It Is Unlikely That This Discrepancy Is Due to Other Factors. the Neuromagnetometer Was

Appendix e-6.

In some centers, only patients with many spikes are studied with MSI. Spikes near the zone of seizure origin usually are the most frequent and abundant spikes.36 Thus in other centers there may be a natural and beneficial tendency to record the most relevant spikes. This enhances the specificity of the technique but unfortunately reduces the sensitivity, since not all patients have frequent spikes. We studied all patients who did not meet “skip” criteria, including those with few spikes because our aim was to obtain useful information in all of the more complicated patients - those patients who require ICEEG. It is reasonable to expect that in some patients with more than one spike focus, a spike focus distant from the seizure zone might occasionally dominate a study due to temporal sampling error. This seems to be the most likely explanation of our findings reported here.

It is unlikely that this discrepancy is due to other factors. The neuromagnetometer was accurate with an error less than 1.0 centimeter with 95% confidence. This small error does not explain our cases where the MSI showed dipole clusters in the frontal or parietal lobe when the seizure onset was in the temporal lobe. It is unlikely that our protocol caused the difference. Although there is some variability across centers in the pre-surgical evaluation protocol of patients with suspected non-mesial temporal lobe epilepsy, most centers use ICEEG to evaluate non-lesional neocortical epilepsy. The MSI findings here would be expected to be relevant for most centers, which tend to follow a conservative protocol as outlined above in Methods. The patient population would be unlikely to explain the discrepancy. More patients with more complicated seizure disorders are referred, i.e. those in which clear cut mesial temporal lobe epilepsy is not present and our population is skewed towards the more complex patients. Most other centers, however, also use MSI primarily for these more difficult patients, who often are neocortical.

There is a solution for the problem of recording the most relevant spike type by combining information from MSI and EEG. MSI has limited time for recording; however, EEG is frequently recorded continuously over days for inpatient Video-EEG intensive monitoring telemetry. The EEG could be used to identify the most relevant spike type due to its chronic recording capability. Thus, in the presence of more than one spike location, the EEG can screen for the most relevant spike type. The four characteristics of spikes near the zone of seizure origin thus could be detected by the EEG: frequency and abundance, 36 sharpness or spiky-ness,37 autonomy or similarity of abundance across the wake-sleep cycle 38 and propagation patterns.39 Then the region of the relevant spike type apparent in EEG could be further analyzed by MSI to estimate the exact location of this spike type.