Interictal high‐frequency oscillations (80–500 Hz) are an indicator of seizure onset areas independent of spikes in the human epileptic brain

Abstract

SUMMARYPurpose: High-frequency oscillations (HFOs) known as ripples and fast ripples (250-500 Hz) can be recorded from macroelectrodes inserted in patients with intractable focal epilepsy. They are most likely linked to epileptogenesis and have been found in the seizure onset zone (SOZ) of human ictal and interictal recordings. HFOs occur frequently at the time of interictal spikes, but were also found independently. This study analyses the relationship between spikes and HFOs and the occurrence of HFOs in nonspiking channels. Methods: Intracerebral EEGs of 10 patients with intractable focal epilepsy were studied using macroelectrodes. Rates of HFOs within and outside spikes, the overlap between events, event durations, and the percentage of spikes carrying HFOs were calculated and compared according to anatomical localization, spiking activity, and relationship to the SOZ.Results: HFOs were found in all patients, significantly more within mesial temporal lobe structures than in neocortex. HFOs could be seen in spiking as well as nonspiking channels in all structures. Rates and durations of HFOs were significantly higher in the SOZ than outside. It was possible to establish a rate of HFOs to identify the SOZ with better sensitivity and specificity than with the rate of spikes. Discussion: HFOs occurred to a large extent independently of spikes. They are most frequent in mesial temporal structures. They are prominent in the SOZ and provide additional information on epileptogenicity independently of spikes. It was possible to identify the SOZ with a high specificity by looking at only 10 min of HFO activity.

Automatic detection of fast oscillations (40–200Hz) in scalp EEG recordings

“…The four cycle minimum duration has been adopted in our previous studies involving FOs (AndradeValença et al, 2011) and HFOs (Jacobs et al, 1893;Zelmann et al, 2009Zelmann et al, , 2010). …”

“…Recent studies suggest that they may play an important role in normal and pathological brain function. In epileptic patients high frequency oscillations were found in intracranial EEG recordings, both with microelectrodes (Bragin et al, 1999a,b;Staba et al, 2002) and macroelectrodes during ictal (Jirsch et al, 2006) and interictal intervals (Urrestarazu et al, 2007;Jacobs et al, 1893;Worrell et al, 2008;Crepon et al, 2010). The high frequency oscillations appear to be a good indicator of the seizure onset zone , and are possibly related to the outcome of epilepsy surgery .…”

Advances of Intracranial Electroencephalography in Localizing the Epileptogenic Zone

“…A significant proportion of ripples and fast ripples cooccur with spikes, but many spikes also occur independently [19,29,30]. In addition, fast oscillations in the gamma and low ripple range (30-100 Hz) have been found to precede interictal epileptiform spike discharges (IEDs), and these so called gamma-IEDs are strongly associated with the SOZ in TLE [31].…”

Transformation of neuronal modes associated with low-Mg2 + /high-K + conditions in an in vitro model of epilepsy

“…On the other hand, the hippocampus becomes sensitized by high-frequency stimulation, as in the case of long-term potentiation (LTP) [24], which implies the action of a rate-dependent (differentiating) process. Abnormal high-frequency activity is also associated with the epileptiform condition [25,26]. In the kindling model of epilepsy, the system threshold for initiating a seizure-like event becomes lower as the number of SLEs is increased [27], until kindling is no longer required for SLEs to occur.…”