Rationale: Intracranial electroencephalography (iEEG) is crucial for determining indications for resection of seizure onset zone and defining extent of resection in surgical treatment of medically intractable epilepsy. The placement of intracranial electrodes causes acute pathological changes in brain parenchyma that may affect excitability of the brain. These changes evolve and subside over the course of postsurgical recovery, and it is not clear whether they might affect the diagnostic value of seizure recorded at different phases of the monitoring period. In this study, we investigated whether the timing after the electrode implantation affects diagnosis of the location of seizure onset. Methods: A total of 62 patients underwent iEEG at University of Iowa Hospitals & Clinics from 2006 until 2019, had clinical seizures during the iEEG monitoring, underwent resection of epileptogenic zones, and were followed more than a year. The resection site was finalized mainly based on the onset of habitual seizures recorded with iEEG. We examined the seizure onset sites that were recorded within three days (early) and after four days (late) following electrode implantation in relation to the resection sites and seizure control outcomes. Results: The duration of iEEG monitoring was 12.1 ± 3.5 days. The concordance of the seizure foci and the resection sites was significantly higher with the late seizures (p < 0.01), and 54 patients with favorable outcomes were also significantly more concordant in the late period (p < 0.01). Next we analyzed the patients who had seizures in both periods. Twenty-four patients had seizures at both periods and 22 of them had favorable outcomes. In these cases, the concordance between the seizure focus and the resection site was significantly higher in the late period (p = 0.02). Conclusions: Seizures that occurred after four days had more diagnostic value than early seizures in determining the location and the extent of resection. Funding: Please list any funding that was received in support of this abstract.: No funding
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