FULL-LENGTH ORIGINAL RESEARCH
Interictal high frequency oscillations correlating with seizure outcome in patients with widespread epileptic networks in tuberous sclerosis complex *Tohru Okanishi, †Tomoyuki Akiyama, *Shin-Ichi Tanaka, *Ellen Mayo, *Ayu Mitsutake, *Cyrus Boelman, *Cristina Go, *O. Carter Snead 3rd, ‡James Drake, ‡James Rutka, *Ayako Ochi, and *Hiroshi Otsubo Epilepsia, 55(10):1602–1610, 2014 doi: 10.1111/epi.12761
SUMMARY
Tohru Okanishi is interested in the development of the treatment for children with medically refractory epilepsy.
Objective: Multiple tubers in patients with tuberous sclerosis complex (TSC) often are responsible for drug-resistant epilepsy. The complexity of the epileptic network formed by multiple tubers complicates localization of the epileptogenic zone that is needed to design a surgical treatment strategy. High frequency oscillations (HFOs) on intracranial video-electroencephalography (IVEEG) may be a valuable surrogate marker for the localization of the epileptogenic zone. The purpose of this study was to test the hypothesis that high occurrence rate (OR) of interictal HFOs can guide the localization of the epileptogenic zone. Methods: We analyzed the OR of interictal HFOs at 80–200 Hz (ripples) and >200 Hz (fast ripples, FRs). We divided OR of interictal HFOs between high and low rates by thresholding. We analyzed the correlation between seizure outcomes using Engel classification and the resection ratio of the seizure onset zone (SOZ), and high-OR HFOs using ordinal logistic regression analysis. Results: We collected 10 patients. The seizure outcomes resulted in Engel classification I in three patients, II in four, III in one, and IV in two. High-OR ripples (5–57 [mean 29] channels, 1–4 [2.8] lobes) and high-OR FRs (9–66 [mean 27] channels, 1–4 [2.6] lobes) were widely distributed. The resection ratio of SOZ did not show statistically significant correlation with the seizure outcome. The resection ratio of high-OR ripples showed statistically significant correlation with the seizure outcome (p = 0.038). The resection ratio of high-OR FRs showed statistically significant correlation with the seizure outcome (p = 0.048). Significance: The multiple extensive zones with high-OR HFOs suggest a complex and widespread epileptic network in patients with TSC. In a subset of TSC patients with drug-resistant epilepsy, resection of cortex with both interictal high-OR FRs and ripples on IVEEG correlated with a good seizure outcome. KEY WORDS: High frequency oscillations, Epilepsy surgery, Potential epileptogenic zone, Complex epileptic network, Epileptogenesis, Multiple cortical tubers.
Accepted July 22, 2014; Early View publication September 5, 2014. *Division of Neurology, The Hospital for Sick Children, Toronto, Ontario, Canada; †Department of Child Neurology, Okayama University Hospital, Okayama, Japan; and ‡Division of Neurosurgery, The Hospital for Sick Children, Toronto, Ontario, Canada Address correspondence to Hiroshi Otsubo, Division of Neurology, The Hospital for Sick Children, 555 University Avenue, Toronto, ON, Canada M5G 1X8. E-mail: [email protected]; [email protected] Wiley Periodicals, Inc. © 2014 International League Against Epilepsy
Tuberous sclerosis complex (TSC) is a neurocutaneous disorder caused by mutations in either TSC1 or TSC2 genes, which encode the protein products of hamartin and tuberin, respectively. Phenotypically this disorder is characterized by tumors in multiple organs, including the brain.1 Sixty percent to 90% of patients with TSC develop epilepsy.1,2 The multiple cortical tubers associated with this disorder can cause generalized and multifocal seizures, and epileptic
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1603 Interictal HFOs in Tuberous Sclerosis spasms.3–6 Fifty percent to 80% of TSC patients with epilepsy are refractory to treatment with multiple antiepileptic medications.7 Resective surgery may be a therapeutic option for a subset of TSC patients with drug-resistant seizures. Seizure freedom has been reported in 56% after resective surgery.8 In patients with intractable epilepsy secondary to TSC, seizures can arise independently from multiple cortical tubers.4,6 Epileptogenic zones have been defined to occur around tubers with both intracranial video-EEG (IVEEG) and magnetoencephalography (MEG).5,9–11 Furthermore, it is expected that alterations in cortical-subcortical system connectivity may lead to wide epileptic network spreading to nonlesional areas in patients with TSC.12 Therefore, removal of the cortical tuber (tuberectomy) alone may not be sufficient to interrupt the complex epileptic network. High frequency oscillations (HFOs), subdivided into ripples (80–200 Hz) and fast ripples (FRs, >200 Hz), are biomarkers of epileptic brain activity. Bragin et al.13 found HFOs in entorhinal cortex and hippocampus in patients with mesial temporal lobe epilepsy. Epileptic HFOs were also found in neocortical seizure onset zones (SOZs).14 Pathologically interconnected neurons produce hypersynchronous bursts of 200–300 Hz.13 Neocortical inhibitory neurons generate fast-spiking activity, with a frequency of up to 800 Hz.15 Firing in phase synchronously and out of phase creates 100–400 Hz HFOs.16 Disrupted neuronal networks are composed of pathologic clusters that generate FRs.17 The resection of brain regions with interictal ripples and FRs recorded on IVEEG correlated with better seizure outcomes.14,18,19 The purpose of this study was to test the hypothesis that high occurrence rate (OR) interictal HFOs can guide the localization of the epileptogenic zone pursuant to its surgical resection in patients with TSC. We hypothesized further that an extensive and discontinuous distribution of SOZ and HFOs is characteristic of a widespread and complex epileptic network in patients with TSC.
Methods Patients We retrospectively identified 13 children with TSC and drug-resistant epilepsy, who underwent IVEEG recordings using subdural and depth electrodes followed by resective surgery between June 2006 and March 2012 at The Hospital for Sick Children. We excluded two patients with continuous seizures over 1 h and one child who had significant artifacts on the interictal IVEEG. We used the data from the remaining 10 patients. The follow-up period ranged from 19 to 76 months, with a mean of 58 months. The seizure outcomes were Engel classification I in three patients, II in four patients, III in one patient, and IV in two patients. All patients underwent presurgical evaluation including scalp video-EEG (SVEEG), magnetic resonance imaging (MRI),
and magnetoencephalography (MEG), and had postsurgical follow-up >12 months. From clinical records, we reviewed the range and mean age at seizure onset, duration of seizure history, age at surgery, and the number of antiepileptic medications over the clinical course before surgery and at the time of surgery. IVEEG recording Based on seizure semiology and ictal/interictal data on SVEEG, prominent lesions on MRI, and the lateralization and distributions of interictal MEG dipole sources, we determined the hemisphere and location to place the subdural grid electrodes. We performed IVEEG with onesheet subdural grid EEG electrodes and some strip/depth electrodes. Five patients had 8–12 strip electrodes in the hemisphere contralateral to the surgical side. We performed functional mapping using direct cortical stimulation to define eloquent cortices and sensory evoked potentials. The technique of implantation of intracranial grid, strip and depth electrodes, and intraoperative and extraoperative functional mapping of eloquent cortex was described previously.20,21 Inter-electrode distance in the subdural grid electrodes was 9–11 mm. The diameter of subdural electrodes was 4 mm with exposure of 2.3 mm (effective surface area: 4.2 mm2) and the surface area of depth electrodes was 8.3 mm2 (Ad-Tech Medical Instrument, Racine, WI, U.S.A.). For IVEEG recordings, we used HARMONY 5.4 (Stellate, Montreal, PQ, Canada). The IVEEG recordings were sampled at 1 kHz with the anti-aliasing filter at 300 Hz, or 2 kHz with the anti-aliasing filter at 880 Hz. IVEEG was recorded for 24–216 h (mean 80 h). We analyzed the SOZ and HFOs in the unilateral hemisphere of the surgical side, because the contralateral strip electrodes were much fewer in number than the grid electrodes and were not placed to cover multiple lobes. Resective surgery The area of cortical resection was determined by clinical neurophysiologists based on the localization of the SOZ, ictal symptomatogenic zone, prominent interictal zone, and the location of eloquent cortices on IVEEG data. The SOZ was identified as low amplitude fast waves or rhythmic change from background activity at the ictal EEG onset period. When we removed the grid, the resection margin was marked by the electrodes that included SOZ and active interictal epileptiform discharges, with consideration of MEG, MRI, and neuropsychologic findings.22 Because the evaluation of HFOs in this study was retrospectively performed, the real-time surgical decision making process was not related to the results of this study. Evaluation of HFOs We selected 10 epochs of 2-min interictal EEG during sleep from IVEEG, so that they were remote from each other and from seizures by at least 1 h. Interictal EEG during Epilepsia, 55(10):1602–1610, 2014 doi: 10.1111/epi.12761
1604 T. Okanishi et al. non–rapid eye movement (non-REM) sleep was selected because the non-REM sleep shows HFOs more frequently than other states.23,24 We visually inspected each EEG epoch with a high-pass filter at 0.5 and 200 Hz to ensure that they were not contaminated by significant artifacts, such as environmental or muscle artifacts.25 We used bipolar montage with pairs of two adjacent EEG electrodes successively connected. We called “channel” using the bipolar montage. The channels including reference electrodes and those with significant artifacts were excluded. We evaluated the 10 EEG epochs. The automated detection of HFOs was performed using MATLAB (The MathWorks, Natick, MA, U.S.A.) described in the previous paper.18 In this analysis, we calculated the OR (per minute) of HFOs (ripples and FRs) on each channel. The analysis of 10 epochs of EEG recording yielded 10 values for the OR of HFOs for each channel. From the results, we chose the median values of the OR of HFOs for each channel. Subsequently, we determined the channels with high-OR and low-OR of HFOs for each patient by calculating the threshold using Kittler’s method (Appendix S1).26 Because of the small samples, we applied the bootstrap method to better estimate the threshold (Appendix S2). We generated a bootstrap sample by sampling with replacement from the
original dataset. The size of the bootstrap sample was set to be the same as that of the original dataset (the number of channels). We repeated this process 1,000 times to obtain 1,000 bootstrap samples. Thereafter we calculated Kittler’s threshold for each of 1,000 bootstrap samples. We applied the mean of the 1,000 Kittler’s thresholds as the final threshold to separate the channels with high-OR of HFOs from those with low-OR of HFOs. Quantitative measures We reviewed the total number of electrodes used for IVEEG, the number of electrodes on the surgical side, and the number of electrodes within the resection margin. For analysis of the distribution of SOZ and high-OR HFOs, we applied “segment” for contiguous channels with SOZ and high-OR HFO. We included contiguous channels in one segment. For example, two segments with high-OR FRs in patient 1 meant one segment with 24 channels and the other segment with one channel (Fig. 1). The number of channels inside the resection margin or SOZ was counted as follows: (1) if both electrodes in bipolar montage were inside of the resection margin or SOZ, the channel was counted as “1”; (2) if only one of two electrodes in a bipolar montage was inside of the resection margin or SOZ, the channel was counted as “0.5.”
Figure 1. Seizure onset zone (SOZ, yellow lines), topographic maps of high occurrence rate (OR) ripples (orange), high-OR fast ripples (FRs, red), and resection margins (blue) in a seizure-free patient (patient 1) and a residual seizure patient (patient 9). Epilepsia ILAE Epilepsia, 55(10):1602–1610, 2014 doi: 10.1111/epi.12761
1605
Single or multiple
L R L L R R R R R R Bilateral Bilateral Bilateral Bilateral Bilateral Bilateral Bilateral Bilateral Unilateral (R) Unilateral (R)
Cortical tubers on MRI
Statistical analysis We examined the relationships between the seizure outcome and the number of IVEEG electrodes in the surgical side, the number of resected IVEEG electrodes, the resection ratio of SOZ, or the resection ratio of high-OR HFOs. We used univariate ordinal logistic regression analysis. The significance level was set to p < 0.05.
Unilateral or bilateral
Largest tubers
Resection ratio of SOZ or high-OR HFOs ¼ Resected channels of SOZ or high-OR HFOs total channels of SOZ or high-OR HFOs
Multiple Multiple Multiple Multiple Multiple Multiple Multiple Multiple Multiple Single
Surgical prognosis (Engel classification)
We analyzed the resection ratio of SOZ and high-OR HFOs, using following formula:
I I I II II II II III IV IV
Interictal HFOs in Tuberous Sclerosis
L L R L R R L R R (UD) L L R L R R L R R R Male Female Male Female Male Male Male Male Female Female 1 2 3 4 5 6 7 8 9 10
PSz, partial seizure; ES, epileptic spasm; 2G, secondary generalization; L, left; R, right; UD, undetermined.
2 2 2 4 1 3 2 1 2 2 5 6 3 5 6 5 4 5 7 10 54 91 113 96 131 216 156 22 81 38 PSz ES PSz, 2G ES, PSz ES PSz ES ES PSz, 2G PSz 2 13 72 8 6 1.5 5 10 1 2
Seizure type at onset Age at seizure onset (months) Age at surgery (years old) Gender
4.7 8.7 15.4 8.7 11.4 18.4 13.4 2.7 6.8 3.3
Ictal Interictal At surgery In the course
No. of antiepileptic medications
Duration of seizure history (months) Pt no.
IVEEG profiles Table 2 describes a summary of IVEEG, SOZ, HFOs, and resective surgery. Figure 1 described SOZ and zones of high-OR ripples and FRs in patients 1 and 9, respectively. We applied 96–124 IVEEG electrodes (mean 115), including 92–120 IVEEG electrodes (109) in the surgical side. There was no significant correlation between the number of IVEEG electrodes in the surgical side and the seizure
Table 1. Clinical profiles
Clinical information Table 1 describes the patients’ clinical profiles including gender, age at surgery and at seizure onset, number of antiepileptic medications in the clinical course and at surgery, seizure type at the onset, duration of seizure history, predominant hemisphere of interictal and ictal epileptiform discharge on SVEEG, MRI findings, and seizure outcome. The age at surgery ranged from 2.7 to 18.4 years, with a mean of 10 years. Duration of epilepsy before surgery ranged from 22 to 216 months, with a mean of 100 months. The number of antiepileptic medications throughout the clinical course ranged from 3 to 10 with a mean of 6. The number of antiepileptic medications at surgery ranged from 1 to 4, with a mean of 2. The follow-up periods ranged from 19 to 76 months, with a mean of 58 months. There was a heterogeneity of seizure types with partial seizures in six patients, epileptic spasms in five, and secondarily generalized seizures in two. The predominant hemisphere with interictal discharges was left in four patients and right in six. The predominant hemisphere with ictal discharges was left in four patients, right in five, and undetermined in one. MRI revealed multiple cortical tubers in nine patients and a single tuber in one. The tubers were distributed bilaterally in eight patients and in the right hemisphere in two. The largest cortical tubers on MRI was seen in the left hemisphere in three patients and the right hemisphere in seven. The side of the largest tuber was concordant with surgical side in seven patients (patients 1, 4–6, 8–10).
Predominance of epileptiform discharge on scalp video- EEG
Results
Epilepsia, 55(10):1602–1610, 2014 doi: 10.1111/epi.12761
Epilepsia, 55(10):1602–1610, 2014 doi: 10.1111/epi.12761
L L
R
L R
R
L R R
R
1 2
3
4 5
6
7 8 9
10
120
119 120 120
103
104 96
118
122 124
120
111 108 120
103
92 96
118
110 112
1
1 1 3
5
2 3
4
1 3
No. of types ES ES, PSz, EEGSz PSz x2, EEGSz x2 PSz x2 PSz x2, PSz2G PSz, PSz2G, EEGSz x3 PSz PSz PSz x2, PSz2G ES
Seizure types
Seizures on IVEEG monitoring
52
4.5 8 8
8
11 15.5
14.5
48.5 6.5
Total no. of channelsa
2
3 3 3
4
7 6
3
4 6
Total no. of segments
Seizure onset zones
FTP
FP F F
FPO
FTP FP
TO
FTPO FPO
Distributed lobes
57
10 5 15
36
36 30
46
7 44
Total no. of channels
2
4 2 9
5
4 5
6
2 4
Total no. of segments
FTP
FT F FPO
FPO
FTO FTP
TPO
TPO FTPO
Distributed lobes
High occurrence rate ripples
66
13 9 17
31
17 19
36
25 41
Total no. of channels
2
5 6 5
5
4 4
9
2 7
Total no. of segments
FTP
FTP F FP
FPO
FP FTP
PO
TPO FTPO
Distributed lobes
High occurrence rate FRs
FTP
FTP F FT
FPO
FTPO FP
TPO
TPO FTPO
Resection lobes
42
32 33 29
41
65 35
47
69 83
Resected EEG electrodes
IVEEG, intracranial video-EEG; FRs, fast ripples; L, left; R, right; ES, epileptic spasms; PSz, partial seizure; PSz2G, partial seizure secondarily generalized; EEGSz, EEG seizure; F, frontal; T, temporal; P, parietal; O, occipital. a Channel: using bipolar montage.
Surgery side
Pt no.
Total no. of IVEEG electrodes
No. of IVEEG electrodes on surgery side
Table 2. Profiles of intracranial video-EEG and resective surgery
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100 96 74 74 66 71 65 78 24 52 25 39.5 12.5 26.5 12.5 22 8.5 7 4 34.5 25 41 17 36 19 31 13 9 17 66 100 93 100 66 55 67 85 100 37 60 1 2 3 4 5 6 7 8 9 10
SOZ, seizure onset zone; HFOs, high frequency oscillations; FRs, fast ripples.
45 6.5 11 14.5 15.5 8 4 8 7 35
94 100 100 100 100 100 89 100 93 67
7 44 36 46 30 36 10 5 15 57 48.5 6.5 11 14.5 15.5 8 4.5 8 8 52
Pt #
7 41 36 30.5 16.5 24 8.5 5 5.5 34
Resection ratio (%) # of resected channels
High occurrence rate FRs
Total # of channels Resection ratio (%) # of resected channels
High occurrence rate ripples
Total # of channels
Widespread and disrupted epileptic network in TSC The multiple segments of SOZs, interictal high-OR ripples, and FRs in our patients suggest that there are widespread and disrupted epileptic networks in TSC (Fig. 2).
Resection ratio (%)
Our TSC patients with intractable epilepsy showed multiple segments of SOZs, high-OR ripples, and FRs. This multiplicity of epileptic areas with SOZ, ripples, and FRs reflects widespread, disrupted, and complicated epileptic networks in TSC, thus validating our second hypothesis. The resected electrodes, resection of area with high-OR ripples, and FRs showed correlation with good seizure outcome. These data may support the hypothesis that HFOs can be a surrogate biomarker to detect the epileptogenic zones and target surgical resection in TSC patients with intractable epilepsy. This is the first report of interictal HFO analysis for TSC surgical patients demonstrating high-OR HFOs relating to widespread epileptogenic zone on the intracranial electrodes.
# of resected channels
Discussion
Total # of channels
Resection ratio of SOZ and high-OR HFOs Table 3 describes the profiles of resected channels and resection ratios of SOZ and high-OR HFO channels. The resection ratios of SOZ, high-OR ripples, and high-OR FRs were 67–100% (mean: 94%), 37–100% (76%), and 24–100% (70%), respectively. SOZ data did not show significant correlation with the seizure outcome (Table S1). The resection ratio of high-OR ripples showed a significant correlation with the seizure outcome (p = 0.038; standard regression coefficient 1.63). The resection ratio of high-OR FRs showed a significant correlation with the seizure outcome (p = 0.048; standard regression coefficient 3.54).
Seizure onset zones
outcome (Table S1). Five patients (1, 2, 4, 7, and 8) needed contralateral strips electrodes to record the IVEEG. During IVEEG, four patients (1, 7, 8, and 10) showed single type of seizure. The other six patients showed multiple types of seizures, including different types of ictal EEG changes. The thresholds of high-OR ripples ranged between 1.2 and 42.0/min (Table S2). The thresholds of high-OR FRs ranged between 1.8 and 18.6/min. Total numbers of channels of SOZ, high-OR ripples, and FRs were 4.5–52 (mean: 18), 5–57 (29), and 9–66 (27), respectively. Total numbers of segments with SOZ, high-OR ripples, and high-OR FRs were 2–7 (4.1), 2–9 (4.3), and 2–9 (4.9), respectively. The channels with SOZ, high-OR ripples, and high-OR FRs in surgical hemisphere were distributed in 1–4 (mean 2.4) lobes, 1–4 (2.8), and 1–4 (2.6) lobes, respectively. The resected IVEEG electrodes ranged between 29 and 83 (mean 48). The resected IVEEG electrodes showed significant correlation with seizure outcome (p = 0.048; standard regression coefficient = 1.78; Table S1).
Table 3. Resected channels and resection ratios of SOZ and high occurrence rate HFOs channels, and ratios of unresected high-occurrence rate HFOs channels outside SOZ
Interictal HFOs in Tuberous Sclerosis
Epilepsia, 55(10):1602–1610, 2014 doi: 10.1111/epi.12761
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Figure 2. Schema of widespread epileptic networks in the brain with TSC. Multiple tubers alter widespread epileptic network in the corticalsubcortical regions, with short or long, various range interconnections. Interictal HFOs can exist in both the SOZ and potential epileptogenic zone. Epilepsia ILAE
These aberrant neuronal networks in TSC have been reported to alter cortical-subcortical system connectivity.12 Seizure entrainment on interconnectivities among the multiple epileptogenic foci caused by the cortical tubers4,6 provoke a wide epileptic network.27 The cortex surrounding cortical tubers in patients with TSC produce epileptic discharges and contribute to this aberrant network,9,10 as does cortex remote from the tubers.9,12,28 Neurophysiologic examinations including EEG and MEG can detect epileptic networks in TSC that are more complex than that suggested by lesions seen with conventional neuroimaging techniques. Ohmori et al.29 reported that SVEEG identified two or three seizure foci at the seizure onset of partial seizures in 4 of 15 TSC patients with epilepsy. Multifocal interictal discharges were observed frequently in 25 of 27 TSC patients with localization-related epilepsy.30 Segments of SOZ, interictal high-OR ripples, and FRs ranged between two and nine in our IVEEG using subdural grids with >100 electrodes. Although the IVEEG cannot cover the entire bilateral hemispheres, the subdural grid at least covers the presumed epileptogenic zone, as indicated by SVEEG, MRI, and MEG. High spatiotemporal resolution using >100 electrodes and 1–2 kHz sampling rate can reveal the complex, widespread, and disrupted epileptic networks in patients with TSC. Extensive resection of the complex epileptic network in TSC The cerebral tissues surrounding cortical tubers appear to provoke epileptic seizures in patients with TSC.4,9,31 If a single primary epileptogenic tuber is identified in the patients with TSC, surgical removal of the single epileptogenic tuber results in a good seizure outcome.3,28,32–34 However, the identification of multiple epileptogenic zones in Epilepsia, 55(10):1602–1610, 2014 doi: 10.1111/epi.12761
patients with multiple tubers is challenging because the multiple epileptogenic zones can demonstrate extensive complex relationships among the lesions.3,12,28,35 In our study, nine patients had multiple tubers and one patient had a single tuber, but all ten had multiple segments of SOZ, high-OR ripples and FRs. HFO analysis using the interictal IVEEG data can detect the epileptogenic zones in epilepsy surgery cases. Jacobs et al.14 described that the proportion of resection area with FRs in patients with good seizure outcomes was higher than that in patients with poor seizure outcomes. Akiyama et al.18 analyzed the correlation between resection ratio of SOZ, interictal high-OR ripples, and FRs in 28 patients with epilepsy. Kerber et al.19 showed that not only FRs, but also ripples, significantly correlated with seizure-free outcomes. Similarly, in our study, the higher resection ratio of both high-OR ripples and FRs correlated with better seizure outcome in the patients with TSC. Even for TSC patients with multiple seizure types secondary to multiple cortical tubers, the HFOs can be a surrogate biomarker to identify the epileptogenic zones that can be included in the resection to improve seizure outcomes. Potential epileptogenesis in TSC A potential epileptogenic zone is an area of cortex that may generate seizures after the presurgically identified SOZ has been resected.36 Currently, there is no diagnostic modality available to directly measure the entire epileptogenic zone. This is because we cannot exclude the existence of a potential epileptogenic zone that would only become clinically apparent postoperatively. The epileptogenic zone may sometimes be more extensive than the SOZ. In such cases, the total resection of the SOZ does not result in seizure freedom. When a patient has areas with different seizure threshold within the epileptogenic zone, the SOZ must have the lowest threshold to generate seizures. In our study, the resection ratio of electrodes with high-OR HFOs channels was a better predictor of seizure outcome than the resection ratio of SOZ. This result might indicate that the electrodes with high-OR HFOs reflect the potential epileptogenic zones better than the electrodes of SOZ. Not only the complete resection of SOZ, but also the maximum resection of cortices with high-OR HFOs was important to achieve good seizure outcome in TSC. TSC patients with intractable epilepsy secondary to multiple tubers may have multiple potential epileptogenic zones, since multiple tubers may contribute to and alter a widespread epileptic network in the cortical-subcortical regions, with short or long, various range interconnections.12 The patients with residual seizures probably had lower threshold in the unresected potential epileptogenic zone. The areas with interictal high-OR HFOs can reveal the potential epileptogenic zone in the widespread epileptic network (Fig. 2).
1609 Interictal HFOs in Tuberous Sclerosis In the future, if automated and online HFO analysis becomes sufficiently accurate to delineate the epileptogenic zone, this prospective judgment will succeed in guiding resection of the entire epileptogenic zone with good seizure outcomes. However, it should be emphasized that HFO analysis and any other modalities using intracranial electrodes are imperfect because the grid and depth electrodes do not cover and record the whole brain. This study has some limitations. The number of the patients is small. If a larger number of patients with TSC can be studied in the future, we may clarify the correlation of seizure outcome with the resection ratio of high-OR HFOs, adjusting the effect of the resection size. To determine the channels with high-OR of HFOs, we applied Kittler’s method to binarize low-OR and high-OR HFOs. This binarization may be practical to distinguish the brain region frequently generating HFOs from the other area. However, it should be noted that the threshold is based on the EEG data from only the brain region covered by the electrodes. In addition, when the occurrence rate of HFOs does not fit well to double Gaussian distributions, which Kittler’s thresholding method assumes, the threshold could become biased (Appendix S2).
Conclusions The interictal IVEEG showed multiple distributions of SOZ and HFOs in patients with intractable epilepsy secondary to TSC. The multiplicity of SOZ and HFOs indicate complex and widespread epileptic networks in TSC. In a subset of TSC patients with drug-resistant epilepsy, resection of cortex with both interictal high-OR FRs and ripples on IVEEG correlated with a good seizure outcome.
Acknowledgments Tohru Okanishi was supported by Ontario Brain Institute, The Waksman Foundation of Japan, and Japan Brain Foundation. We thank members of the Facebook Organization of R Users for Medical Statistics in Japan (FORUMS-J) for advising on statistical analyses.
Disclosure None of the authors has any conflict of interest to disclose. We confirm that we have read the Journal’s position on issues involved in ethical publication and affirm that this report is consistent with those guidelines.
References 1. Yates JR, Maclean C, Higgins JN, Tuberous Sclerosis 2000 Study Group, et al. The Tuberous Sclerosis 2000 Study: presentation, initial assessments and implications for diagnosis and management. Arch Dis Child 2011;96:1020–1025. 2. Holmes GL, Stafstrom CE, Tuberous Sclerosis Study Group. Tuberous sclerosis complex and epilepsy: recent developments and future challenges. Epilepsia 2007;48:617–630.
3. Weiner HL, Carlson C, Ridgway EB, et al. Epilepsy surgery in young children with tuberous sclerosis: results of a novel approach. Pediatrics 2006;117:1494–1502. 4. Jacobs J, Rohr A, Moeller F, et al. Evaluation of epileptogenic networks in children with tuberous sclerosis complex using EEGfMRI. Epilepsia 2008;49:816–825. 5. Wu JY, Salamon N, Kirsch HE, et al. Noninvasive testing, early surgery, and seizure freedom in tuberous sclerosis complex. Neurology 2010;74:392–398. 6. Carlson C, Teutonico F, Elliott RE, et al. Bilateral invasive electroencephalography in patients with tuberous sclerosis complex: a path to surgery? J Neurosurg Pediatr 2011;7:421–430. 7. Evans LT, Morse R, Roberts DW. Epilepsy surgery in tuberous sclerosis: a review. Neurosurg Focus 2012;32:E5. 8. Fallah A, Guyatt GH, Snead OC 3rd, et al. Predictors of seizure outcomes in children with tuberous sclerosis complex and intractable epilepsy undergoing resective surgery: an individual participant data meta-analysis. PLoS ONE 2013;8:e53565. 9. Iida K, Otsubo H, Mohamed IS, et al. Characterizing magnetoencephalographic spike sources in children with tuberous sclerosis complex. Epilepsia 2005;46:1510–1517. 10. Sugiyama I, Imai K, Yamaguchi Y, et al. Localization of epileptic foci in children with intractable epilepsy secondary to multiple cortical tubers by usingsynthetic aperture magnetometry kurtosis. J Neurosurg Pediatr 2009;4:515–522. 11. Major P, Rakowski S, Simon MV, et al. Are cortical tubers epileptogenic? Evidence from electrocorticography. Epilepsia 2009;50:147–154. 12. Curatolo P. No abstract available. Intractable epilepsy in tuberous sclerosis: is the tuber removal not enough? Dev Med Child Neurol 2010;52:987. 13. Bragin A, Engel J Jr, Wilson CL, et al. High-frequency oscillations in human brain. Hippocampus 1999;9:137–142. 14. Jacobs J, Zijlmans M, Zelmann R, et al. High-frequency electroencephalographic oscillations correlate with outcome of epilepsy surgey. Ann Neurol 2010;67:209–220. 15. Timofeev I, Grenier F, Steriade M. The role of chloride-dependent inhibition and the activity of fast-spiking neurons during cortical spike-waveelectrographic seizures. Neuroscience 2002;114:1115– 1132. 16. Ibarz JM, Foffani G, Cid E, et al. Emergent dynamics of fast ripples in the epileptic hippocampus. J Neurosci 2010;30:16249–16261. 17. Jefferys JG, Menendez de la Prida L, Wendling F, et al. Mechanisms of physiological and epileptic HFO generation. Prog Neurobiol 2012;98:250–264. 18. Akiyama T, McCoy B, Go CY, et al. Focal resection of fast ripples on extraoperative intracranial EEG improves seizure outcome in pediatric epilepsy. Epilepsia 2011;52:1802–1811. 19. Kerber K, D€umpelmann M, Schelter B, et al. Differentiation of specific ripple patterns helps to identify epileptogenic areas for surgical procedures. Clin Neurophysiol 2014;125:1339–1345. 20. Pang EW, Gaetz W, Drake JM, et al. Patient with postcentral gyrectomy demonstrates reliable localization of hand motor area using magnetoencephalography. Pediatr Neurosurg 2009;45:311–316. 21. Benifla M, Sala F Jr, Jane J, et al. Neurosurgical management of intractable rolandic epilepsy in children: role of resection in eloquent cortex. Clinical article. J Neurosurg Pediatr 2009;4:199–216. 22. Ochi A, Otsubo H, Donner EJ, et al. Dynamic changes of ictal highfrequency oscillations in neocortical epilepsy: using multiple band frequency analysis. Epilepsia 2007;48:286–296. 23. Staba RJ, Wilson CL, Bragin A, et al. Quantitative analysis of highfrequency oscillations (80–500 Hz) recorded in human epileptic hippocampus and entorhinal cortex. J Neurophysiol 2002;88: 1743–1752. 24. Bagshaw AP, Jacobs J, LeVan P, et al. Effect of sleep stage on interictal high-frequency oscillations recorded from depth macroelectrodes in patients with focal epilepsy. Epilepsia 2009;50:617–628. 25. Otsubo H, Ochi A, Imai K, et al. High-frequency oscillations of ictal muscle activity and epileptogenic discharges on intracranial EEG in a temporal lobe epilepsy patient. Clin Neurophysiol 2008;119:862–868. 26. Kittler J, Illingworth J. Minimum error thresholding. Pattern Recognit 1986;19:41–47. Epilepsia, 55(10):1602–1610, 2014 doi: 10.1111/epi.12761
1610 T. Okanishi et al. 27. Spencer SS. Neural networks in human epilepsy: evidence of and implications for treatment. Epilepsia 2002;43:219–227. 28. Weiner HL. Tuberous sclerosis and multiple tubers: localizing the epileptogenic zone. Epilepsia 2004;45(Suppl. 4):41–42. 29. Ohmori I, Ohtsuka Y, Ohno S, et al. Analysis of ictal EEGs of epilepsy associated with tuberous sclerosis. Epilepsia 1998;39:1277–1283. 30. Ohtsuka Y, Ohmori I, Oka E. Long-term follow-up of childhood epilepsy associated with tuberous sclerosis. Epilepsia 1998;39: 1158–1163. 31. Jansen FE, van Huffelen AC, Algra A, et al. Epilepsy surgery in tuberous sclerosis: a systematic review. Epilepsia 2007;48:1477–1484. 32. Avellino AM, Berger MS, Rostomily RC, et al. Surgical management and seizure outcome in patients with tuberous sclerosis. J Neurosurg 1997;87:391–396. 33. Guerreiro MM, Andermann F, Andermann E, et al. Surgical treatment of epilepsy in tuberous sclerosis: strategies and results in 18 patients. Neurology 1998;51:1263–1269. 34. Koh S, Jayakar P, Dunoyer C, et al. Epilepsy surgery in children with tuberous sclerosis complex: presurgical evaluation andoutcome. Epilepsia 2000;41:1206–1213. 35. Wong M. Mechanisms of epileptogenesis in tuberous sclerosis complex and related malformations of cortical developmentwith abnormal glioneuronal proliferation. Epilepsia 2008;49:8–21.
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36. Rosenow F, L€uders H. Presurgical evaluation of epilepsy. Brain 2001;124:1683–1700.
Supporting Information Additional Supporting Information may be found in the online version of this article: Table S1. Correlations between surgical prognosis and intracranial video-EEG data. Table S2. Thresholds of high-occurrence rate of ripples and FRs. Appendix S1. Kittler’s method to determine the threshold separating the channels with high occurrence rate of HFOs from those with low occurrence rate of HFOs. Appendix S2. Bootstrapping to determine the final threshold.
This document is a scanned copy of a printed document. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material.
Interictal high frequency oscillations correlating with seizure outcome in patients with widespread epileptic networks in tuberous sclerosis complex *Tohru Okanishi, †Tomoyuki Akiyama, *Shin-Ichi Tanaka, *Ellen Mayo, *Ayu Mitsutake, *Cyrus Boelman, *Cristina Go, *O. Carter Snead 3rd, ‡James Drake, ‡James Rutka, *Ayako Ochi, and *Hiroshi Otsubo Epilepsia, 55(10):1602–1610, 2014 doi: 10.1111/epi.12761
SUMMARY
Tohru Okanishi is interested in the development of the treatment for children with medically refractory epilepsy.
Objective: Multiple tubers in patients with tuberous sclerosis complex (TSC) often are responsible for drug-resistant epilepsy. The complexity of the epileptic network formed by multiple tubers complicates localization of the epileptogenic zone that is needed to design a surgical treatment strategy. High frequency oscillations (HFOs) on intracranial video-electroencephalography (IVEEG) may be a valuable surrogate marker for the localization of the epileptogenic zone. The purpose of this study was to test the hypothesis that high occurrence rate (OR) of interictal HFOs can guide the localization of the epileptogenic zone. Methods: We analyzed the OR of interictal HFOs at 80–200 Hz (ripples) and >200 Hz (fast ripples, FRs). We divided OR of interictal HFOs between high and low rates by thresholding. We analyzed the correlation between seizure outcomes using Engel classification and the resection ratio of the seizure onset zone (SOZ), and high-OR HFOs using ordinal logistic regression analysis. Results: We collected 10 patients. The seizure outcomes resulted in Engel classification I in three patients, II in four, III in one, and IV in two. High-OR ripples (5–57 [mean 29] channels, 1–4 [2.8] lobes) and high-OR FRs (9–66 [mean 27] channels, 1–4 [2.6] lobes) were widely distributed. The resection ratio of SOZ did not show statistically significant correlation with the seizure outcome. The resection ratio of high-OR ripples showed statistically significant correlation with the seizure outcome (p = 0.038). The resection ratio of high-OR FRs showed statistically significant correlation with the seizure outcome (p = 0.048). Significance: The multiple extensive zones with high-OR HFOs suggest a complex and widespread epileptic network in patients with TSC. In a subset of TSC patients with drug-resistant epilepsy, resection of cortex with both interictal high-OR FRs and ripples on IVEEG correlated with a good seizure outcome. KEY WORDS: High frequency oscillations, Epilepsy surgery, Potential epileptogenic zone, Complex epileptic network, Epileptogenesis, Multiple cortical tubers.
Accepted July 22, 2014; Early View publication September 5, 2014. *Division of Neurology, The Hospital for Sick Children, Toronto, Ontario, Canada; †Department of Child Neurology, Okayama University Hospital, Okayama, Japan; and ‡Division of Neurosurgery, The Hospital for Sick Children, Toronto, Ontario, Canada Address correspondence to Hiroshi Otsubo, Division of Neurology, The Hospital for Sick Children, 555 University Avenue, Toronto, ON, Canada M5G 1X8. E-mail: [email protected]; [email protected] Wiley Periodicals, Inc. © 2014 International League Against Epilepsy
Tuberous sclerosis complex (TSC) is a neurocutaneous disorder caused by mutations in either TSC1 or TSC2 genes, which encode the protein products of hamartin and tuberin, respectively. Phenotypically this disorder is characterized by tumors in multiple organs, including the brain.1 Sixty percent to 90% of patients with TSC develop epilepsy.1,2 The multiple cortical tubers associated with this disorder can cause generalized and multifocal seizures, and epileptic
1602
1603 Interictal HFOs in Tuberous Sclerosis spasms.3–6 Fifty percent to 80% of TSC patients with epilepsy are refractory to treatment with multiple antiepileptic medications.7 Resective surgery may be a therapeutic option for a subset of TSC patients with drug-resistant seizures. Seizure freedom has been reported in 56% after resective surgery.8 In patients with intractable epilepsy secondary to TSC, seizures can arise independently from multiple cortical tubers.4,6 Epileptogenic zones have been defined to occur around tubers with both intracranial video-EEG (IVEEG) and magnetoencephalography (MEG).5,9–11 Furthermore, it is expected that alterations in cortical-subcortical system connectivity may lead to wide epileptic network spreading to nonlesional areas in patients with TSC.12 Therefore, removal of the cortical tuber (tuberectomy) alone may not be sufficient to interrupt the complex epileptic network. High frequency oscillations (HFOs), subdivided into ripples (80–200 Hz) and fast ripples (FRs, >200 Hz), are biomarkers of epileptic brain activity. Bragin et al.13 found HFOs in entorhinal cortex and hippocampus in patients with mesial temporal lobe epilepsy. Epileptic HFOs were also found in neocortical seizure onset zones (SOZs).14 Pathologically interconnected neurons produce hypersynchronous bursts of 200–300 Hz.13 Neocortical inhibitory neurons generate fast-spiking activity, with a frequency of up to 800 Hz.15 Firing in phase synchronously and out of phase creates 100–400 Hz HFOs.16 Disrupted neuronal networks are composed of pathologic clusters that generate FRs.17 The resection of brain regions with interictal ripples and FRs recorded on IVEEG correlated with better seizure outcomes.14,18,19 The purpose of this study was to test the hypothesis that high occurrence rate (OR) interictal HFOs can guide the localization of the epileptogenic zone pursuant to its surgical resection in patients with TSC. We hypothesized further that an extensive and discontinuous distribution of SOZ and HFOs is characteristic of a widespread and complex epileptic network in patients with TSC.
Methods Patients We retrospectively identified 13 children with TSC and drug-resistant epilepsy, who underwent IVEEG recordings using subdural and depth electrodes followed by resective surgery between June 2006 and March 2012 at The Hospital for Sick Children. We excluded two patients with continuous seizures over 1 h and one child who had significant artifacts on the interictal IVEEG. We used the data from the remaining 10 patients. The follow-up period ranged from 19 to 76 months, with a mean of 58 months. The seizure outcomes were Engel classification I in three patients, II in four patients, III in one patient, and IV in two patients. All patients underwent presurgical evaluation including scalp video-EEG (SVEEG), magnetic resonance imaging (MRI),
and magnetoencephalography (MEG), and had postsurgical follow-up >12 months. From clinical records, we reviewed the range and mean age at seizure onset, duration of seizure history, age at surgery, and the number of antiepileptic medications over the clinical course before surgery and at the time of surgery. IVEEG recording Based on seizure semiology and ictal/interictal data on SVEEG, prominent lesions on MRI, and the lateralization and distributions of interictal MEG dipole sources, we determined the hemisphere and location to place the subdural grid electrodes. We performed IVEEG with onesheet subdural grid EEG electrodes and some strip/depth electrodes. Five patients had 8–12 strip electrodes in the hemisphere contralateral to the surgical side. We performed functional mapping using direct cortical stimulation to define eloquent cortices and sensory evoked potentials. The technique of implantation of intracranial grid, strip and depth electrodes, and intraoperative and extraoperative functional mapping of eloquent cortex was described previously.20,21 Inter-electrode distance in the subdural grid electrodes was 9–11 mm. The diameter of subdural electrodes was 4 mm with exposure of 2.3 mm (effective surface area: 4.2 mm2) and the surface area of depth electrodes was 8.3 mm2 (Ad-Tech Medical Instrument, Racine, WI, U.S.A.). For IVEEG recordings, we used HARMONY 5.4 (Stellate, Montreal, PQ, Canada). The IVEEG recordings were sampled at 1 kHz with the anti-aliasing filter at 300 Hz, or 2 kHz with the anti-aliasing filter at 880 Hz. IVEEG was recorded for 24–216 h (mean 80 h). We analyzed the SOZ and HFOs in the unilateral hemisphere of the surgical side, because the contralateral strip electrodes were much fewer in number than the grid electrodes and were not placed to cover multiple lobes. Resective surgery The area of cortical resection was determined by clinical neurophysiologists based on the localization of the SOZ, ictal symptomatogenic zone, prominent interictal zone, and the location of eloquent cortices on IVEEG data. The SOZ was identified as low amplitude fast waves or rhythmic change from background activity at the ictal EEG onset period. When we removed the grid, the resection margin was marked by the electrodes that included SOZ and active interictal epileptiform discharges, with consideration of MEG, MRI, and neuropsychologic findings.22 Because the evaluation of HFOs in this study was retrospectively performed, the real-time surgical decision making process was not related to the results of this study. Evaluation of HFOs We selected 10 epochs of 2-min interictal EEG during sleep from IVEEG, so that they were remote from each other and from seizures by at least 1 h. Interictal EEG during Epilepsia, 55(10):1602–1610, 2014 doi: 10.1111/epi.12761
1604 T. Okanishi et al. non–rapid eye movement (non-REM) sleep was selected because the non-REM sleep shows HFOs more frequently than other states.23,24 We visually inspected each EEG epoch with a high-pass filter at 0.5 and 200 Hz to ensure that they were not contaminated by significant artifacts, such as environmental or muscle artifacts.25 We used bipolar montage with pairs of two adjacent EEG electrodes successively connected. We called “channel” using the bipolar montage. The channels including reference electrodes and those with significant artifacts were excluded. We evaluated the 10 EEG epochs. The automated detection of HFOs was performed using MATLAB (The MathWorks, Natick, MA, U.S.A.) described in the previous paper.18 In this analysis, we calculated the OR (per minute) of HFOs (ripples and FRs) on each channel. The analysis of 10 epochs of EEG recording yielded 10 values for the OR of HFOs for each channel. From the results, we chose the median values of the OR of HFOs for each channel. Subsequently, we determined the channels with high-OR and low-OR of HFOs for each patient by calculating the threshold using Kittler’s method (Appendix S1).26 Because of the small samples, we applied the bootstrap method to better estimate the threshold (Appendix S2). We generated a bootstrap sample by sampling with replacement from the
original dataset. The size of the bootstrap sample was set to be the same as that of the original dataset (the number of channels). We repeated this process 1,000 times to obtain 1,000 bootstrap samples. Thereafter we calculated Kittler’s threshold for each of 1,000 bootstrap samples. We applied the mean of the 1,000 Kittler’s thresholds as the final threshold to separate the channels with high-OR of HFOs from those with low-OR of HFOs. Quantitative measures We reviewed the total number of electrodes used for IVEEG, the number of electrodes on the surgical side, and the number of electrodes within the resection margin. For analysis of the distribution of SOZ and high-OR HFOs, we applied “segment” for contiguous channels with SOZ and high-OR HFO. We included contiguous channels in one segment. For example, two segments with high-OR FRs in patient 1 meant one segment with 24 channels and the other segment with one channel (Fig. 1). The number of channels inside the resection margin or SOZ was counted as follows: (1) if both electrodes in bipolar montage were inside of the resection margin or SOZ, the channel was counted as “1”; (2) if only one of two electrodes in a bipolar montage was inside of the resection margin or SOZ, the channel was counted as “0.5.”
Figure 1. Seizure onset zone (SOZ, yellow lines), topographic maps of high occurrence rate (OR) ripples (orange), high-OR fast ripples (FRs, red), and resection margins (blue) in a seizure-free patient (patient 1) and a residual seizure patient (patient 9). Epilepsia ILAE Epilepsia, 55(10):1602–1610, 2014 doi: 10.1111/epi.12761
1605
Single or multiple
L R L L R R R R R R Bilateral Bilateral Bilateral Bilateral Bilateral Bilateral Bilateral Bilateral Unilateral (R) Unilateral (R)
Cortical tubers on MRI
Statistical analysis We examined the relationships between the seizure outcome and the number of IVEEG electrodes in the surgical side, the number of resected IVEEG electrodes, the resection ratio of SOZ, or the resection ratio of high-OR HFOs. We used univariate ordinal logistic regression analysis. The significance level was set to p < 0.05.
Unilateral or bilateral
Largest tubers
Resection ratio of SOZ or high-OR HFOs ¼ Resected channels of SOZ or high-OR HFOs total channels of SOZ or high-OR HFOs
Multiple Multiple Multiple Multiple Multiple Multiple Multiple Multiple Multiple Single
Surgical prognosis (Engel classification)
We analyzed the resection ratio of SOZ and high-OR HFOs, using following formula:
I I I II II II II III IV IV
Interictal HFOs in Tuberous Sclerosis
L L R L R R L R R (UD) L L R L R R L R R R Male Female Male Female Male Male Male Male Female Female 1 2 3 4 5 6 7 8 9 10
PSz, partial seizure; ES, epileptic spasm; 2G, secondary generalization; L, left; R, right; UD, undetermined.
2 2 2 4 1 3 2 1 2 2 5 6 3 5 6 5 4 5 7 10 54 91 113 96 131 216 156 22 81 38 PSz ES PSz, 2G ES, PSz ES PSz ES ES PSz, 2G PSz 2 13 72 8 6 1.5 5 10 1 2
Seizure type at onset Age at seizure onset (months) Age at surgery (years old) Gender
4.7 8.7 15.4 8.7 11.4 18.4 13.4 2.7 6.8 3.3
Ictal Interictal At surgery In the course
No. of antiepileptic medications
Duration of seizure history (months) Pt no.
IVEEG profiles Table 2 describes a summary of IVEEG, SOZ, HFOs, and resective surgery. Figure 1 described SOZ and zones of high-OR ripples and FRs in patients 1 and 9, respectively. We applied 96–124 IVEEG electrodes (mean 115), including 92–120 IVEEG electrodes (109) in the surgical side. There was no significant correlation between the number of IVEEG electrodes in the surgical side and the seizure
Table 1. Clinical profiles
Clinical information Table 1 describes the patients’ clinical profiles including gender, age at surgery and at seizure onset, number of antiepileptic medications in the clinical course and at surgery, seizure type at the onset, duration of seizure history, predominant hemisphere of interictal and ictal epileptiform discharge on SVEEG, MRI findings, and seizure outcome. The age at surgery ranged from 2.7 to 18.4 years, with a mean of 10 years. Duration of epilepsy before surgery ranged from 22 to 216 months, with a mean of 100 months. The number of antiepileptic medications throughout the clinical course ranged from 3 to 10 with a mean of 6. The number of antiepileptic medications at surgery ranged from 1 to 4, with a mean of 2. The follow-up periods ranged from 19 to 76 months, with a mean of 58 months. There was a heterogeneity of seizure types with partial seizures in six patients, epileptic spasms in five, and secondarily generalized seizures in two. The predominant hemisphere with interictal discharges was left in four patients and right in six. The predominant hemisphere with ictal discharges was left in four patients, right in five, and undetermined in one. MRI revealed multiple cortical tubers in nine patients and a single tuber in one. The tubers were distributed bilaterally in eight patients and in the right hemisphere in two. The largest cortical tubers on MRI was seen in the left hemisphere in three patients and the right hemisphere in seven. The side of the largest tuber was concordant with surgical side in seven patients (patients 1, 4–6, 8–10).
Predominance of epileptiform discharge on scalp video- EEG
Results
Epilepsia, 55(10):1602–1610, 2014 doi: 10.1111/epi.12761
Epilepsia, 55(10):1602–1610, 2014 doi: 10.1111/epi.12761
L L
R
L R
R
L R R
R
1 2
3
4 5
6
7 8 9
10
120
119 120 120
103
104 96
118
122 124
120
111 108 120
103
92 96
118
110 112
1
1 1 3
5
2 3
4
1 3
No. of types ES ES, PSz, EEGSz PSz x2, EEGSz x2 PSz x2 PSz x2, PSz2G PSz, PSz2G, EEGSz x3 PSz PSz PSz x2, PSz2G ES
Seizure types
Seizures on IVEEG monitoring
52
4.5 8 8
8
11 15.5
14.5
48.5 6.5
Total no. of channelsa
2
3 3 3
4
7 6
3
4 6
Total no. of segments
Seizure onset zones
FTP
FP F F
FPO
FTP FP
TO
FTPO FPO
Distributed lobes
57
10 5 15
36
36 30
46
7 44
Total no. of channels
2
4 2 9
5
4 5
6
2 4
Total no. of segments
FTP
FT F FPO
FPO
FTO FTP
TPO
TPO FTPO
Distributed lobes
High occurrence rate ripples
66
13 9 17
31
17 19
36
25 41
Total no. of channels
2
5 6 5
5
4 4
9
2 7
Total no. of segments
FTP
FTP F FP
FPO
FP FTP
PO
TPO FTPO
Distributed lobes
High occurrence rate FRs
FTP
FTP F FT
FPO
FTPO FP
TPO
TPO FTPO
Resection lobes
42
32 33 29
41
65 35
47
69 83
Resected EEG electrodes
IVEEG, intracranial video-EEG; FRs, fast ripples; L, left; R, right; ES, epileptic spasms; PSz, partial seizure; PSz2G, partial seizure secondarily generalized; EEGSz, EEG seizure; F, frontal; T, temporal; P, parietal; O, occipital. a Channel: using bipolar montage.
Surgery side
Pt no.
Total no. of IVEEG electrodes
No. of IVEEG electrodes on surgery side
Table 2. Profiles of intracranial video-EEG and resective surgery
1606 T. Okanishi et al.
1607
100 96 74 74 66 71 65 78 24 52 25 39.5 12.5 26.5 12.5 22 8.5 7 4 34.5 25 41 17 36 19 31 13 9 17 66 100 93 100 66 55 67 85 100 37 60 1 2 3 4 5 6 7 8 9 10
SOZ, seizure onset zone; HFOs, high frequency oscillations; FRs, fast ripples.
45 6.5 11 14.5 15.5 8 4 8 7 35
94 100 100 100 100 100 89 100 93 67
7 44 36 46 30 36 10 5 15 57 48.5 6.5 11 14.5 15.5 8 4.5 8 8 52
Pt #
7 41 36 30.5 16.5 24 8.5 5 5.5 34
Resection ratio (%) # of resected channels
High occurrence rate FRs
Total # of channels Resection ratio (%) # of resected channels
High occurrence rate ripples
Total # of channels
Widespread and disrupted epileptic network in TSC The multiple segments of SOZs, interictal high-OR ripples, and FRs in our patients suggest that there are widespread and disrupted epileptic networks in TSC (Fig. 2).
Resection ratio (%)
Our TSC patients with intractable epilepsy showed multiple segments of SOZs, high-OR ripples, and FRs. This multiplicity of epileptic areas with SOZ, ripples, and FRs reflects widespread, disrupted, and complicated epileptic networks in TSC, thus validating our second hypothesis. The resected electrodes, resection of area with high-OR ripples, and FRs showed correlation with good seizure outcome. These data may support the hypothesis that HFOs can be a surrogate biomarker to detect the epileptogenic zones and target surgical resection in TSC patients with intractable epilepsy. This is the first report of interictal HFO analysis for TSC surgical patients demonstrating high-OR HFOs relating to widespread epileptogenic zone on the intracranial electrodes.
# of resected channels
Discussion
Total # of channels
Resection ratio of SOZ and high-OR HFOs Table 3 describes the profiles of resected channels and resection ratios of SOZ and high-OR HFO channels. The resection ratios of SOZ, high-OR ripples, and high-OR FRs were 67–100% (mean: 94%), 37–100% (76%), and 24–100% (70%), respectively. SOZ data did not show significant correlation with the seizure outcome (Table S1). The resection ratio of high-OR ripples showed a significant correlation with the seizure outcome (p = 0.038; standard regression coefficient 1.63). The resection ratio of high-OR FRs showed a significant correlation with the seizure outcome (p = 0.048; standard regression coefficient 3.54).
Seizure onset zones
outcome (Table S1). Five patients (1, 2, 4, 7, and 8) needed contralateral strips electrodes to record the IVEEG. During IVEEG, four patients (1, 7, 8, and 10) showed single type of seizure. The other six patients showed multiple types of seizures, including different types of ictal EEG changes. The thresholds of high-OR ripples ranged between 1.2 and 42.0/min (Table S2). The thresholds of high-OR FRs ranged between 1.8 and 18.6/min. Total numbers of channels of SOZ, high-OR ripples, and FRs were 4.5–52 (mean: 18), 5–57 (29), and 9–66 (27), respectively. Total numbers of segments with SOZ, high-OR ripples, and high-OR FRs were 2–7 (4.1), 2–9 (4.3), and 2–9 (4.9), respectively. The channels with SOZ, high-OR ripples, and high-OR FRs in surgical hemisphere were distributed in 1–4 (mean 2.4) lobes, 1–4 (2.8), and 1–4 (2.6) lobes, respectively. The resected IVEEG electrodes ranged between 29 and 83 (mean 48). The resected IVEEG electrodes showed significant correlation with seizure outcome (p = 0.048; standard regression coefficient = 1.78; Table S1).
Table 3. Resected channels and resection ratios of SOZ and high occurrence rate HFOs channels, and ratios of unresected high-occurrence rate HFOs channels outside SOZ
Interictal HFOs in Tuberous Sclerosis
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1608 T. Okanishi et al.
Figure 2. Schema of widespread epileptic networks in the brain with TSC. Multiple tubers alter widespread epileptic network in the corticalsubcortical regions, with short or long, various range interconnections. Interictal HFOs can exist in both the SOZ and potential epileptogenic zone. Epilepsia ILAE
These aberrant neuronal networks in TSC have been reported to alter cortical-subcortical system connectivity.12 Seizure entrainment on interconnectivities among the multiple epileptogenic foci caused by the cortical tubers4,6 provoke a wide epileptic network.27 The cortex surrounding cortical tubers in patients with TSC produce epileptic discharges and contribute to this aberrant network,9,10 as does cortex remote from the tubers.9,12,28 Neurophysiologic examinations including EEG and MEG can detect epileptic networks in TSC that are more complex than that suggested by lesions seen with conventional neuroimaging techniques. Ohmori et al.29 reported that SVEEG identified two or three seizure foci at the seizure onset of partial seizures in 4 of 15 TSC patients with epilepsy. Multifocal interictal discharges were observed frequently in 25 of 27 TSC patients with localization-related epilepsy.30 Segments of SOZ, interictal high-OR ripples, and FRs ranged between two and nine in our IVEEG using subdural grids with >100 electrodes. Although the IVEEG cannot cover the entire bilateral hemispheres, the subdural grid at least covers the presumed epileptogenic zone, as indicated by SVEEG, MRI, and MEG. High spatiotemporal resolution using >100 electrodes and 1–2 kHz sampling rate can reveal the complex, widespread, and disrupted epileptic networks in patients with TSC. Extensive resection of the complex epileptic network in TSC The cerebral tissues surrounding cortical tubers appear to provoke epileptic seizures in patients with TSC.4,9,31 If a single primary epileptogenic tuber is identified in the patients with TSC, surgical removal of the single epileptogenic tuber results in a good seizure outcome.3,28,32–34 However, the identification of multiple epileptogenic zones in Epilepsia, 55(10):1602–1610, 2014 doi: 10.1111/epi.12761
patients with multiple tubers is challenging because the multiple epileptogenic zones can demonstrate extensive complex relationships among the lesions.3,12,28,35 In our study, nine patients had multiple tubers and one patient had a single tuber, but all ten had multiple segments of SOZ, high-OR ripples and FRs. HFO analysis using the interictal IVEEG data can detect the epileptogenic zones in epilepsy surgery cases. Jacobs et al.14 described that the proportion of resection area with FRs in patients with good seizure outcomes was higher than that in patients with poor seizure outcomes. Akiyama et al.18 analyzed the correlation between resection ratio of SOZ, interictal high-OR ripples, and FRs in 28 patients with epilepsy. Kerber et al.19 showed that not only FRs, but also ripples, significantly correlated with seizure-free outcomes. Similarly, in our study, the higher resection ratio of both high-OR ripples and FRs correlated with better seizure outcome in the patients with TSC. Even for TSC patients with multiple seizure types secondary to multiple cortical tubers, the HFOs can be a surrogate biomarker to identify the epileptogenic zones that can be included in the resection to improve seizure outcomes. Potential epileptogenesis in TSC A potential epileptogenic zone is an area of cortex that may generate seizures after the presurgically identified SOZ has been resected.36 Currently, there is no diagnostic modality available to directly measure the entire epileptogenic zone. This is because we cannot exclude the existence of a potential epileptogenic zone that would only become clinically apparent postoperatively. The epileptogenic zone may sometimes be more extensive than the SOZ. In such cases, the total resection of the SOZ does not result in seizure freedom. When a patient has areas with different seizure threshold within the epileptogenic zone, the SOZ must have the lowest threshold to generate seizures. In our study, the resection ratio of electrodes with high-OR HFOs channels was a better predictor of seizure outcome than the resection ratio of SOZ. This result might indicate that the electrodes with high-OR HFOs reflect the potential epileptogenic zones better than the electrodes of SOZ. Not only the complete resection of SOZ, but also the maximum resection of cortices with high-OR HFOs was important to achieve good seizure outcome in TSC. TSC patients with intractable epilepsy secondary to multiple tubers may have multiple potential epileptogenic zones, since multiple tubers may contribute to and alter a widespread epileptic network in the cortical-subcortical regions, with short or long, various range interconnections.12 The patients with residual seizures probably had lower threshold in the unresected potential epileptogenic zone. The areas with interictal high-OR HFOs can reveal the potential epileptogenic zone in the widespread epileptic network (Fig. 2).
1609 Interictal HFOs in Tuberous Sclerosis In the future, if automated and online HFO analysis becomes sufficiently accurate to delineate the epileptogenic zone, this prospective judgment will succeed in guiding resection of the entire epileptogenic zone with good seizure outcomes. However, it should be emphasized that HFO analysis and any other modalities using intracranial electrodes are imperfect because the grid and depth electrodes do not cover and record the whole brain. This study has some limitations. The number of the patients is small. If a larger number of patients with TSC can be studied in the future, we may clarify the correlation of seizure outcome with the resection ratio of high-OR HFOs, adjusting the effect of the resection size. To determine the channels with high-OR of HFOs, we applied Kittler’s method to binarize low-OR and high-OR HFOs. This binarization may be practical to distinguish the brain region frequently generating HFOs from the other area. However, it should be noted that the threshold is based on the EEG data from only the brain region covered by the electrodes. In addition, when the occurrence rate of HFOs does not fit well to double Gaussian distributions, which Kittler’s thresholding method assumes, the threshold could become biased (Appendix S2).
Conclusions The interictal IVEEG showed multiple distributions of SOZ and HFOs in patients with intractable epilepsy secondary to TSC. The multiplicity of SOZ and HFOs indicate complex and widespread epileptic networks in TSC. In a subset of TSC patients with drug-resistant epilepsy, resection of cortex with both interictal high-OR FRs and ripples on IVEEG correlated with a good seizure outcome.
Acknowledgments Tohru Okanishi was supported by Ontario Brain Institute, The Waksman Foundation of Japan, and Japan Brain Foundation. We thank members of the Facebook Organization of R Users for Medical Statistics in Japan (FORUMS-J) for advising on statistical analyses.
Disclosure None of the authors has any conflict of interest to disclose. We confirm that we have read the Journal’s position on issues involved in ethical publication and affirm that this report is consistent with those guidelines.
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Supporting Information Additional Supporting Information may be found in the online version of this article: Table S1. Correlations between surgical prognosis and intracranial video-EEG data. Table S2. Thresholds of high-occurrence rate of ripples and FRs. Appendix S1. Kittler’s method to determine the threshold separating the channels with high occurrence rate of HFOs from those with low occurrence rate of HFOs. Appendix S2. Bootstrapping to determine the final threshold.
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