For patients with focal epilepsy, surgery is often the only chance for freedom from seizures. Successful surgery depends on the surgeon's ability to localize the source of seizures in the brain. Finding that source generally requires a patient to be monitored by EEG in the hospital for days or even weeks, in hopes that a habitual seizure will be recorded.
Mayo Clinic researchers are conducting clinical trials of a procedure that eventually could streamline that evaluation. In a single operating room procedure, intraoperative EEG would locate biomarkers of the epileptic brain that are present whether a seizure is occurring or not. "Theoretically, you wouldn't need to wait for a seizure to occur," explains Squire (Matt) M. Stead, M.D., Ph.D., a pediatric neurologist at Mayo Clinic in Rochester, Minn.
The goal is for patients to have a single procedure to localize seizures and then, if possible, to resect the affected area of brain. "This procedure would take hours, rather than days. It would be much less arduous for the patient," says Gregory A. Worrell, M.D., Ph.D., an epilepsy specialist at Mayo Clinic in Minnesota, who collaborates with Dr. Stead.
Intraoperative EEG uses microelectrodes thinner than a human hair. Capable of recording activity in a single cortical column, intraoperative EEG has helped the Mayo research team uncover two possible biomarkers for focal epilepsy.
The first is microseizures, seizure-like events in microdomains of the brain. Microseizures may occur frequently in the epileptic brain, but they are usually subclinical events. "They're occurring in such a small area that we only see it within a square millimeter of the electrode," Dr. Stead says. "The electrodes around it are still showing normal EEG, whereas one electrode in the middle is showing a seizure."
The second biomarker is high-frequency oscillation. "These oscillations are very brief — tiny buzzes that last for a couple hundred milliseconds," Dr. Stead explains. High-frequency oscillations occur virtually constantly, and although some are normal, others are not. The Mayo researchers are working to differentiate the two. "We get on the order of a million high-frequency oscillations over the course of recording for a patient," Dr. Stead says. Computer analysis of the oscillations can pinpoint the regions of the brain where seizures are likely to originate.
The current clinical study involves adult patients who undergo standard seizure localization testing, as well as intraoperative EEG. If intraoperative EEG produces similar results to the standard tests, it could eventually be used for both children and adults with epilepsy, the researchers say. "Our current best tool for localizing seizures is highly invasive," Dr. Worrell says. "We want to make that evaluation much less invasive for patients."