8 Analysis of EEG in Medically Intractable Epilepsy
185
identify and localize epileptogenic zones in presurgical evaluation of intractable
epilepsy patients.
HFOs are subdivided into ripples ranging from 80 to 250 Hz and fast ripples
>250 Hz [16]. Cellular and network mechanisms generating ripples and fast ripples remain unclear. Ripples reflect fast inhibitory postsynaptic potentials (IPSPs)
on the soma of pyramidal cells. Ripples are observed in non-epileptic hippocampi
and entorhinal cortices, and may play a role in memory consolidation by phase synchronization [20]. On the other hand, fast ripples are found only in the epileptogenic
region and may reflect pathological hypersynchronous population spikes of bursting
pyramidal cells.
8.3.1 What Are HFOs and How to Recognize HFOs
HFOs are defined as all physiologic and pathologic oscillatory EEG activities within
a limited frequency band from 80 to 500 Hz range that clearly stand out from the
baseline and persist for at least four oscillation cycles [49].
To record HFOs, there should be special considerations in terms of both hardware
and software. In the beginning of HFOs discovery, HFOs were detected using microelectrodes (40 µm wires) implanted in the hippocampus and entorhinal cortex. Then,
it has been also reported that HFOs can be picked up by macro-intracranial electrodes
such as subdural and depth electrodes, which are clinically used to localize SOZ in
presurgical evaluation. HFOs can be recorded even with scalp EEG. Therefore, it
seems that any clinically used electrodes can now record HFOs.
The sampling rate of EEG amplifiers should be at least 4 times higher than the
upper frequency of interest. For identifying ripples (80–250 Hz), proper sampling
rate of amplifiers should be at least 1000 Hz, and a sampling rate of 2000 Hz or
above should be used to record HFOs over 250 Hz [48]. Software for reviewing data
should have a capability of a high-order finite impulse response (FIR) filter, and of
extended time scale up to 0.6 s/page, and of amplitude scale changeable to 1 uV/mm
[48, 49].
8.3.2 Interictal HFOs
The first recording of HFOs during interictal period in human mesial temporal
epilepsy with or without hippocampal sclerosis was performed by the University
of California at Los Angeles group [39, 40]. Depth electrodes were inserted in both
hippocampi and entorhinal cortices (i.e., epileptic side and healthy side). They found
two distinct oscillatory groups composed of the lower-frequency ripple group having a mean frequency of 96 Hz and the higher-frequency fast ripple group having
a mean frequency of 262 Hz. The ratio of the number of fast ripples to ripples was
significantly higher in the sites ipsilateral to seizure onset with hippocampal atrophy
185
identify and localize epileptogenic zones in presurgical evaluation of intractable
epilepsy patients.
HFOs are subdivided into ripples ranging from 80 to 250 Hz and fast ripples
>250 Hz [16]. Cellular and network mechanisms generating ripples and fast ripples remain unclear. Ripples reflect fast inhibitory postsynaptic potentials (IPSPs)
on the soma of pyramidal cells. Ripples are observed in non-epileptic hippocampi
and entorhinal cortices, and may play a role in memory consolidation by phase synchronization [20]. On the other hand, fast ripples are found only in the epileptogenic
region and may reflect pathological hypersynchronous population spikes of bursting
pyramidal cells.
8.3.1 What Are HFOs and How to Recognize HFOs
HFOs are defined as all physiologic and pathologic oscillatory EEG activities within
a limited frequency band from 80 to 500 Hz range that clearly stand out from the
baseline and persist for at least four oscillation cycles [49].
To record HFOs, there should be special considerations in terms of both hardware
and software. In the beginning of HFOs discovery, HFOs were detected using microelectrodes (40 µm wires) implanted in the hippocampus and entorhinal cortex. Then,
it has been also reported that HFOs can be picked up by macro-intracranial electrodes
such as subdural and depth electrodes, which are clinically used to localize SOZ in
presurgical evaluation. HFOs can be recorded even with scalp EEG. Therefore, it
seems that any clinically used electrodes can now record HFOs.
The sampling rate of EEG amplifiers should be at least 4 times higher than the
upper frequency of interest. For identifying ripples (80–250 Hz), proper sampling
rate of amplifiers should be at least 1000 Hz, and a sampling rate of 2000 Hz or
above should be used to record HFOs over 250 Hz [48]. Software for reviewing data
should have a capability of a high-order finite impulse response (FIR) filter, and of
extended time scale up to 0.6 s/page, and of amplitude scale changeable to 1 uV/mm
[48, 49].
8.3.2 Interictal HFOs
The first recording of HFOs during interictal period in human mesial temporal
epilepsy with or without hippocampal sclerosis was performed by the University
of California at Los Angeles group [39, 40]. Depth electrodes were inserted in both
hippocampi and entorhinal cortices (i.e., epileptic side and healthy side). They found
two distinct oscillatory groups composed of the lower-frequency ripple group having a mean frequency of 96 Hz and the higher-frequency fast ripple group having
a mean frequency of 262 Hz. The ratio of the number of fast ripples to ripples was
significantly higher in the sites ipsilateral to seizure onset with hippocampal atrophy