In March 2010, part of my Left Temporal Lobe was removed in an attempt to reduce my seizures. In a previous post, I described the Wada test and how I “failed” it. Basically, my doctor froze each side of my brain and found speech was dominant on both sides. So before any part of my left temporal lobe could be removed, they would need to create a map. Below I have used epilepsy.com to define brain mapping, and have included actual video from my brain mapping experience.
What is 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.
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. 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.
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.
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 very irritable, the patient is often given a powerful anti-seizure 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.
epilepsy.com/ Topic Editor: Howard L. Weiner, M.D.
