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Summary: Purpose: Objectively to evaluate whether independent spike detection by human interpreters is clinically valid in magnetoencephalography (MEG) and to characterize detection differences between MEG and scalp electroencephalography (EEG).
Methods: We simultaneously recorded scalp EEG and MEG data from 43 patients with intractable focal epilepsy. Raw EEG and MEG waveforms were reviewed independently by two experienced epileptologists, one for EEG and one for MEG, blinded to the other modality and to the clinical information. The number and localization of spikes detected by EEG and/or MEG were compared in relation to clinical diagnosis based on postoperative seizure freedom.
Results: Interictal spikes were captured in both EEG and MEG in 31, in MEG alone in eight, in EEG alone in one, and in neither modality in three patients. The number of detections ranged widely with no statistical difference between modalities. A median of 25.7% of total spikes was detectable by both modalities. Spike localization was similarly consistent with the epilepsy diagnosis in 85.2% (EEG) and 78.1% (MEG) of the patients. Inaccurate localization occurred only in those cases with very few spikes detected, especially when the detections were in one modality alone.
Conclusions: Interictal epileptiform discharges are easily perceived in MEG. Independent spike identification in MEG can provide clinical results comparable, but not superior, to EEG. Many spikes were seen in only one modality or the other; therefore the use of both EEG and MEG may provide additional information.
Despite advances in imaging (both anatomic and functional), the most indispensable tools during diagnostic evaluation of epilepsy are the recording and interpretation of “brain waves.” These brain waves arise from summated dipolar electric currents generated in the cerebral cortex. Two fundamental methods exist to record brain waves—electroencephalography (EEG) and magnetoencephalography (MEG); each measures complementary manifestations of the same phenomenon.
The EEG is traditionally “read” by experienced interpreters, by using an appreciation of waveform appearance to discriminate epileptiform activity from background and other nonepileptiform activities based on waveform morphology, distinctness, and experiential knowledge (1,2). No independent confirmation of true positives or true negatives yet exists, and human interpretation is still the gold standard for the identification of spikes (2). The spatial correlation of the irritative zone (area from where interictal spikes arise) with the epileptogenic zone can help to define whether an EEG transient is epileptogenic. A gold standard for the correct localization of a spike also is difficult to find in vivo, but the elimination of seizures after resection of a cortical region strongly supports the notion that the tissue removed included the spike focus. The same limitations apply to the interpretation of MEG. Therefore as with EEG, it is reasonable to identify MEG spikes by independent analysis of the MEG alone, and then to correlate the findings with relevant clinical information. Surprisingly, very few studies have addressed this issue critically (3–5).
MEG has been increasingly used for localization of epileptic activity (6–17). Most of those articles show only “abstractions of the activity” as maps or data points superimposed on the magnetic resonance imaging (18). The original data (waveforms) and the method of spike detection are sometimes only perfunctorily described. Without presentation of the raw data and all of the well-known nuances that accompany visual interpretation, the neurophysiology community at large has been having difficulty assessing the usefulness or reliability of MEG relative to EEG (18). MEG is ordinarily used more precisely to estimate the source of spikes that have already been identified in the simultaneously recorded EEG. Some reports, however, describe the presence of spikes seen on MEG that were not captured with scalp EEG (6,15,19,20). The fact that several authors have reported some spikes seen in one modality but not in the other begs the question of whether MEG is superior for the detection of some epileptic transients or whether it simply picks up more nonepileptic transients. It is, therefore, important to clarify whether either modality is superior in terms of detection of epileptiform activity, what characterizes the different abnormalities detected by each modality, and whether these findings provide additional, clinically relevant information. To compare their detection capabilities, it also is necessary to know whether independent review of MEG provides clinically valid or erroneous results.
Although the retrospective study of consecutive intractable focal epilepsy patients cannot provide comprehensive answers to all of these questions, our primary purpose is to ascertain whether independent MEG analysis (without knowledge of the clinical information) is valid clinically. The second purpose is to investigate qualitative difference in the detection of spikes by EEG and MEG (if any).