Epilepsy surgery involves more than just identifying the seizure focus in the brain. The preoperative evaluation must show that it is safe to remove that portion of the brain without causing excessive harm. While all parts of the brain are important, it also is true that some parts are more important than others. We place particular importance on preserving language and motor skills. The goal of epilepsy surgery is to cure epilepsy without causing the patient new or additional problems. Several tests are available for functional brain mapping including Wada, functional MRI, and grid stimulation studies.
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Figure 1. Functional magnetic resonance imaging study of a normal brain, arrows indicate activation of mesial temporal structures with memory task. |
Wada (Intracarotid Amobarbital Testing)
The Wada test helps the epilepsy surgery team to lateralize language (to determine the side of the brain in the patient's language function resides) and is valuable for testing memory function. It is most often used in candidates for temporal lobe surgery.
Wada is performed in the radiology area by a team that includes an epilepsy specialist, neuropsychologist, and angiography radiologist. A catheter is placed into the large blood vessel near the groin. The tip is maneuvered into the internal carotid artery in the neck.
A small amount of very short-acting medication, which has the effect of sedating half of the brain is administered into the catheter. The neuropsychologist tests language and memory functions to determine how effectively the other half of the brain (the unsedated half) is working. This is an excellent way to test the effect of surgery without performing an operation.
Functional Magnetic Resonance Imaging (fMRI)
The Barrow Epilepsy Center uses functional magnetic resonance imaging to perform non-invasive functional brain mapping in candidates for epilepsy surgery.
The active use of various brain regions slightly increases blood flow and can be noninvasively imaged by MRI. For example, asking the patient to wiggle his or her toes activates the "leg region" of the cerebral cortex. The fMRI study then shows exactly where this region is located. Many functions relating to motor movements, sensation, language, vision, and memory can be tested (Figure 1).
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| Figure 2. During Grid Stimulation Studies, a small amount of electrical current is induced in recording electrodes. |
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Grid Stimulation Studies
The same sheet of electrodes (the "grid") used to record seizures during video-EEG monitoring can be used to map the function of brain regions underneath the grid.
A small amount of electrical current is induced in recording electrodes (Figure 2). The current has the effect of "numbing" the cerebral cortex immediately underneath the electrode contacts being tested. This type of test is painless because the surface of the brain has no sensory fibers. The "numbing" effect only lasts for the duration of the electrical stimulation, which typically is applied 1 to 5 seconds. For example, if electrodes over the language expression region are stimulated while the patient is reading aloud, the patient will stop reading for the duration of the stimulation and then immediately resume once the stimulation stops.
Various brain regions can be tested for the critical functions that need to be preserved including language use or motor movements. Sometimes intracranial grids are placed surgically for mapping studies only, for example, in a brain tumor patient who does not have epilepsy but who has a tumor located near the language area. This mapping information helps the neurosurgeon perform the best operation possible to minimize unwanted side effects of surgery.
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Figure 3. EEG indicating focal spike actrivity on the right side. |
Electroencephalography (EEG)
Electroencephalography (EEG) records electrical activity in the brain, much like electrocardiography (EKG) records electrical activity in the heart. During an EEG, about 20 electrodes are placed evenly on the head. Ideally, the EEG is conducted while the patient is fully awake and asleep. The physician may ask patients to deprive themselves of sleep the night before the test.
A routine EEG evaluates brain activity between seizures because seizures often occur during the test (the test typically lasts 20 to 30 minutes). See Figure 3.
Most patients with epilepsy have one of two types of abnormalities on their EEG study: focal or generalized.
Focal abnormalities are spike activity or abnormal slowing over a certain spot or region of the head. This finding usually is consistent with focal (also called partial) seizures, in which the seizure event begins at a localized spot (focus) in the brain. Sometimes this spot is easy to identify and sometimes it is not.
Generalized abnormalities appear on both sides of the head at the same instant. This finding usually is consistent with generalized seizures, in which seizure events start on both sides of the brain.
For a thorough description of EEG, please visit the Electroencephalography page.
