The purpose of mapping is to outline areas of vital function such as language and the senses before a resection. This may be done in the operating room (intraoperatively) or prior to surgery (extraoperatively), in the Epilepsy Monitoring unit.
Once the area of seizure origin has been determined it is necessary to ensure that the surgical resection can be performed without compromising vital areas of language and motor. Mapping of language areas is conducted by stimulating parts of the brain with a low-voltage stimulator that does not damage brain tissue. If performed during surgery just before the resection, electrocorticography (ECOG) is used during the cortical stimulations. ECOG is the recording of the electroencephalogram (EEG) directly from the brain during surgery.
The ECOG identifies areas of damaged cortex or brain tissue that show increased abnormal activity associated with seizures. This abnormal activity is mapped before the resection. At Cedars-Sinai, a team which includes a surgeon, neurophysiologist, neuropsychologist and EEG technologists participate in this type of testing.
Because patients are awakened during the surgery for testing purposes, education is necessary and is provided for the patient. It is crucial that a patient be thoroughly informed as to exactly what he/she may expect during this process. The more the patient knows about the process the better. It is not easy for patients to wake during the surgery. It can be disorienting, frightening and daunting. It is crucial that a patient understand the entire process clearly for effective testing and a favorable outcome.
A pre-test examination is required before surgery for comparison. During the testing process, the patient is asked to speak or name objects during the stimulation. Speech arrest occurs when the speech area is stimulated. It is common for patients to experience seizures if stimulation is performed in areas that are near the seizure origin or in the proximity of the area of seizure involvement.
In epilepsy surgery, when the seizures originate from the frontal or parietal areas, it is important to know the location of the motor areas. This limits the resection as the motor areas must be spared. Motor-strip mapping can guide the surgical procedure to spare these vital areas. This is done by stimulating a nerve at the wrist. The use of somatosensory evoked potentials or stimulations of the median nerve typically produce a response in the motor/sensory areas that identifies and localizes the motor area of the hand as in the image to the right. This technique is fairly straightforward and takes about 10 to 15 minutes.