PurposeInfantile epileptic spasms syndrome (IESS) with epileptic spasms as the main seizure type, is treated with adrenocorticotropic hormone (ACTH). This study, for the first time, examines the effects of epileptic spasms and ACTH on blood-brain barrier (BBB) permeability in patients with IESS of unknown etiology. MethodsWe prospectively evaluated the changes in BBB permeability in patients with IESS of unknown etiology at the Saitama Children's Medical Center between February 2012 and February 2024. We compared the levels of serum-albumin, cerebrospinal fluid (CSF)-albumin, Q-albumin, and CSF-neuron-specific enolase (NSE) before and after ACTH therapy. We also assessed the correlation between the frequency of epileptic spasms and these markers. ResultsOverall, 16 patients with IESS (8 males) were included in the study. The median age at IESS onset was 5 (range, 2–9) months. The median duration between the epileptic spasms onset and the serum and CSF sample examination before ACTH therapy was 26 (range, 1–154) days. After ACTH therapy, CSF-albumin and Q-albumin levels significantly decreased (CSF-albumin: 13.5 (9.0–32.0) mg/dL vs 11.0 (7.0–19.0) mg/dL, p = 0.001. Q-albumin: 3.7× 10−3 (2.2 × 10−3-7.3 × 10−3) vs 2.8× 10−3 (1.9 × 10−3-4.5 × 10−3), p = 0.003). No correlation was observed between the epileptic spasms frequency and levels of serum-albumin, CSF-albumin, Q-albumin, and CSF-NSE (Spearman's coefficient: r = 0.291, r = 0.141, r = 0.094, and r = −0.471, respectively). ConclusionACTH therapy is one of the factors that play a role in restoring BBB permeability in patients with IESS of unknown etiology. Our findings may be useful in elucidating the mechanism of ACTH action and IESS pathophysiology.