Sensing the body electric: biomarkers of epileptic brain.

Commentary Localization of epileptic brain is the cornerstone of successful epilepsy surgery. In the final decades of the 20th century, advances in neuroimaging led to significant improvements in epilepsy surgery efficacy (1). Unfortunately, the gains in epilepsy surgery efficacy have reached a plateau. In particular , patients with normal MRI, diffuse or multifocal imaging abnormalities remain a significant challenge. There is an intense effort to better localize epileptic brain for epilepsy surgery. Electrophysiology was the breakthrough that drove the initial development of epilepsy surgery (1). While not the first to utilize intraoperative electrocorticography to map epileptic brain, Penfield and Jasper at the Montreal Neuro-logical Institute are notable for demonstrating the benefits of close collaboration between neurosurgeon and electro-physiologist (2). They recorded epileptiform spikes from the neocortex to map the epileptogenic brain and guide focal resection (2). The current conceptual approach to epilepsy surgery defines the epileptogenic zone (EZ) as the brain region which must be resected for seizure freedom (3). The brain region generating interictal spikes, called the irritative zone (IZ), provides an interictal map of epileptic brain but is generally more widespread than the region generating spontaneous seizures, the ictal onset zone (IOZ). The relationship between the IZ, IOZ, and EZ remains poorly defined in practice. For this reason, chronic intracranial EEG recording (iEEG) to capture habitual seizures remains the gold standard for localizing epileptic brain and guiding surgery. Unfortunately, iEEG extending over days is associated with significant cost, morbidity, and patient discomfort. In addition, the need for iEEG is unclear in many patients. Patients with a clear epileptogenic structural lesion on MRI may not require iEEG (4) but still benefit from limited duration intraoperative electrocorticography recordings (ECoG) to guide surgery. Many epilepsy centers, particularly in pediatric epilepsy surgery, utilize ECoG rather than chronic iEEG recordings, but the data are still limited (5, 6). A robust interictal signature of the EZ could potentially eliminate the need for multi-day iEEG in a wider spectrum of patients. Thus, the search for electrophysiological biomarkers of the EZ remains an active and potentially high impact area of clinical research. Perhaps the most promising candidate for interic-tal EZ localization is the use of pathological high frequency oscillations (pHFO) (7, 8). The study highlighted here is important because they use intraoperative ECoG to localize epileptic brain using interictal pHFO (9). This is motivated by over a decade of accumulating evidence that pHFO are useful electrophysi-BACKGROUND: …