What is brain mapping?
Different brain regions have specific functions. Before performing any surgery on the brain, including epilepsy surgery, the surgeon wants to understand how the brain areas near the seizure onset function. This helps your team know how much of the seizure focus can be removed safely.
Brain mapping is a procedure that can help identify what different regions of the brain do.
- The exact location of certain functions (like movement, speech, vision, and more) differs quite a bit from person to person. The presence of tumors, seizures, or other brain abnormalities may change what parts of the brain control certain functions. General rules may not apply.
- A "map" of each person’s brain can be made by stimulating certain brains areas. The map tells the doctors just what parts of the brain are responsible for critical functions such as movement, sensation, or speech.
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- The brain is stimulated by applying a very low electrical current to a small area on the surface of the brain.
- The current is not painful and cannot be felt.
- The current interferes with how the area normally works. When this happens, the doctors can see which brain functions are affected.
- The doctor starts by giving the lowest current. Gradually the current is increased until the highest safe amount is reached or until a response is seen.
- Once the current stops, that part of the brain resumes its usual activity. The changes during stimulation are only temporary.
What functions are mapped electrically?
Typically, areas that have language, motor, sensory, or vision are mapped. By applying current to just one area at a time, the doctor learns what each area does.
- If a current causes the person to stop speaking, or to speak in a way that can't be understood, then that area is likely important for language.
- If a person’s limbs, trunk, or face start and stop moving with the current, the area being tested is responsible for movement.
- If the person feels tingling, numbness, or a sensation in a body part when current is applied, a sensory area has been discovered.
- On rare occasions, brain mapping can be used to test for other functions and in other areas.
How is electrical brain mapping performed?
Electrical brain mapping to guide brain surgery for epilepsy or a tumor can be performed in two ways:
- Invasive electrodes are placed (called a two-stage procedure)
- During surgery to remove the seizure focus or tumor
How is the two-stage procedure done?
- During the first stage, a surgeon creates an opening in the skull and exposes the surface of the brain.
- No brain tissue is removed, but small electrical contacts, or electrodes, are placed over the surface of the brain. Once these electrodes are in place, the scalp is closed.
- The person returns to a hospital bed and is closely monitored.
- The electrodes record the person’s seizures and can be used to map the brain.
- During this kind of mapping (called extraoperative brain mapping because it happens outside the operating room), the person is awake and conscious.
- The second stage of surgery is often done several days later. The surgeon performs another operation and removes the same area of skull. He then takes out the abnormal brain tissue. The information from the first mapping helps the surgeon avoid brain that controls important functions.
How is the brain mapped during surgery?
Electrical brain mapping can also be performed during any surgery that exposes part of the brain. This is called intraoperative brain mapping because it occurs during an operation.
- When mapping is done during surgery, often the person is awake and able to talk. They are given medicines to keep them comfortable.
- Using a small electrical probe, the surgeon tests locations on the brain’s surface one after another to create a map of functions. Any number of locations can be tested.
- Mapping of some functions, like movement, can be done even if the person is asleep from anesthesia.
- Mapping during surgery is often done when earlier mapping or tests found important functions very close to the area targeted for surgery.
How long will the brain mapping procedure take?
- The length of the mapping procedure depends on many things, such as the brain area to be removed or where seizures arise, how many areas need to be tested, and what those areas of brain normally do.
- Mapping may last anywhere from an hour to several hours.
What are the risks of electrical brain mapping?
Brain mapping has few risks.
- The main risk, especially for people with epilepsy, is that a seizure may be triggered. The areas being mapped are usually close to where seizures begin. Electrical currents applied in this location can set off a seizure.
- An EEG (electroencephalograph) watches the person’s brainwaves during the stimulation. If electrical discharges that could build up to a seizure are seen, stimulation is stopped. Some of these discharges can be stopped immediately by giving an additional brief pulse of current to the same area.
- If the area being mapped is showing a lot of epilepsy waves, the person may be given seizure medication before receiving any more electrical stimulation. If a person does have a seizure, mapping is stopped until the person has fully recovered.
- There is a relatively small risk of pain during electrical stimulation. Even though the brain itself does not feel the currents or any pain, an electrode occasionally touches the membranes surrounding the brain. The person may feel pain or a tingling sensation when this happens. Since the doctor always starts at a low current, these contacts are easily identified and avoided.
How do people react to brain mapping?
People usually find the mapping process interesting. Imagine your arm lifting without any effort on your part or feeling a tickle on the bottom of your foot when nothing is touching it. Sometimes an electrode may stimulate an area that makes specific visions turn on and off with the current. At other locations, the currents may trigger giggling or laughter. When the current stops, you can’t understand why you were laughing.
Remember that brain mapping with electrical stimulation is done to increase the benefits of epilepsy surgery and lower the risks. If you have questions about what to expect and how you might respond, talk with your epilepsy doctor and nurse.
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