OBJECTIVE: For patients with infantile epileptic spasms syndrome (IESS) who have achieved remission of epileptic spasms (ES), indicators of how well the electroencephalographic (EEG) state should be maintained during follow-up are not available. We hypothesized that the time course of the Burden of Amplitudes and Epileptiform Discharges (BASED) score after ES remission is associated with ES relapse. This study aimed to investigate the association between ES relapse and BASED scores at the time of initial ES remission and during the subsequent follow-up period. METHODS: We collected clinical and digital EEG data at four hospitals from patients with IESS who achieved initial ES remission. The BASED score was evaluated using EEGs obtained before remission, during remission, and after remission. The scoring was performed independently by the three reviewers, who were blinded to each other's scores. We analyzed the associations between clinical factors, BASED score, maximum BASED score, and ES relapse. RESULTS: Data were collected from 44 patients aged 4-8 months at disease onset, which included 33 nonrelapsed and 11 relapsed patients. We assessed the BASED score of 262 EEGs. Structural etiology (p = .002) and the development of seizure other than ES (p = .04) were positively associated with ES relapse. Whereas BASED score before and at remission was not associated with relapse, a maximum BASED score of ≥3 during the postremission period was significantly associated with relapse (p <
.001, odds ratio = 100). In multivariate analysis, only maximum BASED score ≥3 during this period remained significantly associated with relapse (p = .001, odds ratio = 76). The interrater reliability of the BASED score was high, with a quadratically weighted kappa coefficient of .93. SIGNIFICANCE: A maximum BASED score of ≥3 during postremission indicates a risk for ES relapse in patients with IESS.