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

Abstract

Objective: 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.

Methods: We retrospectively analysed a cohort of 59 consecutive MRI-negative surgical candidates, who underwent SEEG recordings followed by cortectomy between 2000 and 2016.

Results: 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.

Conclusions: 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.

Significance: Some specific seizure-onset patterns seem to be predictive of the underlying histology and may help to detect an MRI-invisible FCD.

Keywords: Epilepsy; Epileptogenic zone; Focal cortical dysplasia; MRI-negative; SEEG; Seizure-onset.