Electroencephalography (EEG) is a non-invasive technique that records electrical activity in the cerebral cortex, providing critical insights into brain function. It is instrumental in detecting ischemia, seizure activity, and assessing the impact of anesthetic agents on the brain. Typically, EEG electrodes are placed on the scalp in a standardized array before surgery, each electrode continuously recording spontaneous brain activity from a superficial area of 2 to 3 cm in diameter. In certain cases, such as during seizure surgery, awake brain mapping, or specific tumor resections, electrodes may be placed directly on the brain's surface intraoperatively (ECoG).
EEG is highly sensitive to changes in cerebral blood flow (CBF). When ischemia occurs, a progressive decrease in synaptic activity leads to a loss of high-frequency activity and power, ultimately resulting in EEG silence. This makes EEG particularly valuable during procedures like carotid endarterectomy, where monitoring cerebral perfusion during carotid cross-clamping is crucial. EEG slowing or asymmetry between recordings from the operative and contralateral sides can indicate ischemia.
However, EEG has limitations; surface recordings cannot detect ischemia in subcortical regions. Additionally, during pharmacologic metabolic suppression, EEG can confirm burst suppression but cannot monitor for ischemia. Intraoperative ECoG is used during seizure surgery to identify epileptogenic foci for resection, often performed with the patient awake under local anesthesia or light general anesthesia. This method also helps identify seizures evoked by cortical stimulation.
For patients with focal seizures refractory to medical therapy, stereotactic electroencephalography, a minimally invasive method involving implanted leads, helps localize seizure foci. These leads can be implanted using stereotactic techniques or robotic assistance under general anesthesia.
EEG is also crucial for monitoring the depth of anesthetic effect, which is essential to prevent accidental awareness during general anesthesia. Overall, EEG provides invaluable real-time data, enhancing the safety and effectiveness of neuromonitoring in anesthesia practice.
