Different brain regions have specific functions. Before performing any surgery on the brain, including surgery for the treatment of epilepsy, the surgeon seeks to understand the functions of the areas affected by the seizures or of the lesion (abnormal tissue). All of the surgical planning is done to preserve important functions such as speech, comprehension, sight, movement, or sensation, and to lessen the risk of loss of function from the surgery.
The task of identifying the functions of different regions of the brain is called brain mapping. Over about the last hundred years, scientists and physicians have gathered large amounts of information, or maps, of the brain regions generally involved in specific functions. At first they learned from studying the functioning of people who had injuries in certain areas of their brain (from accidents or strokes, for instance). The exact location of various functions differs quite a bit from person to person, however, and the presence of tumors, seizures, or other brain abnormalities may distort these maps so that general rules do not apply. Therefore, surgeons and physicians need a way to identify the precise locations of functions within each patient’s brain. This is now done by using direct electrical simulation, a process called electrical brain mapping.
Electrical brain mapping is a technique by which physicians use direct stimulation of the brain to determine the specific function of a particular area. During the procedure, a current is applied to a very small area of the brain’s surface. The current is not painful and cannot be felt by the patient, but it interferes with how the area normally works. Once the current stops, that part of the brain resumes its usual activity.
At each location within a brain region, the physician starts by applying the lowest current. Gradually the current is increased until a preset maximum is reached, or until a significant response is seen. Then a new location in that region is chosen and the physician starts over with the lowest current. Each location is tested in this way to create an accurate "map" of functions present within that region of the patient’s brain.
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. For example, 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 a motor area and is responsible for movement. If the patient feels tingling, numbness, or a sensation of movement 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.
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.
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 (thereby allowing physicians 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 completeness of the final map is limited only by the number and location of the electrodes that were placed on the brain’s surface.
The second stage of surgery, which may be several days later, is when the surgeon performs a second operation and removes the abnormal brain tissue, using the information gathered from the electrical recordings and the brain mapping.
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.
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.
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 very irritable, the patient is often given a powerful 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 sense the currents, 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.
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.
Topic Editor: Howard L. Weiner, M.D.
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