirmed during complex partial, generalized tonic-clonic, and absence seizures. In this review we will discuss these three seizure types and will focus on possible mechanisms by which decreased default mode network activity occurs. Although the specific mechanisms of onset and propagation differ considerably across these seizure types, we propose that the resulting loss of consciousness in all three types of seizures is due to active inhibition of subcortical arousal systems that normally maintain default mode network activity in the awake state. Further, we suggest that these findings support a general “network inhibition hypothesis”, by which active inhibition of arousal systems by seizures in certain cortical regions leads to cortical deactivation in other cortical areas. This may represent a push-pull mechanism similar to that seen operating between cortical networks under normal conditions.

Neocortical thinning in "benign" mesial temporal lobe epilepsy

PURPOSE

In refractory mesial temporal lobe epilepsy (MTLE) extrahippocampal and neocortical abnormalities have been described in patients with or without mesial temporal sclerosis (MTS). Recently we observed gray matter reductions in regions outside the hippocampus in benign MTLE with or without MTS. Cortical thickness has been proposed as a viable methodologic alternative for assessment of neuropathologic changes in extratemporal regions. Herein, we aimed to use this technique to describe cortical abnormalities in patients with benign TLE.

METHODS

Whole-brain cortical thickness analysis (FreeSurfer) was performed in 32 unrelated patients with benign TLE [16 patients with signs of MTS on magnetic resonance imaging (MRI), pMTLE; 16 without, nMTLE] and 44 healthy controls.

KEY FINDINGS

In the pMTLE group, the most significant thinning was found in the sensorimotor cortex bilaterally but was more extensive in the left hemisphere (false discovery rate, p < 0.05). Other areas were localized in the occipital cortex, left supramarginal gyrus, left superior parietal gyrus, left paracentral sulcus, left inferior/middle/superior frontal gyrus, left inferior frontal sulcus, right cingulate cortex, right superior frontal gyrus, right inferior parietal gyrus, right fusiform gyrus, and cuneus/precuneus. In the nMTLE, a similar neurodegenerative pattern was detected, although not surviving correction for multiple comparisons. Direct comparison between pMTLE and nMTLE did not reveal significant changes.

SIGNIFICANCE

Patients with either benign pMTLE or nMTLE showed comparable cortical thinning, mainly confined to the sensorimotor cortex. This finding that is not appreciated on routine MRI supports the hypothesis that similar to refractory MTLE, even in benign MTLE, pathology in neocortical regions maybe implicated in the pathophysiology of this syndrome.

Quantitative multi-compartmental SPECT image analysis for lateralization of temporal lobe epilepsy

This study assesses the utility of compartmental analysis of SPECT data in lateralizing ictal onset in cases of a putative mesial temporal lobe epilepsy (mTLE). An institutional archival review provided 46 patients (18M, 28F) operated for a putative mTLE who achieved an Engel class Ia postoperative outcome. This established the standard to assure a true ictal origin. Ictal and interictal SPECT images were separately coregistered to T1-weighted (T1W) magnetic resonance (MR) image using a rigid transformation and the intensities matched with an l(1) norm minimization technique. The T1W MR image was segmented into separate structures using an atlas-based automatic segmentation technique with the hippocampi manually segmented to improve accuracy. Mean ictal-interictal intensity difference values were calculated for select subcortical structures and the accuracy of lateralization evaluated using a linear classifier. Hippocampal SPECT analysis yielded the highest lateralization accuracy (91%) followed by the amygdala (87%), putamen (67%) and thalamus (61%). Comparative FLAIR and volumetric analyses yielded 89% and 78% accuracies, respectively. A multi-modality analysis did not generate a higher accuracy (89%). A quantitative anatomically compartmented approach to SPECT analysis yields a particularly high lateralization accuracy in the case of mTLE comparable to that of quantitative FLAIR MR imaging. Hippocampal segmentation in this regard correlates well with ictal origin and shows good reliability in the preoperative analysis.

Utility of Ictal Single Photon Emission Computed Tomography in Mesial Temporal Lobe Epilepsy With Hippocampal Atrophy

BACKGROUND:

The development of newer diagnostic technologies has reduced the need for invasive electroencephalographic (EEG) studies in identifying the epileptogenic zone, especially in adult patients with mesial temporal lobe epilepsy and hippocampal sclerosis (MTLE-HS).

OBJECTIVE:

To evaluate ictal single photon emission computed tomography (SPECT) in the evaluation and treatment of patients with MTLE-HS.

METHODS:

MTLE patients were randomly assigned to those with (SPECT, n = 124) and without ictal SPECT (non-SPECT, n = 116) in an intent-to-treat protocol. Primary end points were the proportion of patients with invasive EEG studies, and those offered surgery. Secondary end points were the length of hospital stay and the proportion of patients with secondarily generalized seizures (SGS) during video-EEG, postsurgical seizure outcome, and hospital cost.

RESULTS:

The proportion of patients offered surgery was similar in the SPECT (85%) and non-SPECT groups (81%), as well as the proportion that had invasive EEG studies (27% vs 23%). The mean duration of hospital stay was 1 day longer for the SPECT group (P &lt; 0.001). SGS occurred in 51% of the SPECT and 26% of the non-SPECT group (P &lt; 0.001). The cost of the presurgical evaluation was 35% higher for the SPECT compared with the non-SPECT group (P &lt; 0.001). The proportion of patients seizure-free after surgery was similar in the SPECT (59%) compared with non-SPECT group (54%).

CONCLUSION:

Ictal-SPECT did not add localizing value beyond what was provided by EEG-video telemetry and structural MRI that altered the surgical decision and outcome for MTLE-HS patients. Ictal-SPECT increased hospital stay was associated with increased costs and a higher chance of SGS during video-EEG monitoring. These findings support the notion that a protocol including ictal SPECT is equivalent to one without SPECT in the presurgical evaluation of adult patients with MTLE-HS.

Impaired consciousness in temporal lobe seizures: role of cortical slow activity

Impaired consciousness requires altered cortical function. This can occur either directly from disorders that impair widespread bilateral regions of the cortex or indirectly through effects on subcortical arousal systems. It has therefore long been puzzling why focal temporal lobe seizures so often impair consciousness. Early work suggested that altered consciousness may occur with bilateral or dominant temporal lobe seizure involvement. However, other bilateral temporal lobe disorders do not impair consciousness. More recent work supports a 'network inhibition hypothesis' in which temporal lobe seizures disrupt brainstem-diencephalic arousal systems, leading indirectly to depressed cortical function and impaired consciousness. Indeed, prior studies show subcortical involvement in temporal lobe seizures and bilateral frontoparietal slow wave activity on intracranial electroencephalography. However, the relationships between frontoparietal slow waves and impaired consciousness and between cortical slowing and fast seizure activity have not been directly investigated. We analysed intracranial electroencephalography recordings during 63 partial seizures in 26 patients with surgically confirmed mesial temporal lobe epilepsy. Behavioural responsiveness was determined based on blinded review of video during seizures and classified as impaired (complex-partial seizures) or unimpaired (simple-partial seizures). We observed significantly increased delta-range 1-2 Hz slow wave activity in the bilateral frontal and parietal neocortices during complex-partial compared with simple-partial seizures. In addition, we confirmed prior work suggesting that propagation of unilateral mesial temporal fast seizure activity to the bilateral temporal lobes was significantly greater in complex-partial than in simple-partial seizures. Interestingly, we found that the signal power of frontoparietal slow wave activity was significantly correlated with the temporal lobe fast seizure activity in each hemisphere. Finally, we observed that complex-partial seizures were somewhat more common with onset in the language-dominant temporal lobe. These findings provide direct evidence for cortical dysfunction in the form of bilateral frontoparietal slow waves associated with impaired consciousness in temporal lobe seizures. We hypothesize that bilateral temporal lobe seizures may exert a powerful inhibitory effect on subcortical arousal systems. Further investigations will be needed to fully determine the role of cortical-subcortical networks in ictal neocortical dysfunction and may reveal treatments to prevent this important negative consequence of temporal lobe epilepsy.

New observations may inform seizure models: very fast and very slow oscillations

Epilepsy is a common and disabling disorder which fails to respond to drug treatment in about 30% of cases. The key feature of epilepsy is the apparently unexpected occurrence of seizures, accompanied by typical high-amplitude synchronised oscillations recorded with scalp electroencephalogram (EEG), or occasionally with intracranial EEG in patients undergoing neurosurgery. A wide variety of theoretical and computational models of epilepsy has provided some insights into how seizures might emerge. However, these models have mostly been based on classical EEG phenomena and experimental animal models, and may not capture all the relevant features of human epilepsy. In particular, evidence has accumulated showing that very localised high-frequency oscillations may be characteristic of seizure onsets, and that seizure susceptibility fluctuates at long timescales of hours or days. These aspects are discussed from a cross-disciplinary perspective, with the aim that future collaborations between theoreticians, experimentalists and clinicians may greatly improve seizure models and the future treatment of this devastating disorder.

Imaging in the surgical treatment of epilepsy

Medically refractory focal epilepsy is potentially curable by surgery. This Review considers the application of recent advances in structural and functional brain imaging to increase the number of patients with epilepsy who are treated surgically, and to reduce the risk of complications arising from such intervention. Current optimal MRI of brain structure can identify previously undetectable lesions, with voxel-based and quantitative analyses further increasing the diagnostic yield. If MRI proves unremarkable, PET (with (18)F-fluorodeoxyglucose) and single-photon emission CT of ictal-interictal cerebral blood flow might identify the brain region that contains the epileptic focus. Magnetoencephalography plus simultaneous EEG and functional MRI can map the location of interictal epileptic discharges, thereby facilitating placement of intracranial recording electrodes to define the site of seizure onset. Functional MRI can also lateralize language and localize primary motor, somatosensory and language areas, and shows promise for predicting the effects of temporal lobe resection on memory. Tractography can visualize the main cerebral white matter tracts, thereby predicting and reducing surgery risk. Currently, displays of the optic radiation and pyramidal tracts are the most relevant for epilepsy surgery. Reliable integration of structural and functional data into surgical image-guidance systems is being pursued, and promises safer neurosurgery for epilepsy in the future.

Update on neuroimaging in epilepsy

Neuroimaging in epilepsy is a very large and growing field. Researchers in this area have quickly adopted new methods, resulting in a lively literature. Basic features of common epilepsies are well known, but, outside of the specific area of epilepsy surgery evaluation, new methods evolving in the last few years have had limited new beneficial clinical impact. Here, an overview of the epilepsy neuroimaging literature of the last 5 years, with an emphasis on mesial temporal lobe epilepsy, idiopathic generalized epilepsies, presurgical evaluation and new developments in functional MRI is presented. The need for attention to clinical translation, as well as immediate opportunities and future trends in this field, are discussed.

Brain SPECT in mesial temporal lobe epilepsy: comparison between visual analysis and SPM

OBJECTIVE: To compare the accuracy of SPM and visual analysis of brain SPECT in patients with mesial temporal lobe epilepsy (MTLE). METHOD: Interictal and ictal SPECTs of 22 patients with MTLE were performed. Visual analysis were performed in interictal (VISUAL(inter)) and ictal (VISUAL(ictal/inter)) studies. SPM analysis consisted of comparing interictal (SPM(inter)) and ictal SPECTs (SPM(ictal)) of each patient to control group and by comparing perfusion of temporal lobes in ictal and interictal studies among themselves (SPM(ictal/inter)). RESULTS: For detection of the epileptogenic focus, the sensitivities were as follows: VISUAL(inter)=68%; VISUAL(ictal/inter)=100%; SPM(inter)=45%; SPM(ictal)=64% and SPM(ictal/inter)=77%. SPM was able to detect more areas of hyperperfusion and hypoperfusion. CONCLUSION: SPM did not improve the sensitivity to detect epileptogenic focus. However, SPM detected different regions of hypoperfusion and hyperperfusion and is therefore a helpful tool for better understand pathophysiology of seizures in MTLE.

Current themes in neuroimaging of epilepsy: brain networks, dynamic phenomena, and clinical relevance

Brain scanning methods were first applied in patients with epilepsy more than 30years ago. A very substantial literature now exists in this field, which is exponentially increasing. Contemporary neuroimaging studies in epilepsy reflect new concepts in the epilepsies, as well as current methodological developments. In particular, this area is emphasising the role of networks in epileptogenicity, the existence of dynamic phenomena which can be captured by imaging, and is beginning to validate the implementation of neuroimaging in the clinic. Here, recent studies of the last 5years are reviewed, covering the full range of neuroimaging methods with SPECT, PET and MRI in epilepsy.

Glossopharyngeal Neuralgia Triggered by Non-Noxious Stimuli at Multiple Cephalic and Extracephalic Sites

Glossopharyngeal neuralgia (GN) triggered by non-noxious stimuli at multiple cephalic and extracephalic sites with positron emission tomography (PET) evidence for involvement of the upper brainstem has never been reported. We present such a patient, a 73-year-old man who since the age of 50 had suffered from GN with a high recurrence rate and very severe unilateral, non-familial GN episodes with very easy trigger zones widely extending beyond the n IX territory. Extensive neuroimaging and neurophysiological tests detected no precise underlying cause. PET scan revealed activation in the upper brainstem on extracephalic triggers. Single-fibre electromyography data will be discussed. We hypothesize that deficient inhibition as seen in trigeminal nociceptive reflexes on the level of brainstem interneurons, a functional lesion in the primary somatosensory cortex-sensory thalamic nuclei circuit and the dorsal column-thalamic pathway both activated by light touch may in part be involved in the extra- cephalic triggering.

Cortical deactivation induced by subcortical network dysfunction in limbic seizures

Normal human consciousness may be impaired by two possible routes: direct reduced function in widespread cortical regions or indirect disruption of subcortical activating systems. The route through which temporal lobe limbic seizures impair consciousness is not known. We recently developed an animal model that, like human limbic seizures, exhibits neocortical deactivation including cortical slow waves and reduced cortical cerebral blood flow (CBF). We now find through functional magnetic resonance imaging (fMRI) that electrically stimulated hippocampal seizures in rats cause increased activity in subcortical structures including the septal area and mediodorsal thalamus, along with reduced activity in frontal, cingulate, and retrosplenial cortex. Direct recordings from the hippocampus, septum, and medial thalamus demonstrated fast poly-spike activity associated with increased neuronal firing and CBF, whereas frontal cortex showed slow oscillations with decreased neuronal firing and CBF. Stimulation of septal area, but not hippocampus or medial thalamus, in the absence of a seizure resulted in cortical deactivation with slow oscillations and behavioral arrest, resembling changes seen during limbic seizures. Transecting the fornix, the major route from hippocampus to subcortical structures, abolished the negative cortical and behavioral effects of seizures. Cortical slow oscillations and behavioral arrest could be reconstituted in fornix-lesioned animals by inducing synchronous activity in the hippocampus and septal area, implying involvement of a downstream region converged on by both structures. These findings suggest that limbic seizures may cause neocortical deactivation indirectly, through impaired subcortical function. If confirmed, subcortical networks may represent a target for therapies aimed at preserving consciousness in human temporal lobe seizures.

Remote effects of hippocampal damage on default network connectivity in the human brain

In the healthy human brain the hippocampus is known to work in concert with a variety of cortical brain regions. It has recently been linked to the default network of the brain, with the precuneus being its core hub. Here we studied the remote effects of damage to the hippocampus on functional connectivity patterns of the precuneus. From 14 epilepsy patients with selective, unilateral hippocampal sclerosis and 8 healthy control subjects, we acquired functional MRI data during performance of an object-location memory task. We assessed functional connectivity of a functionally defined region in the precuneus, which showed the typical properties of the default network: significant task-related deactivation, which was reduced in patients compared to control subjects. In control subjects, a largely symmetrical pattern of functional coherence to the precuneus emerged, including canonical default network areas such as ventral medial prefrontal cortex, inferior parietal cortex, and the hippocampi. Assessment of group differences within the default network areas revealed reduced connectivity to the precuneus in ipsilesional middle temporal gyrus and hippocampus in left hippocampal sclerosis patients compared to controls. Furthermore, left hippocampal sclerosis patients showed lower connectivity than right hippocampal sclerosis patients in left middle temporal gyrus, ventral medial prefrontal cortex, and left amygdala. We report remote effects of unilateral hippocampal damage on functional connectivity between distant brain regions associated with the default network of the human brain. These preliminary results underline the impact of circumscribed pathology on functionally connected brain regions.

SPM analysis of ictal-interictal SPECT in mesial temporal lobe epilepsy: relationships between ictal semiology and perfusion changes

A combination of temporo-limbic hyperperfusion and extratemporal hypoperfusion was observed during complex partial seizures (CPS) in temporal lobe epilepsy (TLE). To investigate the clinical correlate of perfusion changes in TLE, we analyzed focal seizures of increasing severity using voxel-based analysis of ictal SPECT. We selected 26 pre-operative pairs of ictal-interictal SPECTs from adult mesial TLE patients, seizure-free after surgery. Ictal SPECTs were classified in three groups: motionless seizures (group ML, n=8), seizures with motor automatisms (MA) without dystonic posturing (DP) (group MA, n=8), and seizures with DP with or without MA (DP, n=10). Patients of group ML had simple partial seizures (SPS), while others had CPS. Groups of ictal-interictal SPECT were compared to a control group using statistical parametric mapping (SPM). In ML group, SPM analysis failed to show significant changes. Hyperperfusion involved the anteromesial temporal region in MA group, and also the insula, posterior putamen and thalamus in DP group. Hypoperfusion was restricted to the posterior cingulate and prefrontal regions in MA group, and involved more widespread associative anterior and posterior regions in DP group. Temporal lobe seizures with DP show the most complex pattern of combined hyper-hypoperfusion, possibly related both to a larger spread and the recruitment of more powerful inhibitory processes.

The importance of latency in the focality of perfusion and oxygenation changes associated with triggered afterdischarges in human cortex

The spatiotemporal dynamics of neurovascular coupling during epilepsy are not well understood, and there are little data from studies of the human brain. We investigated changes in total hemoglobin (Hbt) and hemoglobin oxygenation in patients undergoing epilepsy surgery with intraoperative intrinsic optical spectroscopy (IOS) during triggered afterdischarges (ADs). We found an early (approximately 0.5 secs) focal dip in hemoglobin oxygenation, arising precisely in the stimulated gyrus that lasted for 11.5+/-10.0 secs, approximately the length of the AD (10.4+/-4.4 secs). A later oxygen overshoot and increase in blood volume occurred in the adjacent surrounding gyri. After a significant delay (approximately 20 to 30 secs), the overshoot and blood volume signal became extremely focal to the area of the onset of the AD. A smaller very late undershoot, the last phase of the 'triphasic' response, was also identified, although localization was inconsistent. In this study, we show that a 'late focal overshoot' and late Hbt signal may be extremely useful, in addition to the early dip, for the localization of seizure onset. It is likely that a separate mechanism underlies the persistent focal increase in cerebral blood volume after a long-duration cortical stimulation, compared with the nonspecific mechanism that causes the initial increase in cerebral blood flow.

Clinical use of ictal SPECT in secondarily generalized tonic-clonic seizures

Partial seizures produce increased cerebral blood flow in the region of seizure onset. These regional cerebral blood flow increases can be detected by single photon emission computed tomography (ictal SPECT), providing a useful clinical tool for seizure localization. However, when partial seizures secondarily generalize, there are often questions of interpretation since propagation of seizures could produce ambiguous results. Ictal SPECT from secondarily generalized seizures has not been thoroughly investigated. We analysed ictal SPECT from 59 secondarily generalized tonic-clonic seizures obtained during epilepsy surgery evaluation in 53 patients. Ictal versus baseline interictal SPECT difference analysis was performed using ISAS (http://spect.yale.edu). SPECT injection times were classified based on video/EEG review as either pre-generalization, during generalization or in the immediate post-ictal period. We found that in the pre-generalization and generalization phases, ictal SPECT showed significantly more regions of cerebral blood flow increases than in partial seizures without secondary generalization. This made identification of a single unambiguous region of seizure onset impossible 50% of the time with ictal SPECT in secondarily generalized seizures. However, cerebral blood flow increases on ictal SPECT correctly identified the hemisphere (left versus right) of seizure onset in 84% of cases. In addition, when a single unambiguous region of cerebral blood flow increase was seen on ictal SPECT, this was the correct localization 80% of the time. In agreement with findings from partial seizures without secondary generalization, cerebral blood flow increases in the post-ictal period and cerebral blood flow decreases during or following seizures were not useful for localizing seizure onset. Interestingly, however, cerebral blood flow hypoperfusion during the generalization phase (but not pre-generalization) was greater on the side opposite to seizure onset in 90% of patients. These findings suggest that, with appropriate cautious interpretation, ictal SPECT in secondarily generalized seizures can help localize the region of seizure onset.

Theories of impaired consciousness in epilepsy

Although the precise mechanisms for control of consciousness are not fully understood, emerging data show that conscious information processing depends on the activation of certain networks in the brain and that the impairment of consciousness is related to abnormal activity in these systems. Epilepsy can lead to transient impairment of consciousness, providing a window into the mechanisms necessary for normal consciousness. Thus, despite differences in behavioral manifestations, cause, and electrophysiology, generalized tonic-clonic, absence, and partial seizures engage similar anatomical structures and pathways. We review prior concepts of impaired consciousness in epilepsy, focusing especially on temporal lobe complex partial seizures, which are a common and debilitating form of epileptic unconsciousness. We discuss a "network inhibition hypothesis" in which focal temporal lobe seizure activity disrupts normal cortical-subcortical interactions, leading to depressed neocortical function and impaired consciousness. This review of the major prior theories of impaired consciousness in epilepsy allows us to put more recent data into context and to reach a better understanding of the mechanisms important for normal consciousness.

Widespread neocortical abnormalities in temporal lobe epilepsy with and without mesial sclerosis

PURPOSE

Extrafocal structural abnormalities have been consistently described in temporal lobe epilepsy (TLE) with mesial temporal lobe sclerosis (TLE-MTS). In TLE without MTS (TLE-no) extrafocal abnormalities are more subtle and often require region of interest analyses for their detection. Cortical thickness measurements might be better suited to detect such subtle abnormalities than conventional whole brain volumetric techniques which are often negative in TLE-no. The aim of this study was to seek and characterize patterns of cortical thinning in TLE-MTS and TLE-no.

METHODS

T1 weighted whole brain images were acquired on a 4 T magnet in 66 subjects (35 controls, 15 TLE-MTS, 16 TLE-no). Cortical thickness measurements were obtained using the FreeSurfer software routine. Group comparisons and correlation analyses were done using the statistical routine of FreeSurfer (FDR, p=0.05).

RESULTS

TLE-MTS and TLE-no showed both widespread temporal and extratemporal cortical thinning. In TLE-MTS, the inferior medial and posterior temporal regions were most prominently affected while lateral temporal and opercular regions were more affected in TLE-no. The correlation analysis showed a significant correlation between the ipsilateral hippocampal volume and regions of thinning in TLE-MTS and between inferior temporal cortical thickness and thinning in extratemporal cortical regions in TLE-no.

CONCLUSION

The pattern of thinning in TLE-no was different from the pattern in TLE-MTS. This finding suggests that different epileptogenic networks could be involved in TLE-MTS and TLE and further supports the hypothesis that TLE-MTS and TLE-no might represent two distinct TLE syndromes.

Consciousness and epilepsy: why are complex-partial seizures complex?

Why do complex-partial seizures in temporal lobe epilepsy (TLE) cause a loss of consciousness? Abnormal function of the medial temporal lobe is expected to cause memory loss, but it is unclear why profoundly impaired consciousness is so common in temporal lobe seizures. Recent exciting advances in behavioral, electrophysiological, and neuroimaging techniques spanning both human patients and animal models may allow new insights into this old question. While behavioral automatisms are often associated with diminished consciousness during temporal lobe seizures, impaired consciousness without ictal motor activity has also been described. Some have argued that electrographic lateralization of seizure activity to the left temporal lobe is most likely to cause impaired consciousness, but the evidence remains equivocal. Other data correlates ictal consciousness in TLE with bilateral temporal lobe involvement of seizure spiking. Nevertheless, it remains unclear why bilateral temporal seizures should impair responsiveness. Recent evidence has shown that impaired consciousness during temporal lobe seizures is correlated with large-amplitude slow EEG activity and neuroimaging signal decreases in the frontal and parietal association cortices. This abnormal decreased function in the neocortex contrasts with fast polyspike activity and elevated cerebral blood flow in limbic and other subcortical structures ictally. Our laboratory has thus proposed the "network inhibition hypothesis," in which seizure activity propagates to subcortical regions necessary for cortical activation, allowing the cortex to descend into an inhibited state of unconsciousness during complex-partial temporal lobe seizures. Supporting this hypothesis, recent rat studies during partial limbic seizures have shown that behavioral arrest is associated with frontal cortical slow waves, decreased neuronal firing, and hypometabolism. Animal studies further demonstrate that cortical deactivation and behavioral changes depend on seizure spread to subcortical structures including the lateral septum. Understanding the contributions of network inhibition to impaired consciousness in TLE is an important goal, as recurrent limbic seizures often result in cortical dysfunction during and between epileptic events that adversely affects patients' quality of life.

Remote effects of focal hippocampal seizures on the rat neocortex

Seizures have both local and remote effects on nervous system function. Whereas propagated seizures are known to disrupt cerebral activity, little work has been done on remote network effects of seizures that do not propagate. Human focal temporal lobe seizures demonstrate remote changes including slow waves on electroencephalography (EEG) and decreased cerebral blood flow (CBF) in the neocortex. Ictal neocortical slow waves have been interpreted as seizure propagation; however, we hypothesize that they reflect a depressed cortical state resembling sleep or coma. To investigate this hypothesis, we performed multimodal studies of partial and secondarily generalized limbic seizures in rats. Video/EEG monitoring of spontaneous seizures revealed slow waves in the frontal cortex during behaviorally mild partial seizures, contrasted with fast polyspike activity during convulsive generalized seizures. Seizures induced by hippocampal stimulation produced a similar pattern, and were used to perform functional magnetic resonance imaging weighted for blood oxygenation and blood volume, demonstrating increased signals in hippocampus, thalamus and septum, but decreases in orbitofrontal, cingulate, and retrosplenial cortex during partial seizures, and increases in all of these regions during propagated seizures. Combining these results with neuronal recordings and CBF measurements, we related neocortical slow waves to reduced neuronal activity and cerebral metabolism during partial seizures, but found increased neuronal activity and metabolism during propagated seizures. These findings suggest that ictal neocortical slow waves represent an altered cortical state of depressed function, not propagated seizure activity. This remote effect of partial seizures may cause impaired cerebral functions, including loss of consciousness.

[Epileptogenic and non-epileptogenic zones: blood flow studies of temporo-limbic seizures]

To assess the contribution of ictal SPECT to the definition of the epileptogenic zone (EZ) prior to surgery in focal drug-resistant epilepsies, we investigated the effect of the timing of injection and seizure semiology on patterns of perfusion and cerebral blood flow changes (CBF) beyond the EZ. In the rat model of amygdala-kindled seizures, we measured CBF changes with the quantitative [(14)C]-iodoantipyrine autoradiographic method during secondary generalized (SGS, n=26 fully-kindled rats) and focal seizures (FS, n=19 partially kindled rats), according to sequential timing of injection with respect to seizure onset. During SGS, the correct lateralization and rough localization of the focus within limbic structures was only possible at the early ictal and post-ictal times, in between we observed widespread rCBF increases. The switch from hyper to hypoperfusion occurred at the time of late ictal injection. The accurate localization of the EZ was obtained in the study of the more subtle FS (stage 0). At stage 1 of the kindling, there was already a remote widespread spreading of hyperperfusion. In patients surgically cured from a mesio-temporal lobe epilepsy (mean post-operative follow-up: 66 months), we retrospectively studied 26 pairs of ictal and interictal pre-operative SPECTs, classified in 3 groups according to the progression of ictal semiology. Using visual analysis of subtracted SPECTs (SISCOM) and group comparisons with a control group (using SPM), we observed more widespread combined hyper and hypoperfusion with the increasing complexity of seizures. In simple partial seizures, the SISCOM analysis allowed a correct localization of the focus in 4/8 patients, whereas the SPM analysis failed to detect significant changes, due to individual variation, spatial normalization and small magnitude of CBF changes. In complex partial seizures with automatisms, SISCOM and SPM analysis showed antero-mesial temporal hyperperfusion (overlapping the EZ), extending to the insula, basal ganglia, and thalamus in the group of patients having dystonic posturing (DP group) in addition to automatisms. Ictal hypoperfusion involved pre-frontal and parietal regions, the anterior and posterior cingulate gyri, to a greater extent in the DP group. In both human and animals studies, we observed a correlation between the extent of composite patterns of hyper/hypoperfusion and the severity of seizures, and the recruitment of remote sub-cortical structures. Hypoperfused areas belong to neural networks involved in perceptual decision making and motor planning, whose transient disruption could support purposeless actions, i.e. motor automatisms.

Regional neocortical thinning in mesial temporal lobe epilepsy

PURPOSE

To determine the nature and extent of regional cortical thinning in patients with mesial temporal lobe epilepsy (MTLE).

METHODS

High-resolution volumetric MRIs were obtained on 21 patients with MTLE and 21 controls. Mean cortical thickness was measured within regions of interest and point-by-point across the neocortex using cortical reconstruction and parcellation software.

RESULTS

Bilateral thinning was observed within frontal and lateral temporal regions in MTLE patients relative to controls. The most striking finding was bilateral cortical thinning in the precentral gyrus and immediately adjacent paracentral region and pars opercularis of the inferior frontal gyrus, extending to the orbital region. Within the temporal lobe, bilateral thinning was observed in Heschl's gyrus only. Ipsilateral only thinning was observed in the superior and middle temporal gyri, as well as in the medial orbital cortex. Greater asymmetries in cortical thickness were observed in medial temporal cortex in patients relative to controls. Individual subject analyses revealed that this asymmetry reflected significant ipsilateral thinning of medial temporal cortex in 33% of patients, whereas it reflected ipsilateral thickening in 20% of MTLEs.

DISCUSSION

Patients with MTLE show widespread, bilateral pathology in neocortical regions that is not appreciated on standard imaging. Future studies are needed that elucidate the clinical implications of neocortical thinning in MTLE.

Temporal lobe interictal epileptic discharges affect cerebral activity in "default mode" brain regions

A cerebral network comprising precuneus, medial frontal, and temporoparietal cortices is less active both during goal-directed behavior and states of reduced consciousness than during conscious rest. We tested the hypothesis that the interictal epileptic discharges affect activity in these brain regions in patients with temporal lobe epilepsy who have complex partial seizures. At the group level, using electroencephalography-correlated functional magnetic resonance imaging in 19 consecutive patients with focal epilepsy, we found common decreases of resting state activity in 9 patients with temporal lobe epilepsy (TLE) but not in 10 patients with extra-TLE. We infer that the functional consequences of TLE interictal epileptic discharges are different from those in extra-TLE and affect ongoing brain function. Activity increases were detected in the ipsilateral hippocampus in patients with TLE, and in subthalamic, bilateral superior temporal and medial frontal brain regions in patients with extra-TLE, possibly indicating effects of different interictal epileptic discharge propagation.

Autoradiography reveals selective changes in serotonin binding in neocortex of patients with temporal lobe epilepsy

The main goal of the present study was to evaluate binding to serotonin in the neocortex surrounding the epileptic focus of patients with mesial temporal lobe epilepsy (MTLE). Binding to 5-HT, 5-HT(1A), 5-HT(4), 5-HT(7) receptors and serotonin transporter (5-HTT) in T1-T2 gyri of 15 patients with MTLE and their correlations with clinical data, neuronal count and volume were determined. Autopsy material acquired from subjects without epilepsy (n=6) was used as control. The neocortex from MTLE patients demonstrated decreased cell count in layers III-IV (21%). No significant changes were detected on the neuronal volume. Autoradiography experiments showed the following results: reduced 5-HT and 5-HT(1A) binding in layers I-II (24% and 92%, respectively); enhanced 5-HT(4) binding in layers V-VI (32%); no significant changes in 5-HT(7) binding; reduced 5-HTT binding in all layers (I-II, 90.3%; III-IV, 90.3%, V-VI, 86.9%). Significant correlations were found between binding to 5-HT(4) and 5-HT(7) receptors and age of seizure onset, duration of epilepsy and duration of antiepileptic treatment. The present results support an impaired serotoninergic transmission in the neocortex surrounding the epileptic focus of patients with MTLE, a situation that could be involved in the initiation and propagation of seizure activity.

The precuneus and consciousness

This article reviews the rapidly growing literature on the functional anatomy and behavioral correlates of the precuneus, with special reference to imaging neuroscience studies using hamodynamic techniques. The precuneus, along with adjacent areas within the posteromedial parietal cortex, is among the most active cortical regions according to the "default mode" of brain function during the conscious resting state, whereas it selectively deactivates in a number of pathophysiological conditions (ie, sleep, vegetative state, drug-induced anesthesia), and neuropsychiatric disorders (ie, epilepsy, Alzheimer's disease, and schizophrenia) characterized by impaired consciousness. These findings, along with the widespread connectivity pattern, suggest that the precuneus may play a central role in the neural network correlates of consciousness. Specifically, its activity seems to correlate with self-reflection processes, possibly involving mental imagery and episodic/autobiographical memory retrieval.

Ictal hyperperfusion patterns in relation to ictal scalp EEG patterns in patients with unilateral hippocampal sclerosis: a SPECT study

PURPOSE

The aims of the present study were to explore the relation between ictal scalp EEG patterns and ictal hyperperfusion patterns in patients with unilateral hippocampal sclerosis-associated mesial temporal lobe epilepsy (HS-MTLE) by using semiquantitative single-photon emission computed tomography (SPECT) analysis and to assess clinical significance of ictal hyperperfusion patterns.

METHODS

We studied retrospectively 39 consecutive patients with surgically proven HS-MTLE. All had both interictal and ictal SPECTs with the tracer injection during a complex partial seizure (CPS) typical of MTLE semiology. According to initial ictal discharge (IID) frequency on scalp EEG, two lateralizing patterns were identified: (a) a sustained regular 5- to 9-Hz rhythm with a restricted temporal or subtemporal distribution (group 1); and (b) an irregular 2- to 5-Hz rhythm with a widespread distribution (group 2). We performed group analysis by using statistical parametric mapping (SPM) of paired ictal-interictal SPECTs to identify regions of significant ictal hyperperfusion and compared clinical characteristics, tracer-injection time, semiology, pathologic HS grade, and surgical outcome between two groups.

RESULTS

Of the 39 patients, 19 patients (10 males, nine right HS) were designated as group 1, and the remaining 20 patients (eight males, seven right HS), group 2. Group 1 showed hyperperfusion mainly confined to the ipsilateral temporal lobe, whereas group 2 showed widespread hyperperfusion in the extratemporal structures such as ipsilateral basal ganglia, brainstem, and bilateral thalamus, in addition to the ipsilateral temporal lobe. No significant difference was found between two groups in clinical characteristics, injection time, pathologic HS grade, and surgical outcome. Among semiologic features, dystonic limb posturing was more frequently observed in group 2 (p = 0.006).

CONCLUSIONS

Scalp EEG IID frequency in HS-MTLE can be an important determining factor of ictal hyperperfusion patterns. The lack of difference in surgical outcome between two groups implies that different hyperperfusion patterns, according to their IID frequencies, reflect only preferential pathways of ictal propagation rather than intrinsic epileptogenic region.