Changes in cerebral blood flow velocities during childhood absence seizures

Ocular Perfusion Characteristics of Children with Newly Diagnosed Epilepsy

BACKGROUND

To investigate the vascular changes of the optic nerve head(ONH) and macula by using optical coherence tomography angiography(OCT-A), and also the choroidal vascular structure by using an image binarization tool in children with newly diagnosed epilepsy and to then compare these parameters with healthy subjects.

METHODS

Forty-one epilepsy children and 36 healthy controls were included in this prospective and cross-sectional study.The radial peripapillary capillary(RPC) vessel density(VD) and macular foveal,parafoveal,perifoveal of superficial capillary plexus(SCP),deep capillary plexus(DCP) and choriocapillaris(CC) VD, and CC flow area were analyzed.Enhanced depth imaging(EDI) OCT scans of the macula were obtained and the images were binarized using the ImageJ software (National Institutes of Health, Bethesda, MD, USA).The subfoveal choroidal thickness(SFCT),the area of choroidal, luminal, and interstitial and the percentage of luminal area in the choroid(Choroidal vascular index (CVI)) were analyzed.We also evaluated the thickness of the peripapillary retinal nerve fiber layer(RNFL),the macular ganglion cell layer(GCL), and the inner plexiform layer(IPL).

RESULTS

There was a significant decrease in the VD of the CC and the CC flow area in children with epilepsy compared to healthy subjects(p<0.05).However, the VD of the RPC, and of the SCP and DCP of the macula were similar between the two groups(p>0.05).The SFCT,choroidal area,luminal area, and CVI were statistically significant lower in children with newly diagnosed epilepsy compared to healthy subjects.

CONCLUSION

This study has demonstrated that the choroidal perfusion from the microcirculation is lower in children with newly diagnosed epilepsy.The pathophysiology of epilepsy and neurodegenerative processes may also include this vascular dysfunction as one of the factors.

Current advances in childhood absence epilepsy

BACKGROUND

Childhood absence epilepsy is an age-dependent, idiopathic, generalized epilepsy with a characteristic seizure appearance. The disorder is likely to be multifactorial, resulting from interactions between genetic and acquired factors, but the debate is still open. We review recent studies on different aspects of childhood absence epilepsy and also to describe new concepts.

METHODS

Data for this review were identified using Medline and PubMed survey to locate studies dealing with childhood absence epilepsy. Searches included articles published between 1924 and 2013.

RESULTS

The diagnosis comprises predominant and associated seizure types associated with other clinical and electroencephalographic characteristics. Many studies have challenged the prevailing concepts, particularly with respect to the pathophysiological mechanisms underlying the electroencephalographic seizure discharges. Childhood absence epilepsy fits the definition of system epilepsy as a condition resulting from the persisting susceptibility of the thalamocortical system as a whole to generate seizures. This syndrome, if properly defined using strict diagnostic criteria, has a good prognosis. In some cases, it may affect multiple cognitive functions determining risk for academic and functional difficulties; the detection of children at risk allows tailored interventions. Childhood absence epilepsy is usually treated with ethosuximide, valproate, lamotrigine, or levetiracetam, but the most efficacious and tolerable initial empirical treatment has not been well defined.

CONCLUSIONS

We review recent studies and new concepts on the electroclinical features and pathophysiological findings of childhood absence epilepsy in order to highlight areas of consensus as well as areas of uncertainty that indicate directions for future research.

Neurovascular coupling and epilepsy: hemodynamic markers for localizing and predicting seizure onset

Hemodynamic surrogates of epileptic activity are being used to map epileptic foci with PET, SPECT, and fMRI. However, there are few studies of neurovascular coupling in epilepsy. Recent data indicate that cerebral blood flow, although focally increased at the onset of a seizure, may be temporarily inadequate to meet the metabolic demands of both interictal and ictal epileptic events. Transient focal tissue hypoxia and hyperperfusion may be excellent markers for the epileptic focus and may even precede the onset of the ictal event.

Lateralization of cerebral blood flow in juvenile absence seizures

Cerebrovascular blood flow in absence seizures and flow patterns during the ictal period have not been thoroughly investigated. We aimed to evaluate cerebral blood flow changes in typical juvenile absence seizures during the ictal and postictal phases. Seizures were recorded in three patients (mean age: 21 +/- 1 years) with multiple daily typical absence seizures. Simultaneous video electroencephalography and bilateral middle cerebral artery transcranial Doppler ultrasonography recordings were conducted during seizures. Basal, ictal, and postictal blood flow velocities were recorded bilaterally, and offline analyses were performed in relation with generalized spike and wave discharges. Total of 43 seizures were recorded. Ictal increase and postictal decrease of cerebral blood flow velocities were significant for both sides (P < 0.001). The interhemispheric asymmetry in the ictal velocity increase was significant (P < 0.05). The interhemispheric asymmetry in the postictal velocity decrease was not significant (P > 0.05). The blood flow velocity increase after seizure onset indicates a vascular coupling mechanism. A sudden and then a gradual decrease in blood flow velocity, which lasted even after the seizure ceased, might suggest a preventive mechanism to avoid excessive seizure duration or even an absence status epilepticus. Significant asymmetries in increase and a symmetrical decrease may support the cortical focus theory.

Functional neuroimaging of spike-wave seizures

Generalized spike-wave seizures are typically brief events associated with dynamic changes in brain physiology, metabolism, and behavior. Functional magnetic resonance imaging (fMRI) provides a relatively high spatiotemporal resolution method for imaging cortical-subcortical network activity during spike-wave seizures. Patients with spike-wave seizures often have episodes of staring and unresponsiveness which interfere with normal behavior. Results from human fMRI studies suggest that spike-wave seizures disrupt specific networks in the thalamus and frontoparietal association cortex which are critical for normal attentive consciousness. However, the neuronal activity underlying imaging changes seen during fMRI is not well understood, particularly in abnormal conditions such as seizures. Animal models have begun to provide important fundamental insights into the neuronal basis for fMRI changes during spike-wave activity. Work from these models including both fMRI and direct neuronal recordings suggest that, in humans, specific cortical-subcortical networks are involved in spike-wave, while other regions are spared. Regions showing fMRI increases demonstrate correlated increases in neuronal activity in animal models. The mechanisms of fMRI decreases in spike-wave will require further investigation. A better understanding of the specific brain regions involved in generating spike-wave seizures may help guide efforts to develop targeted therapies aimed at preventing or reversing abnormal excitability in these brain regions, ultimately leading to a cure for this disorder.

Focal Increases in Perfusion and Decreases in Hemoglobin Oxygenation Precede Seizure Onset in Spontaneous Human Epilepsy

PURPOSE

Optical recording of intrinsic signals provides the highest combined spatial and temporal resolution with broad spatial sampling for measuring cerebral blood volume (CBV) and hemoglobin oxygenation in cerebral cortex. Few opportunities arise to apply this laboratory method to record spontaneous seizures in unanesthetized human brain during neurosurgery. We report such a rare opportunity in a man with recurrent focal epilepsy arising from a cavernous malformation.

METHODS

We recorded intrinsic optical signals (IOS) from human cortex intraoperatively during spontaneous seizures arising from brain surrounding a small cavernous malformation in an awake patient using only local anesthesia with simultaneous electrocorticography. The IOS was recorded at two wavelengths, one an isosbestic point for hemoglobin to measure CBV (570 nm) and the other at a wavelength more sensitive to deoxygenated hemoglobin (Hbr) (610 nm). A modified Beer-Lambert calculation was used on two separate but similar seizures to approximate changes in Hbr, CBV as well as oxygenated hemoglobin (HbO(2)).

RESULTS

Electrographically recorded seizures (n = 3) elicited a focal increase in both Hbr and CBV that lasted for the duration of the seizure, indicating that perfusion was inadequate to meet metabolic demand. Remarkably, these hemodynamic changes preceded the onset of the seizures by approximately 20 s and occurred focally over the known location of the lesion and the seizure onsets.

DISCUSSION

These findings demonstrate that the hemoglobin becomes deoxygenated in spite of large increase in CBV during spontaneous human focal seizures and that optically recorded hemodynamic events can be used both to predict and localize human focal epilepsy. Such data may someday be useful to assist in the presurgical evaluation of patients considered for epilepsy surgery and to predict the timing and location of seizure onsets.

Coupled oxygenation oscillation measured by NIRS and intermittent cerebral activation on EEG in premature infants

Electroencephalography of premature neonates shows a physiological discontinuity of electrical activity during quiet sleep. Near infrared spectroscopy (NIRS) shows spontaneous oscillations of hemoglobin oxygenation and volume. Similar oscillations are visible in term neonates and adults, with NIRS and other functional imaging techniques (fMRI, Doppler, etc.), but are generally thought to result from vasomotion and to be a physiological artifact of limited interest. The origin and possible relationship to neuronal activity of the baseline changes in the NIRS signal have not been established. We carried out simultaneous EEG-NIRS recordings on six healthy premature neonates and four premature neonates presenting neurological distress, to determine whether changes in the concentration of cerebral oxy- and deoxy- and total hemoglobin were related to the occurrence of spontaneous bursts of cerebral electric activity. Bursts of electroencephalographic activity in neonates during quiet sleep were found to be coupled to a transient stereotyped hemodynamic response involving a decrease in oxy-hemoglobin concentration, sometimes beginning a few seconds before the onset of electroencephalographic activity, followed by an increase, and then a return to baseline. This pattern could be either part of the baseline oscillations or superimposed changes to this baseline, influencing its shape and phase. The temporal patterns of NIRS parameters present an unique configuration, and tend to be different between our healthy and pathological subjects. Studies of physiological activities and of the effects of intrinsic regulation on the NIRS signal should increase our understanding of these patterns and EEG-NIRS studies should facilitate the integration of NIRS into the set of clinical tools used in neurology.

EEG–fMRI of idiopathic and secondarily generalized epilepsies

We used simultaneous EEG and functional MRI (EEG-fMRI) to study generalized spike wave activity (GSW) in idiopathic and secondary generalized epilepsy (SGE). Recent studies have demonstrated thalamic and cortical fMRI signal changes in association with GSW in idiopathic generalized epilepsy (IGE). We report on a large cohort of patients that included both IGE and SGE, and give a functional interpretation of our findings. Forty-six patients with GSW were studied with EEG-fMRI; 30 with IGE and 16 with SGE. GSW-related BOLD signal changes were seen in 25 of 36 individual patients who had GSW during EEG-fMRI. This was seen in thalamus (60%) and symmetrically in frontal cortex (92%), parietal cortex (76%), and posterior cingulate cortex/precuneus (80%). Thalamic BOLD changes were predominantly positive and cortical changes predominantly negative. Group analysis showed a negative BOLD response in the cortex in the IGE group and to a lesser extent a positive response in thalamus. Thalamic activation was consistent with its known role in GSW, and its detection in individual cases with EEG-fMRI may in part be related to the number and duration of GSW epochs recorded. The spatial distribution of the cortical fMRI response to GSW in both IGE and SGE involved areas of association cortex that are most active during conscious rest. Reduction of activity in these regions during GSW is consistent with the clinical manifestation of absence seizures.

Brain imaging in idiopathic generalized epilepsies

While it is generally accepted that there is no neuroimaging abnormality in idiopathic generalized epilepsy (IGE), image processing and quantitative magnetic resonance imaging (MRI) studies suggest that there may be subtle structural abnormalities. Magnetic resonance spectroscopy indicates neuronal dysfunction with differing abnormalities in the IGE subsyndromes, and high concentrations of glutamate and glutamine have been inferred in the frontal lobes, and low gamma-aminobutyric acid levels in the occipital lobe. Studies of cerebral blood distribution at the time of absences have given complex results. The general consensus is of an increase in the thalamus and broad decreases in the neocortex, reflecting a suppression of neuronal activity, but with some increases, that may indicate focal areas of activation. Positron emission tomography (PET) ligand studies with an opioid tracer have inferred neocortical release of endogenous opioids at the time of serial absences. In combination with neurophysiological methods, PET studies with specific ligands have the potential to clarify the functional anatomy and neurochemical circuits that underlie IGE.

A unifying concept of seizure onset and termination

Recent discoveries in molecular biology and human genetics have contributed greatly to an understanding of the nature of seizure (ictal) activity. However, two questions of fundamental clinical importance continue to resist scientific inquiry: when and why does a seizure begin; and when and why does a seizure end? This paper cites evidence from the medical literature in support of two counterintuitive concepts that address this issue. First, that despite the diversity of conditions that are associated with seizures, the ictal response results from disturbances of a mitochondrial metabolic pathway that is common to them all. Second, that the seizure is not inherently harmful but is, instead, associated with massive intracerebral circulatory changes that are intended to restore impaired mitochondrial function. We hypothesize that the protogenic pathophysiological condition leading to neuronal hyperexitability and seizures results from inadequate mitochondrial energy production due to hypoxia or a hypoxia-equivalent state. Failure to generate sufficient adenosine triphosphate compromises ionic pump function and the ability to maintain neuronal homeostasis and stability. The seizure cascade is a heroic effort to perfuse the brain when local mechanisms fail to restore energy production and ionic equilibrium. In summary, a seizure starts when the neuron's aerobic machinery fails to maintain effective ionic pump function and terminates when increased cerebral perfusion, associated with the seizure response, restores adequate supplies of metabolic nutrients required for mitochondrial respiration. This unorthodox unifying concept that views ictogenesis as part of a restorative process rather than as a life threatening event may provide the basis for a much needed paradigm shift in the management of seizures. Current antiepileptic drugs are associated with many serious side effects, including death, and fail to control seizures in 20% of patients with primary generalized epilepsy and 35% of patients with partial epilepsy. We propose that efforts to prevent and control seizures should be directed away from pharma-chemical suppression towards removing the causes of disturbed neuronal energy production and developing methods and bioactive agents that promote an optimized physiological milieu within the brain.

Imaging idiopathic generalized epilepsy

It has been traditionally held that there is no radiological abnormality in patients with idiopathic generalized epilepsy (IGE). Sophisticated image processing and quantitative magnetic resonance imaging (MRI) studies suggest that, in some cases, there may be a subtle structural abnormality. Magnetic resonance spectroscopy indicates neuronal dysfunction with differing patterns of abnormality in the IGE sub-syndromes, and high levels of glutamate and glutamine have been suggested in the frontal lobes, and low GABA levels in the occipital lobe. Studies reflecting cerebral blood flow at the time of absences have given complex results. The principal consensus is of an increase in the thalamus and broad decreases in the neocortex, reflecting a suppression of neuronal activity, but with the possibility of some increases, that could perhaps reflect focal areas of neuronal activation. PET ligand studies with an opioid tracer have implied neocortical release of endogenous opioids at the time of serial absences. In combination with high time-resolution neurophysiological methods, static and dynamic PET studies with specific ligands have the potential to elucidate the functional anatomy and neurochemical circuits that under-lie IGE.

Dynamic fMRI and EEG recordings during spike-wave seizures and generalized tonic-clonic seizures in WAG/Rij rats

Generalized epileptic seizures produce widespread physiological changes in the brain. Recent studies suggest that "generalized" seizures may not involve the whole brain homogeneously. For example, electrophysiological recordings in WAG/Rij rats, an established model of human absence seizures, have shown that spike-and-wave discharges are most intense in the perioral somatosensory cortex and thalamus, but spare the occipital cortex. Is this heterogeneous increased neuronal activity matched by changes in local cerebral blood flow sufficient to meet or exceed cerebral oxygen consumption? To investigate this, we performed blood oxygen level-dependent functional magnetic resonance imaging (fMRI) measurements at 7T with simultaneous electroencephalogram recordings. During spontaneous spike-wave seizures in WAG/Rij rats under fentanylhaloperidol anesthesia, we found increased fMRI signals in focal regions including the perioral somatosensory cortex, known to be intensely involved during seizures, whereas the occipital cortex was spared. For comparison, we also studied bicuculline-induced generalized tonic-clonic seizures under the same conditions, and found fMRI increases to be larger and more widespread than during spike-and-wave seizures. These findings suggest that even in regions with intense neuronal activity during epileptic seizures, oxygen delivery exceeds metabolic needs, enabling fMRI to be used for investigation of dynamic cortical and subcortical network involvement in this disorder.

Corticothalamic modulation during absence seizures in rats: a functional MRI assessment

PURPOSE

Functional magnetic resonance imaging (fMRI) was used to identify areas of brain activation during absence seizures in an awake animal model.

METHODS

Blood-oxygenation-level-dependent (BOLD) fMRI in the brain was measured by using T2*-weighted echo planar imaging at 4.7 Tesla. BOLD imaging was performed before, during, and after absence seizure induction by using gamma-butyrolactone (GBL; 200 mg/kg, intraperitoneal).

RESULTS

The corticothalamic circuitry, critical for spike-wave discharge (SWD) formation in absence seizure, showed robust BOLD signal changes after GBL administration, consistent with EEG recordings in the same animals. Predominantly positive BOLD changes occurred in the thalamus. Sensory and parietal cortices showed mixed positive and negative BOLD changes, whereas temporal and motor cortices showed only negative BOLD changes.

CONCLUSIONS

With the BOLD fMRI technique, we demonstrated signal changes in brain areas that have been shown, with electrophysiology experiments, to be important for generating and maintaining the SWDs that characterize absence seizures. These results corroborate previous findings from lesion and electrophysiological experiments and show the technical feasibility of noninvasively imaging absence seizures in fully conscious rodents.

Ictogenesis: the origin of seizures in humans. A new look at an old theory

Seizure (ictal) behavior in humans has been observed and recorded since ancient times. A satisfactory solution to this vexing problem continues to elude medical science. Antiepileptic drug (AED) therapy fails to control seizures in 20% of patients with primary generalized epilepsy and 35% of patients with partial epilepsy and has many side effects, including death. This paper cites evidence from the current literature that supports a plausible hypothesis of seizure genesis that was published in 1942, but somehow escaped recognition. It presents a concept that challenges contemporary thinking and may provide the basis for a much needed paradigm shift in the understanding of the nature of seizures and an approach to their management. The theory views a seizure as a natural reflex defense response to a lethal threat to the brain. Although capable of inflicting bodily injury due to falls, drowning, etc., the seizure is not considered inherently harmful to the brain and may be associated with beneficial circulatory changes. Efforts to control and prevent seizures should be directed away from pharma-chemical suppression towards developing methods and bioactive agents that promote neuroplasticity, neurogenesis, and an optimized physiological milieu within the brain.

Epilepsy patients treated with antiepileptic drug therapy exhibit compromised ocular perfusion characteristics

PURPOSE

Reduced cerebral blood flow and decreased cerebral glucose metabolism have been identified in patients with epilepsy treated with antiepileptic drug (AED) therapy. The purpose of this study was to determine whether ocular haemodynamics are similarly reduced in patients with epilepsy treated with AEDs.

METHODS

Scanning laser Doppler flowmetry was used to measure retinal capillary microvascular flow, volume, and velocity in the temporal neuroretinal rim of 14 patients diagnosed with epilepsy (mean age, 42.0 +/- 0.9 years). These values were compared with those of an age- and gender-matched normal subject group (n = 14; mean age, 41.7 +/- 0.3 years). Student's unpaired two-tailed t tests were used to compare ocular blood-flow parameters between the epilepsy and normal subject groups (p < 0.05; Bonferroni corrected).

RESULTS

A significant reduction in retinal blood volume (p = 0.001), flow (p = 0.003), and velocity (p = 0.001) was observed in the epilepsy group (13.52 +/- 3.75 AU, 219.14 +/- 76.61 AU, and 0.77 +/- 0.269 AU, respectively) compared with the normal subject group (19.02 +/- 5.11 AU, 344.03 +/- 93.03 AU, and 1.17 +/- 0.301 AU, respectively). Overall, the percentage mean difference between the epilepsy and normal groups was 36.31% for flow, 28.92% for volume, and 34.19% for velocity.

CONCLUSIONS

Patients with epilepsy exhibit reduced neuroretinal capillary blood flow, volume, and velocity compared with normal subjects. A reduction in ocular perfusion may have implications for visual function in people with epilepsy.

Estimation and significance testing of cross-correlation between cerebral blood flow velocity and background electro-encephalograph activity in signals with missing samples

Cross-correlation between cerebral blood flow (CBF) and background EEG activity can indicate the integrity of CBF control under changing metabolic demand. The difficulty of obtaining long, continuous recordings of good quality for both EEG and CBF signals in a clinical setting is overcome, in the present work, by an algorithm that allows the cross-correlation function (CCF) to be estimated when the signals are interrupted by segments of missing data. Methods are also presented to test the statistical significance of the CCF obtained in this way and to estimate the power of this test, both based on Monte Carlo simulations. The techniques are applied to the time-series given by the mean CBF velocity (recorded by transcranial Doppler) and the mean power of the EEG signal, obtained in 1 s intervals from nine sleeping neonates. The peak of the CCF is found to be low (< or = 0.35), but reached statistical significance (p < 0.05) in five of the nine subjects. The CCF further indicates a delay of 4-6s between changes in EEG and CBF velocity. The proposed signal-analysis methods prove effective and convenient and can be of wide use in dealing with the common problem of missing samples in biological signals.