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K.-Y. Jung
In another study, ictal source localization using high-density scalp EEG data (128or 256-electrode) was reported in 14 patients with focal epilepsy (eight male, mean
age 26.3 years, range 10–45 years) [32]. Among them, 8 patients underwent epilepsy
surgery and were followed up for at least 1 year after surgery. Dominant frequency of
EEG rhythm was filtered with narrow band (±1 Hz) from epochs of early rhythmic
ictal changes (mean 2 s, range 1.5–2.5 s), which was used for the source localization.
The locally spherical model with anatomical constraints (LSMAC) for head model
and LORETA for inverse method were used in this study. LORETA was performed
at each time point of the filtered ictal EEG data and then averaged across the defined
early ictal epoch. Six of eight patients who underwent epilepsy surgery were seizure
free. Five of them showed the maximum current density of ictal ESI within the
resected area, indicating ictal ESI correctly localized the SOZ in the resection area.
On the other hand, two of the operated patients who had a less favorable outcome
revealed that the maximal ictal ESI solution was not included in the resected area.
Remarkably, the agreement between interictal and ictal ESI were concordant (9) or
partially concordant (4) in 13 of 14 patients (93%).
Although there are still limited number of studies, ictal ESI provides more direct
information about SOZ compared with interictal ESI (providing irritative zone), and
it is feasible in clinical practice. Larger studies are needed to determine the clinical
feasibility of ictal ESI as a valuable presurgical evaluation tool. Standardization and
automation of processing steps, and multimodal approaches combining electrophysiology and neuroimages are strongly required for wider application of ictal ESI in
clinical practice.
8.3 High Frequency Oscillation (HFOs)
The frequencies of conventional EEG recordings range from 0.5 to 70 Hz. However,
high frequency EEG oscillatory activities (HFOs) above 70 Hz have been recognized
during physiological and pathological conditions in intracranial recordings [9, 15].
HFOs refer to distinct types of brain activity occurring in a frequency band range
between 80 and 500 Hz. Since HFO was discovered in epileptogenic hippocampus
in patients with mesial temporal lobe epilepsy and in animal model of epilepsy using
microelectrodes [6, 7], it has been considered to be a new biological marker for
potential epileptogenecity although more convincing evidences are still needed.
Conventionally, IEDs have been considered as biological markers of epilepsy
because almost all patients with epilepsy show IEDs at some time points. However,
it is not uncommon that IEDs can be seen in healthy people or in some patients with
neuropsychiatric disorders such as attention deficit hyperkinetic disorder, autism
spectrum disorder, and developmental disorders. In addition, IEDs represent only
irritative zones of cerebral cortex, neither epileptogenic zones nor SOZs. The identification of HFOs in epileptogenic tissues is one of the major discoveries in epilepsy
research over the past two decades [21], because no clearly identified biological
marker for epilepsy has been developed yet. HFOs are now increasingly used to
K.-Y. Jung
In another study, ictal source localization using high-density scalp EEG data (128or 256-electrode) was reported in 14 patients with focal epilepsy (eight male, mean
age 26.3 years, range 10–45 years) [32]. Among them, 8 patients underwent epilepsy
surgery and were followed up for at least 1 year after surgery. Dominant frequency of
EEG rhythm was filtered with narrow band (±1 Hz) from epochs of early rhythmic
ictal changes (mean 2 s, range 1.5–2.5 s), which was used for the source localization.
The locally spherical model with anatomical constraints (LSMAC) for head model
and LORETA for inverse method were used in this study. LORETA was performed
at each time point of the filtered ictal EEG data and then averaged across the defined
early ictal epoch. Six of eight patients who underwent epilepsy surgery were seizure
free. Five of them showed the maximum current density of ictal ESI within the
resected area, indicating ictal ESI correctly localized the SOZ in the resection area.
On the other hand, two of the operated patients who had a less favorable outcome
revealed that the maximal ictal ESI solution was not included in the resected area.
Remarkably, the agreement between interictal and ictal ESI were concordant (9) or
partially concordant (4) in 13 of 14 patients (93%).
Although there are still limited number of studies, ictal ESI provides more direct
information about SOZ compared with interictal ESI (providing irritative zone), and
it is feasible in clinical practice. Larger studies are needed to determine the clinical
feasibility of ictal ESI as a valuable presurgical evaluation tool. Standardization and
automation of processing steps, and multimodal approaches combining electrophysiology and neuroimages are strongly required for wider application of ictal ESI in
clinical practice.
8.3 High Frequency Oscillation (HFOs)
The frequencies of conventional EEG recordings range from 0.5 to 70 Hz. However,
high frequency EEG oscillatory activities (HFOs) above 70 Hz have been recognized
during physiological and pathological conditions in intracranial recordings [9, 15].
HFOs refer to distinct types of brain activity occurring in a frequency band range
between 80 and 500 Hz. Since HFO was discovered in epileptogenic hippocampus
in patients with mesial temporal lobe epilepsy and in animal model of epilepsy using
microelectrodes [6, 7], it has been considered to be a new biological marker for
potential epileptogenecity although more convincing evidences are still needed.
Conventionally, IEDs have been considered as biological markers of epilepsy
because almost all patients with epilepsy show IEDs at some time points. However,
it is not uncommon that IEDs can be seen in healthy people or in some patients with
neuropsychiatric disorders such as attention deficit hyperkinetic disorder, autism
spectrum disorder, and developmental disorders. In addition, IEDs represent only
irritative zones of cerebral cortex, neither epileptogenic zones nor SOZs. The identification of HFOs in epileptogenic tissues is one of the major discoveries in epilepsy
research over the past two decades [21], because no clearly identified biological
marker for epilepsy has been developed yet. HFOs are now increasingly used to