Background: Abnormal late responses to single pulse electrical stimulation (SPES) in patients with intracranial recordings can identify epileptogenic cortex. We aimed to investigate the presence of neuropathological abnormalities in abnormal SPES areas and to establish if removal of these areas improved postsurgical seizure control.
Methods: We studied abnormal responses to SPES during chronic intracranial recordings in 40 consecutive patients who were thereafter operated on because of refractory epilepsy and had a follow-up period of at least 12 months.
Findings: 22 patients had abnormal responses to SPES exclusively located in resected regions (96% with favourable outcome), seven had abnormal responses to SPES located in resected and non-resected regions (71% with favourable outcome), three had abnormal responses to SPES exclusively outside the resected region (none with favourable outcome), and eight did not have abnormal responses to SPES (62.5% with favourable outcome). Surgical outcome was significantly better when areas with abnormal responses to SPES were completely resected compared with partial or no removal of abnormal SPES areas (p=0.006). Neuropathological examination showed structural abnormalities in the abnormal SPES areas in 26 of the 29 patients in whom these regions were resected, despite the absence of clear MRI abnormalities in nine patients.
Interpretation: Abnormal responses to SPES are functional markers of epileptogenic structural abnormalities, and can identify epileptogenic cortex and predict surgical outcome, especially when a frontal or temporal focus is suspected.