Abstracts

Rationale: The current international 10-20 system was standardized by H.H. Jasper and the International Federation of Societies for Electroencephalography and Clinical Neurophysiology (IFSECN) Committee on Methods of Clinical Examination in 1958. The 10-20 system is a standardized electrode placement system based on anatomy, with a naming system based on anatomy to help facilitate communication between physicians. However, it is well known that the standard 10-20 placement may not optimally cover certain regions of the brain, particularly the mesial and basal temporal regions. This has led to several proposals for extra electrode coverage in suspected temporal lobe epilepsies. The midline cortical regions are neurophysiologically complex regions because electrical discharges may project ipsilaterally, contralaterally, or be electrographically silent, in cases of deep sources. This study examines whether extra midline 10-10 electrodes help to better lateralize epilepsies originating in these midline regions. Methods: The subjects were nine patients with suspected midline epilepsies. These included patients with left or right frontal, central, parietal, and occipital seizures near the midline. Extra midline 10-10 electrodes were placed. The electrodes chosen were at the discretion of the neurophysiologist taking care of the patient. A single seizure from each patient was then clipped with all patient identifiers, video, and clinical markers removed. The EEGs were blindly interpreted by two neurophysiologists and instructed to identify the region of ictal onset, using only montages involving the standard 10-20 electrodes. A third neurophysiologist was given the same seizures and instructed to identify the region of ictal onset, using the extra 10-10 electrodes. All interpreters were instructed to indicate 'right/left hemisphere' if the seizure appeared to start in an entire hemisphere, 'generalized' if the seizure onset appeared generalized, or 'indeterminate' if they felt the EEG onset could not reliably be defined based on the data.Results: When comparing the EEG interpretations between the first neurophysiologist using only 10-20 electrodes and the third neurophysiologist using extra 10-10 electrodes, the latter physician disagreed with the lateralization of the seizures in 3/9 cases (33.3%). This consisted of cases where the latter physician picked the opposite hemisphere from the first physician, as well as cases where he was able to lateralize a seizure when the first physician was not able to. Comparing the results of the second neurophysiologist using 10-20 only electrodes and third neurophysiologist using extra 10-10 electrodes yielded similar results (3/9 cases were discordant).Conclusions: This study is suggestive that utilizing extra 10-10 electrodes in midline epilepsies yields extra information with regards to lateralization than the standard 10-20 electrodes alone. This study is limited by a small sample size, and may benefit in the future from a larger study population.