OBJECTIVE: This study was undertaken to develop a standardized grading system based on expert consensus for evaluating the level of confidence in the localization of the epileptogenic zone (EZ) as reported in published studies, to harmonize and facilitate systematic reviews in the field of epilepsy surgery. METHODS: We conducted a Delphi study involving 22 experts from 18 countries, who were asked to rate their level of confidence in the localization of the EZ for various theoretical clinical scenarios, using different scales. Information provided in these scenarios included one or several of the following data: magnetic resonance imaging (MRI) findings, invasive electroencephalography summary, and postoperative seizure outcome. RESULTS: The first explorative phase showed an overall interrater agreement of .347, pointing to large heterogeneity among experts' assessments, with only 17% of the 42 proposed scenarios associated with a substantial level of agreement. A majority showed preferences for the simpler scale and single-item scenarios. The successive Delphi voting phases resulted in a majority consensus across experts, with more than two thirds of respondents agreeing on the rating of each of the tested single-item scenarios. High or very high levels of confidence were ascribed to patients with either an Engel class I or class IA postoperative seizure outcome, a well-delineated EZ according to all available invasive EEG (iEEG) data, or a well-delineated focal epileptogenic lesion on MRI. MRI signs of hippocampal sclerosis or atrophy were associated with a moderate level of confidence, whereas a low level was ascribed to other MRI findings, a poorly delineated EZ according to iEEG data, or an Engel class II-IV postoperative seizure outcome. SIGNIFICANCE: The proposed grading system, based on an expert consensus, provides a simple framework to rate the level of confidence in the EZ reported in published studies in a structured and harmonized way, offering an opportunity to facilitate and increase the quality of systematic reviews and guidelines in the field of epilepsy surgery.

APHM Timone Hospital Epileptology and Cerebral Rhythmology Marseille France

Brno Epilepsy Center Department of Neurology St Anne's University Hospital Brno Czech Republic

Clinical Neuroscience Department Carol Davila University of Medicine and Pharmacy Epilepsy Monitoring Bucharest Romania

Department for Epileptology and Clinical Neurophysiology Medical Faculty Sigmund Freud University Vienna Austria

Department of Clinical Medicine Aarhus University Aarhus Denmark

Department of Clinical Neurophysiology Danish Epilepsy Center and Aarhus University Hospital Dianalund Denmark

Department of Clinical Neuroscience Institute of Neuroscience and Physiology Sahlgrenska Academy at Gothenburg University Gothenburg Sweden

Department of Clinical Neurosciences Centre Hospitalier Universitaire Vaudois and Université de Lausanne Lausanne Switzerland

Department of Epilepsy Movement Disorders and Physiology Kyoto University Graduate School of Medicine Kyoto Japan

Department of Epileptology University Hospital Bonn Bonn Germany

Department of Functional Neurology and Epileptology Hospices Civils de Lyon and Lyon 1 University Lyon France

Department of Neurology and Biomedical Engineering Duke University Durham North Carolina USA

Department of Neurology Northwestern University Chicago Illinois USA

Department of Neurology Tel Aviv Sourasky Medical Center; Faculty of Medicine Tel Aviv University Tel Aviv Israel

Department of Neurology University of Texas Health Science Center Houston Texas USA

Department of Neurosciences Mater Misericordiae Hospital University of Queensland Brisbane Queensland Australia

Department of Neurosurgery Sahlgrenska University Hospital Gothenburg Sweden

Division of Neurology Centre Hospitalier de l'Université de Montréal Montreal Quebec Canada

EEG and Epilepsy Unit Department of Clinical Neurosciences Geneva University Hospital and University of Geneva Geneva Switzerland

Emergency University Hospital Bucharest Bucharest Romania

Epilepsia Helsinki Helsinki University Hospital Helsinki Finland

Epilepsy Center Neurocenter University Medical Center University of Freiburg Freiburg Germany

Epilepsy Monitoring Unit Department of Neurology and Medical Research Institute Hospital del Mar Barcelona Spain

Epilepsy Unit Department of Neurology National Institute of Mental Health Neurology and Neurosurgery Budapest Hungary

Faculty of Health and Life Sciences Universitat Pompeu Fabra Barcelona Spain

Karl Landsteiner Institute for Clinical Epilepsy Research and Cognitive Neurology Vienna Austria

Kokilaben Dhirubhai Ambani Hospital and Medical Research Center Mumbai India

Member of the ERN EpiCARE

Neuroscience Department Meyer Children's Hospital IRCCS Florence Italy

School of Medicine Masaryk University Brno Czech Republic

University of Florence Florence Italy

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