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What is invasive EEG monitoring?

Invasive EEG monitoring is a type of electroencephalography (EEG) that is recorded intracranially, or directly from the brain. Intracranial EEG is the gold standard for localizing where in the brain seizures start (epileptogenic zone), as it is more specific than a noninvasive (scalp) EEG. For invasive EEG, a neurosurgeon places the recording electrodes during an operation. Electrode placement occurs under general anesthesia through a hole in the skull or a craniotomy. The location of recording will be determined by previous EEG studies, MRI, and other imaging such as Positron Emission Tomography (PET) or Single-Photon Emission Computerized Tomography (SPECT).

There are two methods of invasive EEG monitoring: 

Subdural Grids and Strips: Subdural grids and strips are used when larger areas of the brain need to be covered during invasive monitoring. The electrode grids and strips are made up of small disc electrodes that are embedded in soft rubber called silastic. Configurations and the number of electrodes required vary from case to case and can range from as few as 8 electrodes up to 64 electrodes.  
Because of their larger recording area, a craniotomy is required to place subdural grid electrodes. Grids are often used in conjunction with strip electrodes, which are arranged linearly and are best for recording from under the temporal or frontal lobes. These strips can be placed via a hole in the skull or during the same craniotomy as the grid electrode.  

Stereotactic EEG (sEEG): Stereotactic EEG (sEEG) is thought of as less invasive than subdural grids and strips, requiring only small incisions in the scalp and skull for electrode placement. sEEG electrodes are made up of thin, flexible plastic with embedded electrodes that can record from deep parts of the brain. The electrodes are placed stereotactically, meaning the electrode placement is guided by three-dimensional imaging of your brain. Because of this minimally invasive approach, electrodes can be implanted in both hemispheres of the brain.  

Who is a candidate for invasive EEG monitoring?

Invasive EEG monitoring is part of the evaluation for epilepsy surgery. For some surgery candidates, additional intracranial EEG monitoring is not required because the epileptogenic zone was clearly defined during early testing, from the video EEG, MRI, PET, SPECT, or MEG.  
Invasive EEG monitoring is generally reserved for patients with medically refractory epilepsy, or epilepsy that has not responded to medication or treatment, whose early evaluation does not give a clear enough picture to localize seizure onset for surgical intervention. Invasive monitoring may also be necessary when there is a need to define brain function near the epilepsy surgery areas.  

What can I expect during invasive EEG monitoring?

Once the electrodes are placed in the operating room, you will have a CT scan to ensure proper placement on the brain. Then you will be taken back to your hospital room to begin EEG monitoring. The goal of invasive EEG monitoring is to record your typical seizures to better determine the epileptogenic zone. To increase the likelihood of seizures, your seizure medications may be adjusted or reduced during this time. The duration of monitoring is dependent on the frequency of your seizures.  

If the epileptogenic zone is found to be close to the areas of the brain related to speech, motor and sensory function, or vision, you may have functional mapping completed during your invasive EEG monitoring. This will provide the neurosurgeon with a map of areas to avoid during any future brain surgery.  

 After your invasive EEG monitoring is completed, you will return to the operating room for the electrodes to be removed. Before you are discharged from the hospital, you will return to your usual seizure medication dosage and schedule. 

What are the benefits of invasive EEG monitoring?

As invasive EEG monitors directly from the brain, this allows for the detection of seizures earlier and more often than scalp electrodes. More abnormalities can be detected between seizures (interictally) as well. If you are a candidate for invasive EEG monitoring, this enables the neurology and neurosurgery team to create a surgical plan that will be safe and effective in controlling your seizure activity.

What are the risks of invasive EEG monitoring?

The risks of invasive EEG monitoring are similar to other neurosurgeries, and may include:  

  • Infection 
  • Hemorrhage 
  • Cerebral edema 

Next Steps

The additional EEG data gathered during your stay will be presented at the Epilepsy Patient Management Conference (PMC), where epileptologists and other epilepsy health care providers will discuss the best path forward for seizure control.  

This information is for educational purposes only. It is not intended to replace the advice of your health care providers. If you have any questions, talk with your doctor or others on your health care team. If you are a Gillette patient with urgent questions or concerns, please contact Telehealth Nursing at 651-229-3890.