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The Role of Investigative Modalities in Epilepsy Work-up

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Author(s): Helen Nayeri

Journal: Iranian Journal of Radiology
ISSN 1735-1065

Volume: 6;
Issue: S1;
Start page: 117;
Date: 2009;
Original page

ABSTRACT
"nEpilepsy is a common disorder, affecting 50 million people worldwide. The prevalence of epilepsy has significant medical, social, and economic implications both for the individual and for the society. "nIn evaluating the epilepsy patient, it is helpful to be familiar with the etiologies commonly associated with this disease. Identifiable causes of partial epilepsy have been divided into 5 categories, namely neoplasms, vascular abnormalities, mesial temporal sclerosis (MTS), non-vascular developmental disorders and indeterminate substrates (consisting mostly of non-specific gliosis, traumatic, atrophic or inflammatory abnormalities). In studies of surgical epilepsy patients, hippocampal sclerosis is the most common cause (50% to 70%). "nLocalization of the epileptogenic focus is the major task in preoperative evaluation of surgical candidates. Presently, no single technique can precisely identify the epileptogenic zone. In the past, EEG was essentially the only method of localizing the seizure focus. Accuracy of the noninvasive scalp EEG is limited and often inadequate for preoperative evaluation. "nComputed tomography is the appropriate modality to evaluate the underlying cause of new-onset seizures in the emergent setting. It has little or no role in the evaluation of patients with intractable seizures. In cases of refractory seizures, MR has significantly greater sensitivity for lesion detection than does CT. Because of its ability to depict neuroanatomy, MR is ideally suited for identifying focal brain abnormalities, and it can detect structural lesions with a high degree of sensitivity. The accuracy of MR in determining the substrate category in intractable epilepsy has been reported to be 88%. "nMR and video monitoring EEG are widely available and are the most critical noninvasive studies in the evaluation of the epileptogenic zone. There is an approximately 70% correlation of MRI findings with EEG abnormality for patients with temporal lobe epilepsy. In such patients, there is an approximately 97% satisfactory post-operative outcome. In cases with discordant EEG and MR findings or when confirmation of the epileptogenicity of the MR-identified lesion is lacking, intracranial EEG recording using subdural or parenchymal depth electrodes are warranted. Invasive electrophysiological studies are also indicated in cases with more than one MR abnormality, when MR shows a large atrophic region or developmental abnormality, and when functional mapping of the brain is indicated based on MR findings or other reasons. "nOther modalities for preoperative evaluation of surgical candidates with intractable epilepsy involve functional neuroimaging (include SPECT, PET, magnetoencephalography) as well as neuropsychological assessment and lateralization of language and memory functions by Amytal procedure (IAP or Wada test). "nMR imaging protocols for epilepsy vary. The protocol must be able to detect hippocampal sclerosis, as well as foreign tissue lesions (i.e., neoplasms, infections, etc) and developmental abnormalities. The MR imaging of amygdala and hippocampus is best performed in a slightly oblique coronal plane, perpendicular to the long axis of hippocampus. The study requires evaluation of both morphological and signal abnormalities. High resolution FSE T2W and IR sequences are important for depiction of hippocampal signal and architectural changes. Either a coronal T1W gradient volume acquisition (e.g., SPGR or MP-RAGE sequences) or inversion recovery (IR) can be used to help evaluate cortical dysplasia, as well as to assess subtle hippocampal asymmetry. High-resolution T2 and axial inversion recovery FSE imaging is also routinely used. Coronal, T1-weighted, three dimensional volume, gradient echo sequences not only provide excellent gray-white matter differentiation, but also thin slices (1.0 to 1.6 mm thick), which may improve the detection of subtle malformations of cortical development. Volume loss and T2 signal changes can be assessed quantitatively as well as by visual inspection. Quantitative analysis requires volumetric acquisition sets using either 3D gradient recalled sequences or T2-weighted fast spin-echo or turbo spin-echo sequences. Quantitative data may increase diagnostic accuracy and reliability to between 80% and 100%. With small FOV phased array surface coils significant improvements in resolution are realized. "nThe diffusion-weighted sequence and calculated apparent diffusion coefficient (ADC) maps and diffusion tensor imaging have been used to identify diffusion abnormalities in cases of previously MR-normal-appearing hippocampal sclerosis and refractory epilepsy. "nMRS is helpful in resolving some of the issues involved in the treatment of MTS that are difficult by MRI alone. "nMR tractography in the future will map the spread of abnormal electrical seizure activity from its initial focus through the epileptogenic zone. "nFunctional MRI (fMRI) detects brain activity, which can be co-registered to conventional MR images for exquisite anatomic localization of brain activity. "nWe will continue to witness the increasing confluence of structural, metabolic, and functional imaging of epilepsy all under the rubric of MR imaging.  
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