Interictal regional cerebral blood flow during non specific activation test in partial epilepsy

Advances in Brain Imaging Techniques for Patients With Intractable Epilepsy

Intractable epilepsy, also known as drug resistance or refractory epilepsy, is a major problem affecting nearly one-third of epilepsy patients. Surgical intervention could be an option to treat these patients. Correct identification and localization of epileptogenic foci is a crucial preoperative step. Some of these patients, however, have no abnormality on routine magnetic resonance imaging (MRI) of the brain. Advanced imaging techniques, therefore, can be helpful to identify the area of concern. Moreover, a clear delineation of certain anatomical brain structures and their relation to the surgical lesion or the surgical approach is essential to avoid postoperative complications, and advanced imaging techniques can be very helpful. In this review, we discuss and highlight the use of advanced imaging techniques, particularly positron emission tomography (PET)–MRI, single-photon emission computed tomography, functional MRI, and diffusion tensor imaging–tractography for the preoperative assessment of epileptic patients.

An interictal measurement of cerebral oxygen extraction fraction in MRI-negative refractory epilepsy using quantitative susceptibility mapping

PURPOSE

Oxygen extraction fraction (OEF) can be a factor to identify brain tissue's disability in epileptic patients. This study aimed to assess the OEF's level measurement in refractory epileptic patients (REPs) using a quantitative susceptibility mapping (QSM) method and to determine whether the OEF parameters change.

METHODS

QSM-OEF maps of 26 REPs and 16 healthy subjects were acquired using 3T MRI with a 64-channel coil. Eighteen regions-of-interest (ROIs) were chosen around the cortex in one appropriate slice of the brain and the mean QSM-OEF for each ROI was obtained. The correlations of QSM-OEF among different clinical characteristics of the disease, as well as between the patients and normal subjects, were also investigated.

RESULTS

QSM-OEF was shown to be significantly higher in REPs (44.9 ± 5.8) than that in HS (41.9 ± 6.2) (p < 0.05). Mean QSM-OEF was statistically lower in the ipsilateral side (44.5 ± 6.6) compared to the contralateral side (46.4 ± 6.8) (P < 0.01). QSM-OEF was illustrated to have a strong positive correlation with the attack duration (r = 0.6), and a moderate negative correlation with the attack frequency (r = -0.3). Using an optimized support vector machine algorithm, we could predict the disease in subjects having abnormal OEF values in the brain-selected-ROIs with sensitivity, specificity, AUC, and the precision of 0.96, 1, 0.98, and 1, respectively.

CONCLUSIONS

The results of this study revealed that QSM-OEF of the REPs' brain is higher than that of HS, which indicates that QSM-OEF is associated with disease activity.

Late-onset epilepsy and occult cerebrovascular disease

The interface between cerebrovascular disease (CVD) and epilepsy is complex and multifaceted. Late-onset epilepsy (LOE) is increasingly common and is often attributed to CVD, and is indeed associated with an increased risk of stroke. This relationship is easily recognizable where there is a history of stroke, particularly involving the cerebral cortex. However, the relationship with otherwise occult, subcortical CVD is currently less well established yet causality is often invoked. In this review, we consider the diagnosis of LOE in clinical practice--including its behaviour as a potential mimic of acute ischemic stroke and transient ischemic attack; evidence for an association between occult CVD and LOE; and potential mechanisms of epileptogenesis in occult CVD, including potential interrelationships between disordered cerebral metabolism and perfusion, disrupted neurovascular unit integrity, blood-brain barrier dysfunction, and inflammation. We also discuss recently recognized issues concerning antiepileptic drug treatment and vascular risk and consider a variety of less common CVD entities associated with seizures.

Occult cerebrovascular disease and late-onset epilepsy: could loss of neurovascular unit integrity be a viable model?

Late-onset epilepsy (LOE) first occurs after 60 years of age and may be due to occult cerebrovascular disease (CVD) which confers an increased risk of stroke. However, patients with late-onset epilepsy are not currently consistently investigated or treated for cerebrovascular risk factors. We discuss how abnormalities of neurovascular unit function, namely, changes in regional cerebral blood flow and blood brain barrier disruption, may be caused by occult cerebrovascular disease but present clinically as late-onset epilepsy. We describe novel magnetic resonance imaging methods to detect abnormal neurovascular unit function in subjects with LOE and controls. We hypothesise that occult CVD may cause LOE as a result of neurovascular unit dysfunction.

Photic driving in the electroencephalogram of children and adolescents: harmonic structure and relation to the resting state

In order to identify latent bioelectrical oscillators, 15 normal subjects (aged 9-17 years, 8 males, 7 females) were subjected to intermittent photic stimulation. The EEG amplitude spectra corresponding to the 11 fixed frequencies of stimulation presented (3-24 Hz) were combined to form "profiles" of the driving reaction in the right occipital area. The driving response varied with frequency, and was demonstrable in 70-100% of cases (using as criterion peak amplitudes 20% larger than those of the neighbors). The strongest responses were observed at the frequency closest to the alpha peak of the resting EEG. A secondary profile maximum was in the theta band. In 10 subjects, this maximum exceeded half the alpha peak (with an average of 72.4% of the alpha peak), while in the resting spectra, theta amplitudes were much lower than the alpha maxima. This responsiveness in theta activity seems to be characteristic of prepubertal and pubertal subjects. The profiles and resting EEG spectra showed a highly significant Pearson's correlation, with the peak in the theta band of the profiles being the main difference observed between them. The correlation coefficient was significantly correlated with the ratio of the maxima in the theta and alpha bands (R = -0.77, P<0.001). The correlation coefficient between profile and resting spectrum may be a useful indicator in screening methods used to reveal latent cerebral oscillators. Profiles for the second and third harmonics were correlated with those of the first harmonic (fundamental frequency), when considering the corresponding EEG frequencies. Peak frequencies in all three profiles were close to those of the individual's background alpha rhythm, and peak amplitudes in higher harmonics were not much lower than those of the fundamental frequency (mean values of 84 and 63%, for second and third harmonics, respectively).

Clinical brain radionuclide imaging studies

A recent survey of the knowledge and practice of both positron-emission tomography (PET) and single-photon emission computed tomography (SPECT) of the brain among referring physicians in Europe (neurologists and psychiatrists) showed a disquieting lack of knowledge of the potential of these methodologies in the investigation and management of patients of their own specialities. The need to bring the knowledge of the potential of these techniques to the practicing physicians is paramount. It is imperative that the methodologies and concepts that preside over the application of these techniques in neurology and psychiatry must become more uniform if an impact is to be felt at a clinical level. There is clear improvement in the instrumentation available with the new state-of-the-art tomographic devices and with the development of new technetium-based radiopharmaceuticals for the study of cerebral perfusion. The constant progress made with ligands that permit the study of neurotransmission, tumor metabolism, and turnover do expand our capability to improve the knowledge concerning neurophysiology, neuropathology, and neuropharmacology of a variety of disease states. PET and SPECT will be progressively included in protocols aimed at stratifying patients with dementia, monitoring therapeutic trials, and improving our ability to determine outcome. Clinical usefulness of PET and SPECT begin to emerge in cerebral vascular disease, in the identification of cerebral death, in epilepsy, in cerebral trauma, in the investigation of HIV-positive patients with cerebral involvement, and in the monitoring of tumor recurrence and postirradiation damage. This review article outlines a current perspective of SPECT and PET as practiced in Europe, its potential, and its limitations.

Effect of non-steady-state perfusion on xenon-133 cerebral blood flow measurements: an analytical study

Activation studies employing the noninvasive xenon-133 technique are widely used to investigate the cerebral circulation. Typical examples are the investigation of hemispheral specialization of higher cortical function with cognitive activation or the assessment of the hemodynamic reserve in occlusive cerebrovascular disease by CO2 inhalation. Traditionally, in studies using this technique, there is the requirement of a circulatory steady state during the measurement. Due to limitations in the duration of the stimulus or habituation to the stimulus, the basic assumption is often violated. In this study we investigated with the aid of a computer model to what extent blood flow measurement results are affected by non-steady-state blood flow. The findings indicate that cortical activation need not extend throughout the whole measurement to be detectable. Maintenance of activation for at least 5 min is sufficient for a successful measurement. In addition, the results show that the activation should be fully established when the measurement starts to achieve maximal sensitivity. Delay in activating the circulation will result in attenuated responses, especially if the stimulus is delayed beyond 2 min.

Ictal/postictal SPECT in the pre-surgical localisation of complex partial seizures

Single photon emission computed tomography (SPECT) used in conjunction with HM-PAO (Ceretec-Amersham International) was used to image regional cerebral blood flow (rCBF) in 28 patients with medically intractable complex partial seizures during or soon after a seizure, and interictally. Changes from interictal rCBF were seen in 26/28 (93%) patients. The main findings were; 1) During the seizure--hyperperfusion of the whole temporal lobe; 2) Up to 2m postically--hyperperfusion of the hippocampus with hypoperfusion of lateral temporal structures; 3) From 2-15m postically--hypoperfusion of the whole temporal lobe. When compared with EEG and MRI data, correct localisation to one temporal lobe was obtained in 23 patients. In one further patient bilateral temporal foci, and in a further two patients frontal foci, were correctly identified. There were no disagreements between EEG and SPECT localisation. Temporal lobe surgery was successful (by the criterion of at least 90% reduction in seizure frequency) in all but one of the 23 patients operated on. It is concluded that ictal/postictal SPECT is a reliable technique for the presurgical localisation of complex partial seizures. The data indicate a likely sequence of changes in rCBF during and after complex partial seizures of temporal lobe origin.

Neurological stamp. Hermann Boerhaave 1668-1738

Images

Interictal and postictal focal hypermetabolism on positron emission tomography

Decreased glucose utilization in the epileptogenic zone is typically observed interictally on positron emission tomography (PET), whereas ictal PET studies reveal complex patterns of increased and decreased metabolism. PET findings of 7 children, ages 2 months to 16 years, are described and demonstrate small focal regions of hypermetabolism in the absence of clinical or electrographic seizure during the 2-deoxy-2[18F]fluoro-D-glucose (FDG) uptake period. Magnetic resonance imaging scans were nonlocalizing in 5 of 7 children. In 4 children, seizures had not occurred for at least several hours prior to PET. Electroencephalography during PET disclosed active spike-and-wave activity on the side of interictal hypermetabolism in these 4 children. The remaining 3 children had seizures within 15 min prior to FDG injection and were considered postictal; their PET images revealed focal hypermetabolism. Results indicated the need for electroencephalographic monitoring during functional neuroimaging studies of all epileptic patients. The biochemical basis of interictal hypermetabolism is probably related to increased energy consumption by an active epileptogenic focus, whereas postictal hypermetabolism is likely due to energy expenditure for the restoration of resting membrane potentials and chemical homeostasis following an epileptic event.

A review of interictal cerebral blood flow in the evaluation of patients for epilepsy surgery

The literature on cerebral blood flow (CBF) studies in patients with epilepsy is critically reviewed. Two methods are specifically addressed, radioactive xenon delivered through inhalation or vascularly, and single photon emission computed tomography (SPECT) using various tracers. Both regional and global blood flow were used in these studies to assess focal and generalized seizures. Electroencephalogram (EEG) foci were determined by various techniques, including interictal and ictal recordings from scalp or intracranial electrodes. All studies reported a positive concordance between EEG foci and CBF. However, there was a high incidence of false positive and false negative results. The methodology of almost all of the studies was inadequate to assess accurately the sensitivity and specificity of CBF to localize the seizure focus. Even when conservative estimates were made, the sensitivity and specificity of CBF was too low to be used as a diagnostic test for epilepsy, and inadequate to localize the EEG focus in the evaluation of patients for epilepsy surgery.