Rachel Cottier ^1,2,3 Farbod Niazi ^1,2 Keshav Goël ⁴ Catherine Korman ^1,5 Tiphaine Porte ⁶ Thierry Ducruet ⁶ Giulia Cossu ³ Christina Briscoe Abath ⁷ Avantika Singh ⁸ Chellamani Harini ⁷ George Ibrahim ^10,11,9 Aria Fallah ⁴ Alexander Gregory Weil ^1,12,13,2 Aristides Hadjinicolaou ^1,13,5*

• ¹ CHU Sainte Justine Research Center, University of Montreal, Montréal, Canada
• ² Division of Neurosurgery, CHU Sainte-Justine, Montreal, Quebec, Canada
• ³ Department of Neuroscience, Section of Neurosurgery, University Hospital of Lausanne and University of Lausanne, Lausanne, Switzerland, Lausanne, Switzerland
• ⁴ Department of Neurosurgery, David Geffen School of Medicine, University of California, Los Angeles, Los Angeles, California, United States
• ⁵ Division of Neurology, Department of Paediatric, CHU Sainte-Justine, Montreal, Ontario, Canada
• ⁶ Institut de Recherche en Santé Publique, Université de Montréal, Montreal, Ontario, Canada
• ⁷ Epilepsy Center, Department of Neurology, Boston Children’s Hospital, Harvard Medical School, Boston, Massachusetts, United States
• ⁸ Department of Neurology, Division of Pediatric Neurology, Children's Wisconsin, Medical College of Wisconsin, Milwaukee, WI, USA, Milwaukee, United States
• ⁹ Neuroscience and Mental Health, Hospital for Sick Children, Toronto, Ontario, Canada
• ¹⁰ Division of Neurosurgery, Hospital for Sick Children, University of Toronto, Toronto, Ontario, Canada
• ¹¹ Institute of Medical Science, Temerty Faculty of Medicine, University of Toronto, Toronto, Ontario, Canada
• ¹² Division of Neurosurgery, Department of Surgery, University of Montreal Hospital Centre, Montréal, QC, Canada, Montreal, Canada
• ¹³ Department of Neuroscience, University of Montréal, Montréal, QC, Canada, Montreal, Canada

Epileptic spasms (ES) are a unique seizure type typically presenting in the form of infantile epileptic spasms syndrome (IESS) with characteristic hypsarrhythmia on scalp EEG and a preponderance with developmental delay or regression. While pharmacotherapy is the mainstay of treatment, disconnective or resective surgical procedures are increasingly recognized as viable therapeutic options for recurrent or persistent ES. However, limited data on safety, effectiveness, and prognostic factors hinder informed decision-making regarding surgery indications, timing, and intervention type. We performed a systematic review and an individual patient data meta-analysis (IPDMA) in accordance with PRISMA guidelines, focusing on surgical interventions for ES and reporting seizure outcomes using the Engel or ILAE scales. Twenty-six studies encompassing 358 ES patients undergoing resection/callosotomy were included. Participants undergoing other approaches (e.g. multiple subpial transections) or multimodality approaches were excluded from analysis. The median age at spasm onset was 6 months (IQR = 3.0-15.6), with a median age at surgery of 37 months (IQR = 17.2 -76.8). Most patients (74.1%) exhibited additional seizure types. A total of 136 patients (35.8%) underwent corpus callosotomy (CC), of whom 125 (91.9%) had a complete callosotomy, while 11 (8.1%) had a partial callosotomy. Resective surgery was performed on 222 patients (58.4%), of which 109 (49.1%) had both lesional MRI findings and lateralized EEG abnormalities. Overall, 201 patients (56.1%) remained spasm-free at a median postoperative follow-up of 36 months (interquartile range, IQR = 21-60), including 52 (38.2%) from the callosotomy group and 149 (67.1%) from the resective surgery group. In the resective surgery cohort, patients with MRI-confirmed lesions (p=0.026; HR=0.53, 95% CI = 0.31-0.93) and those who underwent hemispherectomy (p=0.026, HR=0.46, 95% CI = 0.23 -0.91) had better seizure outcomes. Only a minority (24.4%) underwent invasive EEG monitoring prior to ES surgery. Surgical treatment of ES proves effective, with two thirds of patients undergoing resective surgery and a third undergoing CC becoming spasm free. Post-operative developmental improvement was observed in 44 participants (65.7% of those with available data). The presence of lesional MRI and more extensive resection/disconnection (e.g. hemispherectomy) emerged as significant prognostic factors for spasm freedom and can inform clinical decision-making.