Comparison of dense array EEG with simultaneous intracranial EEG for Interictal spike detection and localization
Introduction
It is important to understand the inherent limitations of noninvasive MEG and dEEG in predicting the brain activity seen by icEEG. Several studies have examined MEG measures taken simultaneously with implanted icEEG recordings. Mikuni et al. (1997), Oishi et al. (2002) and Huiskamp et al. (2010) reported that MEG was less sensitive in detecting and accurate in localizing in mesial temporal lobe compared to lateral temporal or frontal lobe. However, Kaiboriboon et al. (2010) reported good success in localizing medial temporal spikes with MEG dipole analysis to the resected side in 13 of 19 patients.
Long-term monitoring with dEEG has recently become feasible, and dEEG recordings of seizure onset have shown good predictive ability for localization obtained by long-term monitoring ECoG (Holmes et al., 2010). Although interictal events do not invariably localize to the seizure onset zone, a number of studies have shown interictal source localization with dEEG to be useful in neurosurgical planning in epilepsy (Michel et al., 2004).
In order to establish the clinical usefulness of dEEG, it is important to evaluate the sensitivity and accuracy of dEEG source analysis. For this purpose, we compared 256-channel dEEG source analysis of interictal activity and simultaneously recorded ECoG which covered both the lateral and mesial temporal lobe.
We received approval for this study from Seirei Hamamatsu General Hospital Human Subject Committee and informed consent was obtained from all patients.
Section snippets
Patients
We studied seven patients, each of whom had suffered from intractable epilepsy for at least 2 years. Each was a surgical candidate and underwent a presurgical workup including conventional long-term EEG monitoring, magnetic resonance imaging (MRI), 125-Iomazenil (IMZ)-single-photon emission tomography (SPECT), 18-F fluorodeoxyglucose (FDG)-positron emission tomography (PET) and neuropsychological testing. Clinical informations for these patients are summarized in Table 1.
Intracranial EEG (icEEG) recording
For each patient, from
Dense array EEG spike detection
A total of 760 intracranial icEEG spikes was recorded from mesial temporal regions in the 30–40 min recordings. A total of 339 spikes (45%) were clearly distinguished from the background activity by inspection of the dEEG whole-head topoplot. With the dEEG reduced to at 19-channel 10-20 display, 169 spikes (22%) were detected (Figure 1, Figure 2). Spike detection rates in each patient ranged from 37 to 59% for dEEG and 16 to 27% for the downsampled 19-channel 10-20 display (Fig. 3A).
Number of involved subdural electrodes and detection rate
Fig. 3B
Dense array EEG spike detection
Mesial temporal spikes confirmed by icEEG were detected 45% in 256-channel dEEG and 22% in downsampled 19-channel 10-20 display. In comparison, Mikuni et al. (1997) reported that MEG detected 18% of mesial temporal spikes, and Oishi et al. (2002) reported that MEG detected 53% of lateral frontal spikes but only 26% of mesial temporal spikes. Huiskamp et al. (2010) also reported that only 28% of icEEG spikes in mesial temporal lobe were detected by MEG, whereas 70% of icEEG spikes in lateral
Conclusion
Our study demonstrates that 256-channel dEEG can reliably detect mesial temporal spikes and localize them reasonably accurately. The 256-channel dEEG may provide more precise information for the localization of interictal epileptiform discharges than conventional EEG or MEG in patients with deep spike foci and may be clinically useful in the presurgical work-up for epilepsy, improving accuracy in the placement of intracranial electrodes.
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