Epilepsy is a complex neurological disorder defined by recurrent, unprovoked seizures, which are essentially temporary electrical disturbances in the brain. For many patients, standard medication can control these electrical storms, but for a significant number, seizures continue despite treatment. Stereoelectroencephalography, or SEEG, is an advanced, minimally invasive diagnostic procedure reserved for these complex cases of drug-resistant epilepsy. The procedure aims to identify the precise area of the brain where a person’s seizures begin, which is a necessary step before considering curative surgical options.
The Role of SEEG in Epilepsy Diagnosis
The primary purpose of SEEG is to generate a highly detailed, three-dimensional electrical map of the brain to locate the Seizure Onset Zone (SOZ). The SOZ is the small region of brain tissue where abnormal electrical activity originates. Unlike a standard electroencephalogram (EEG), which records electrical activity from electrodes placed on the scalp, SEEG utilizes thin, wire-like depth electrodes placed directly into the brain.
These depth electrodes are strategically positioned to monitor areas inaccessible to surface recording methods, including structures deep within the brain like the hippocampus or insula. Recording electrical signals from multiple locations allows the clinical team to observe the exact moment and location where a seizure starts and track its spread. This intracranial recording provides the precision needed to differentiate the SOZ from surrounding healthy tissue, which is fundamental for treatment planning. The electrodes also allow for electrical stimulation, which helps map functional areas (e.g., those controlling language or movement) ensuring they can be preserved during future procedures.
When Standard EEG Monitoring is Not Enough
SEEG is recommended for patients whose seizures have not been controlled by at least two appropriate anti-seizure medications. For many of these individuals, non-invasive tests like scalp EEG, magnetic resonance imaging (MRI), or positron emission tomography (PET) scans have failed to clearly identify the source of the seizures. This ambiguity often arises when the seizure focus is located deep within the brain, where surface electrodes cannot detect the electrical signals with sufficient clarity.
Localization can also be complicated when imaging shows no obvious structural abnormality, which occurs in 20% to 40% of patients with refractory focal epilepsy. Patients may also exhibit multiple potential seizure sites, or the non-invasive data may suggest a focus close to critical functional areas. In these complex scenarios, SEEG provides the anatomical and electrical evidence required for targeted treatment planning.
Mapping the Brain: The SEEG Procedure
The SEEG process begins with extensive pre-operative planning, which involves fusing high-resolution imaging, typically a CT scan and an MRI, to create a detailed, three-dimensional model of the patient’s brain. The epilepsy team uses this model to meticulously plot the precise trajectory for each electrode, ensuring they target the suspected SOZ while safely navigating around blood vessels and functional brain regions. This planning phase determines the accuracy and safety of the procedure.
During the surgery, which is performed under general anesthesia, a specialized stereotactic frame or a robotic assistance system, such as ROSA, is often used to guide the surgeon. This technology ensures the electrodes are implanted with sub-millimeter accuracy along the pre-planned trajectories. Tiny holes, approximately the width of a piece of spaghetti, are drilled into the skull, through which the thin electrodes are gently inserted into the brain tissue. Neurosurgeons typically implant between 10 to 20 electrodes, each containing multiple contacts, to cover the suspected seizure network.
Following implantation, the patient is moved to a specialized Epilepsy Monitoring Unit (EMU) for the monitoring phase, which typically lasts for several days to a week. During this time, anti-seizure medications are often reduced to allow seizures to occur, which are then recorded simultaneously with video and the electrical activity from the SEEG electrodes. Once the team has captured sufficient seizure activity to confidently localize the SOZ and map functional areas, the electrodes are removed in a short, separate procedure under light sedation.
From Data to Treatment: Planning Epilepsy Surgery
The extensive data collected during the SEEG monitoring period is analyzed by the epilepsy team to form a consensus on the exact location and extent of the SOZ. This localization links diagnosis and therapeutic intervention. The primary goal of SEEG is to determine if the patient is a candidate for curative epilepsy surgery, which may involve resection, laser ablation, or the placement of neuromodulation devices.
If the SOZ is found in an area that can be safely removed without causing significant neurological deficits, the surgeon can proceed with a targeted resection or laser interstitial thermal therapy (LITT) to destroy the problematic tissue. The SEEG data, including functional mapping via electrical stimulation, allows for maximum precision during removal, maximizing the chance of seizure freedom while preserving motor, sensory, and language functions. For patients whose seizures originate from a non-resectable area, the SEEG data can still guide the placement of responsive neurostimulation (RNS) or deep brain stimulation (DBS) devices, offering an alternative pathway to reduce seizure frequency.