Intracranial electroencephalography (iEEG) is a specialized diagnostic procedure used to evaluate drug-resistant epilepsy, a condition where seizures cannot be controlled by medication. This invasive test involves surgically placing electrodes directly inside the skull to record the brain’s electrical activity. The fundamental purpose of iEEG is to precisely locate the seizure onset zone, the specific area of brain tissue where epileptic activity begins. Pinpointing this location with high accuracy determines if a patient is a suitable candidate for epilepsy surgery and guides the neurosurgeon’s plan for tissue removal.
How Intracranial EEG Differs from Standard Monitoring
Standard scalp electroencephalography (EEG) uses electrodes placed on the patient’s skin, which must record electrical signals that have passed through the scalp, skull, and intervening tissue. This signal path causes a smearing or blurring effect, resulting in lower spatial resolution and a limited ability to detect activity from deep brain structures. Intracranial EEG bypasses these obstacles, placing the recording contacts either on the brain’s surface or within the tissue itself. This proximity to the neural sources provides a higher signal-to-noise ratio and spatial resolution, allowing neurologists to detect subtle electrical patterns invisible to non-invasive methods.
iEEG captures the unfiltered, high-frequency oscillations that are often directly linked to the seizure-generating tissue. This allows for a clearer distinction between the area where the seizure begins and the areas where the electrical activity merely spreads. When non-invasive test results, such as MRI or scalp EEG, are conflicting or inconclusive, iEEG offers the definitive clarity needed to proceed with surgery.
The Procedure: Implantation and Monitoring
The iEEG procedure begins with extensive pre-surgical planning, which frequently involves fusing high-resolution magnetic resonance imaging (MRI) and computed tomography (CT) scans. This detailed imaging helps the surgical team create a precise, three-dimensional roadmap for electrode placement, ensuring the contacts cover all suspected seizure areas. The actual implantation is performed under general anesthesia by a neurosurgeon, utilizing one of two primary methods depending on the suspected location of the seizure focus.
One method involves a craniotomy, where a section of the skull is temporarily removed to allow the placement of subdural grids or strips, which are thin sheets of flexible material embedded with electrodes that sit directly on the brain’s surface. Alternatively, Stereo-EEG (sEEG) involves drilling small burr holes into the skull to insert fine, wire-like depth electrodes into the brain parenchyma. These depth electrodes allow for the exploration of activity in deeper brain structures, such as the insula or hippocampus, without a large opening.
Once the electrodes are secured, the patient is transferred to the hospital’s Epilepsy Monitoring Unit (EMU) for the recording phase, which typically lasts a few days to two weeks. During this period, the patient’s anti-seizure medication is gradually reduced or temporarily stopped under medical supervision to provoke and capture their habitual seizures. Continuous video and EEG monitoring ensures that multiple seizures are recorded. The wires from the implanted electrodes connect to external recording equipment, allowing the patient a degree of mobility while remaining under constant observation.
Diagnostic Precision: Mapping Seizure Networks
The continuous recording in the EMU allows the epileptologist to precisely delineate the Seizure Onset Zone (SOZ) and the entire epileptic network. This data is considered the gold standard for defining the tissue that must be removed for seizure freedom. Analyzing the propagation patterns of the electrical signals helps distinguish the SOZ from areas that only become involved as the seizure spreads.
Functional brain mapping is performed while the electrodes are in place. This involves passing a very low level of electrical current through specific electrode contacts to temporarily stimulate the adjacent brain tissue. If stimulation in a particular area causes a patient to momentarily lose the ability to speak or experience a twitch in their hand, that area is identified as responsible for functions like speech or motor control. This mapping creates a detailed risk assessment, ensuring the surgical resection of the SOZ avoids tissue responsible for language, movement, or sensation.
Risks and Recovery
As an invasive surgical procedure, iEEG carries risks. Potential complications include surgical site infection, intracranial hemorrhage (bleeding within the skull), and brain swelling. Though rare, there is also a slight risk of temporary or permanent neurological deficits related to the electrode placement or the surgical process itself.
Following the implantation surgery, patients typically spend a brief time in an intensive care or neurocritical care unit for observation, then move to the EMU for monitoring. Headaches and discomfort at the surgical sites are common and managed with pain medication. Once sufficient seizure data has been collected and analyzed, a second procedure is performed to remove the electrodes. The patient is returned to their normal anti-seizure medication schedule before being discharged, with a recovery period of several weeks before the final epilepsy surgery is scheduled.