Magnetoencephalography: Tie-breaker vs Confounding Data for Epilepsy Localization

Abstract & Commentary

Source: Pataraia E, et al. Does magnetoencephalography add to scalp video-EEG as a diagnostic tool in epilepsy surgery? Neurology. 2004;62:943-948.

Medication resistance is a fact of life for 30-40% of patients with epilepsy. Non-pharmacologic treatment options include resective epilepsy surgery, vagal nerve stimulation, the ketogenic diet, and experimental protocols. Of these, epilepsy surgery offers the greatest chance of curing the patient’s epilepsy. To achieve this degree of success, it is critical to localize the epileptogenic zone as accurately as possible. Pataraia and colleagues attempt to evaluate the use of MEG in evaluating patients for epilepsy surgery.

Pataraia et al analyzed the data from 82 of 113 consecutive patients evaluated for epilepsy surgery. All patients underwent video-EEG (vEEG) monitoring and eventually proceeded to surgery. Each patient had a 30-minute recording session in which both MEG and EEG were recorded simultaneously. The sensitivity of MEG in detecting epileptiform discharges was 79%. Localizing data from interictal and ictal vEEG and interictal MEG were compared with the area of resection and classified as completely overlapping, partially overlapping, and nonoverlapping. Using such criteria, MEG and vEEG results were equivalent 32% of the time. Pataraia et al also estimate that MEG provided additional localizing information in 40% of their patients.

Commentary

MEG detects the magnetic currents induced by the electrical field potentials of the dendritic arbor. The chief advantage of the method vs EEG is that the meninges, skull, and scalp are all "transparent" relative to the magnetic field (ie, there is no distortion of the signal by these tissues). The mathematical modeling of the source of the magnetic field is, therefore, much simpler (but does not yield a unique "inverse solution"). The magnitude of the brain’s magnetic currents is in the femtotesla range, making this a very expensive technique because one needs to have the MEG machine housed in a magnetically shielded room. Further disadvantages of MEG involve characteristics of the equivalent current dipole: deep sources are difficult to detect, and radial dipoles are completely undetectable.

One confusing part of Pataraia et al’s analysis is that they do not provide information regarding why the resection zone did not completely overlap with the ictal vEEG results. There are at least 3 potential reasons for such a discrepancy: 1) Interictal EEG data that were discordant with ictal EEG were compelling enough to warrant extending the resection zone; 2) Ictal EEG was non-localizing; and 3) The resection had to be tailored to avoid eloquent cortex. Without having details of these cases, it is difficult to interpret the sensitivity and specificity data.

Another criticism of this study involves the fact that there is only cursory information regarding the outcome of epilepsy surgery. The use of any localization technique should be discussed in the context of the gold standard of seizure-free outcome. If one diagnostic test localizes the epileptogenic zone to the same area as another method, is it because they are both equally good or equally bad? To determine whether one method is superior to another, or at least qualifies as a "tie-breaker" when data conflict, it is preferable to gauge the localization relative to seizure outcome.

MEG is not yet a fully mature technology in localizing the epileptogenic zone in patients undergoing evaluation for epilepsy surgery. Pataraia et al do bring us closer to the routine use of MEG by providing preliminary sensitivity and specificity information on the largest number of patients thus far studied. — Andy Dean, MD, Assistant Professor of Neurology and Neuroscience; Director of the Epilepsy Monitoring Unit, Department of Neurology, New York Presbyterian Hospital-Cornell Campus and Assistant Editor of Neurology Alert.