Electroencephalography (EEG) measures the electrical activity of the brain using sensors placed on the scalp, providing a non-invasive view of neural function. Invasive EEG (iEEG), conversely, is a specialized, high-resolution diagnostic method that requires the temporary surgical implantation of electrodes directly onto or into the brain tissue itself. This procedure is reserved for complex cases where non-invasive methods, such as scalp EEG or magnetoencephalography (MEG), have failed to precisely locate the source of abnormal neurological activity. By placing electrodes closer to the source, iEEG dramatically improves the spatial and temporal resolution of the recording, which is necessary for planning highly targeted neurosurgery.
Primary Applications of Invasive EEG
The primary use of iEEG is the pre-surgical evaluation of patients with refractory epilepsy, which is a form of the disorder where seizures are not controlled by medication. For these individuals, surgery to remove the seizure-generating area may offer the best chance for seizure freedom. The central purpose of the iEEG procedure is to precisely identify the epileptogenic zone, the specific area of the brain where seizures originate.
This diagnostic clarity is particularly important when the suspected seizure focus is located near eloquent cortex, areas responsible for critical functions. The high-resolution data from iEEG allows the medical team to delineate the boundaries of the epileptogenic zone with high accuracy. The procedure also helps to map functional areas, like those controlling speech or motor function, that lie adjacent to the target zone. This detailed mapping ensures that any subsequent surgical resection can be performed while avoiding damage to essential brain functions, maximizing both safety and surgical success.
Types of Intracranial Electrodes and Placement
The implantation of intracranial electrodes is a neurosurgical procedure performed under general anesthesia. The specific type of electrode is chosen based on the suspected location of the seizure focus. Electrodes fall into two main categories: subdural electrodes, which record activity from the brain’s surface or cortex, and depth electrodes.
Subdural electrodes come as either strips or grids, consisting of small metallic contacts embedded in a flexible material. Strip electrodes are linear arrays of contacts that can be inserted through small bore holes, often used to cover basal or interhemispheric regions. Grid electrodes are larger sheets of contacts placed directly onto the exposed cortical surface, requiring a more extensive procedure called a craniotomy.
Depth electrodes, also known as Stereoelectroencephalography (sEEG) electrodes, are thin wires inserted deep into the brain substance. This technique uses robotic guidance and stereotactic methods to precisely target deep structures like the hippocampus or amygdala, which are often involved in temporal lobe epilepsy. The sEEG procedure is generally less invasive than grid placement, requiring only small bore holes for insertion and allowing for three-dimensional sampling of activity. The choice between subdural and depth electrodes, or a combination, is tailored to each patient’s unique suspected seizure network.
The Monitoring Phase and Functional Mapping
Following the surgical placement of the electrodes, the patient is transferred to an Epilepsy Monitoring Unit for the monitoring phase, which typically lasts between 5 and 17 days. During this inpatient stay, the patient is under continuous video and electroencephalographic surveillance. Anti-seizure medication is gradually reduced to intentionally provoke the patient’s typical seizures.
The primary goal of this phase is seizure localization, where the medical team analyzes the recorded data to identify the exact electrode contacts where the seizure activity begins and how it propagates through the brain. The high resolution of iEEG allows for the detection of subtle seizure onset patterns impossible to capture with scalp electrodes. This information is synthesized with pre-implantation imaging to create a clear map of the seizure-generating zone.
A second goal is functional mapping, which uses the implanted electrodes to identify areas of eloquent cortex. Brief, low-current electrical stimulation is delivered through individual or pairs of electrodes, and the patient’s response is observed. Stimulating the motor cortex might cause a twitch, while stimulating the language area can temporarily disrupt speech. This mapping provides a safety margin for the neurosurgeon, clearly indicating which areas must be preserved during any future resection.
Post-Procedure Considerations and Risks
Once sufficient data has been collected to localize the epileptogenic zone and map the functional areas, the patient returns to the operating room for electrode removal. This procedure involves carefully extracting the strips, grids, or depth electrodes, often followed by the second stage of surgery to resect the seizure focus if the data supports it. The recovery period for the initial iEEG procedure usually takes one to two weeks.
The placement of iEEG electrodes is a significant neurosurgical procedure that carries associated risks. The most common adverse events include intracranial hemorrhage and infection, both superficial and neurological. Studies indicate that the prevalence of intracranial hemorrhage is around 4.0%, and neurological infection is about 2.3%.
Other potential complications include brain swelling (cerebral edema) and cerebrospinal fluid leakage. The risk of these adverse events can increase with the number of electrodes implanted and the duration of the monitoring period. The benefits of precisely locating the seizure source are generally considered to outweigh the risks for patients with medically refractory epilepsy.