Functional neuroimaging with positron emission tomography

The Evaluation of Dynamic FDG-PET for Detecting Epileptic Foci and Analyzing Reduced Glucose Phosphorylation in Refractory Epilepsy

Aims: Static fluorodeoxyglucose (FDG)-positron emission tomographic (PET) imaging plays an important role in the localization of epileptic foci. Dynamic FDG PET allows calculation of kinetic parameters. The aim of this study was to investigate whether kinetic parameters have potential for identifying epileptic foci, and to assess the correlation of parameters asymmetry indexes (ASYM) between dynamic and static FDG PET for understanding the pathophysiology of hypometabolism within intractable epilepsy.

Methods: Seventeen patients who had refractory epilepsy correctly localized by static FDG PET with good outcome after foci resection were included. Eight controls were also studied. We performed dynamic and static FDG PET scan before operation. Images of both scans were coregistered to the montreal neurological institute space, regional time activity curves and activity concentration (AC) were obtained by applying the automated anatomical labeling template to the two spatially normalized images, respectively. Kinetic parameters were obtained using a two-tissue non-reversible compartmental model with an image-derived input function. AC from the static scan was used. Side-to-side ASYM of both static AC and kinetic parameters were calculated and analyzed in the hypometabolic epileptogenic regions and non-epileptogenic regions.

Results: Higher values of ASYM from both kinetic parameters and static AC were found in the patients compared to the controls from epileptogenic regions. In the non-epileptogenic regions, no ASYM differences were seen between patients and controls for all parameters. In patients, static AC showed larger ASYM than influx (K₁) and efflux (k₂) of capillaries, but there were no statistical differences of ASYM between net metabolic flux (K_i) or the phosphorylation (k₃) and static AC. ASYM of static AC positively correlated with ASYM of k₃.

Conclusion: Dynamic FDG PET can provide equally effective in detecting the epileptic foci compared to static FDG PET in this small cohort. In addition, compared to capillary influx, the hypometabolism of epileptic foci may be related to reduced glucose phosphorylation.

Neurochemistry and epileptology

This paper gives an account of the global evolution of (neuro-)chemistry in epileptology with an emphasis on the role of the International League Against Epilepsy (ILAE), which declared in its constitution a mission "to make the epilepsy-problem the object of special study and to make practical use of the results of such study." As Epilepsia is the scientific journal of the ILAE, the review emphasizes papers published in the journal.

Nuclear Medicine in Pediatric Neurology and Neurosurgery: Epilepsy and Brain Tumors

In pediatric drug-resistant epilepsy, nuclear medicine can provide important additional information in the presurgical localization of the epileptogenic focus. The main modalities used are interictal (18)F-fluorodeoxyglucose positron emission tomography (FDG-PET) and ictal regional cerebral perfusion study with single-photon emission computed tomography (SPECT). Nuclear medicine techniques have a sensitivity of approximately 85% to 90% in the localization of an epileptogenic focus in temporal lobe epilepsy; however, in this clinical setting, they are not always clinically indicated because other techniques (eg, icterictal and ictal electroencephalogram, video telemetry, magnetic resonance imaging [MRI]) may be successful in the identification of the epileptogenic focus. Nuclear medicine is very useful when MRI is negative and/or when electroencephalogram and MRI are discordant. A good technique to identify the epileptogenic focus is especially needed in the setting of extra-temporal lobe epilepsy; however, in this context, identification of the epileptogenic focus is more difficult for all techniques and the sensitivity of the isotope techniques is only 50% to 60%. This review article discusses the clinical value of the different techniques in the clinical context; it also gives practical suggestions on how to acquire good ictal SPECT and interictal FDG-PET scans. Nuclear medicine in pediatric brain tumors can help in differentiating tumor recurrence from post-treatment sequelae, in assessing the response to treatment, in directing biopsy, and in planning therapy. Both PET and SPECT tracers can be used. In this review, we discuss the use of the different tracers available in this still very new, but promising, application of radioisotope techniques.

Diffusion tensor imaging in medial temporal lobe epilepsy with hippocampal sclerosis

Interictal diffusion imaging studies in patients with medial temporal lobe epilepsy (MTLE) accompanied by hippocampal sclerosis (HS) have shown an increased diffusivity in the epileptogenic hippocampus. In this study, we wanted to explore the whole brain in order to determine if MTLE could have an impact on the organization and the architecture of a large cerebral network and to identify clinical factors that could mediate diffusion abnormalities. Diffusion tensor imaging (DTI) and statistical parametric mapping of the entire brain were performed in 35 well-defined MTLE patients and in 36 healthy volunteers. SPM analyses identified three abnormal areas: an increased diffusivity was detected in the epileptic hippocampus and the ipsilateral temporal structures associated with a decreased anisotropy along the temporal lobe, a decreased diffusivity was found in the contralateral non-sclerotic hippocampus, the amygdala, and the temporal pole, and finally, a decreased anisotropy was noted ipsilaterally in posterior extratemporal regions. Duration of epilepsy, age at onset, and the frequency of generalized tonic-clonic seizures or partial complex seizures did not correlate with the presence of diffusion abnormalities. Region of interest analysis in the hippocampus/parahippocampus demonstrated a correlation between lower ipsilateral diffusivity values and occurrence of epigastric aura and between higher anisotropy values in both hemispheres and history of febrile seizures. In conclusion, this study showed that diffusion abnormalities are not restricted to the pathologic hippocampus and involve a larger network. This pattern may indirectly reflect the epileptogenic network and may be interpreted as a cause or a consequence of epilepsy.

Primidone is associated with interictal depression in patients with epilepsy

Depressive symptoms are common in epilepsy. To determine associations between depression and demographic, clinical, and pharmacological factors among epileptic patients, we conducted a cross-sectional survey. We evaluated 241 epileptic outpatients at a neurological center in a 6-month period. Depressive syndrome was diagnosed when both the Montgomery-Asberg Scale and the Beck Depression Inventory were rated above the standard cutoff points. Bivariate and multivariate analyses were performed to assess the differences between depressed and nondepressed patients with respect to demographic, clinical, and pharmacological features. Depressive syndrome was diagnosed in 42.7% of patients (n=103). Factors associated in the bivariate analysis were: cryptogenic etiology, posttraumatic epilepsy, use of primidone, and inadequate seizure control. After logistic regression, inadequate seizure control (OR 3.08, 95% CI 1.40-6.77, P=0.005) and use of primidone (OR 4.08, 95% CI 2.09-7.98; P<0.001) remained significantly associated. Depression was common and associated with inadequate seizure control and use of primidone.

The current place of single photon emission computed tomography in epilepsy evaluations

Single photon emission computed tomography is appropriate for peri-ictal imaging inpatients with focal epilepsy being considered for surgical treatment.

The role of positron emission tomography with [18F]fluorodeoxyglucose in the evaluation of the epilepsies

Cerebral glucose metabolic mapping using positron emission tomography (PET) and 2-[18F]fluoro-2-deoxyglucose (FDG) has been extensively studied in the epilepsies. Regions of interictal glucose hypometabolism are highly associated with cerebral sites of seizure generation-propagation in focal epilepsies. The volume of reduced glucose metabolism is often widespread and even bilateral in focal epilepsies, although ictal onset zones typically are located at the sites of most severe hypometabolism within a larger volume of hypometabolism.

Surgical treatment for epilepsy

Nearly one-third of patients with newly diagnosed epilepsy will develop medically refractory seizure disorders. The initial response to antiepileptic drug therapy is highly predictive of long-term outcome. Patients with intractable epilepsy may have a progressive disorder that is medically, physically, and socially disabling. Surgical resection of the epileptogenic zone or lesional pathology, or both, may significantly reduce seizure tendency in selected patients. The present review supports the position that early and effective epilepsy surgery may not only render the patient with intractable partial epilepsy seizure-free, but also allow the individual to become a participating and productive member of society. Patients with surgically remediable epileptic syndromes should be identified early in the evaluation and treatment of their seizure disorders. Favorable candidates for focal cortical resection include individuals with medial temporal lobe epilepsy and partial seizures related to selected lesional pathology, e.g. primary brain tumor or vascular anomalies. In conclusion, surgical treatment of intractable partial epilepsy has been shown to compare favorably to antiepileptic drug therapy. Individuals rendered seizure-free may experience a significant improvement in quality of life. Patients who fail to respond to initial antiepileptic drug therapy should be "triaged" to a presurgical evaluation. Ictal semiology combined with structural magnetic resonance imaging and the electroclinical correlation may permit identification of candidates for early and effective surgical treatment.

Positron emission tomography and single photon emission computed tomography in epilepsy care

Radiopharmaceutical brain imaging is clinically applied in planning resective epilepsy surgery. Cerebral sites of seizure generation-propagation are highly associated with regions of hyperperfusion during seizures, and with glucose hypometabolism interictally. For surgical planning in epilepsy, the functional imaging modalities currently established are ictal single photon emission computed tomography (SPECT) with [(99m)Tc]technetium-hexamethylpropyleneamine oxime (HMPAO) or with [(99m)Tc]technetium-ethylene cysteine dimer (ECD), and interictal positron emission tomography (PET) with 2-[(18)F]fluoro-2-deoxyglucose (FDG). Ictal SPECT and interictal FDG PET can be used in presurgical epilepsy evaluations to reliably: (1) determine the side of anterior temporal lobectomy, and in children the area of multilobar resection, without intracranial electroencephalographic recording of seizures; (2) select high-probability sites of intracranial electrode placement for recording ictal onsets; and, (3) determine the prognosis for complete seizure control following anterior temporal lobe resection. Coregistration of a patient's structural (magnetic resonance) and functional images, and statistical comparison of a patient's data with a normal data set, can increase the sensitivity and specificity of these SPECT and PET applications to the presurgical evaluation.

Advances in neuroimaging: non-substrate-directed partial epilepsy

Contemporary neuroimaging studies using structural and functional techniques are critical in the evaluation of patients with localization-related epilepsy. Imaging procedures may be used to localize the epileptic brain tissue or determine the likely pathologic findings underlying the epileptogenic zone, or both. The diagnostic yield of magnetic resonance imaging (MRI) has been demonstrated in patients with partial epilepsy. The identification of an MRI epileptogenic lesion is almost invariably a reliable indicator of the site of seizure onset. Peri-ictal single photon emission computed tomography (SPECT) may be of particular benefit in patients with normal MRI studies. The use of neuroimaging in the care and management of patients with partial epilepsy is discussed here.

PET with 11C-deuterium-deprenyl and 18F-FDG in focal epilepsy

OBJECTIVES

This study compares positron emission tomography (PET) using 11C-deuterium-deprenyl (DED) with PET using 18F-fluorodeoxyglucose(18F-FDG) for examining epileptogenic regions in patients with focal epilepsy.

MATERIAL AND METHODS

Twenty-three patients undergoing evaluation for epilepsy surgery were subjected to PET with DED. Fourteen patients had mesial temporal lobe epilepsy (TLE) and 9 patients had seizures of neocortical origin. In addition, 6 healthy control subjects were examined. Pixel-by-pixel analysis was used to generate graphical images of tracer distribution volume (intercept) and the accumulation rate (slope). Asymmetries with respect to relative intercept and slope were compared in patients with temporal lobe epilepsy (TLE), in patients with extra-temporal lobe epilepsy (exTLE), and in the control subjects. The results were compared with 18F-FDG-PET.

RESULTS

Among the patients with TLE, significant differences between the epileptogenic and the contralateral lobe were found with DED intercept and FDG-uptake. No significant differences were found with DED slope. The exTLE and the control groups showed no significant differences between sides or lobes.

CONCLUSIONS

This study indicates that PET with 11C-deuterium-deprenyl is a useful method for identifying TLE and is equivalent to PET with 18F-FDG in this sense. The method has little localizing value in seizures originating from neocortical structures.

Relationship between neuronal loss and interictal glucose metabolism during the chronic phase of the lithium-pilocarpine model of epilepsy in the immature and adult rat

The lithium-pilocarpine (Li-Pilo) model of epilepsy reproduces most of the features of human temporal lobe epilepsy. After having studied the metabolic changes occurring during the silent phase, in the present study, we explored the relationship between interictal metabolic changes and neuronal loss during the chronic phase following status epilepticus (SE) induced by Li-Pilo in 10-day-old (P10), 21-day-old (P21), and adult rats. Rats were observed and their EEG was recorded to detect the occurrence of spontaneous recurrent seizures (SRS). Local cerebral glucose utilization was measured during the interictal period of the chronic phase, between 2 and 7 months after SE, by the [(14)C]2-deoxyglucose method in rats subjected to SE at P10, P21, or as adults. Neuronal damage was assessed by cell counting on adjacent cresyl violet stained sections. When SE was induced at P10, rats did not become epileptic, did not develop lesions and cerebral glucose utilization was in the normal range 7 months later. When SE was induced in adult rats, they all became epileptic after a mean duration of 25 days and developed lesions in the forebrain limbic areas, which were hypometabolic during the interictal period of the chronic phase, 2 months after SE. When SE was induced in P21 rats, 24% developed SRS, and in 43% seizures could be triggered (TS) by handling, after a mean delay of 74 days in both cases. The remaining 33% did not become epileptic (NS). The three groups of P21 rats developed quite comparable lesions mainly in the hilus of the dentate gyrus, lateral thalamus, and entorhinal cortex; at 6 months after SE, the forebrain was hypometabolic in NS and TS rats while it was normo- to slightly hypermetabolic in SRS rats. These data show that interictal metabolic changes are age-dependent. Moreover, there is no obvious correlation, in this model, between interictal hypometabolism and neuronal loss, as reported previously in human temporal lobe epilepsy.

Planned ictal FDG PET imaging for localization of extratemporal epileptic foci

PURPOSE

This work demonstrates the feasibility of planned ictal positron emission tomography (PET) with [18F]fluoro-2-deoxy-glucose (FDG) for localization of epileptic activity in patients with frequent partial seizures of extratemporal origin.

METHODS

Ictal PET imaging was performed in four patients (two men and two women, ages 28-61) with continuous or very frequent (every 3-15 min) partial seizures. All patients had abnormalities apparent on magnetic resonance (MR) or computed tomographic (CT) imaging, two with extensive brain lesions that precluded precise localization of the seizure focus with interictal PET or single-photon emission tomography (SPECT) imaging.

RESULTS

Ictal PET imaging demonstrated a restricted area of focal hypermetabolism concordant with surface electroencephalographic (EEG) recording in all cases. The PET images were registered to MR imaging data for further anatomic localization of hypermetabolic regions in three cases. The ictal PET data were used to guide neurosurgical intervention in one case.

CONCLUSIONS

We conclude that planned ictal PET imaging may be a useful and potentially superior approach to ictal SPECT for identifying the epileptic focus in a selected group of patients with continuous or frequent simple partial seizures.

An H(2) (15)O-PET study of cerebral blood flow changes during focal epileptic discharges induced by intracerebral electrical stimulation

Partial epileptic seizures are known to cause a focal increase in cerebral blood flow (CBF). However, quantified studies of ictal CBF changes under intracranial EEG control are still needed to assess the relationships in time and space between CBF changes and electrical discharges. Ten patients undergoing an intracerebral stereotaxic EEG (stereo-EEG) investigation for epilepsy surgery were prospectively studied for local perfusion changes. These were measured by H(2)(15)O-PET during 12 subclinical or mild symptomatic focal epileptic discharges induced by intracerebral electrical stimulation of the hippocampus (eight), amygdala (two), temporal pole (one) and fusiform gyrus (one). This study aimed to assess whether a significant focal blood flow change reflected the geographical extent of the underlying coincident epileptic discharge, as measured by this method at seizure onset. No significant CBF change was observed on test-retest at rest or during ineffective electrical stimulations outside the epileptogenic area. Compared with the resting condition, a significant focal perfusion increase of 16-55% occurred during eight discharges, there was no CBF change in three and a significant CBF decrease in one. Ictal CBF increases were mostly associated with low-voltage fast activity, but their magnitude had no obvious link with the duration of the discharge (range 8-106 s). Regional analysis of ictal PET was performed in 10 anatomical areas during each of the 12 discharges. Of all the 120 regions, 59 were not explored by intracerebral electrodes and 14 (24%) of these demonstrated ictal CBF changes. In 43 of the 61 regions explored by stereo-EEG (70.5%), PET and depth EEG findings converged, showing either a CBF change in a discharging area or no CBF change in a region unaffected by the discharge. Areas of increased CBF indicated an underlying epileptic discharge in almost 100% of the cases. Conversely, of the 18 regions showing discrepancies between intracerebral recordings and PET data, 17 were discharging regions showing no ictal CBF changes. Thus, a focal CBF increase, when detected at the seizure onset concomitantly with the initial low-voltage fast activity, was a reliable marker of an underlying epileptic discharge. It emphasizes the importance of injecting blood-flow tracers as soon as possible after detection of the discharge in routine clinical studies, even at a subclinical stage of the seizure. However, the extent of significant ictal CBF changes can be more restricted than that of the electrical discharge, thus limiting the reliability of ictal CBF images for outlining the contours of a tailored cortectomy.

Brain waves and brain wiring: the role of endogenous and sensory-driven neural activity in development

Neural activity is critical for sculpting the intricate circuits of the nervous system from initially imprecise neuronal connections. Disrupting the formation of these precise circuits may underlie many common neurodevelopmental disorders, ranging from subtle learning disorders to pervasive developmental delay. The necessity for sensory-driven activity has been widely recognized as crucial for infant brain development. Recent experiments in neurobiology now point to a similar requirement for endogenous neural activity generated by the nervous system itself before sensory input is available. Here we use the formation of precise neural circuits in the visual system to illustrate the principles of activity-dependent development. Competition between the projections from lateral geniculate nucleus neurons that receive sensory input from the two eyes shapes eye-specific connections from an initially diffuse projection into ocular dominance columns. When the competition is altered during a critical period for these changes, by depriving one eye of vision, the normal ocular dominance column pattern is disrupted. Before ocular dominance column formation, the highly ordered projection from retina to lateral geniculate nucleus develops. These connections form before the retina can respond to light, but at a time when retinal ganglion cells spontaneously generate highly correlated bursts of action potentials. Blockade of this endogenous activity, or biasing the competition in favor of one eye, results in a severe disruption of the pattern of retinogeniculate connections. Similar spontaneous, correlated activity has been identified in many locations in the developing central nervous system and is likely to be used during the formation of precise connections in many other neural systems. Understanding the processes of activity-dependent development could revolutionize our ability to identify, prevent, and treat developmental disorders resulting from disruptions of neural activity that interfere with the formation of precise neural circuits.

NMDA-receptor activity visualized with (S)-[N-methyl-11C]ketamine and positron emission tomography in patients with medial temporal lobe epilepsy

PURPOSE

To determine whether neurochemical activation of the N-methyl-D-aspartate (NMDA) receptor-gated ion channel shows quantitative changes, measured as binding of 11C-labeled (S)-[N-methyl]ketamine, in patients with medial temporal lobe epilepsy (MTLE).

METHODS

Eight patients with MTLE who were evaluated regarding epilepsy surgery underwent positron emission tomography (PET) with (S)-[N-methyl-11C]ketamine. The presurgical investigations included magnetic resonance imaging (MRI), PET with 18F-fluoro-deoxyglucose (18FDG), and seizure monitoring by using video-EEG. The uptake of (S)-[N-methyl-11C]ketamine in the temporal lobe of ictal onset was compared with the contralateral side and correlated to changes in regional glucose metabolism measured by PET with 18FDG.

RESULTS

(S)-[N-methyl-11C]ketamine rapidly reached the brain, and high radioactivities were measured in the striatum, thalamic nuclei, and cortical regions. Overall the brain uptake and regional binding potentials of (S)-[N-methyl-11C]ketamine were similar to measurements observed previously in healthy controls. However, 20 min after administration, when blood flow influence was negligible, a side-to-side comparison revealed a 9-34% reduction of tracer radioactivity in the temporal lobes of ictal onset. At earlier times, the differences in binding potentials were less pronounced, 9-21%. The magnitude and distribution of the reduction were similar to the metabolic pattern seen on PET scans with 18FDG.

CONCLUSIONS

Radioactivity uptake of intravenously administered (S)-[N-methyl-11C]ketamine was reduced in temporal lobes of ictal in patients with TLE. This may reflect reduced NMDA-receptor density, reduced perfusion, focal atrophy, or other factors.

Cerebral hemodynamic response to generalized spike-wave discharges

PURPOSE

Data in the literature concerning metabolic demand during generalized spike-wave activity (gSW) are conflicting. We investigated instantaneous changes in cerebral blood flow velocities (CBFV) in both middle cerebral arteries (MCAs) by transcranial Doppler sonography (TCD) during gSW paroxysms recorded by scalp EEG.

METHODS

In 13 patients, CBFVs in both MCAs were averaged, time-locked to the occurrence of the gSW; respiratory rate (RR) and end-expiratory pco2 were measured in one patient.

RESULTS

Nine patients showed significant changes in CBFV during gSW. Four had biphasic flow changes with an initial increase (p < 0.05) and a subsequent decrease (p < 0.01). This was partially paralleled by an increase in RR (p < 0.01) and a decrease in pco2 (p < 0.01). In three patients, an increase in CBFV that preceded the onset of gSW by several seconds was observed, followed by a decrease in CBFV. Two patients showed a significant decrease only of CBFV. Only gSWs of a median duration of >0.8 s were associated with significant changes in CBFV.

CONCLUSIONS

We were able to demonstrate that gSWs of several seconds duration lead to cortical perfusion changes. We suggest that the initial increase of CBFV demonstrated in some patients reflects neuronal activation, whereas the subsequent decrease might in part be due to hyperventilation-induced hypocapnia.

Functional imaging of the brain in the evaluation of drug response and its application to the study of aging

Functional neuroimaging techniques including single photon emission computerised tomography (SPECT), positron emission tomography (PET) and functional magnetic resonance imaging (FMRI) can provide insight into the functional connectivity of the human brain in both health and disease, including the effects of aging and drugs on brain function. Neuroimaging measurement techniques can either be direct, using radio-specific ligands, or indirect, using the neurophysiological consequences of pharmacological interventions. Both approaches can be combined with sensorimotor or cognitive activation to examine the interaction between the targeted receptor function and the sensorimotor or cognitive process implicit in the study design. Using radionuclides, PET can provide absolute measurement of cerebral blood flow to regions of interest and can measure changes in cerebral metabolism using labelled fluorodeoxyglucose. PET offered the first opportunity to image brain activation caused by a variety of stimuli and hence to measure the effect of drugs on brain activation. PET also enables the study of drug disposition within the brain. SPECT has been used to study relative changes in cerebral blood flow associated with disease processes and also receptor occupancy. FMRI, by contrast, does not involve ionising radiation and has better spatial and temporal resolution. It is still a relatively new technique and limited by its ability to only measure haemodynamic changes through the blood oxygen level-dependent (BOLD) signal. The effects of aging on drug responsiveness and the effects of drug treatment of diseases associated with old age are relatively unexplored areas of functional neuroimaging research.

Dynamic [18F]fluorodeoxyglucose positron emission tomography and hypometabolic zones in seizures: reduced capillary influx

We performed dynamic [18F]fluorodeoxyglucose ([18F]FDG) positron emission tomographic (PET) analyses in 8 patients. Rate constants of influx (K1*), efflux (k2*), phosphorylation (k3*), and dephosphorylation (k4*) were derived for the regions of interest (ROIs), which included (1) the hypometabolic epileptogenic regions and (2) the homologous regions in the contralateral hemispheres. In addition, the four constants were determined from at least one clearly defined (control) ROI from the same plane and its homologous contralateral ROI. Influx (K1*) in the epileptogenic region was reduced in comparison with the contralateral ROI. Reductions in influx (K1*), which averaged 18 +/- 13% (mean +/- SD), [18F]FDG phosphorylation (k3*) (25 +/- 20%), and brain glucose utilization rates (26 +/- 10%) were observed in the epileptogenic region. Reductions in efflux were not statistically significant (k2* = 13 +/- 28%) but were comparable in magnitude to the average reduction in K1*. No ipsilateral versus contralateral differences were seen for any rate constants measured outside the epileptogenic region. Influx correlated highly with phosphorylation in the epileptogenic region. Our data suggest that the hypometabolic epileptogenic focus seen in [18F]FDG-PET studies is also a region of reduced blood-brain barrier glucose transporter activity and that reductions in phosphorylation are proportional to reductions in [18]FDG influx.

The current state of epilepsy surgery

Epilepsy surgery is an effective therapy for many patients with refractory partial seizures. The results of epilepsy surgery have improved with advances in the evaluation of patients for surgery and the care of patients during and after surgery. This review looks at recent innovations in imaging modalities to identify the candidates for surgery who will benefit most, to identify subtle pathology, to resect the epileptogenic focus more precisely, and to preserve functionally important cortex in order to achieve the best possible outcome. Patient selection and the identification and removal of the epileptogenic focus, in a safe manner, are crucial for good outcome.

Brain hypometabolism in a model of chronic focal epilepsy in rat neocortex

PURPOSE

Metabolic mapping of the human brain has become a widely used method for identifying and localizing epileptic foci. A reduction of glucose consumption usually is found interictally in the area of the focus. By contrast, animal models of acute epilepsy show a hypermetabolism in the epileptic focus. Here we investigated how metabolism is altered in an animal model of chronic epilepsy caused by focal injection of tetanus toxin into rat neocortex.

METHODS

A total of 27 male Wistar rats were anesthetized and injected into the motor or sensory cortex either with dissolved tetanus toxin or with the solvent only. Animals recovered for 7, 14, or 30 days and then were anesthetized again for quantitative 14C-deoxyglucose autoradiography. Data were analyzed with an imaging program, and regional cerebral glucose metabolism (rCMRGlc) was determined.

RESULTS

Injection of tetanus toxin into the motor cortex caused a focal hypometabolism which was confined to the cytoarchitectonic boundaries of the injected area, whereas sensory cortex injection caused a more widespread hypometabolism in all sensory cortical and connected, areas. None of the animals displayed focal hypermetabolism and we observed no significant time-dependent alteration of brain metabolism.

CONCLUSIONS

Tetanus toxin injection into the cortex of the rat induces chronic epileptic activity accompanied by a focal hypometabolism. The data suggest that the spread of the metabolic alterations depends on the connectivity of the injected cortical area.