Intracranial electroencephalography (iEEG) is a specialized procedure that records electrical activity directly from the human brain. This direct monitoring provides a detailed view of how brain cells communicate, allowing medical professionals to observe precise patterns of brain activity. This helps guide the diagnosis and treatment of complex neurological conditions.
What is Intracranial EEG?
Intracranial EEG involves a surgical procedure where electrodes are placed either on the brain’s surface or deep within its structures. These small, conductive sensors detect subtle electrical signals generated by neurons. This direct placement offers a level of precision non-invasive methods, such as standard scalp EEG, cannot achieve. Scalp EEG records brain activity through electrodes on the scalp, picking up signals from a wider area but with less spatial detail due to the skull and skin barriers.
In contrast, iEEG bypasses these barriers, allowing for recordings with millisecond and millimeter precision directly from specific neuronal populations. This direct contact enables the identification of subtle electrical patterns, such as those associated with a seizure focus, which might be undetectable or unclear with external recordings. The purpose of iEEG is to capture and analyze these highly localized electrical signals to understand brain function and dysfunction.
Why Intracranial EEG is Performed
Intracranial EEG becomes a necessary diagnostic step when non-invasive tests, such as standard scalp EEG, MRI, or PET scans, do not provide enough information to understand a patient’s condition fully. This is particularly relevant for individuals with epilepsy where medications have not been effective in controlling seizures, a condition known as drug-resistant epilepsy.
In such cases, iEEG helps pinpoint the exact origin of seizures within the brain, which is called the seizure onset zone. Precisely localizing this zone is important for determining if a patient is a suitable candidate for epilepsy surgery, which aims to remove the seizure-generating brain tissue.
Beyond seizure localization, iEEG is also performed for presurgical brain mapping. This involves identifying and mapping “eloquent cortex,” which are brain areas responsible for vital functions like language, movement, and sensation. Knowing the precise location of these functional areas allows surgeons to plan their approach to minimize the risk of damaging them during any potential surgical intervention.
The Intracranial EEG Procedure and Monitoring
The iEEG procedure begins with surgical implantation of electrodes under general anesthesia. Neurosurgeons place different types of electrodes based on the suspected seizure location and monitoring needs. These include subdural grids (flexible mats on the brain’s surface), subdural strips (strips with fewer contact points), or depth electrodes (thin wires inserted into deeper structures). Depth electrodes require small burr holes, while grids may need a craniotomy.
After securing the electrodes, wires are tunneled through the scalp to external recording equipment. Patients then transfer to an epilepsy monitoring unit for continuous video-EEG monitoring, lasting several days to weeks. During this period, patients are continuously observed, and brain activity is recorded 24 hours a day.
Anti-seizure medications may be gradually reduced to encourage seizures, helping the medical team capture onset and propagation patterns. Once sufficient data is collected, electrodes are surgically removed, usually under anesthesia, and patients are monitored for several days before discharge.
Insights from Intracranial EEG
The data gathered from iEEG provides detailed insights into the brain’s electrical activity. It helps medical teams pinpoint the exact seizure onset zone and understand how seizures spread through brain networks, a process known as seizure propagation. iEEG is also instrumental in mapping functional areas of the brain, such as those controlling language, motor skills, and memory. During monitoring, electrical stimulation through the implanted electrodes can temporarily disrupt these functions, helping to create a detailed map of their locations. This functional mapping, combined with seizure onset localization, guides surgical planning to minimize the risk of neurological deficits following brain surgery.