tests before surgery

What testing is done during an evaluation for epilepsy surgery?

Testing for epilepsy surgery is aimed at finding out if a person can benefit from surgery to treat their seizures. Testing for epilepsy surgery considers:

Some tests are common for everyone. Other more specialized tests may be needed for some people. Your epilepsy team will explain why they are recommending the test and how it will help.

A number of tests done before surgery (called a “pre-surgical evaluation”) are done in the outpatient setting. For other tests, a hospital stay will be required.

Outpatient testing helps to:

  • Assess a person’s baseline neurologic status
  • Make an accurate diagnosis of seizure type
  • Assess seizure frequency and impact on a person’s life
  • Review past treatment trials
  • Determine if a person is drug resistant or if more medication trials and other therapies (such as diet) should be considered
  • Determine if routine EEGs (electroencephalographs) are able to show where seizures begin
  • Determine if a structural abnormality (lesion) is present on brain imaging (MRI, CT) and if it is the cause of seizures
  • Determine if there are problems with memory and thinking
  • Evaluate if a person has difficulty with depression or anxiety
  • Assess if mood problems are related to seizures or anti-seizure medications
  • Assess a person’s day-to-day functional status and quality of life
  • Assess a person’s understanding of their epilepsy, their prior treatments, and their readiness to consider surgery

Tell Me More About the Testing

Often people have had several tests done by their outpatient neurologist before they go to an epilepsy center. Your epileptologist (a neurologist who specializes in epilepsy) will determine if this testing needs to be repeated and what new testing should be done.

Evaluation for epilepsy surgery takes place over weeks to months. Although the testing can feel like it is taking a long time, careful evaluation is important. This time also allows you to get to know your epilepsy team, ask questions, and tell them how you feel about possible treatment with surgery.

One way to think about the pre-surgical evaluation is to divide the timeline of testing into phases.

Phase 1: Pre-Surgical Evaluation

tests before surgery

This phase provides a baseline neurological exam, including visual field (vision) testing, and aims to determine the where seizure activity in the brain starts.

  • This testing is considered non-invasive testing because no surgery is required.
  • It can include outpatient EEG monitoring (person wears an EEG recording device at home) over a number of days.
  • In almost all cases, inpatient EEG monitoring and videotaping (called a video EEG) is done to record a person’s typical seizures and compare them with the changes seen on EEG.
    • During this testing, usually seizure medication is lowered or stopped so seizures can be recorded and the location where they arise can be found.
    • Length of hospital stay can vary from 3 to 10 days and in some cases longer.
  • Imaging during this phase may include fMRI, PET, SPECT, or MEG studies.
  • Neuropsychology testing assesses language, memory, and cognition.
  • A psychiatrist evaluates any mood problems, how this may be related to your epilepsy, and how to treat it.
  • A therapist or social worker may help assess how you are coping with epilepsy and the possibility of surgery, as well as what family and friend support you have in your life.

Phase 1 testing will vary for each person.

One person may require 3 tests and another person may require 6 tests. It all depends on the person’s type of seizures and what information your epilepsy team feels is needed to make the best decisions about moving forward with the surgical evaluation and surgery.

Phase 1 Tests

EEG (Electroencephalography)

An EEG records electrical activity in the brain to find where any abnormal activity starts. It records the brain’s electrical activity (brain waves) through electrodes placed on the scalp. The electrodes are connected to a computer recording the electrical activity. Learn more.

Continuous Video EEG Monitoring

Video cameras are used to record a person having seizures as they occur. At the same time, EEG electrodes on the person’s scalp record the electrical activity of the brain. This can help identify a seizure focus (the part of the brain where the seizure starts). The person's behavior during a seizure is matched to the timing and location of abnormal brain waves showing seizure activity on the EEG. Learn more.

High Density EEG and Source Localization

This type of EEG uses a much larger number (128 or 256) of sensors (electrodes) placed on the scalp to record brain waves. This information can be used to better identify seizures and to locate more precisely the region of the brain where a seizure begins by using source localization algorithms.

Special Electrodes

  • Sphenoidal electrodes are thin wires placed through the skin of the cheek towards the base of the skull. The electrodes do not go into the skull or brain. They can be placed closer to important brain areas where seizures may arise. Typically, one electrode is placed on each side. These electrodes may help improve recording of seizure activity from the tip of the temporal lobe. The electrodes can be placed in the EEG lab by an epileptologist.
  • Foramen ovale electrodes can be placed even closer to the tip of the temporal lobe (may be called mesial temporal lobe). The electrodes are placed by a neurosurgeon through an opening in the skull called the foramen ovale. Imaging of the brain are done at the same time to guide their placement. The electrodes sit under the bone or skull but do not go into the brain.
  • The use of sphenoidal and foramen ovale electrodes has decreased markedly with advanced imaging and other testing, such as high resolution MRI and high density EEG.

CT Scan (Computed Tomography Scan)

This test is special X-ray images processed by a computer to create a picture of the brain. It is also sometimes referred to as a CAT scan. Learn more.

MRI (Magnetic Resonance Imaging)

MRI uses magnetic fields and a computer to create a picture of the structure of the brain with high resolution and detail. This type of picture can give information about abnormalities in the brain that a CT scan cannot detect. Learn more.

PET/MRI (Positron Emission Tomography/Magnetic Resonance Imaging)

This technology allows for a single imaging session to obtain both PET and MRI images. The test gives a better understanding of what the brain looks like and how it functions in one set of images. Learn more.

MEG (Magnetoencephalography)

This is a technique that identifies brain activity by measuring small magnetic fields in the brain. A map of the brain’s magnetic fields and the structure of the brain can be seen. This information can help find the area of the brain where seizures begin. Learn more.

MRS (Magnetic Resonance Spectroscopy)

MRS uses a magnetic field like MR. Different computer software is used to measure chemical components of brain tissue. MRS can be used to measure the chemical component of a specific brain area.

Neuropsychology Testing

Neuropsychology testing is used to measure how the brain works or functions. It can give important information about specific brain areas or pathways in the brain. Tests can assess memory, language, attention, visual, and executive function. Tests are done before surgery to provide a baseline and are repeated after surgery to look for changes in cognitive functioning. This testing is completed in an office setting with a neuropsychologist or their assistant. The testing usually takes between 3 and 6 hours. The test results help the epilepsy team learn where seizures may start and other important questions. The results can also help the person living with epilepsy and their family learn about cognitive strengths and difficulties and what help may be needed. Learn more.

fMRI (functional Magnetic Brain Imaging)

fMRI measures brain activity by detecting changes associated with blood flow. It can be used to map brain activity in areas that control speech, movement, sensation, memory, and vision.

FDG-PET (Fluorodeoxyglucose-Positron Emission Tomography)

PET is a scanning technique that measures the metabolic activity in brain cells and how they work. Before the PET scan, a special tracer chemical (FDG) is given to the person. The tracer binds to glucose or sugar in the body. The glucose helps to identify areas of metabolism. A series of images are taken that measure the amount of glucose being used by the brain. If an area has decreased usage, it may suggest an area where seizures start, a “seizure focus.”

SPECT (Single Photon Emission Computed Tomography)

This test shows blood flow in the brain. A small amount of radioactive substance is injected into the person’s bloodstream. After several hours, a scan is done to see how the brain has absorbed the material. A SPECT scan can be done during a seizure or between seizures. If it is done during a seizure, it can show areas with increased blood flow. This information helps to find where seizure activity begins in the brain. If it is done between seizures, there will be decreased blood flow in regions of the brain where frequent seizures arise. Learn more.

SISCOM (Subtraction Ictal SPECT co-registered with MRI)

This is an imaging tool that combines functional information from a SPECT scan with the high-resolution images of MRI to help find a seizure focus.


This test is done to determine which hemisphere (side) of the brain is most responsible for functions such as speech and memory. The test is done in the interventional radiology department. During the test, each side of the brain is injected with a medication to cause brain activity in that hemisphere to decrease (“fall asleep”). While one side of the brain is asleep, the awake side of the brain is tested for memory, speech, and ability to understand speech. This allows doctors to know which side of a person’s brain controls those important functions. Learn more.

Phase 2: Intracranial Monitoring

After careful review of Phase 1 testing, the epileptologist will talk with the person and their family about findings and possible next steps. For some people, more testing may be needed. This next step is called Phase 2 testing.

Phase 2 testing uses surgically placed electrodes to find out exactly where seizures begin in the brain. Because these tests involve surgery, you can expect to have a pre-operative evaluation that includes:

  • Meeting with an anesthesiologist
  • A general medical history and physical exam
  • A chest X-ray
  • An electrocardiogram (ECG)
  • Blood and urine tests may also be done

Phase 2 Tests

Intracranial Depth Electrodes (also called Stereo-EEG or SEEG)

Testing with Intracranial depth electrodes is also called Stereo-EEG or SEEG. These electrodes are placed in the brain by a neurosurgeon. They can be put in deeper areas of the brain to find the precise region where seizure activity starts. The electrodes are fine, flexible plastic attached to wires that carry electrical activity from the brain to a recording computer. Imaging studies (CT and MRI) are used to help plan where to place the electrodes. They can also be used to stimulate different areas of the brain to map motor and language functions before surgery. Once the electrodes are in place, they can record seizures for up to 2 to 3 weeks.

Subdural Strip and Subdural Grid Electrodes

These are a series of EEG electrodes mounted on thin plastic that can record electrical activity from the surface of the brain. These electrodes are placed in the operating room. They can be made in a strip or as a square or rectangular grid to cover a larger area of brain surface. Sometimes grid and strip electrodes are used together to cover large areas of the brain when the specific site or lobe of the brain where seizures arise is not clear.

Learn more about implanted EEG electrodes.

ECoG (Electrocorticography) or iEEG (intracranial Electroencephalography)

ECoG testing involves mapping of brain function that is done in the operating room. This technique uses electrode grids to record activity directly from the surface of the brain (cortex). ECoG/iEEG can help find the specific area of the brain where seizures start and how much to remove during surgery. These electrodes also help find the exact areas responsible for important functions (such as speech, memory, and movement). The spatial resolution of ECoG/iEEG is much higher than scalp EEG recording.

Depending on a person’s individual situation, sometimes this testing is done during phase 2 testing or as a part of the final epilepsy surgery. Your epileptologist and neurosurgeon will decide the best timing for each phase to occur.

Surgical Decision Making

It is important to remember each person’s journey to epilepsy surgery will be different. Your entire epilepsy team will meet and have a “surgical conference” that is dedicated to just your case. They will review all of your testing prior to deciding what epilepsy surgery is best for you.

Examples of how testing decisions may vary include:

  • Not all people require Phase 2 or 3 testing.
  • Sometimes a brain imaging test (MRI) shows a structural brain lesion (such as a scar from an injury, a tumor, abnormal blood vessels, or an abnormal cluster of neurons) that is in an area of the brain matching their seizure symptoms. In this case, the surgery may be done without extensive EEG monitoring.
  • If the neurologic history and exam, imaging studies, and non-invasive EEG do not all point to one area of the brain where seizures start, then Phase 2 monitoring becomes necessary.

The epilepsy team has made a decision. What’s next?

Once your epilepsy team makes their recommendation, you and your loved ones will have time to ask questions and discuss with your doctors and nurses any concerns you may have. It is important to know that in some cases surgery is not possible after a review of all the testing. However, other treatment options may be suggested.

Remember, each member of your epilepsy team is available to support you throughout your journey. Excellent communication with your epilepsy team, comprehensive evaluation and testing, careful surgical planning, and caregiver and family support along the way will all help to make your epilepsy surgery journey a success.

Authored By: 
Elaine Kiriakopoulos MD, MSc
Authored Date: 
Reviewed By: 
Mohamad Koubeissi MD
Sandra Dewar PhD, RN, MS
Monday, October 15, 2018