What is brain mapping?

Different brain regions have specific functions. Before performing any surgery on the brain, including surgery for epilepsy, the surgeon wants to understand how the brain areas affected by the seizures and surrounding brain areas function.  Plans for surgery aim to: remove as much of the seizure focus as possible to control seizures; preserve important functions such as speech, understanding, vision, movement, or sensation; and lessen the risk of complications from the surgery.

Brain mapping is a procedure that can help identify the functions of different regions of the brain.  

  • The exact location of various functions differs quite a bit from person to person. The presence of tumors, seizures, or other brain abnormalities may distort the usual location of some functions. General rules may not a apply. 
  • Mapping the brain by stimulating certain brains areas can help create a map for that person. The map tells the doctors just what parts of the brain are responsible for critical functions such as movement, sensation, speech or other functions. 

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  • The brain is stimulated by applying a very small electrical current to a small area on the surface of the brain.  
  • The current is not painful and cannot be felt by the patient.
  • The current interferes with how the area normally works. When this happens, the doctors can see functions are affected.
  • At each location within a brain region, the physician starts by applying the lowest current. Gradually the current is increased until a certain amounts is reached, or until a response is seen.
  • Once the current stops, that part of the brain resumes its usual activity.

 

What functions are mapped electrically?

Typically areas that have language, motor, sensory, or visual function are mapped. By applying current to just one area at a time, the doctor learns what each area does.

  • If a current causes the patient to stop speaking, or to speak in a way that can't be understood, then that area is likely to be important for language function.
  • If a patient’s limbs, trunk, or face start and stop moving with the current, the area being tested is responsible for movement.
  • If the patient 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 customized 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: in a two-stage procedure (the most common way) or directly during the surgery itself.

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 patient then returns to a hospital bed and is closely monitored.
  • The electrodes not only record the patient’s seizures electrically (to identify where the seizures start), but also allow mapping of brain areas under the electrodes.
  • During this kind of mapping (called extraoperative brain mapping because it happens outside the operating room), the patient is always awake and conscious. He or she is able to participate fully during the mapping. 
  • The second stage of surgery is often done several days later. The surgeon performs another operation and removes the abnormal brain tissue, using the information gathered from the electrical recordings and the brain mapping.

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.

  • 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.
  • In this kind of mapping, areas involved with movement can be identified electrically even if the patient is under anesthesia.
  • To map areas that have functions such as language, sensation, or vision, however, the patient must actively participate. If this kind of mapping is needed, the patient is awakened from anesthesia and is given enough medications to stop pain.
  • Intraoperative mapping is often done when previous extraoperative mapping 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 how much brain tissue is targeted for surgery, how many locations need to be tested, and what kind of functions are expected in those areas.
  • 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 the patient’s seizures ordinarily begin. Electrical currents applied in this location can set off a seizure.
    • Physicians pay close attention to the patient’s brainwaves during the stimulation. If electrical discharges that could build up to a seizure are seen, stimulation is immediately 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 patient is often given antiseizure medication before receiving any further electrical stimulation. If a patient does have a seizure, mapping is temporarily stopped until the patient 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 makes contact with the membranes surrounding the brain. At these locations, the patient may feel pain or a tingling sensation when the current is applied. Since the physician always starts at a low current, these contacts are easily identified and avoided.

How do patients react to brain mapping?

Patients 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 you know nothing is touching it. Sometimes an electrode may stimulate an area that produces specific visual images that turn on and off with the current. At other locations, the currents may trigger giggling or laughter; when current is applied, you suddenly find the situation very funny. When the current stops, you can’t understand why you were laughing.

Authored by: Howard L. Weiner, MD | Joseph I. Sirven, MD
Reviewed by: Joseph I. Sirven, MD | Patricia O. Shafer, RN, MN on 8/2013
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