Detection and significance of focal, interictal, slow-wave activity visualized by magnetoencephalography for localization of a primary epileptogenic region

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Object. Magnetoencephalography (MEG) is a novel noninvasive diagnostic tool used to determine preoperatively the location of the epileptogenic zone in patients with epilepsy. The presence of focal slowing of activity recorded by electroencephalography (EEG) is an additional indicator of an underlying pathological condition in cases of intractable mesial temporal lobe epilepsy (MTLE). In the present study the authors examined the significance of focal, slow-wave and interictal spike activity detected using MEG in 29 patients who suffered from MTLE that was not associated with structural brain lesions.

Methods. All patients underwent resective surgery after MEG and EEG monitoring. Equivalent single-dipole modeling was applied to focal low-frequency magnetic activity (LFMA) and interictal paroxysmal activity. Lateralized LFMA was defined as trains of rhythmic activity over the temporal area, with frequencies lower than 7 Hz, which were easily distinguished from background activity.

Lateralized LFMA was found in 17 patients (58.6%); it always occurred on the side ipsilateral to the side of resection and displayed a maximum amplitude over the temporal area. Dipolar sources of magnetic flux computed during slow-wave trains were found in the majority of cases in the posterior superior temporal region and, occasionally, in mesial temporal structures that were subsequently resected. With respect to lateralization there was never disagreement between LFMA and MEG interictal spike sources. Thus, in patients with MTLE that is not associated with a mass lesion LFMA is topographically related to the epileptogenic area and, therefore, has value for reliable determination of the side and, possibly, the location of this area.

Conclusions. Although focal slowing of EEG background activity is generally considered to be a nonspecific sign of functional disturbance, interictal LFMA in patients with MTLE should be conceptualized as a distinct electrographic phenomenon that is directly related to the epileptogenic abnormality. Analyzing the interictal MEG distribution of LFMA and sharp activity improves the diagnostic utility of MEG in patients with suspected TLE who are undergoing surgical evaluation.

Article Information

Address reprint requests to: Hideaki Ishibashi, M.D., Ph.D., Vivian L. Smith Foundation for Neurological Research, Division of Clinical Neurosciences, Department of Neurosurgery, The University of Texas Houston Health Science Center, 6431 Fannin Street, Suite 7.154, Houston, Texas 77030. email: Hideaki.Ishibashi@uth.tmc.edu.

© AANS, except where prohibited by US copyright law.

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Figures

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    Traces obtained simultaneously using MEG over both temporal areas (upper) and EEG in a bipolar montage (lower). A 1- to 70-Hz filter was used. On the left temporal area the MEG recording depicts multiple instances of low-frequency activity with a reversal field polarity, whereas such active areas are less apparent on simultaneous EEG recordings. ECG = electrocardiogram.

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    Traces demonstrating the same waveforms shown in Fig. 1 that were obtained when a different filter (1–7 Hz) was used. Slow-wave activity is much more apparent in the left temporal area.

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    Magnetic resonance images obtained in a patient showing a cluster of ECDs of interictal spike activity (yellow circles) and interictal slow-wave activity (red circles) concentrated mainly on the surface of the upper part of the posterior temporal lobe. Note the localization of slow-wave dipole clusters is slightly deeper than that of the interictal ones. Ant = anterior; post = posterior.

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