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Non-Invasive Work-Up in Presurgical Evaluation of Extratemporal Lobe Epilepsy*

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Non-Invasive Work-Up in Presurgical Evaluation of Extratemporal Lobe Epilepsy* BEL_Inhalt_3_2010_6.0 29.09.2010 11:11 Uhr Seite 151 Non-Invasive Work-up in Presurgical Evaluation of Extratemporal Lobe Epilepsy* Margitta Seeck 1, Christoph M. Michel1,2 and Serge Vulliémoz1 1 Neurology Clinics, University Hospital of Geneva 2 Department of Fundamental Neurosciences, University Medical School, University of Geneva Summary des epileptogenen Fokus sowie der evozierten Potenzia- le, EEG-getriggertes funktionelles MRI, sowie die Ko-Re- Surgery of extratemporal epilepsy is still a challenge, gistrierung aller Bildgebungsverfahren mit dem indivi- due to difficulties to define precisely the epileptogenic duellen MRI des Patienten. zone and the presence of vital cortex, such as motor, sensory, visual or language cortex. Both may be per- Schlüsselwörter: Extratemporale Epilepsie, prächirurgi- ceived as too close, preventing the complete resection sche Abklärung, Lokalisation, elektrische Quellenanaly- of the epileptic focus and thus, leading to a less favor- se, EEG-fMRI able result of extratemporal lobe epilepsy surgery as compared to temporal lobe epilepsy surgery. In most cases, invasive monitoring with implanted electrodes is Bilan non-invasif dans l'évaluation préchirurgicale required. However, pre-operative comprehensive loca- de l'épilepsie extra-temporale lization of the focus as well as essential cortex can be done also with non-invasive tools. Here, we discuss the Le traitement chirurgical de l'épilepsie extra-tem- most advanced techniques, readily applied in extratem- porale reste un challenge important, en raison des diffi- poral surgical epilepsy, i.e. electric source imaging (ESI) cultés pour définir précisément la zone épileptogène et of the focus and evoked potentials, simultaneous EEG- la localisation du cortex éloquent, tel les cortex moteur, fMRI recordings, and co-registration of the different sensitif, visuel ou langagier. Ceux-ci peuvent être jugés imaging modalities with the individual patient's MRI. trop proches du foyer épileptique, empêchant sa résec- tion totale avec, comme conséquence, un résultat post- Epileptologie 2010; 27: 151 – 163 opératoire moins favorable que dans le cas de l'épilepsie du lobe temporal. Dans la plupart des cas, un enregis- Key words: Extratemporal lobe epilepsy, presurgical trement EEG invasif avec des électrodes implantées est evaluation, localisation, EEG source imaging, EEG-fMRI nécessaire. Toutefois, une localisation pré-opérative détaillée du foyer et du cortex éloquent peut être ac- complie aussi avec des outils non-invasifs. Dans cet ar- ticle, nous discutons des techniques de pointes ac- Zusammenfassung tuellement appliquées dans le bilan préchirurgical de l'épilepsie extra-temporale : L'imagerie de source EEG Die chirurgische Therapie der extratemporalen Epi- (ESI) du foyer et du cortex éloquent, l'enregistrement lepsien ist immer noch eine Herausforderung, da es simultané EEG et IRM fonctionnelle (EEG-fMRI) et le schwierig ist, die epileptogene Zone und den vitalen recalage des différentes images avec IRM structurelle Kortex, wie zum Beispiel den sensorischen, motori- du patient individuel. schen, visuellen oder Sprachkortex, genauestens zu lo- kalisieren. Beide können als zu nahe wahrgenommen Mots clés : Epilepsie extratemporale, évaluation werden, was die komplette Resektion des epileptoge- préchirurgicale, localisation, imagerie de source électri- nen Fokus verhindert, und damit – im Gegensatz zu den que, IRMf-EEG Temporallappen-Epilepsien – ein weniger gutes operati- ves Resultat erreicht wird. In den meisten Fällen ist ein *Acknowledgements invasives Monitoring mit implantierten Elektroden er- The authors are supported by the Swiss National Science forderlich. Die umfassende prä-operative Lokalisation Foundation (grants # 32-111783, 320030-122073, des Fokus sowie des essenziellen Kortex kann jedoch 32-113766, SPUM 33CM30-124089) and by the Biome- heute auch mit nicht-invasiven Methoden durchgeführt werden. Hier diskutieren wir die neuesten Techniken, dical Imaging Center (CIBM) Geneva-Lausanne. We die oft bei Patienten mit extratemporaler Epilepsie an- thank Dr. Laurent Spinelli for the image co-registration, gewandt werden: die elektrische Quellenanalyse (ESI) and Drs Verena Brodbeck and Agustina Lascano for data acquisition and analysis. Non-Invasive Work-up in Presurgical Evaluation of Extratemporal Lobe Epilepsy | M. Seeck, C. M. Michel, S. Vulliémoz Epileptologie 2010; 27 151 BEL_Inhalt_3_2010_6.0 29.09.2010 11:11 Uhr Seite 152 Introduction high density EEG, evoked potentials for localization of essential cortex, simultaneous EEG-fMRI recordings, An essential task in the surgical treatment of epilep- and finally, the co-registration of the results from diffe- sy is the definition of resection margins that allows the rent imaging modalities. removal of the epileptogenic zone as complete as pos- sible while avoiding neurological deficits. To achieve this goal, the pre-surgical work-up includes precise loca- Electric source imaging (ESI) lization of the epileptogenic cortex and of the functio- nally surrounding eloquent cortical areas. While this re- In contrast to fMRI, PET and SPECT, ESI directly mea- quires often invasive evaluation with intracranial elec- sures neuronal activity [6, 7]. It is based on the record- trodes, recent research showed that a large amount of ing of the electric potential field on the scalp using elec- work-up can be done with non-invasive tools. troencephalography (EEG), and similar to any other It has to be kept in mind, that even with intracranial imaging technique, its yield is higher with “sensors”, or monitoring, the chances to become seizure-free when scalp electrodes allowing a more extensive coverage of suffering from extratemporal lobe epilepsy are still less the brain [8]. The high temporal resolution (millise- likely than in temporal lobe epilepsy. Most studies re- conds) allows tracing the flow of neuronal activity in port 50-70 % of extratemporal patients with post-ope- the whole brain in real time. This is important for a dis- rative complete seizure control, and if the MRI is nor- ease like epilepsy where fast propagation is regularly mal, only around 40 % can be expected to become sei- encountered during interictal and ictal activity. Magne- zure-free after surgical intervention [1]. toencephalogram (MEG) relies on the same electrophy- The risks and benefits of invasive monitoring siological phenomena, i.e. temporally synchronized and have to be carefully balanced against each other. Up to spatially aligned postsynaptic potentials, however, EEG 10 % of all patients with intracranial monitoring experi- – when recorded with a sufficient high number of elec- ence more or less significant complications. Reported trodes – has a higher sensitivity than MEG because it risk factors are the following: greater number of in- “sees” neuronal activity independent of the dipole ori- tracranial electrodes, longer duration of monitoring, entation and it is more susceptible for deeper sources older age at implantation, left hemisphere implanta- [9]. Given that MEGs require a larger financial invest- tion, and experience of the neurological and neurosur- ment and maintenance, and that it is difficult to carry gical team. In the Cleveland clinic experience, in the out ictal recordings with MEG, we are focusing here on- first 10 years of their activity, 42 % of all implanted pa- ly EEG/ESI. tients had complications, which deceased to 19 % after > Our own studies showed that > 100 electrodes are 10 years of teamwork [2]. Most frequent problems are necessary to perform ESI with good to excellent preci- bleeding (subdural, epidural or intracerebral hemato- sion. ESI is reliable and provides valid results only if ba- mas) and infections, requiring constant clinical monito- sal temporal, frontal and occipital regions are covered ring by an experienced staff. Moreover, intracranial elec- [10] . Due to lower conductivity than previously estima- trodes cover only a small amount of surrounding tissue ted, in particular in young children or newborns (thin- (in the order of several cm3) [3], which means that the ner skull), >250 electrodes are theoretically necessary to placement of electrodes needs careful elaboration of obtain correctly localizing results [11, 12 ]. Fortunately, hypothesis of possible seizure origins. If the intracranial this requirement is now met by several recent systems, electrodes are not correctly placed, the yield of this which are commercially available and allow recordings costly and physically demanding exam is markedly re- of up to 256 electrodes [6, 13], probably more in the duced. near future. Fast application of electrode caps with As indicated above, not only the focus but also large number of electrodes is now readily possible, the vital cortex needs to be localized before an eventual making high density EEG well feasible in clinical resection. Again, this may require intracranial electrode routine. placement, with consecutive electrical stimulation (or Most of the clinical ESI studies analyzed interictal electro-corticography, ECOG), but ECOG alone as gold spike activity. Simultaneous intracranial and scalp re- standard for functional localization has been also ques- cordings showed that the analysis of scalp spikes pro- tioned [4, 5]. vides more reliable localizing information about the Given the significant impact of proper localization epileptogenic zone than the analysis of ictal activity of the focus and adjacent eloquent cortex on the surgi- [14]. This is principally due to less extensive
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