BACKGROUND: This study evaluates the electroclinical features of infantile epileptic spasms syndrome (IESS) suddenly appearing in previously normal patients, aiming to describe clinical outcomes and independent predictors. METHOD: We retrospectively selected a homogeneous group of patients with IESS from two Italian centers. All patients had normal development prior to IESS onset and a follow-up period lasting at least one year. Patients with clinically relevant risk factors, other seizure types, brain structural abnormalities or known genetic diseases were excluded. The BASED score was used to standardize interictal EEG patterns. RESULTS: Forty-three patients were enrolled, with a median age at IESS onset of 6 months
median follow-up was 43 months. At onset, 65.11 % exhibited mild behavioral changes, including irritability and poor social smile. At firstEEG, epileptic encephalopathy (EE) was prevalent during wakefulness (69.76 %
median BASED score 4) and sleep (81.40 %
median BASED score 5). Within 15 days of treatment, 83.72 % achieved seizure freedom, primarily with ACTH depot (90.70 %). After six months, all patients were seizure- and EE-free. At the last follow-up, 81.40 % had normal cognitive functioning
in the remaining, specific neurodevelopmental disorders, predominantly involving language were reported. No statistically significant differences were found in the electroclinical presentation and neuropsychological outcome. CONCLUSION: We describe a subgroup of IESS patients with prompt response to treatment, long-term seizure freedom, and absence of severe neurodevelopmental impact. Our data suggest that within the IESS spectrum, there is a distinctive subgroup with global favorable outcome. Key clinical features predictors of good outcome could include normal development prior to IESS and early response to treatment.