FULL-LENGTH ORIGINAL RESEARCH

Role of ictal baseline shifts and ictal high-frequency oscillations in stereo-electroencephalography analysis of mesial temporal lobe seizures *Shasha Wu, †Hari Prasad Kunhi Veedu, ‡Samden D. Lhatoo, §Mohamad Z. Koubeissi, ¶Jonathan P. Miller, and ‡Hans O. L€ uders Epilepsia, 55(5):690–698, 2014 doi: 10.1111/epi.12608

SUMMARY

Dr. Shasha Wu is an assistant professor of neurology at the University of Chicago.

Objective: To assess the role of ictal baseline shifts (IBS) and ictal high-frequency oscillations (iHFOs) in intracranial electroencephalography (EEG) presurgical evaluation by analysis of the spatial and temporal relationship of IBS, iHFOs with ictal conventional stereo-electroencephalography (icEEG) in mesial temporal lobe seizures (MTLS). Methods: We studied 15 adult patients with medically refractory MTLS who underwent monitoring with depth electrodes. Seventy-five ictal EEG recordings at 1,000 Hz sampling rate were studied. Visual comparison of icEEG, IBS, and iHFOs were performed using Nihon-Kohden Neurofax systems (acquisition range 0.016–300 Hz). Each recorded ictal EEG was analyzed with settings appropriate for displaying icEEG, IBS, and iHFOs. Results: IBS and iHFOs were observed in all patients and in 91% and 81% of intracranial seizures, respectively. IBS occurred before (22%), at (57%), or after (21%) icEEG onset. In contrast, iHFOs occurred at (30%) or after (70%) icEEG onset. The onset of iHFOs was 11.5 s later than IBS onset (p < 0.0001). All of the earliest onset of IBS and 70% of the onset of iHFOs overlapped with the ictal onset zone (IOZ). Compared with iHFOs, interictal HFOs (itHFOs) were less correlated with IOZ. In contrast to icEEG, IBS and iHFOs had smaller spatial distributions in 70% and 100% of the seizures, respectively. An IBS dipole was observed in 66% of the seizures. Eighty-seven percent of the dipoles had a negative pole at the anterior/medial part of amygdala/hippocampus complex (A-H complex) and a positive pole at the posterior/lateral part of the A-H complex. Significance: The results suggest that evaluation of IBS and iHFOs, in addition to routine icEEG, helps in more accurately defining the IOZ. This study also shows that the onset and the spatial distribution of icEEG, IBS, and iHFOs do not overlap, suggesting that they reflect different cellular or network dynamics. KEY WORDS: Depth intracranial monitoring, Ripples, Fast ripples, Infraslow activity, DC shifts.

Accepted February 21, 2014; Early View publication April 11, 2014. *Department of Neurology, Adult Epilepsy Center, University of Chicago Medical Center, Chicago, Illinois, U.S.A.; †Department of Neurology, Metro Health Medical Center, Cleveland, Ohio, U.S.A.; ‡Department of Neurology, Epilepsy Center, University Hospitals, Case Medical Center, Cleveland, Ohio, U.S.A.; §Department of Neurology, Epilepsy Center, George Washington University, Washington, District of Columbia, U.S.A.; and ¶Department of Neurosurgery, University Hospitals, Case Medical Center, Cleveland, Ohio, U.S.A Address correspondence to: Hans O. L€uders, MD, Department of Neurology, Epilepsy Center, University Hospital, Case Medical Center, 11100 Euclid Avenue, Cleveland, OH 44106, U.S.A. E-mail: hans.luders@ uhhospitals.org Wiley Periodicals, Inc. © 2014 International League Against Epilepsy

Epilepsy and seizures affect up to 3 million Americans. Approximately 70–80% of medicated individuals with epilepsy enter remission, whereas the remaining 20–30% become medically resistant.1 Epilepsy surgery provides seizure freedom in 40–70% of patients with medically refractory focal epilepsy,2 and surgical failures, by definition, are due to difficulties in delineating the epileptogenic zone. Therefore, it is crucial to find a reliable method to identify the location and extent of the epileptogenic zone.

690

691 IBS and HFOs in SEEG Analysis Intracranial electroencephalography (EEG) recordings provide direct information on the seizure onset and propagation if scalp EEG findings are equivocal. However, conventional intracranial EEG (icEEG) can sample only limited brain areas and only permits analysis of activities from 1 to 70 Hz. Wide-band EEG systems, however, allow us to visualize waveforms under 0.1 Hz and above 100 Hz. Ictal baseline shifts (IBS) have been described in chemically induced seizures in experimental animals since the 1960s.3–6 Several studies have shown that IBS have considerably smaller electrical fields than conventional frequencies and that they may better define seizure onset.7–12 At the other end of the EEG frequency spectrum, high frequency oscillations (HFOs) have been reported to occur primarily in the epileptogenic zone.13–15 HFOs include ripples (80–200 Hz) and fast ripples (FRs, 200–500 Hz). Ripples have been observed in the hippocampus, parahippocampal structures, and neocortex of rodents, primates, and humans in both normal and epileptic brain tissue.16–18 FRs were recorded mainly in brain tissue capable of generating seizures, and there are reports that they are relatively specific markers of the epileptogenic zone.19–22 More recently it has been reported that surgical resections of cortical areas showing HFOs can increase postsurgical seizure-free outcomes.23 In this study, we assess the role of IBS and ictal HFOs (iHFOs) in intracranial EEG presurgical evaluation by analysis of their spatial and temporal relationships with icEEG using intracranial depth electrodes in 15 patients with medically- refractory mesial temporal lobe seizures (MTLS).

Patients and Methods Patients The study was conducted at the Epilepsy Center of University Hospitals Case Medical Center after institutional review board (IRB) approval. We retrospectively evaluated 15 patients with medically intractable MTLS who were implanted with intracranial depth electrodes. Their mean age was 38 years (21–58). Thirteen patients had unilateral and two patients had bilateral electrode placements. All patients had brain magnetic resonance imaging (MRI), long-term scalp video-EEG monitoring, interictal fluorodeoxyglucose positron emission tomography (FDG-PET) scans, and neuropsychological testing as part of their presurgical evaluation. Eight patients had normal MRIs and seven patients had abnormal MRIs, including three patients with unilateral hippocampal sclerosis (HS), two patients with postsurgical changes from previous failed epilepsy surgery, one patient with postencephalitic left anterior temporal lobe encephalomalacia and gliosis, and one patient with a lesion in the left amygdala. Of the three patients with unilateral HS, one patient had preserved memory function on the ipsilateral side of the sclerosis. In this patient, depth electrodes were used to rule out an extrahippocampal seizure onset and

to confirm his surgical candidacy. Another patient with unilateral HS had extratemporal interictal epileptiform discharges and seizure semiology suggestive of frontal lobe epilepsy during noninvasive monitoring. The third patient with unilateral HS had bilateral seizure onsets on scalp EEG. Depth electrodes implantation and stereoelectroencephalography recording The number and location of the invasive EEG electrodes were based on findings of the presurgical evaluation as described elsewhere.24 Regions targeted varied from patient to patient but included the amygdala (AM), hippocampal head (HH), hippocampal body (HB), anterior temporal pole/ temporal neocortex, posterior temporal lobe/temporooccipital region, anterior and posterior insula, posterior cingulate gyrus, and mesial/orbital frontal regions. Implantation of electrodes was performed stereotactically using a Leksell frame, and trajectories were previously planned using the iPlan 3.0 workstation (Brainlab, Inc., Westchester, IL, U.S.A.). Typically, 7–14 depth electrodes with 12 contacts each were used. Each depth electrode consisted of 12 platinum-iridium cylinders measuring 1.1 mm in diameter and 2.3 mm in length, evenly spaced at 5 mm intervals (Integra Life Sciences, Plainsville, NJ, U.S.A.). The implanted electrodes allowed recordings from both deep and superficial cortical areas. The tip (deepest) contact of each electrode was numbered as one and the most superficial contact was labeled as 12. Under general anesthesia, a twist drill was used to make a small hole through which each electrode was advanced to target under fluoroscopic guidance and secured with an anchor bolt. Subdural electrodes were subsequently implanted via craniotomy in one patient. A head computerized tomography (CT) scan was obtained postoperatively to verify the actual location of the electrodes by coregistration with a presurgical volumetric brain MRI. Monitoring was performed for 5–10 days, after which all electrodes were removed under general anesthesia. Analysis methods Retrospective visual analyses were performed using a Nihon-Kohden Neurofax system (Nihon Kohden America, Foothill Ranch, CA, U.S.A.) at a sampling rate of 1,000 Hz and alternating current (AC) amplifiers. The presence of icEEG, IBS, and iHFOs was determined by visual inspection using a reference montage. The electrode/contact with the least likelihood of involvement in the seizure onset and with the least artifact was selected as a reference. A multimedia monitor system allowed analysis of each ictal EEG in different settings on four different monitors simultaneously. Seventy-five seizures (62 clinical and 13 electrographic) were analyzed. Eleven of 75 seizures had secondary generalization. Seizure onset was defined as the earliest occurrence of rhythmic sinusoidal activity or repetitive spikes that clearly were distinctive Epilepsia, 55(5):690–698, 2014 doi: 10.1111/epi.12608

692 S. Wu et al. from background activity and evolved in frequency and morphology.25 EEG seizures were defined as electrographic seizures with no associated clinical signs or symptoms. Clinical seizures included auras (psychic, nonspecific, and olfactory auras), automotor seizures, and dialeptic seizures with or without secondary generalization.26 The ictal onset zone (IOZ) was defined as the contacts that showed the seizure onset. The icEEG was recorded using an input filter of 5–120 Hz, 15 s/page timescale, and sensitivity of 100 lV/ mm. IBS onset consisted of slow potential shifts occurring in association with the seizures with input filter of 0.016– 30 Hz, 1–5 min/page timescale, and sensitivity of 100 lV/ mm. Only baseline shifts >100 lV in amplitude and >1.5 s in duration were considered.10 HFOs were analyzed by visual inspection and defined as sinusoidal rhythms of frequencies >80 Hz and lasting longer than 300 msec.23 HFOs in the range of 80–120 Hz were first identified with conventional invasive EEG settings, and then the HFOs were confirmed using a 2–5 s/page timescale and a high sensitivity (15 lV/mm). HFOs >120 Hz were identified with input filters of 120–300 Hz, 2–5 s/page timescale, and a sensitivity of 15 lV/mm. To differentiate ictal and interictal HFOs (itHFOs), we defined the iHFOs as HFOs that were observed in a repetitive or rhythmic pattern with evolving of amplitude, duration, and/or frequency and had a clear association with conventional ictal EEG. The iHFOs onset was the earliest occurrence of iHFOs around the time of occurrence of conventional seizure onset. itHFOs were HFOs had no evolving pattern. The analysis of itHFOs of each patient was performed on 10 randomly selected 15-min segments of slow-wave sleep and wakeful-state artifact-free intracranial EEG recording, which was at least 5 h away from the seizures. Care was taken to differentiate IBS and HFOs from artifacts. Only slow EEG oscillations that had a similar spatial distribution, polarity, and time evolution in two or more seizures in the same patient were identified as IBS. The same criteria have been used to identify HFOs. The only exception was subject 4 who had only one seizure, whose IBS and iHFOs were very similar to the seizures from other MTLS patients in our study. Therefore, we assumed the findings in this patient were real. A 60 Hz notch filter was used in all settings. Surgery Depending on the specific IOZ information obtained in each case, five patients had multiple hippocampal transections (MHT), one patient had MHT plus amygdala and temporal tip resections, one patient had MHT plus lesionectomy and anterior temporal lobectomy (ATL), one had an ATL plus amygdala resection, three had en bloc temporal lobectomy (TL), two had ATL with amygdalohippocampectomy (AHP), one had AHP only, and one patient was found not to be a surgical candidate because of bilateral seizure onset. The patients’ profiles are summarized in Table 1. Epilepsia, 55(5):690–698, 2014 doi: 10.1111/epi.12608

Data analysis Logistic regression was used to compare the incidence of IBS and iHFOs in clinical and EEG seizures. Student’s t-test was used to compare the time difference of iHFOs onset and IBS onset. The exact binomial test was used for comparing IBS polarity orientation.

Results Incidence of IBS, iHFOs, and itHFOs in MTLS patients Sixty-two clinical seizures with EEG ictal pattern and 13 EEG seizures without clinical manifestation were analyzed. All patients had at least one seizure with IBS and/or iHFOs. The incidence of IBS and iHFOs in clinical seizures and EEG seizures are illustrated in Table 2. Logistic regression showed that EEG seizures associated with clinical seizures were, respectively, 4.35 and 3.68 times more likely to have IBS and iHFOs than EEG seizures without clinical seizures. However, this incidence was not statistically significant (p = 0.079, and p = 0.053, respectively). itHFOs were observed in 10/15 patients (67%). All the patients had at least one seizure with iHFOs. The incidence of iHFOs was 81% (61/75) of all seizures. In another words, seizures in patients with itHFOs usually had iHFOs (10 of 10 patients). Moreover, iHFOs were also observed in the patients who had no itHFOs (five patients). We also tried to detect IBS and iHFOs on scalp EEG using the traditional 10–20 system. However, the ictal scalp EEG recordings were often obscured by abundant muscle, and movement artifacts and no clear IBS or iHFOs were observed. Temporal relationships of IBS, iHFOs, and icEEG IBS onset coincided with icEEG onset in 39/68 (57%) seizures; IBS onset preceded icEEG onset by 2–14 s in 15/68 (22%) seizures; and IBS onset started 3–15 s after icEEG onset in 14/68 (21%) seizures (Fig. 1). On the other hand, iHFOs started 5–55 s after icEEG onset in 43/61 (70%) seizures (Fig. S1) and coincided with icEEG onset in only 18/61 (30%) seizures. In 40 seizures, IBS preceded iHFOs. The mean IBS onset time was 0.56 s after icEEG onset, with a standard error of 0.51 s. The mean iHFOs onset time was 13.36 s after icEEG onset, with a standard error of 1.64 s. A t-test showed that the iHFOs onset time was significantly later than the IBS onset time (p < 0.0001). The relationship between IBS and iHFOs was consistent across different seizures in the same patient, with multiple seizures recorded. The offset of icEEG, IBS, and iHFOs were different as well. IBS outlasted icEEG offset for 3–35 s in 55/68 (81%) of the seizures (Fig. 1); in 13/68 (19%) of the seizures it ended 5–60 s before icEEG offset. On the other hand, iHFOs all ended within 1 s before or after the last discharge of the icEEG (Fig. S2). Overall, IBS appeared earlier and lasted longer than iHFOs.

693 IBS and HFOs in SEEG Analysis Table 1. Patient profiles Pt

Age/sex

Seizure number

Semiology

MRI

Invasive recording

Type of surgery

1 2

57/F 22/M

7 6

Automotor Dialeptic

Unremarkable Unremarkable

9 D, R 10 D, L

N/A N/A

3

57/F

4

Unremarkable

11 D, L

4 5

29/M 21/F

4 6

Unremarkable Unremarkable

10 D, L 9 D, L

L MHT L MHT

N/A N/A

6

22/M

4

1 EEG seizure, 3 dialeptic Aura 5 EEG seizure, 1 dialeptic Automotor

R MHT L MHT + AM and temporal tip resection L MHT

Unremarkable

7 D on each side

R ATL+AHP

7

40/F

4

Dialeptic

Unremarkable

8 D, L

L TL

8 9 10 11

44/F 32/M 29/F 39/F

3 1 4 7

Unremarkable L MTS L MTS R MTS

10 D, R 6 D, L 4 D on each side 7 D, R

R TL L MHT Not surgical candidate R TL

12

44/F

8

Dialeptic Automotor 1 aura, 3 dialeptic 1 EEG seizure, 6 automotor 3 aura, 4 automotor

Neocortex: focal CD, Palmini grade1 Extensive focal CD, ILAE type I and neuronal loss in hippocampus MTS N/A N/A Nonspecific findings

L AM lesion

7 D, L and 6 D, R

Hamartoma

13

54/M

4

Dialeptic

9 D, R

14

58/F

4

8 D, L

L AHP

Nonspecific findings

15

26/F

9

1 EEG seizure, 2 aura, 1 dialeptic 5 EEG seize, 3 auras, 1 secondary GTC

Postsurgical changes from previous R TL Postsurgical changes from previous L TL L ATL encephalomalacia and gliosis

L MHT + lesionectomy + ATL R ATL+AHP

5 D, L and one 8 9 8 G, L

L ATL+AM resection

Encephalitis

Pathology

N/A

Nonspecific findings

Pt, patient; D, depth electrodes; R, right; L, left; G, grid; AM, amygdala; TL, en bloc temporal lobectomy; AHP, amygdalohippocampectomy; MHT, multiple hippocampal transaction; MTS, mesial temporal lobe sclerosis; CD, cortical dysplasia; ATL, anterior temporal lobectomy.

Table 2. The incidence of IBS and iHFOs in clinical seizures and EEG seizures IBS

Clinical seizures EEG seizures

With IBS n (%)

No IBS n (%)

58 (94) 10 (78)

4 (6) 3 (22)

iHFOs

OR (95% CI)

p-Value

4.35 (0.84, 22.44)

0.079

With iHFOs n (%)

No iHFOs n (%)

53 (86) 8 (62)

9 (14) 5 (38)

OR (95% CI)

p-Value

3.68 (0.98, 13.81)

0.053

IBS, ictal baseline shifts; iHFOs, ictal high frequency oscillations.

Spatial relationship of IBS, iHFOs, and icEEG In all seizures, the earliest IBS were seen always at the IOZ (68/68 seizures, 100%). The highest amplitude of the IBS was at the IOZ in 41/68 (60%) seizures. In 21 of the remaining 27 seizures, the highest amplitude of the IBS was in proximity to the IOZ or in closely related anatomic regions: The IOZ was at the AM, whereas the maximum amplitude of the IBS was in the hippocampus. The IOZ was in the hippocampus and the maximum amplitude of the IBS was in the posterior insula in four seizures and in the lateral temporal grid in two seizures. The onset of iHFOs also showed a high spatial correlation with the IOZ. In 43/61 (70%) seizures the onset of iHFOs overlapped with the IOZ. However, initial iHFOs were

observed outside the IOZ in 18/61 (30%) of the seizures: In 13 seizures the initial iHFOs were in the HB, whereas the IOZ was in the AM or HH; in four seizures the initial iHFOs were in the posterior temporal neocortex and in one seizure in the posterior insula, whereas the IOZ was in the HB. Most of the seizures started to propagate about 3 s after seizure onset. During the first 3 s of seizure onset, IBS and iHFOs were seen in fewer contacts compared to the distribution of epileptiform discharges on icEEG in 70% (40/57) and 100% (57/57) of the seizures, respectively (Figs 2–4). During the seizure secondary generalization, iHFOs restricted to the electrodes where seizure started when epileptiform discharges on conventional SEEG have already spread to all the surrounding contacts/electrodes (Fig. S2). Epilepsia, 55(5):690–698, 2014 doi: 10.1111/epi.12608

694 S. Wu et al.

Figure 1. An example of IBS occurring 8 s after icEEG onset. IBS outlast icEEG for 35 s. IBS are observed with input filter of 0.016–30 Hz, 5 min/page timescale, and sensitivity of 100 lV/mm. IBS, ictal baseline shifts; icEEG, ictal conventional EEG; TP, temporal pole; AM, amygdala; HH, hippocampal head; HB, hippocampal body. Epilepsia ILAE

Figure 2. An example of the wide distribution of the epileptiform discharges on icEEG at the first 3 s of seizure onset. The seizure starts at HB1, 2. Epileptiform discharges seen at AM1-3, HH1-4, and HB1-3 on icEEG. The red arrow marks the end of the third seconds after the onset of icEEG. icEEG is recorded using an input filter of 5– 120 Hz, 15 s/page timescale, and sensitivity of 100 lV/mm. icEEG, ictal conventional EEG; AM, amygdala; HH, hippocampal head; HB, hippocampal body. Epilepsia ILAE

Compared to iHFOs (onset overlapped with IOZ in 70% seizures), itHFOs had a much broader spatial distribution: itHFOs localized in all the mesial temporal lobe (MTL) contacts, including IOZ in 5 of 10 patients who have itHFOs and localized in mesial temporal structure other than IOZ in one patient (seizure onset was at amygdala and itHFOs was at hippocampus). itHFOs localized in bilateral mesial temporal structure in two patients with bilateral depth electrodes but unilateral seizure onset. itHFOs of only two patients were only seen at the electrodes at seizure onset (20%). Epilepsia, 55(5):690–698, 2014 doi: 10.1111/epi.12608

Figure 3. An example of IBS seen in fewer contacts than icEEG at the first 3 s of seizure onset (IBS and icEEG start simultaneously). The red arrow marks the end of the third seconds after the onset of IBS (icEEG). IBS are seen only at HB1,2 and HH1 at the end of 3 s. IBS are observed with input filter of 0.016–30 Hz, 1 min/page timescale, and sensitivity of 100 lV/mm. AM, amygdala; IBS, ictal baseline shifts; icEEG, ictal conventional EEG; HH, hippocampal head; HB, hippocampal body. Epilepsia ILAE

Figure 4. An example of iHFOs seen in fewer contacts than icEEG at the first 3 s of iHFOs onset (iHFOs start 12 s after icEEG). The red arrow marks the end of the third seconds after the onset of iHFOs. iHFOs are seen only at HB1-2 at the end of 3 s after iHFO onset. iHFOs are visualized with input filters of 120–300 Hz, 5 s/page timescale, and a sensitivity of 15 lV/mm. iHFOs, ictal high frequency oscillations; icEEG, ictal conventional EEG; AM, amygdala; HH, hippocampal head; HB, hippocampal body. Epilepsia ILAE

When directly comparing the distribution of IBS and iHFOs, the IBS had a broader spatial distribution than iHFOs in 37/57 (64.9%) of the seizures, IBS and iHFOs localized to the same area in 11/57 (19.3%) of the seizures, and IBS occupied a smaller area than iHFOs in 9/57 (15.8%) of the seizures. The amplitude and polarity of the IBS A dipole exists when at different electrodes there are both IBS of positive and negative polarity. A monopole means that only positive or negative IBS are observed.

695 IBS and HFOs in SEEG Analysis A

B

Figure 5. Illustration of a dipole of IBS along the longitudinal axis to amygdala/hippocampus complex (A-H complex). A negative pole is seen at the anterior segment of the A-H complex (AM1-2, HH1-2) and a positive pole seen at the posterior of portion of the A-H complex (HB1-3) (A). The sagittal view of the MRI brain showing the location of AM, HH, and HB electrodes and the location of the dipole (B). A similar dipole distribution of the IBS is illustrated in Fig. 1. IBS are observed with input filter of 0.016–30 Hz, 5 min/page timescale, and sensitivity of 100 lV/mm. IBS, ictal baseline shifts; AM, amygdala; HH, hippocampal head; HB, hippocampal body. Epilepsia ILAE

Both negative and positive IBS, as also monopoles and dipoles were observed in this study. The onset of IBS could be positive or negative in polarity. The duration of IBS was 5–180 s. The amplitude of IBS was between 300 lV and 3.4 mV. The mean absolute amplitude of the IBS was 1.7 (0.91) mV. A dipole was observed in 45/68 (66%) seizures (9 of 15 patients), along the axis of the amygdala/ hippocampus complex (A/H complex). Indeed, in 39/45 (87%) seizures a negative pole was observed in the anterior part of the A/H complex (AM1,2 or AM1,2 and HH1,2 or HH1) and a positive pole was identified in the posterior/lateral part of the A/H complex (HB1,2 or HH3-4). HH3-4 was 1–1.5 cm more lateral than HH1. In 6/45 (13%) seizures of one patient, the negative pole was over the temporal tip and the positive pole was over HH and HB. A single negative pole was observed in the anterior part of the A/H complex in 16 of the 23 seizures (70%). In the rest of the seizures, negative IBS was observed in all the mesial contacts (AM1-3, HH1-3, and HB1-3) in one seizure (1/23, 4%), and in four seizures (17%) of one patient who did not have an AM electrode implanted, only a positive pole was seen in the posterior part of the A/H complex. In two seizures from one patient, positive polarity was observed at neocortical subdural electrodes. All together, 52/68 (76%) IBS showed a negative pole in the anterior portion of the A/H complex. This proportion is significantly higher than the chance of 50% (p < 0.001, exact binomial test). This distinct dipole distribution along the axis of the A/H complex on IBS and MRI is illustrated in Fig. 5. Evolution of the frequency of iHFOs during epileptic seizures In 36/61 (59%) seizures, the frequency of the iHFOs increased by at least 20% from the seizure onset through

seizure evolution. At seizure onset, the frequency of iHFOs was between 80 and150 Hz in 29/61 (48%) of the seizures and between 151 and 300 Hz in 32/61 (52%) of the seizures. As the seizure evolved, only 4/61 (7%) seizures had iHFOs frequencies between 80 and 150 Hz; in 57/61 (93%) seizures, the frequency of iHFOs was between 151 and 300 Hz.

Discussion Initially, IBS were recorded using subdural grids with an open low-cut filter or direct current amplifiers (DC amplifiers).4 Later, several groups showed that IBS could be recorded using AC amplifiers with long time constants of many seconds.8,27,28 Because the slow shift activities recorded by AC amplifiers were not real “DC” shifts, Robin et al. suggested using the term infraslow activity (ISA) or IBS instead of DC shifts.29 In this study we used AC amplifiers with an input filter of 0.016–30 Hz. All patients included in this study had at least one seizure that showed an IBS. The incidence of IBS in our study was similar to the one reported by Kim et al.12 using subdural platinum electrodes and Mader et al.10 using depth electrodes. A few publications have also discussed scalp recorded IBS.7,30,31 However, no clear IBS were detected on scalp EEG in our study due to abundant artifacts. Clinical manifestations during seizures was generated during seizure propagation to a symptomatogenic zone. Therefore, seizures with clinical manifestations usually have a larger synchronization area than electrographic seizures only. Although the difference was not statistically significant, the fact that both IBS and iHFOs showed a trend of occurring more frequently during clinical seizures than during electrographic seizures suggests that IBS and iHFOs Epilepsia, 55(5):690–698, 2014 doi: 10.1111/epi.12608

696 S. Wu et al. were probably generated by more extensive neuronal synchronization. icEEG, IBS, and iHFOs were temporally related. In our study, the onset of iHFOs was significantly later than the onset of IBS (average: 11.5 s delay). However, the methodology we used to detect iHFOs could have contributed to the relative delay in the occurrence of iHFOs. IBS preceding iHFOs in all observed seizures were also reported by Imamura27 however, only one patient with 16 seizures was included in their study. There is a debate about the cellular mechanism responsible for generation of IBS and iHFOs. Most authors agree, however, that IBS are an expression of a massive sustained depolarization of glial cells following massive ictal neuronal firing.3,9 The glial response is a more widespread process, produced by the potential fields of glial cells that have a relatively wide spatial extension.32 On the other hand, two theories about the possible mechanism of generation of ripples and fast ripples (FRs) are worth mentioning. One theory is that they are the expression of hypersynchronous action potentials within the epileptogenic region.21,33 Another hypothesis is that they reflect inhibitory postsynaptic potentials of principal neurons occurring as a result of synchronous discharges of the interneuronal network.34 As already mentioned earlier, we would expect that the IBS produced by a slow glial cell response secondary to potassium efflux from neuronal excitation should be observed after the appearance of conventional ictal epileptiform discharges. The fact that IBS were observed as early as 14 s before the icEEG seizure onset and also before the occurrence of iHFOs suggests that the initial massive seizure discharge in the epileptic focus was not sufficiently synchronized to produce a conventional EEG seizure pattern or iHFOs. We also found that iHFOs progressively increased in frequency, from gamma and ripple range to FR range as the seizures evolved. These findings suggest that during most seizures the initial excessive neuronal discharge is poorly synchronized (generating IBS); only after approximately 10–20 s, the discharge becomes sufficiently synchronized to generate gamma and ripple range activities and eventually FRs, as more synchronization occurs at the later phase of the seizure. The fact that the frequency of HFOs accelerates after seizure clinical onset was also reported by Ochi.23 This observation also suggests that ripples and FRs are dynamically connected and further suggests that oscillation frequency alone should not be used to distinguish normal HFOs from pathologic HFOs.35 Some investigators have noted that both ripples and FRs occur most frequently immediately before or at seizure onset.14,36 This is discrepant with our findings and could be related to the recording method (subdural electrodes vs. depth electrodes), the seizure type (temporal lobe seizures vs. extratemporal seizures), or methodologic differences in detecting iHFOs. Epilepsia, 55(5):690–698, 2014 doi: 10.1111/epi.12608

Most studies have focused on itHFOs or did not differentiate between itHFOs and iHFOs; only a few studies have addressed the utility of iHFOs in seizure localization. We observed that iHFOs have a much smaller area of distribution and a much higher correlation with IOZ than itHFOs. This suggests that iHFOs may be a better indicator of seizure localization than itHFOs. Modur et al. reported similar conclusions.25,37 In this sample, itHFOs overlapped with IOZ in only 2 out of 10 patients; this relationship was lower than what has been reported.14,38 This discordance could also be explained by the different seizure type and the different intracranial electrodes used in the studies. Neocortical epilepsy often has a wider IOZ on grid (three to five contiguous electrodes as reported by Worrell et al.14). Therefore, itHFOs were often seen within the IOZ. In our study, the MTL were extensively covered with depth electrodes; the IOZ was often limited within one to two contacts. Only the exact matched IOZ and itHFOs were considered. We observed a close spatial relationship between icEEG, IBS, and iHFOs. In 100% of the seizures the earliest onset IBS were localized in the IOZ. However later on, 40% of the seizures showed IBS at least partially localized outside the IOZ. This suggests that the initial IBS occur usually at the IOZ, but not infrequently the IBS have their maximum amplitude in regions of early seizure propagation. The same finding was also true for the iHFOs. The contacts with iHFOs remained in the center of the IOZ in most of the seizures (70%), even though the conventional EEG epileptiform discharges usually propagate to electrodes outside the IOZ in the later stages of the seizure. In our observations, IBS and iHFOs clearly localized to a more restricted area compared to the icEEG in the first 3 s of ictal onset and did not spread as broadly as the icEEG did during seizure evolution. This suggests that iHFOs and IBS are likely more reliable signatures of potentially epileptogenic neurons than the IOZ. Compared to iHFOs, the distribution of IBS was broader. These findings are concordant with the report from Modur et al.37 from six neocortical epilepsy patients with both subdural and depth electrodes and from Imamura et al.27 from one temporal lobe epilepsy patient with subdural electrodes. Initially, IBS were reported as negative deflections in the center of the seizure focus and positive deflections in the periphery.4 Other studies also showed IBS with bipolar distributions.10 We observed a dipole in MTL epilepsy with a negative pole at the anterior/deeper portion of the A/H complex and a positive pole at the posterior/superficial portion of A/H complex in most of the seizures. The polarity distribution was apparently not related to the polarity of the IBS onset, which could be positive or negative. However, the polarity of the IBS had an anatomic distribution with negativity anteriorly and positivity posteriorly of the temporal lobe. We do not have a clear explanation for this dipolar distribution.

697 IBS and HFOs in SEEG Analysis There are limitations to this study. We used a sampling rate of 1,000 Hz, which only allows recording of HFOs of 300 Hz. In addition, the identification of HFOs was exclusively by visual analysis. No power spectrum was used; therefore, EEG sample selection bias cannot be completely excluded. Another limitation is that the depth electrodes covered only a limited brain area, and, therefore, the real extent of the distribution of the icEEG onset, the iHFO onset, and the IBS onset cannot be assessed in this study. We conclude that IBS, iHFOs, and icEEG have a close temporal and spatial relationship with each other. IBS and iHFOs localize to a smaller area compared to icEEG. Additional studies will be necessary to determine the mechanism of generation of IBS and of iHFOs with more precision and to determine any additional value IBS and iHFOs may have in the definition of the epileptogenic zone.

Acknowledgments The study was supported by Medical and Academic Partnership (Pfizer Fellowships in Epilepsy 2011). We thank Dr. Leo Towle for providing valuable feedback on the manuscript.

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: Figure S1. An example of iHFOs starting 12 sec after the icEEG onset. iHFOs are visualized with input filters of 120 to 300 Hz, 5 sec/page timescale, and a sensitivity of 15 lV/ mm. Both upper and lower figure are intracranial EEG visualized with the setting for iHFOs. Lower figure is five sec after upper figure. icEEG onset marks the time point for the seizure first started. iHFOs, ictal high frequency oscillations; icEEG, ictal conventional EEG; TP, temporal pole; PT, posterior temporal; AM, amygdale; HH, hippocampal head; HB, hippocampal body; AI, anterior insula; PI, posterior insular; MF, medial frontal; CI, cingulate.

Epilepsia, 55(5):690–698, 2014 doi: 10.1111/epi.12608

Figure S2. An example of iHFOs restricted to IOZ during seizure secondary generalization. Upper figures are icEEG and lower figures are iHFOs. Seizure starts at HB2 (A, upper figures) and seizure discharges propagate to HB1-8, HH 1-8, AM1-8 and PI 1-8 during secondary generalization (B,C, upper figures). iHFOs starts HB2 10 sec after icEEG onset (B, lower figures) and remains in the center of the IOZ (HB2) during seizure secondary generation (C, lower figures). icEEG is visualized with input filter 5 to 120 Hz, 15 sec/page timescale, and sensitivity of 100 lV/mm. iHFOs are visualized with input filters of 120 to 300 Hz, five sec/ page timescale, and a sensitivity of 15 lV/mm. iHFOs, ictal high frequency oscillations; icEEG, ictal conventional EEG; IOZ, ictal onset zone; AM, amygdala; HH, hippocampal head; HB, hippocampal body; PI, posterior insular.