Using ictal high-frequency oscillations (80–500 Hz) to localize seizure onset zones in childhood absence epilepsy: A MEG study
Introduction
Childhood absence epilepsy (CAE) is a non-convulsive event characterized by brief (approximately 5–30 s) disruptions in consciousness, as well as 3 Hz bilateral, synchronous spike-and-wave discharges (SWDs) on normal background activity on electroencephalography (EEG) [1]. Although the argument of initialization in CAE has lasted for more than a decade, the exact absence seizure onset zones (SOZ) have not been fully elucidated.
Magnetoencephalography (MEG), a relatively new clinical neuroimaging modality, is well suited for the study of epileptic discharges because MEG can noninvasively detect and localize neuromagnetic signals [2], [3], [4], [5]. Brain activity in a low-frequency range (<80 Hz) has been conventionally studied in epilepsy. Recent advances in brain research have demonstrated that the epileptic brain generates high-frequency oscillations (HFOs, 80–500 HZ); these HFOs can be divided into ripples (80–250 Hz) and fast ripples (FRs) (250–500 Hz) [6], [7], [8]. Reports over the last decade on human epileptic brain with intracranial recording have suggested that HFOs are linked to epileptogenic zones [6], [7], [9], [10]. Studies have demonstrated that good surgical outcome is well correlated with the removal of HFO-generating areas rather than spike-generating areas [10], [11], [12], [13]. Furthermore, the use of HFOs as biomarkers of epileptogenicity in resective procedures improves seizure outcome and represents a novel approach for the treatment of epilepsy.
The objective of the present study was to investigate the spectral and spatial features of HFOs in CAE with MEG, a new clinical neuroimaging modality that can be used to measure very weak magnetic signals from the brain. We hypothesize that the ictal HFOs in CAE will provide novel insight into the initialization of absence seizures in the brain.
Section snippets
Patients
Ten patients (5–11 years old) were recruited from the Department of Neurology of Nanjing Children's Hospital from November 2012 to November 2013 (Table 1).
The inclusion criteria were as follows: (1) clinical diagnosis of typical absence epilepsy consistent with the International League Against Epilepsy Proposal for Revised Classification of Epilepsies and Epileptic Syndromes [1]; (2) clinical EEG recordings with bilateral, synchronous, symmetrical, approximately 3 Hz spike waves, on a normal
Results
None of the 10 children recruited in the study had taken any medicine. Therefore, all subjects were drug naïve and the effect of drugs on MEG data could be eliminated. The clinical details are shown in Table 1. The children are aged 5–11 years, with an average age of 8 years. The seizure frequency ranged from 3 to 15 times per day. We successfully obtained 33 stereotyped ictal MEG recordings from the 10 children.
According to our routine MEG system and environmental noise test, the polarity
Discussion
In this study, HFOs were identified with polarity spectrogram, and the sources for HFOs were localized to the MPFC with dMSI. The results indicate that HFOs in the MPFC may have a primary function in the absence seizures.
Acknowledgements
This study was supported by Key Project of Medical Science and Technology Development Foundation, Nanjing Department of Health (No. ZKX11002, http://www.njh.gov.cn/html/list_83.shtml). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
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