When operating on brain tumors, it is critical to avoid damaging the brain regions responsible for language, motor, and sensory function.
While we know which parts of the brain are responsible for these functions (and where they’re generally located), each person’s brain is unique enough that there are slight variations. Depending how close the tumor is to each of these areas, it may be necessary to make a more precise, patient-specific map of these critical brain regions.
Brain mapping techniques help identify and ultimately preserve vital sites of language, motor, and sensory function. While some information can be gathered prior to surgery (like scans that identify the location of major nerve tracts), additional brain mapping techniques are used during surgery, so that the neurosurgeon can get the most precise, immediate feedback on functional areas of the brain near the tumor. Many of these techniques were pioneered at UCSF by Dr. Mitchel Berger, Director of the UCSF Brain Tumor Center.
Awake Brain Mapping
Awake brain mapping is the most precise way to identify and protect critical brain regions during removal of a tumor. Patients may need this procedure if their tumors are located near language, motor, or sensory regions of the brain. Having the patient awake during a part of the surgery allows the operating team to monitor active responses from the patient. For instance, language mapping requires patients to answer questions and undergo various speech tests. Motor mapping might include awake tasks like asking a patient to wiggle their toes or tap their fingers.
Generally, patients will undergo anesthesia at the beginning and end of the surgery, so that they’re only awake during a short time in the middle. Since there are no pain receptors within the brain itself, patients will not feel any part of the operation (the scalp has pain receptors but remains anesthetized). Throughout the surgery, a neuroanesthesiologist will be monitoring the patient and making sure he/she does not feel pain.
During the awake portion of the surgery, patients will respond to variety of questions while the surgeon applies a small electrical current to the regions of the brain surrounding the tumor. For language mapping, patients are asked to complete tasks like counting, reading, and object naming. Because direct electrical stimulation of language areas temporarily disrupts language-related abilities, the operating team can test whether the patient is speaking and responding properly. For example, if stimulation of a certain area interferes with the patient’s ability to count, then that area is labeled as critical and avoided when the tumor is removed. Likewise, if stimulation of a certain area activates movement or results in a sensation such as tingling, then that area is labeled as critical and protected during the surgery. Often this language or motor testing will continue intermittently, as the surgeon removes parts of the tumor. This allows for surgeons to remove as much tumor as possible, while preserving critical brain functions.
Once the tumor is removed, patients are sedated for the remainder of the procedure.
Asleep Brain Mapping
While awake brain mapping is considered the most precise way to identify and preserve critical brain regions, some types of mapping can be done while the patient is asleep under general anesthesia. Ultimately the recommendation to perform awake or asleep brain mapping depends on many factors, such as size and location of the tumor and patient health.
Motor mapping is the most common type of brain mapping that can be done while the patient is asleep. If the tumor is close to regions of cortex responsible for specific muscle groups (e.g., face, arm, hand, leg), this procedure is critical for identifying those regions and preserving the patient’s motor functions.
In the same way as awake brain mapping, surgeons use a handheld stimulator to apply a small electrical current to the exposed surface of the brain. However, in asleep brain mapping, the effects of direct stimulation can be observed without needing verbal responses from the patient. Instead, direct stimulation to motor areas of cortex results in muscle contractions within the appropriate body part (e.g. leg, arm). These muscle contractions can be visually observed, but is more robustly detected with electromyography (EMG)—a technique that uses electrodes to measure the electrical activity in each muscle. Using these tools, the neurosurgery team is able to remove as much of the brain tumor as possible, while avoiding critical sites in the brain that control body movement.