Subdural electrodes are a form of intracranial electroencephalography (iEEG) used to record brain activity directly from the source. These devices are thin, flexible sheets of biocompatible plastic embedded with small, flat metal discs that act as electrodes. The sheets are shaped as grids or strips, allowing them to be laid directly on the brain’s surface to conform to its curves. This placement provides a more detailed picture of electrical activity than what can be captured through the skull. This method is considered when non-invasive tests have not provided enough information, and their function is to gather data over several days.
Purpose of Subdural Electrode Monitoring
The primary reason for using subdural electrodes is to pinpoint the exact origin of seizures within the brain. This is necessary for individuals with drug-resistant epilepsy, where medications do not effectively control seizure activity. Standard scalp electroencephalography (EEG) often fails to provide the precise location of the seizure onset zone. Subdural electrodes overcome this by recording electrical signals directly from the cortex, filtering out interference from the skull and scalp, which is needed for planning surgical treatments.
Subdural electrode monitoring also serves for functional brain mapping. Before brain surgery, it is important to identify areas controlling functions like language, movement, and sensation. During monitoring, specialists stimulate individual electrodes with a small, painless electrical current. By observing the patient’s responses, such as a muscle twitch or a tingling sensation, they can create a detailed map of these functional areas. This map ensures a surgical plan to remove seizure-causing tissue avoids damaging these regions.
The Implantation and Monitoring Process
The process begins with a surgical procedure to place the electrodes. A neurosurgeon performs a craniotomy, temporarily removing a section of the skull to access the brain’s surface. The subdural electrode grids and strips are then positioned over the areas of the brain suspected of generating seizures. The procedure is planned using results from prior tests like MRI and scalp EEG.
Once the electrodes are in place, their connecting wires are passed through the skin, and the section of skull is secured. The patient is then transferred to an Epilepsy Monitoring Unit (EMU) for a monitoring phase that can last from several days to a few weeks. In the EMU, the patient is under continuous surveillance while the electrodes record brain activity 24 hours a day.
During this time, the primary goal is to record the patient’s typical seizures. To facilitate this, the medical team may gradually reduce or stop anti-seizure medications, intentionally lowering the seizure threshold. This controlled process allows the team to capture several seizures, providing a comprehensive dataset of their onset and spread. While waiting for seizures to occur, the team also performs functional mapping.
Throughout their stay in the EMU, patients are restricted to their room or bed for safety and to protect the monitoring equipment. Continuous observation by medical staff ensures any seizure activity is immediately documented and managed. The data collected provides a complete picture of the electrical disturbances causing the seizures.
Information Gathered and Subsequent Treatment
After the monitoring period, neurophysiologists and surgeons analyze the extensive data recorded by the subdural electrodes. The primary goal is to use this information to create a personalized map of the patient’s brain. This map identifies the precise location of the seizure onset zone—the specific area where seizures originate. It also incorporates the results of functional mapping, distinguishing the seizure-producing areas from brain regions responsible for movement, sensation, and language. This integrated map is fundamental for determining the next steps in treatment.
Based on this map, the most common treatment is resection surgery, where a neurosurgeon removes the identified seizure focus. The high-resolution data allows for a targeted approach, removing only the necessary tissue while preserving surrounding functional areas. This precision increases the likelihood of achieving seizure freedom after surgery.
If monitoring reveals the seizure focus is in an area that cannot be safely removed, other treatments are considered. One alternative is responsive neurostimulation (RNS). This involves implanting a small device to detect and disrupt seizure activity with electrical pulses at the source.
Recovery and Potential Complications
The recovery process occurs in stages, beginning after the initial surgery to implant the electrodes and continuing after the second procedure to remove them. The hospital stay for the monitoring itself can range from a few days to over three weeks. Following the electrode removal surgery, which may also involve the resection of seizure-causing tissue, patients remain in the hospital for several more days for observation. Pain at the incision site is common and is managed with medication, and the timeline for returning to daily activities varies, with most patients needing several weeks to recover fully.
Intracranial monitoring is an invasive procedure and carries potential risks. The most common complications are related to bleeding, such as subdural or epidural hematomas, which are collections of blood near the brain’s surface. These occur in a small percentage of cases and may require further surgical intervention.
Other potential complications include infection, which may require antibiotics, and brain swelling (edema). There is also a risk of new neurological deficits, such as changes in speech or motor function, though these are often temporary. The rate of significant complications is low, and permanent issues are rare.