An invasive electroencephalogram (EEG) records brain activity, differing from a standard scalp EEG. While a conventional EEG involves placing electrodes on the outside of the head, an invasive EEG requires a surgical procedure to position electrodes directly on or within the brain. This direct placement allows for precise measurement of the brain’s electrical signals, offering a detailed view of neuronal activity. The technique provides a clearer, less distorted signal compared to surface recordings, as it bypasses the impedance of the skull and scalp. This enhanced precision is particularly valuable when detailed information about brain electrical patterns is needed for specific medical evaluations.
Purpose of Invasive EEG
The primary reason for performing an invasive EEG is to gain a clear understanding of brain activity when non-invasive tests do not provide sufficient information. This diagnostic approach is most frequently employed for individuals experiencing drug-resistant epilepsy, where seizures persist despite various medication regimens. When scalp EEG recordings and advanced imaging techniques, such as MRI, cannot accurately pinpoint the specific area where seizures originate, an invasive EEG becomes a necessary step. The goal is to precisely localize the “seizure onset zone,” the region of brain tissue generating epileptic seizures. Mapping this zone accurately determines if a patient is a candidate for brain surgery (resection or ablation) to remove or destroy affected tissue. The detailed electrical map guides neurosurgeons in planning interventions for seizure control while preserving healthy brain function. Beyond seizure localization, invasive EEG also serves to perform functional brain mapping. This process identifies areas of the brain responsible for functions like speech, language, and movement, ensuring these areas are not inadvertently damaged during any subsequent surgical procedure.
Types of Invasive EEG
Two primary methods are used for invasive EEG monitoring. Stereoelectroencephalography, commonly known as sEEG, involves drilling small, precise holes, typically 1-2 millimeters in diameter, into the skull. Through these openings, thin, flexible wire electrodes are carefully inserted into specific, deeper structures of the brain. This approach is considered less surgically extensive than other methods and is particularly effective for investigating seizures that may originate from deep within the brain’s folds or structures, which are difficult to access with surface electrodes. sEEG allows for monitoring from multiple points across both hemispheres, providing a three-dimensional view of electrical activity.
The other main method is Electrocorticography, or ECoG, which utilizes grids or strips of electrodes placed directly on the surface of the brain. This procedure requires a craniotomy, a temporary surgical opening where a section of the skull is removed to expose the brain’s surface. ECoG electrodes provide a high-density, detailed view of seizure activity across the cerebral cortex. While requiring a larger surgical opening, ECoG offers superior spatial resolution and a higher signal-to-noise ratio, allowing for precise localization of activity on the brain’s surface.
The Invasive EEG Procedure
An invasive EEG procedure begins with the surgical placement of electrodes. Initially, a neurosurgeon implants either sEEG depth electrodes or ECoG grids and strips. This surgery is conducted under general anesthesia for precise electrode placement. Following the electrode implantation, a CT scan is often performed to confirm the exact positioning of the electrodes within the brain.
The patient then transitions to a specialized Epilepsy Monitoring Unit (EMU) for monitoring. Here, they remain hospitalized for continuous video and EEG surveillance. During this time, seizure medications are often gradually reduced or temporarily stopped in a controlled environment to safely encourage and record the patient’s typical seizures. This controlled triggering of seizures allows the medical team to capture sufficient data on the brain’s electrical patterns during seizure onset and spread.
Once enough information has been collected, usually within 3 to 14 days, the data is thoroughly analyzed by an epilepsy team to create a detailed map of the seizure focus. This analysis informs decisions about potential surgical treatments. The final phase involves a second surgical procedure to remove the implanted electrodes. This removal surgery is also performed under anesthesia, and after the electrodes are taken out, the patient’s usual seizure medication regimen is typically reinstated before discharge.
Risks and Recovery
Undergoing an invasive EEG procedure carries specific risks, similar to those associated with any brain surgery. Potential complications include infection, bleeding (hemorrhage), and cerebral edema (brain swelling). There is also a risk of developing new neurological deficits, such as changes in speech, movement, or sensation, depending on the areas of the brain involved. These risks are inherent to intracranial procedures.
Following electrode removal, patients remain hospitalized for recovery, typically a few days to a week. Common post-operative symptoms include headaches, fatigue, and general discomfort around the surgical sites. Patients may also experience some swelling in the scalp or face. The average recovery time before returning to normal daily activities, such as school or work, is usually about one to two weeks, though this can vary depending on individual healing and the extent of the procedure. Full recovery and return to all pre-procedure activities may take several weeks as the brain continues to heal.