Can histologically normal epileptogenic zone share common electrophysiological phenotypes with focal cortical dysplasia? SEEG-based study in MRI-negative epileptic patients.

We aimed to assess stereoelectroencephalography (SEEG) seizure-onset and interictal patterns associated with MRI-negative epilepsy and investigate their possible links with histology, extent of the epileptogenic zone (EZ) and surgical outcome. We retrospectively analysed a cohort of 59 consecutive MRI-negative surgical candidates, who underwent SEEG recordings followed by cortectomy between 2000 and 2016. Most of the eight distinct seizure-onset patterns could be encountered both in confirmed focal cortical dysplasia (FCD) and in histologically non-specific or normal cases. We found strong correlation (p = 0.008) between seizure-onset pattern and histology for: (1) slow-wave/DC-shift prior to low voltage fast activity (LVFA), associated with normal/non-specific histology, and (2) bursts of polyspikes prior to LVFA, exclusively observed in FCD. Three interictal patterns were identified: periodic slow-wave/gamma burst, sub-continuous rhythmic spiking and irregular spikes. Both "periodic" patterns were more frequent in but not specific to FCD. Surgical outcome depended on the EZ complete removal, regardless electrophysiological features. Histologically normal and FCD-associated epileptogenic zones share distinct interictal and ictal electrophysiological phenotypes, with common patterns between FCD subtypes and between dysplastic and apparently normal brain. Some specific seizure-onset patterns seem to be predictive of the underlying histology and may help to detect an MRI-invisible FCD.