Clinical Utility of Interictal High-Frequency Oscillations Recorded with Subdural Macroelectrodes in Partial Epilepsy

BACKGROUND AND PURPOSE: There is growing interest in high-frequency oscillations (HFO) as electrophysiological biomarkers of the epileptic brain. We evaluated the clinical utility of interictal HFO events, especially their occurrence rates, by comparing the spatial distribution with a clinically determined epileptogenic zone by using subdural macroelectrodes. METHODS: We obtained intracranial electroencephalogram data with a high temporal resolution (2000 Hz sampling rate, 0.05-500 Hz band-pass filter) from seven patients with medically refractory epilepsy. Three epochs of 5-minute, artifact-free data were selected randomly from the interictal period. HFO candidates were first detected by an automated algorithm and subsequently screened to discard false detections. Validated events were further categorized as fast ripple (FR) and ripple (R) according to their spectral profiles. The occurrence rate of HFOs was calculated for each electrode contact. An HFO events distribution map (EDM) was constructed for each patient to allow visualization of the spatial distribution of their HFO events. RESULTS: The subdural macroelectrodes were capable of detecting both R and FR events from the epileptic neocortex. The occurrence rate of HFO events, both FR and R, was significantly higher in the seizure onset zone (SOZ) than in other brain regions. Patient-specific HFO EDMs can facilitate the identification of the location of HFO-generating tissue, and comparison with findings from ictal recordings can provide additional useful information regarding the epileptogenic zone. CONCLUSIONS: The distribution of interictal HFOs was reasonably consistent with the SOZ. The detection of HFO events and construction of spatial distribution maps appears to be useful for the presurgical mapping of the epileptogenic zone.

Intracranial Ictal Electroencephalographic Patterns in Parietal Lobe Epileptic Seizures : Correlation with Surgical Outcome

BACKGROUNDS: Parietal lobe epilepsy is uncommon and its intracranial ictal EEG findings have not been well elucidated. We investigated the ictal EEG patterns of epilepsy originating from the parietal lobe and its adjacent areas recorded by intracranial subdural electrode arrays. METHODS: The following intracranial ictal EEG patterns from twenty patients who underwent epilepsy surgery for medically intractable parietal lobe epilepsy were studied: 1) Anatomical ictal onset area [pure parietal (PP), combined parietal and extraparietal, such as fronto-parietal, temporo-parietal, etc. (EP)], 2) localization within the parietal lobe, 3) extent of the ictal onset zone, 4) the number of ictal onset zones, 5) frequency and waveform of ictal onset rhythms, and 6) type of spreading. RESULTS: Twelve patients (60%) were seizure-free (5 from PP, 7 from EP) and 8 were with persistent seizures (3 from PP, 5 from EP). Eight patients (40%) had ictal onset from PP and 12 were from EP. There were! Significant correlations with surgical outcome (p<0.05) in two variables: extent of ictal onset zone and type of spread. Patients with focal onset zones (n=9) showed better outcomes (eight are seizure-free) than those with wide ictal onset zones (4 out of 7 are seizure-free). Patients with slow spreading patterns showed better outcomes (12 from 16 are seizure-free) than those with fast spreading outcomes (all of them had persistent seizures). CONCLUSIONS: The extent of ictal onset zones correlates with surgical outcomes. The patterns of intracranial ictal EEG are helpful in predicting the surgical outcomes for parietal lobe epilepsy. (J Korean Neurol Assoc 19(5):478~484, 2001)

Combined Bilateral Depth and Subdural Electrode Investigation in Temporal Lobe Epilepsy

BACKGROUND: Depth and subdural electroencephalographic (EEG) recordings are often required to identify an area of the brain for epileptic surgery. We simultaneously compared bilaterally placed depth and subdural electrode EEGs to determine the site of seizure origins from the temporal lobes. METHODS: We included nine consecutive patients with medically refractory temporal lobe epilepsy, whose noninvasive evaluations such as magnetic resonance imaging, scalp and sphenoidal EEG, and other tests had not proved consistent lateralization. All patients had bilateral temporal depth electrodes, anterior and lateral temporal subdural strip electrodes. Thirty-eight clinical seizures and 3 subclinical seizures were evaluated. RESULTS: Seven out of 9 patients (78%) had unitemporal seizures, one patient had bilateral seizures, and the other had lateral temporal seizures in an invasive study. Ictal onset was localized by depth electrodes in 8 patients, and subdural strip electrodes in one. In ictal recordings, the ictal rhythms never spread to the contralateral neocortex before the ipsilateral neocortex. Most of the ictal rhythms began focally with periodic spikes or fast activities in the depth electrode, then spread to the ipsilateral strip electrode after 14 to 90 seconds (mean : 35.2 seconds). If ictal rhythms propagated to the contralateral side, it took 14 to 140 seconds (mean : 64.2 seconds). Subdural strip electrodes were less sensitive than depth electrodes in the detection of seizure onset and subclinical seizures, but were accurate when lateralized. CONCLUSIONS: We conclude that EEG recordings with depth and subdural strip electrodes correctly identify and lateralize temporal lobe seizures more often than subdural electrodes alone.

Comparison of Intracranial Ictal EEG Patterns between Medial and Lateral Temporal Lobe Epilepsy / 대한간질학회지

BACKGROUND AND OBJECTIVES: Temporal lobe epilepsy is the most common uncontrolled epileptic condition and is increasingly treated with surgery. In the absence of definitive results from noninvasive procedures, patients undergo implantation of intracranial electrodes. Intracranial EEG recordings are more accurate than scalp EEG recordings because of minimal artifact and closer approximation to the area of seizure onset. Intracranial EEG patterns between the medial and the lateral temporal lobe epilepsy were thought to have a little differences. METHODS: The authors compared the morphological pattern of seizure onset area, spread pattern, termination pattern and duration of the intracranial EEG manifestation of spontaneous seizures recorded from combined depth and subdural electrodes in 25 intractable temporal lobe epilepsy patients. RESULTS: Sixteen cases were medial temporal onset, six cases were neocortical onset and three cases were multifocal onset. The morphological onset pattern of medial temporal seizures was more likely to have high frequency rhythmic discharge (>13 Hz) and tended to show repetitive spikes prior to the seizure, whereas neocortical seizures were characterized by slow (4-to 10-Hz) and fast frequencies (>35 Hz), without evidence of repetitive spikes. The mean ictal duration at seizure onset of complex partial seizure of medial onset seizure was 121 seconds and was not different from those of neocortical seizures which were 115 seconds. Neocortical seizures take more time to propagate than medial seizures. Propagation to the opposite side of neocortical onset seizure takes 45.53 seconds, whereas medial temporal onset seizure takes 27.92 seconds. In case of second generalization, neocortical seizures continued longer than medial seizures. Of the medial onset temporal lobe seizures, except the simple partial seizures, the 35.2% of seizures initially spread to ipsilateral neocortex, and the 28.2% of seizures initially spread to the contralateral medial temporal lobe and the 25% of seizures simultaneously propagated to the ipsilateral neocortex and contralateral medial temporal lobe. Among the seizures initiated from the neocortex, 79.3% of seizures initially spread to the ipsilateral medial temporal area, but never initially spread to opposite neocortex. The termination pattern of seizures was divided into three types according to their location. In case of medial temporal lobe seizures, the mean of 31% of seizures was diffusely terminated , 38% of seizures terminated at the onset region and 38% of seizures were elsewhere within onset region. On the other hand, 48.6% of neocortical temporal lobe seizure were terminated at seizure onset region. CONCLUSIONS: The pattern of ictal intracranial EEG provides distinguishable differences between the medial temporal seizure and the neocortical temporal seizure.

Surgical Treatment of Postencephalitic Epilepsy

Encephalitis is often followed by chronic intractable epilepsy. Many of these patients pose significant challenges to the localization of seizure generators and to the strateges for management of intractable epilepsy. The authors analysed 17 patients with postencephalitic epilepsy(PEE), who underwent resective surgeries. Most patients had been accompanied by coma, convulsive status epilepticus, and focal motor deficit at the time of encephalitis. MRI studies showed variable degree of brain damage: hippocampal sclerosis only(n=5), neocortical gliosis only(n=6), and both(n=5). Analysis of ictal semiology revealed a predominant temporo-limbic seizure pattern in 7, a variable extralimbic patterns in 6, and unclassified in 4 patients. Surgical resection includes temporal(n=11), frontal(n=3), centroparietal(n=1), multilobar(n=2), and callosotomy(n=2). Surgical outcome was graded as class 1(n=8), class 2(n=2), class 3(n=4), and class 4(n=3). It is concluded that surgical result was promising despite the traditional concerns about localizing problem in the setting of PEE. Surgical treatment should be, therefore, considered if localizing information is persistent. Intracranial EEG recording was very useful to delineate the area of seizure onset. MRI abnormalities were not always correlated with ictal onset zone in the patients with PEE.