Brain Advance Access published online on March 18, 2009
Brain, doi:10.1093/brain/awn351
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High frequency oscillations in intracranial EEGs mark epileptogenicity rather than lesion type
Montreal Neurological Institute and Hospital, McGill University, Montreal, Québec, Canada
Correspondence to:
Julia Jacobs, Montreal Neurological Institute, EEG Department, 3801 University Street, Montreal, Quebec, H3A 2B4 Canada E-mail: julia.jacobs{at}gmx.de
High frequency oscillations (HFOs) called ripples (80–250 Hz) and fast ripples (FR, 250–500 Hz) can be recorded from intracerebral EEG macroelectrodes in patients with intractable epilepsy. HFOs occur predominantly in the seizure onset zone (SOZ) but their relationship to the underlying pathology is unknown. It was the aim of this study to investigate whether HFOs are specific to the SOZ or result from pathologically changed tissue, whether or not it is epileptogenic. Patients with different lesion types, namely mesial temporal atrophy (MTA), focal cortical dysplasia (FCD) and nodular heterotopias (NH) were investigated. Intracranial EEG was recorded from depth macroelectrodes with a sampling rate of 2000 Hz. Ripples (80–250 Hz) and Fast Ripples (250–500 Hz) were visually marked in 12 patients: five with MTA, four with FCD and three with NH. Rates of events were statistically compared in channels in four areas: lesional SOZ, non-lesional SOZ, lesional non-SOZ and non-lesional non-SOZ. HFO rates were clearly more linked to the SOZ than to the lesion. They were highest in areas in which lesion and SOZ overlap, but in patients with a SOZ outside the lesion, such as in NHs, HFO rates were clearly higher in the non-lesional SOZ than in the inactive lesions. No specific HFO pattern could be identified for the different lesion types. The findings suggest that HFOs represent a marker for SOZ areas independent of the underlying pathology and that pathologic tissue changes alone do not lead to high rates of HFOs.
Key Words: high frequency oscillations; focal cortical dysplasia; nodular heterotopia; temporal atrophy; seizure onset zone; intracranial EEG
Abbreviations: EOG, electro-oculogram; EMG, electromyogram; FCD, focal cortical dysplasia; FIR, finite impulse response; FR, fast ripples; HFOs, high frequency oscillations; HSD, honestly significant differences; IEDs, interictal epileptic discharges; MCD, malformations of cortical development; MTA, mesial temporal atrophy; NH, nodular heterotopiaheterotopias; SOZ, seizure onset zone
Received August 26, 2008. Revised November 10, 2008. Accepted November 27, 2008.
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