Interictal high-frequency oscillations generated by seizure onset and eloquent areas may be differentially coupled with different slow waves
Introduction
Extraoperative electrocorticography (ECoG) is commonly utilized as a part of presurgical evaluation for patients with medically-uncontrolled focal seizures in a number of epilepsy centers. The ultimate purpose of extraoperative ECoG is localization of the epileptogenic zone of which surgical resection results in seizure control as well as delineation of the eloquent areas of which preservation minimizes the risk of postoperative functional deficits (Rosenow and Lüders, 2001). The gold-standard electrophysiological biomarkers for estimation of the epileptogenic zone include the seizure-onset zone involved in generation of habitual seizures, namely, the electrode site(s) initially showing the ictal ECoG discharges prior to habitual seizures (Asano et al., 2009). Conversely, significance of seizure-onset zones involved in non-habitual seizures is currently unknown (Kovac et al., 2014). A long period of extraoperative ECoG recording may be undesirably needed to capture habitual seizures, and some patients do not achieve seizure-freedom even after complete resection of the seizure-onset zone (Asano et al., 2009). Thus, investigators are currently looking for alternative biomarkers estimating the extent of the epileptogenic zone.
Promising candidate biomarkers include interictal high-frequency oscillations (HFOs) spontaneously and intermittently generated during resting periods (Jacobs et al., 2012, Zijlmans et al., 2012). Studies of both pediatric and adult populations reported that surgical failure was associated with a high rate of interictal HFOs generated by the non-resected tissues during non-REM sleep and under general anesthesia (Jacobs et al., 2010, Wu et al., 2010, Akiyama et al., 2011, van’t Klooster et al., 2015). A large number of investigators independently reported that electrode sites frequently generating HFOs often turn out to be a part of the seizure-onset zones (Staba et al., 2002, Bagshaw et al., 2009, Crépon et al., 2010, Akiyama et al., 2011, Blanco et al., 2011, Gliske et al., 2016, Sakuraba et al., 2015). Sites showing HFOs>80Hz and HFOs>250Hz are often overlapped in space (Jacobs et al., 2011), but HFOs>80Hz are generated by larger brain regions (Zijlmans et al., 2012). Some studies suggested that the occurrence rate of HFOs>250Hz predicted the postoperative seizure outcome somewhat better than that of HFOs>80Hz (Akiyama et al., 2011, van’t Klooster et al., 2015). Others suggested that HFOs>80Hz may be more useful than HFOs>250Hz, which are difficult to detect in a substantial proportion of patients with neocortical epilepsy (Blanco et al., 2011, Wang et al., 2013).
Despite the efforts of many investigators, the clinical utility of interictal HFOs still remains uncertain (Jobst, 2013), particularly because HFOs are frequently generated by nonepileptic visual, somatosensory, motor, and auditory cortices during resting periods (Blanco et al., 2011, Fukushima et al., 2012, Nagasawa et al., 2012, Melani et al., 2013, Wang et al., 2013, Alkawadri et al., 2014, van’t Klooster et al., 2015). The current recommendation is that one should not determine the resection margin solely based on the occurrence rate or spectral frequency band of interictal HFOs (Engel et al., 2009, Asano et al., 2013, Gliske et al., 2016). A method that can distinguish epileptogenic from physiologic HFOs in an automatic and efficient manner is highly desirable.
Our central hypothesis was that consideration of the spectral frequency band of slow-waves coupled with HFOs would improve the specificity of prediction of seizure-onset zone responsible for generation of habitual seizures. Our preliminary study of interictal ECoG during slow-wave sleep showed that HFOs in the nonepileptic occipital pole were strictly coupled with slow wave at 1 Hz and below, whereas HFOs in the seizure-onset zones coupled with slow wave at 3–4 Hz also (Nagasawa et al., 2012). Another group reported that both nonepileptic visual and sensorimotor areas can frequently generate interictal HFOs independent from interictal spike discharges (Wang et al., 2013). It is plausible that seizure-onset zone could be predicted by measuring in situ modulation index: MI(HFOs)&(slow-wave), a measure that reflects the degree of stability of phase–amplitude coupling between HFO amplitude and slow-wave phase (Canolty et al., 2006). In theory, MI(>XHz)&(YHz) is greater if a larger amplitude of HFOs>XHz are more consistently coupled with a phase of slow-waveYHz. Here, we measured MI at all electrode sites during interictal state on the first evening of extraoperative ECoG recording, using an open source toolbox EEGLAB and its extension Phase–Amplitude Coupling Toolbox (PACT) that incorporates the algorithm and routines for measurement of MI (Miyakoshi et al., 2013).
The first aim of this study was to determine, using receiver operating characteristics (ROC) analysis, how accurately the rates of HFOs>80Hz, HFOs>150Hz, and HFOs>250Hz predicted the seizure-onset sites. The next aim was to determine how accurately, but undesirably, the rates of these HFO rates detected the nonepileptic sensorimotor–visual sites clinically defined by neurostimulation. We subsequently measured MI(HFOs)&(3–4Hz) and MI(HFOs)&(0.5–1Hz) at all electrode sites, and determined how accurately the seizure-onset and nonepileptic sensorimotor–visual sites were detected by MI(HFOs)&(3–4Hz) and MI(HFOs)&(0.5–1Hz), respectively. According to the results of previous studies (Nagasawa et al., 2012, Wang et al., 2013), we expected that seizure-onset sites would be associated with increased MI(HFOs)&(3–4Hz) and that nonepileptic sensorimotor–visual sites would be associated with increased MI(HFOs)&(0.5–1Hz). Specifically, we tested the hypothesis that subtraction-MIHFOs [defined as subtraction of MI(HFOs)&(0.5–1Hz) from MI(HFOs)&(3–4Hz)] would localize the seizure-onset sites with reduced detection of the nonepileptic sensorimotor–visual sites.
Section snippets
Patients
The inclusion criteria consisted of: (i) a two-stage epilepsy surgery using extraoperative subdural ECoG recording in Children’s Hospital of Michigan, Detroit, between October 2013 and September 2014, (ii) ECoG sampling from all four lobes of the affected hemisphere, and (iii) habitual seizures captured during extraoperative ECoG recording. The exclusion criteria consisted of (i) presence of massive brain malformations, such as large porencephaly, perisylvian polymicrogyria, or
Patient profile
A cortical lesion was appreciated on MRI in four patients (Table 1). The remaining nine patients had normal or nonspecific MRI findings, whereas focal or lateralized glucose hypometabolism was noted on PET in all patients. Extraoperative ECoG recording showed that the median number of seizure-onset electrode sites was 10 (range: 1–36; Supplementary Table S1). Neurostimulation mapping localized both somatosensory, motor, and visual sites in all 13 patients (Supplementary Fig. S2).
Clinical implications
The present study demonstrated that HFOs are commonly generated by both ‘seizure-onset sites determined by extraoperative ECoG recording’ and ‘nonepileptic sensorimotor–visual sites defined by neurostimulation mapping’. The novel findings include that the spatial distribution of rate of HFOs was similar to those of MI(HFOs)&(3–4Hz) and MI(HFOs)&(0.5–1Hz). MI(HFOs)&(3–4Hz) localized the seizure-onset sites with a comparable accuracy with the occurrence rate of HFOs and with a better accuracy
Acknowledgements
This work was supported by NIH grant NS64033 (to E. Asano) as well as the intramural grant from Children’s Hospital of Michigan Foundation (to E. Asano). We are grateful to Harry T. Chugani, MD, and Carol Pawlak, REEG/EPT at Children’s Hospital of Michigan, Wayne State University for the collaboration and assistance in performing the studies described above.
Conflict of interest: None of the authors have potential conflicts of interest to be disclosed.
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