Interictal high-frequency oscillations (100–500 Hz) in the intracerebral EEG of epileptic patients

Abstract

Interictal fast oscillations between 100 and 500 Hz have been reported in signals recorded from implanted microelectrodes in epileptic patients and experimental rat models. Oscillations between 250 and 500 Hz, or fast ripples (FR), appeared related to the epileptic focus whereas ripples (80-200 Hz) were not. We report high-frequency oscillations recorded with intracranial macroelectrodes in seven patients with refractory focal epilepsy during slow-wave sleep. We characterize the relation of fast oscillations to the seizure focus and quantify their concordance with epileptiform transients, with which they are strongly associated. The patients were selected because interictal spikes were found within and outside the seizure onset zone. Visual inspection was used to identify and classify the ripples and FRs according to their relation to epileptiform spikes. Continuous-time wavelet analysis was used to compute their power. Ripples were present in all patients while FRs where found in five of the seven patients. Most ripples and FRs occurred at the same time as epileptiform transients. The rate of occurrence of ripples was higher within the seizure onset zone than outside in four of seven patients. The rate of FRs was much higher within the seizure onset zone than outside in four of the five patients with FRs (in these four patients, FRs were almost inexistent outside the seizure onset zone). The power of ripples and FRs tended to be higher in the electrodes where their rate was also higher. These results indicate that FRs were more restricted to the electrodes located within the seizure onset zone, especially to the hippocampus, than ripples. In only one patient, FRs were more frequent outside the seizure onset zone; this patient was the only one with cortical dysplasia and the electrode with a high rate of FRs was inside the lesion. This study demonstrates that interictal ripples and FRs can be recorded with depth macroelectrodes in patients. Most occur at the time of epileptiform spikes but some are isolated. Ripples do not show a clear differentiation between the seizure onset zone and remote areas, whereas FRs have a higher rate and higher power in the seizure onset zone. Our results also suggest a special capacity of the abnormal hippocampus to generate FRs, although they were also recorded in other structures.

Continuous High Frequency Activity: A peculiar SEEG pattern related to specific brain regions

“…HFOs were successively described in stereo-EEG (SEEG) of epileptic patients acquired using clinical macroelectrodes (surface contact: 0.8 mm 2 ), occurring during the ictal phase particularly in the seizure onset zone (SOZ) . Further studies showed that HFOs can be recorded in the interictal phase, still more commonly in the SOZ (Urrestarazu et al, 2007;Jacobs et al, 2008;Worrell et al, 2008); they can be linked to epileptogenic lesions (Staba et al, 2007), being non-specific to a particular type of lesion, representing epileptogenicity per se ; they appear to predict the outcome after epilepsy surgery, in adults (Jacobs et al, 2010) as in children (Wu et al, 2010). HFOs are not confined to mesial temporal structures, occurring also in neocortical regions, usually at lower rates (Urrestarazu et al, 2007).…”

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

“…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 .…”

Ictal and interictal high frequency oscillations in patients with focal epilepsy

“…High frequency oscillations (HFOs) occur in patients with focal epilepsy, mostly during slow wave sleep and at seizure onset (Bagshaw et al, 2009;Bragin et al, 1999bBragin et al, , 2002Fisher et al, 1992;Jirsch et al, 2006;Khosravani et al, 2009;Urrestarazu et al, 2007). They have been associated with epileptogenesis as they occur before the first seizure in the kainic acid rat model (Bragin et al, 2004a).…”

“…HFOs can be divided into ripples and fast ripples (FRs: 250-500 Hz) (Bragin et al, 1999a;Staba et al, 2002;Urrestarazu et al, 2007). Ripples have been thought to be more physiological in nature and involved in memory, while FRs appear to be pathological, except for FRs in sensory evoked potentials (Axmacher et al, 2008;Buzsaki et al, 1992;Curio et al, 1997;Engel et al, 2009;Worrell et al, 2008).…”