You are in: eMedicine Specialties > Neurology > Seizures and Epilepsy Presurgical Evaluation of Medically Intractable EpilepsyArticle Last Updated: May 15, 2006AUTHOR AND EDITOR INFORMATIONSection 1 of 11
  Author: Erasmo A Passaro, MD, Director, Comprehensive Epilepsy Program/Clinical Neurophysiology Lab, Bayfront Medical Center Erasmo A Passaro is a member of the following medical societies: American Academy of Neurology, American Academy of Sleep Medicine, American Clinical Neurophysiology Society, American Epilepsy Society, and American Medical Association Editors: Claude G Wasterlain, MD, Vice-Chairperson, Professor, Department of Neurology, University of California at Los Angeles; Francisco Talavera, PharmD, PhD, Senior Pharmacy Editor, eMedicine; Jose E Cavazos, MD, PhD, Assistant Professor, Departments of Medicine (Neurology), Pharmacology, and Physiology, University of Texas Health Science Center at San Antonio; Selim R Benbadis, MD, Professor, Director of Comprehensive Epilepsy Program, Departments of Neurology and Neurosurgery, University of South Florida School of Medicine, Tampa General Hospital; Nicholas Y Lorenzo, MD, Chief Editor, eMedicine Neurology; Consulting Staff, Neurology Specialists and Consultants Author and Editor Disclosure Synonyms and related keywords: medically refractory epilepsy, refractory epilepsy, epilepsy surgery, seizure localization, intractable, presurgical evaluation of medically intractable epilepsy, seizure evaluation, long-term epilepsy monitoring, AEDs, antiepileptic medications, EEG monitoring for seizures OVERVIEW AND OBJECTIVESSection 2 of 11
  Patients with medically intractable epilepsy are admitted to the long-term epilepsy-monitoring unit, where seizures are recorded with video and simultaneous EEG monitoring. Antiepileptic medications are tapered and the patients may be deprived of sleep to induce seizures. Recorded seizures are reviewed carefully, since the clinical semiology and the interictal and ictal EEG provide localizing and lateralizing information. The goal of the admission is to correlate the clinical features with the EEG findings and to determine whether the patient has a single seizure type with a consistent localization. The semiology of the recorded seizure(s) is reviewed with the patient's family to ensure that the habitual seizure type has been recorded. Occasionally, the recorded events are not representative of the patient's habitual events. In fact, sometimes the patient's habitual disabling event is determined to be nonepileptic, while the epileptic seizures are controlled adequately with medications. For excellent patient education resources, visit eMedicine's Brain and Nervous System Center. Also, see eMedicine's patient education article Epilepsy. Also see Bayfront Medical Center's Epilepsy Information Page. VIDEO-EEG TELEMETRYSection 3 of 11
Clinical semiology The clinical semiology of the seizures usually suggests a localization (see the article Identification of Potential Epilepsy Surgery Candidates); certain clinical features also can have lateralizing value. For example, ictal speech usually is associated with nondominant temporal lobe seizures, as is preserved responsiveness with unilateral manual automatisms. Dystonic limb posturing usually involves flexion of the arm at the elbow with internal or external rotation of the forearm, flexion at the wrist, and extension of the fingers, and is contralateral to the side of temporal lobe seizure onset. Some patients may have ipsilateral head turning and contralateral dystonic posturing. Postictal nose wiping, defined as wiping of the nose with one hand twice in the postictal period, is usually ipsilateral to the temporal lobe of onset. Postictal dysnomia lasting for more than 2 minutes suggests onset in the dominant temporal lobe. Forceful head version immediately prior to a secondarily generalized tonic-clonic seizure lateralizes seizures to the contralateral hemisphere. Asymmetric tonic limb posturing, the "figure four sign," is defined as extension of one arm at the elbow and flexion of the other arm at the elbow during the tonic phase of a secondarily generalized tonic-clonic seizure. The extended limb is usually contralateral to the hemisphere of onset. Furthermore, asymmetric ending of the late clonic phase of a secondarily generalized tonic-clonic seizure is also lateralizing; the limb with the last clonic jerk(s) is ipsilateral to the hemisphere of onset in 80% of cases. However, these lateralizing signs can be less accurate and falsely lateralizing in patients with bilaterally independent, temporal epileptiform discharges. In frontal lobe epilepsy, forced head, eye, and body version suggests onset in the dorsolateral frontal lobe. With dorsolateral frontal lobe seizures, forced head and body version is usually contralateral to the hemisphere of onset. A recent study showed that nonforced head turning at ictal onset without a tonic component or hemifacial clonic twitching was usually ipsilateral to the hemisphere of onset. Gyratory seizures are defined as a rotation around the body axis at least 180 degrees during the seizure. Gyratory seizures that begin with forced head version are usually contralateral, whereas gyratory seizures without forced head turning are usually ipsilateral to the hemisphere of onset. Interictal EEG For each surgical candidate, a baseline 30-minute recording, as well as random awake and sleep samples, are reviewed. Computer-generated spike detection files also are reviewed. Patients with temporal lobe epilepsy (TLE) have epileptiform activity consisting of spikes and/or sharp waves that are usually maximal at the anterior temporal (F7 and F8 electrodes) and the mid temporal regions (T3 and T4 electrodes). Although 20-30% of patients with intractable TLE have bilaterally independent temporal epileptiform activity, most of these patients have a single or predominant localization for their habitual seizures. Occasionally, patients with extratemporal epilepsy of occipital or frontal lobe origin have interictal epileptiform activity at the temporal region, even though seizures do not localize to that region. Patients with occipital lobe epilepsy may not have any occipital epileptiform discharges. In fact, these patients may have only propagated interictal epileptiform discharges in the frontal or temporal regions. Patients with frontal lobe epilepsy may have predominantly bilaterally synchronous or generalized epileptiform activity due to secondary bilateral synchrony. This EEG pattern is observed most commonly in patients with mesial frontal lobe epilepsy. EEG features that help in lateralization of secondary bilateral synchrony include (1) consistent phase reversal over one region, (2) higher amplitude of generalized or bilaterally synchronous interictal epileptiform activity over one hemisphere, (3) a consistent lead in one hemisphere, and (4) persistent lateralized interictal slowing. Midline interictal epileptiform discharges can also be seen. Many patients with extratemporal neocortical epilepsy have no interictal epileptiform discharges. In these patients, localization relies on ictal EEG and structural and functional imaging data. Multifocal independent interictal epileptiform abnormalities do not invariably suggest a poor postoperative prognosis, since focal resections still may be beneficial in well-selected patients with additional evidence for a single epileptogenic zone. Children with refractory infantile spasms may show hypsarrhythmia, which is defined as a highly disorganized background with very high amplitude activity and multifocal independent spikes. Infants with lateralized findings, such as focal loss of fast activity or focal slowing and an MRI or fluoro-deoxyglucose positron emission tomography (FDG-PET) scan showing a focal abnormality, may be candidates for multilobar resection or hemispherectomy. Ictal scalp-sphenoidal EEG In TLE, the ictal EEG typically consists of 5- to 7-Hz rhythmic sharp waves, maximal at one sphenoidal electrode (see Image 1). This pattern may be observed after a nonspecific onset but is equally localizing if it occurs within 30 seconds of the first EEG change. An ictal onset pattern that begins with delta frequency activity may suggest possible temporal neocortical epilepsy. Correlation with clinical and imaging findings is essential in these cases in order to determine whether invasive monitoring is necessary. Extratemporal seizures can begin with generalized voltage attenuation, lateralized slowing, or focal or lateralized beta activity. Extratemporal seizures are more difficult to localize than temporal lobe seizures. In many cases, lateralized ictal EEG activity is sufficient if the clinical semiology and the lesion are concordant with each other. MAGNETIC RESONANCE IMAGINGSection 4 of 11
High-resolution structural MRI High-resolution structural MRI using T1-weighted, spoiled gradient-recall sequences with contiguous slices perpendicular to the long axis of the temporal lobe has the greatest sensitivity (85%) for the detection of unilateral hippocampal atrophy (see Image 2). Hippocampal atrophy on MRI correlates with the presence of hippocampal sclerosis, which is the pathologic substrate of medial TLE in 70% of cases. Hippocampal sclerosis is defined as greater than 30% cell loss in hippocampal regions CA1 and CA3, with relative sparing of the CA2 region. Dentate granule cells also are lost, but principal neurons in the presubiculum are not. Cells also are lost in the hilus, particularly those cells containing neuropeptide Y and somatostatin, together with sprouting of dentate granule cell mossy fibers that produce monosynaptic recurrent excitatory circuits. Approximately 20-30% of patients with hippocampal sclerosis have dual pathology, with the concurrent presence of hamartomas, cortical dysplasia, and heterotopic grey matter. Other MRI findings may include increased signal in the hippocampus on conventional spin echo T2-weighted imaging. MR imaging with fluid attenuated inversion recovery (FLAIR) shows an even greater sensitivity for detecting signal changes within the abnormal sclerotic hippocampus than conventional spin-echo T2-weighted imaging (see Image 3). Surface-coil MRI and 3-D surface rendering may increase the yield in identifying focal areas of cortical thickening. The use of multichannel phased array head coils is preferred over conventional quadrature coils. Other techniques, such as T1-weighted and T2-weighted inversion recovery, also may increase the sensitivity to identify subtle cortical malformations (see Image 4). 3T phased array (PA) MRI can further increase the signal-to-noise ratio 6-8 fold as compared with nonphase array coil 1.5T MRI. A recent study found improvedlesion detection with 3T PA MRI in patients with intractable epilepsy. Novel structural MRI techniques Preliminary data suggest that 3-D preoperative maps of hippocampi can help predict surgical outcome. However, future studies are needed to determine whether this will be an independent predictor of surgical outcome. Novel techniques such as voxel-based morphometry (VBM) where a whole brain gray matter voxel-based comparison is made of the patient and the control group. A z score map is then generated for the patient. This method demonstrates enhanced sensitivity in identifying subtle gray matter abnormalities and for identifying additional areas of gray matter abnormalities in patients with focal cortical dysplasia. HMR spectroscopy This technique is based on the principle that N-acetyl aspartate (NAA) is found primarily within neurons and precursor cells; a reduction in NAA usually is regarded as indicating loss or dysfunction of neurons. Creatinine (CR) and choline (Cho) are present in much higher concentrations in glia than in neurons. Patients with TLE have reductions in the NAA/(Cho + CR) ratio. This reduction has been shown to correlate with the presence of hippocampal sclerosis and to correctly lateralize the side of seizure onset in 97% of patients. About 20-40% of patients have bilateral metabolic disturbances, and preliminary evidence suggests that this finding is associated with a higher probability of surgical failure. Recent developments in multivoxel HMR spectroscopy may be of further value. Functional MRI Functional MRI (fMRI) evaluates cerebral blood flow by looking at the difference between venous oxyhemoglobin and deoxyhemoglobin; this is called the blood oxygen level–dependent (BOLD) contrast technique. During cortical activation, cerebral blood flow to the eloquent cortex increases focally as a response to the stimulus, but oxygen extraction changes little. This causes a relative increased concentration of oxyhemoglobin and a relatively decreased concentration of deoxyhemoglobin draining the activated cortex. Deoxyhemoglobin is paramagnetic; it exerts magnetic susceptibility effects on local tissue, which are detected by T2-weighted imaging as decreased signal intensity. Oxyhemoglobin, on the other hand, is diamagnetic and has little effect on T2-weighted images. Thus, cortical activation results in a relative decrease of the lowered signal intensity produced by the decreased concentration of deoxyhemoglobin, which leads to a relative increase in signal in the activated cortex relative to contiguous cortex. fMRI has been used to map language, motor function, and interictal spikes. It also may be useful for seizure localization and has successfully mapped simple partial seizures. However, capturing seizures with fMRI is difficult, because seizures are unpredictable and complex partial seizures usually are associated with movement that obscures the fMRI image. POSITRON EMISSION TOMOGRAPHYSection 5 of 11
Fluoro-deoxyglucose PET FDG-PET reveals interictal hypometabolism of the epileptogenic temporal lobe in over 85% of cases. This zone of hypometabolism is much larger than the ictal-onset zone defined electrophysiologically and the epileptogenic region defined pathologically. This test is more sensitive when an asymmetry index is calculated comparing the quantitative metabolism of each temporal lobe and prevents misinterpretation due to partial volume averaging artifact. The degree of hypometabolism does not correlate with the degree of cell loss or the degree of hippocampal atrophy identified by MRI. In patients with TLE, unilateral hippocampal atrophy, and concordant EEG data, FDG-PET provides redundant data. However, it may provide additional information in patients whose MRI and EEG findings are discordant and in patients whose MRI findings are normal. Visual analysis of FDG-PET is less sensitive in frontal lobe epilepsy, with fewer than 50% of cases showing localized abnormalities. In these cases, quantitative normalized analysis may improve sensitivity of this test. Newer techniques, such as statistical parametric mapping (SPM) and 3-D stereotactic surface projection (3-DSSP) images, may be more sensitive than conventional FDG-PET analysis. With the 3-DSSP technique, the subject's PET scan is subtracted on a pixel-by-pixel basis from a normalized database of control subjects. This technique may provide localizing information in patients with either extratemporal epilepsy or TLE with a normal MRI. Co-registration with MRI may improve the sensitivity and specificity of FDG-PET by correcting for partial volume effects. 11C-flumazenil PET Preliminary evidence suggests that radioligand PET scans with the benzodiazepine antagonist 11C-flumazenil (FMZ) may have greater sensitivity in identifying the epileptogenic region than FDG-PET. FMZ labels central GABA receptors. Early studies at the University of Michigan showed a reduction in FMZ binding in the temporal lobe of patients with intractable TLE, which is more restricted than the region of hypometabolism seen with FDG-PET. This reduction in FMZ binding correlates with neuron loss in the hippocampus. Newer techniques, using SPM and an MRI-based method for partial-volume effect correction, have shown that the reduction in FMZ binding is greater than what would be expected from volume loss alone. This finding suggests that, in addition to neuronal loss, GABA binding in the epileptogenic hippocampus is reduced. FMZ-PET also shows enhanced sensitivity in patients with malformations of cortical development (MCD). More recent studies have shown either increases or decreases in benzodiazepine receptor density in regions of MCD. However, surgical outcome in patients with a localized abnormality on FMZ-PET and normal MRI findings is not yet known. Alpha methyl L-tryptophan and serotonin receptor PET imaging In view of basic science evidence that serotonin plays a role in epilepsy, PET imaging with serotonin precursors or serotonin agonists have been recently used with the hope of improving the detection of the epileptogenic zone. For example, reduced concentrations of brain serotonin are found in the brains of the genetically epilepsy-prone rat (GEPR). In addition, treatment with agents that facilitate serotonergic transmission inhibit seizures in many animal models of epilepsy. Reduction of brain serotonin concentrations, on the other hand, increases seizure susceptibility in animal models of epilepsy. Furthermore, in human epileptic brain tissue resected for the treatment of epilepsy, increased serotonin was found. Alpha methyl L-tryptophan PET (AMT-PET), like L-tryptophan, is a serotonin precursor that can help measure brain serotonin synthesis rates. Like tryptophan, AMT is metabolized into serotonin, but unlike tryptophan it is not converted into protein. AMT is converted to alpha methyl serotonin, but unlike serotonin, it is not metabolized by monoamine oxidase. Chugani and colleagues used AMT in patients with tuberous sclerosis and found reduced AMT uptake in cortical tubers as compared with normal cortex. However, epileptogenic tubers confirmed by ictal onset region demonstrated increased uptake. Another study by Fedi and colleagues evaluated patients with either cortical dysplasia on MRI or a normal MRI. Increased AMT uptake was identified in 60% of patients with cortical dysplasia and 30% of patients with normal MRI. Juhasz et al reported similar findings. AMT has also been studied in patients who failed epilepsy surgery and was able to identify increased AMT uptake in residual epileptogenic cortex as identified by intracranial EEG. However, it could only identify the epileptogenic region in 43%. Serotonin 5-HT1A receptor binding has also been studied with the PET ligands (18F)FCWAY and (11C)WAY. Toczek et al found reduced 5-HT1A binding in the medial and lateral temporal regions ipsilateral to the epileptogenic temporal lobe. Savic and colleagues reported similar findings but also reported reduced binding in limbic connections such as the cingulate cortex and the insula. ICTAL SPECTSection 6 of 11
Single-photon emission CT (SPECT) with hexamethylpropyleneamine oxime (HMPAO) is performed during the ictal period to help delineate the epileptogenic zone. It is particularly helpful in patients with normal MRI findings, as well as in patients with abnormal MRI findings and a nonlocalizing EEG. Since seizures are associated with increased glucose metabolism (metabolism is closely coupled to cerebral blood flow), ictal SPECT scans show increased perfusion in the region of seizure onset. However, obtaining a true ictal injection is important, particularly for extratemporal lobe seizures, since with late injections, the areas of increased perfusion may represent seizure spread rather than seizure onset. In TLE, ictal SPECT has 90% sensitivity in localizing seizures, with good interobserver reliability. Ictal increased perfusion is seen in both the medial and the lateral temporal lobe. In the immediate postictal period (60 seconds), hyperperfusion of the medial temporal lobe with hypoperfusion of the lateral temporal lobe are noted. In the late postictal period (up to 20 min postictally), perfusion in both the medial and lateral temporal lobes may be decreased. Ictal SPECT is not helpful in localizing seizures in patients with bilaterally independent temporal lobe seizures, since the procedure samples only one seizure at a time. Moreover, false lateralization with ictal SPECT may occur if the seizure ceases in the temporal lobe of origin while continuing in the contralateral temporal lobe at the time of tracer injection. For extratemporal lobe seizure, such as frontal and parietal lobe seizures, ictal SPECT has sensitivity as high as 90% in localizing seizures if ictal injection occurs shortly after ictal onset (ie, within 20 seconds). Subtraction ictal SPECT coregistered to MRI The sensitivity of ictal SPECT is increased significantly when ictal and interictal images are subtracted (see Image 5). This subtracted image is then superimposed on high-resolution MRI, which further increases the sensitivity and specificity of the interpretation. Surgical outcomes in patients whose seizure focus is localized with this technique are under study. More recently, postictal subtraction SPECT coregistered to MRI has been studied as a method of localizing the epileptogenic zone (see Image 6). Newer methods include statistical parametric mapping where a control database of interictal SPECT scans are subtracted from the patient's ictal perfusion pattern and a z score is generated. This is subsequently coregistered to MRI. MAGNETIC SOURCE IMAGINGSection 7 of 11
Magnetoencephalography (MEG) detects the magnetic fields produced by the electrical currents of neuronal activity. Unlike the electrical currents of neuronal activity, which are extracellular, magnetic fields are produced by the intracellular currents of apical dendrites, which are recorded from the scalp by MEG. Unlike conventional EEG that detects radially oriented electrical activity that is attenuated in strength and spatially distorted by tissues between brain and scalp surface, magnetic fields are minimally affected by intervening tissue layers. Furthermore, MEG measures a subset of neuronal activity that is tangential to the scalp. These magnetic dipoles generated by MEG are then superimposed on structural MR images creating magnetic source imaging (MSI). Numerous studies have shown that this technique is helpful in patients with neocortical epilepsies to map interictal epileptiform activity, which in conjunction with other noninvasive structural and imaging data, guide intracranial subdural grid placement to improve surgical outcome. A large series of 455 patients showed that MSI identified the lobe to be treated in 89% of patients. In all extratemporal cases, MSI correctly identified the correct lobe. One might argue that MSI provides redundant data. However, in this study, MSI provided additional information about the epileptogenic zone in 35% and provided crucial information for surgical decision making in 11%. Thus, MSI is a promising modality for seizure localization in that it can (1) confirm the epileptogenic zone along with other functional imaging data, (2) aid in the identification of a subtle cortical abnormality on MRI, and (3) provide localizing information not obtainable from other imaging modalities. In this regard, it can either obviate the need for invasive monitoring in cases with a structural lesion without localizing or lateralizing ictal EEG data or guide intracranial subdural electrode placement to improve localization of the epileptogenic zone and improve seizure-free outcome. The disadvantage to MSI is that it is limited to a few centers; it is performed in the outpatient setting in the United States where AEDs cannot always be tapered or discontinued safely; and recording time is limited, which reduces the chance of obtaining sufficient interictal data with the exception of patients who have frequent interictal activity. Furthermore, although ictal MSI has been recorded and is highly localizing, the chance of capturing a seizure during a study is small. Some centers will partially taper AEDs and/or give clonidine to enhance the yield of identifying interictal epileptiform activity. INTRACRANIAL EEG MONITORING AND WADA TESTINGSection 8 of 11
With the advent of newer imaging techniques, most patients do not require invasive intracranial EEG evaluation to localize their epileptogenic zone. In fact, today only 10-20% of all surgical candidates require this type of evaluation, whereas 50-60% of surgical candidates required this type of monitoring 10 years ago. Clinical indications for intracranial EEG monitoring
Definition of terms Epileptogenic lesion: A lesion that is able to produce seizures, which needs to be included in the resection for the patient to become seizure free Epileptogenic zone: Area of cortex that needs to be resected in order to make the patient seizure free. This zone usually includes the epileptogenic lesion but may be larger than the lesion. Irritative zone: Area of cortex that is involved in generating epileptiform discharges but whose resection is not necessary to make the patient seizure free. This region is usually larger than the epileptogenic lesion and zone. Often, these irritative spikes cease after surgical resection of the epileptogenic lesion and/or zone. Symptomatogenic zone: The region of cortex that is necessary to produce clinical symptoms but whose removal is not necessary to make the patient seizure free. For example, seizures may begin silently in the frontal lobe and produce a typical temporal lobe complex partial seizure when the discharge has spread there. Cortical stimulation studies have shown that often the region producing auras is much larger than the epileptogenic zone. Functional deficit zone: The region of cortex showing hypometabolism on FDG-PET, usually much larger than the epileptogenic lesion and/or zone. In medial TLE, for example, FDG-PET hypometabolism involves both medial and lateral temporal lobe cortex. This functional deficit zone can be in regions outside the epileptogenic zone. With quantitative FDG-PET, some patients with TLE have subtle regions of hypometabolism in the frontal lobe. In addition, patients with TLE usually have material-specific memory impairment that lateralizes to the side of seizure onset. They also can have frontal lobe dysfunction on neuropsychological testing as shown by Hermann et al. At present, whether this functional deficit zone returns to normal after surgical resection is unknown. In most cases when a lesion is present, the epileptogenic zone includes the lesion and the margin surrounding the lesion. This is mostly true for well-circumscribed lesions such as cavernous angiomas and also for well-circumscribed tumors. Other lesions such as encephalomalacia and cortical dysplasia have less clearly defined margins, and the epileptogenic zone may be larger than the lesion(s) visible on MRI. In these cases, intracranial EEG data and electrocorticography guide the resection. Often epileptiform activity is observed in a distribution outside the epileptogenic zone; these spikes represent the irritative zone. For example, an orbitofrontal tumor may generate anterior temporal spikes, even though the temporal lobe is not a part of the epileptogenic zone. Usually, these spikes cease after surgical resection of the epileptogenic zone. Similarly, interictal spikes from supplementary motor cortex can propagate to primary sensorimotor cortex to which it is intimately connected with. The types of electrodes used for intracranial electrode monitoring include depth electrodes, subdural strips, and subdural multicontact grid electrodes. Depth electrodes are helpful in sampling deep structures, such as hippocampus, amygdala, and subcortical heterotopias, but sample only a restricted area. In the temporal lobe, depth electrodes may be implanted orthogonal to the temporal lobe and sample the temporal lobe from medial to lateral. Other centers implant depth electrodes along the anteroposterior axis of the hippocampus, with the most anterior contact in the amygdala and the most posterior contact in the posterior temporo-occipital lobe. Subdural electrodes, on the other hand, record from the cortical surface and can sample larger areas. However, cortical gyri within the depth of a sulcus are not sampled adequately. Neocortical seizures propagate rapidly; thus, if seizure onset is deep within a sulcus, only propagated seizure activity is recorded. For this reason, cortical resections based on invasive EEG data without an MRI abnormality are associated with seizure-free outcome in only 20% of patients. In patients with TLE monitored with depth electrodes, propagation of the seizure discharges to the contralateral temporal lobe in more than 5 seconds correlates with hippocampal neuron cell loss and seizure-free outcome. Lieb et al at UCLA showed that contralateral propagation in less then 5 seconds correlates with surgical failure. In TLE, the typical depth electrode onset consists of either a hypersynchronous discharge with periodic sharp and slow waves followed by low-voltage fast activity, or low-voltage fast activity as the initial change. A less common ictal-onset pattern consists of cessation of ongoing interictal activity. Neocortical EEG onset consists of one of the following:
In general, intracranial electrode investigations are most helpful when a hypothesis about the location of the epileptogenic zone is clearly defined on the basis of noninvasive data. Intracranial electrode implantation is associated with a 2-3% rate of complications, which include hemorrhage or infection. Wada testing Intracarotid amobarbital procedure (IAP) involves the injection of sodium amobarbital into one carotid artery to temporarily inactivate the ipsilateral cerebral hemisphere, allowing independent testing of memory and language function of the contralateral hemisphere. IAP is believed to anesthetize ipsilateral carotid artery distribution, which includes the amygdala and the anterior hippocampus. Injection ipsilateral to the epileptogenic zone assesses the functional adequacy of the contralateral hippocampus to sustain memory. Consequently, the IAP helps to exclude patients who are at risk of developing postoperative amnesia after standard anterior temporal lobectomy (ATL). During intracarotid amobarbital injection, the presence of contralateral hemiparesis and ipsilateral EEG slowing is assessed, since these findings confirm the adequacy of the injection to produce hemispheric inactivation. Memory items are presented while the hemiparesis and EEG slowing persist. Some centers (University of Florida, University of Michigan, and Bayfront Medical Center) use sodium methohexital (Brevital). Its behavioral and neurologic effects are similar to amobarbital; however, its duration is shorter, and it produces less drowsiness. Consequently, behavioral and EEG indices return to baseline more quickly and more completely with methohexital than with amobarbital, allowing several repetitions of the procedure without incremental drowsiness, and the total time taken for the procedure is less with methohexital than with amobarbital. Because of the brief duration of methohexital at least 2 consecutive injections are given. The first dose is 3 mg, and the second dose of 2 mg is given when contralateral motor function begins to return. Additional 2-mg doses have been given to ensure adequate anesthetic effect of the drug if motor power returns before all memory items. Poor memory performance on the IAP with injection contralateral to the epileptogenic zone correlates with severe hippocampal neuron loss. In this regard, the IAP has been used to determine the memory function of the epileptogenic hippocampus and to help predict risk for verbal memory decline in patients undergoing dominant ATL. More recently, the IAP has been used to help predict seizure-free outcome in patients who undergo ATL. For example, an asymmetry score between the left and right hemispheres helps predict seizure-free outcome when the epileptogenic side performs less well than the contralateral side during IAP. In these cases, the surgical decision is never made on the basis of IAP findings alone. However, the IAP provides ancillary predictive information that is to be used in the context of clinical, EEG, imaging, and functional data. Furthermore, this information is useful with regard to preoperative patient counseling regarding language, memory, and seizure-free outcome. MULTIDISCIPLINARY EPILEPSY SURGERY CONFERENCESection 9 of 11
Multidisciplinary epilepsy surgery conferenceNeurologists specializing in epilepsy, epilepsy neurosurgeons, neuropsychologists, epilepsy nurses, speech pathologists, neuroradiologists, and psychiatrists attend this conference. Clinical, EEG, MRI, speech, and neuropsychological data are presented and discussed in detail. Psychosocial factors, such as how the seizures are affecting the patient's quality of life, and, in the case of children, how seizures are impairing normal development, are discussed. The potential cognitive and language ramifications of surgery are discussed when patients with dominant hemisphere epilepsy are presented. For example, patients with relatively preserved verbal memory are at the greatest risk for a significant verbal memory decline after surgery. Patients with high Boston naming scores and a later age of onset of their epilepsy are at greatest risk for decline in naming ability after dominant temporal lobectomy. Patients at risk for postoperative psychiatric complications are followed closely by psychiatry beforeand after epilepsy surgery. If the data are concordant, the patient is scheduled for surgery after the IAP. If the presurgical data presented are discordant, intracranial EEG monitoring is recommended (see Clinical indications for intracranial EEG monitoring in Intracranial EEG Monitoring and Wada Testing). If the epileptogenic region involves eloquent cortex, either intraoperative or extraoperative cortical mapping is recommended. The consensus of the conference is then presented to the patient by his neurologist and the chances for a seizure-free outcome and the potential risks of surgery are discussed. Types of resections offered
SummaryThe presurgical evaluation of patients with medically refractory epilepsy begins with video-EEG monitoring. Here, the ictal semiology is observed closely for localizing and lateralizing information. These clinical data are correlated with ictal EEG data for localization of the epileptogenic zone. In recent years, neuroimaging has developed a central role in the presurgical evaluation. High-resolution MRI can identify hippocampal atrophy in greater than 87% of cases and subtle malformations of cortical development that were previously considered "nonlesional" can now be identified. Newer imaging techniques, such as 3T phase array MRI and voxel-based morphometry of gray matter, can increase the yield of identifying a subtle focal cortical dysplasia that can be correlated with the results from video-EEG monitoring. In cases of negative findings on MRI, FDG-PET can be helpful in identifying the epileptogenic zone particularly when statistical parametric mapping is performed. The use of novel PET tracers, such as flumazenil PET, alpha methyl tyrosine, and serotonin agonists, which are largely investigational at this time, are promising for the future. Ictal SPECT, with early injection times, can help localize the epileptogenic zone. The sensitivity and the specificity of this test is increased when the ictal study is subtracted from the interictal study and then coregistered to MRI. Once the noninvasive data are obtained, the case is presented at the epilepsy surgery conference. If surgery without invasive monitoring is recommended, an angiogram/Wada test is performed to lateralize language and to predict memory outcome after surgery in TLE. Those patients with TLE who do poorly on memory testing after amobarbital injection ipsilateral to the epileptogenic zone are not offered surgery, since anterior temporal resection would lead to permanent amnesia. In some cases, the Wada test is recommended in order to provide ancillary information for lateralization of the epileptogenic zone in TLE. When noninvasive presurgical data are insufficient to proceed to surgical resection, patients are referred for invasive monitoring. These patient groups include, but are not limited to, those with (1) TLE that is not lateralized; (2) dual pathology; (3) lesions and nonlocalizing EEG; and (4) localizing interictal and/or ictal EEG, magnetic source imaging (MSI), and/or functional imaging in the absence of a clearly defined lesion. With improvements in neuroimaging, the need for invasive monitoring has decreased over the last 10 years. On the other hand, invasive monitoring is being considered in some patients who were previously deemed not to be surgical candidates due to improved noninvasive structural and functional imaging data. In the future, fMRI or MSI can be used to lateralize language and assess memory function, making the Wada test no longer necessary. In patients with "nonlesional" epilepsy, the use of multimodality imaging with coregistration can help to delineate the epileptogenic region and improve the seizure-free outcome rate. MULTIMEDIASection 10 of 11
REFERENCESSection 11 of 11
| ||||||||||||||||||||||||||||||||||||||||||
