8 Analysis of EEG in Medically Intractable Epilepsy
187
Taken together, interictal HFOs are more specific and accurate than IEDs to delineate the SOZ. HFOs can be considered as a reliable marker for SOZ, better than IEDs.
8.3.3 HFOs at Seizure Onset (Ictal HFOs)
HFOs occur during preictal and seizure onset periods. It is noted that high-frequency
activity (60–100 Hz) increases significantly in the 20 min prior to neocortical seizure
onset in 62% of all seizures [46].
HFOs were identified at the time of seizure onset in 7 of the 10 patients with focal
epilepsy [20]. HFOs were present in very limited distributions with high frequency
events (100–200 Hz) confined to 1–3 contacts and with very high frequency events
(250–500 Hz) confined to 1–2 contacts of the same depth electrode. The distribution
of HFOs correlates well with the SOZ determined by the comprehensive presurgical
evaluation [20].
Modur et al. demonstrated HFOs (>70 Hz) at seizure onset (ictal HFOs) in six
patients with neocortical epilepsy. Ictal HFOs had a widespread activity at seizure
onset but evolved subsequently in a restricted manner (HFOs+). HFOs+ occurred
earlier and with a smaller distribution than the seizure onset defined by the conventional frequency activity (<40 Hz). Furthermore, five out of six patients (83%)
showed favorable postoperative outcome over a mean follow-up of 27 months, which
included mainly the HFOs+ channels [29].
HFOs show a remarkable reliability as it appears in the same channels during
interictal, preictal and ictal periods in depth electrode recordings. On the other hand,
epileptic spikes are more widespread and occur on different channels in ictal and
interictal periods [47]. Therefore, HFOs are most likely linked to epileptogenesis.
8.3.4 HFOs and Surgical Outcome
The clinical utility of HFOs has been exemplified by better surgical outcome when
resecting cortical areas generating HFOs compared with resecting conventional SOZ.
Cho et al. evaluated the occurrence rate and extent between HFO distribution and the
SOZ in 15 patients with neocortical epilepsy. They found that resection of high-rate
HFO regions was significantly associated with favorable outcome. Interestingly, the
extent/ratio of SOZ or spiking region resection did not differ between seizure-free
and non-seizure-free groups [10].
A meta-analysis which included 11 studies investigated surgical outcomes based
on resection ratio of HFOs, that is the ratio between the number of channels on
which HFOs were detected and, among these, the number of channels that were
inside the resected area [17]. Postoperative good surgical outcome is related to a
higher resection ratio of HFOs compared with poor surgical outcome. The effect
187
Taken together, interictal HFOs are more specific and accurate than IEDs to delineate the SOZ. HFOs can be considered as a reliable marker for SOZ, better than IEDs.
8.3.3 HFOs at Seizure Onset (Ictal HFOs)
HFOs occur during preictal and seizure onset periods. It is noted that high-frequency
activity (60–100 Hz) increases significantly in the 20 min prior to neocortical seizure
onset in 62% of all seizures [46].
HFOs were identified at the time of seizure onset in 7 of the 10 patients with focal
epilepsy [20]. HFOs were present in very limited distributions with high frequency
events (100–200 Hz) confined to 1–3 contacts and with very high frequency events
(250–500 Hz) confined to 1–2 contacts of the same depth electrode. The distribution
of HFOs correlates well with the SOZ determined by the comprehensive presurgical
evaluation [20].
Modur et al. demonstrated HFOs (>70 Hz) at seizure onset (ictal HFOs) in six
patients with neocortical epilepsy. Ictal HFOs had a widespread activity at seizure
onset but evolved subsequently in a restricted manner (HFOs+). HFOs+ occurred
earlier and with a smaller distribution than the seizure onset defined by the conventional frequency activity (<40 Hz). Furthermore, five out of six patients (83%)
showed favorable postoperative outcome over a mean follow-up of 27 months, which
included mainly the HFOs+ channels [29].
HFOs show a remarkable reliability as it appears in the same channels during
interictal, preictal and ictal periods in depth electrode recordings. On the other hand,
epileptic spikes are more widespread and occur on different channels in ictal and
interictal periods [47]. Therefore, HFOs are most likely linked to epileptogenesis.
8.3.4 HFOs and Surgical Outcome
The clinical utility of HFOs has been exemplified by better surgical outcome when
resecting cortical areas generating HFOs compared with resecting conventional SOZ.
Cho et al. evaluated the occurrence rate and extent between HFO distribution and the
SOZ in 15 patients with neocortical epilepsy. They found that resection of high-rate
HFO regions was significantly associated with favorable outcome. Interestingly, the
extent/ratio of SOZ or spiking region resection did not differ between seizure-free
and non-seizure-free groups [10].
A meta-analysis which included 11 studies investigated surgical outcomes based
on resection ratio of HFOs, that is the ratio between the number of channels on
which HFOs were detected and, among these, the number of channels that were
inside the resected area [17]. Postoperative good surgical outcome is related to a
higher resection ratio of HFOs compared with poor surgical outcome. The effect