Candidates for epilepsy surgery undergo an extensive pre-surgery evaluation. The tests help to pinpoint the area in the brain where the seizures begin, called the seizure focus, and to determine whether surgery is feasible. The evaluation also helps to precisely identify important structures in the brain so that the seizure focus may be removed without causing damage to important nearby brain regions.
The pre-surgical evaluation includes an extensive medical history, with a review of seizure activity, including type, frequency, and duration. A complete physical is performed to rule out non-epileptic causes for the seizures, such as those associated with a high fever (febrile seizures).
Because absence seizures are usually quite brief, tend to strike during times of inactivity, and closely resemble daydreaming or "being off in one's own world," they may pass unnoticed by others and go undiagnosed for some time.
Absence seizures fall into two categories: typical and atypical.
Typical absence seizures begin abruptly, last 10 to 30 seconds, and resolve themselves without complication. The person simply stops in his tracks (and/or mid-sentence), and enters a staring, trance-like state...
Once it has been determined that the person's seizures are related to epilepsy and that the patient has not improved after taking medication, a series of tests may be done to help locate the seizure focus and provide the surgeon with important information about the brain that is needed for surgery.
What Tests Are Used Before Epilepsy Surgery?
The specific tests used depend on the type of epilepsy and type of surgery planned, but may include:
Electroencephalography (EEG) -- An EEG records brain waves through electrodes placed on the scalp. An EEG is helpful in diagnosing brain disorders by detecting abnormal electrical activity in the brain.
Magnetic Resonance Imaging (MRI) -- A MRI uses magnetic fields and a computer to create images of the anatomy, or structure, of the brain. A MRI creates a very clear image that can show abnormalities of the brain in great detail.
Magnetic Resonance Spectroscopy (MRS) -- MRS uses the same equipment as a MRI, but uses different computer software, which can measure the chemical components of brain tissue.
Positron Emission Tomography (PET) -- PET is a scanning technique that measures cellular activity (metabolism) in the brain and other organs, providing information about organ function rather than structure. For a PET scan, the patient is given a tracer chemical prior to scanning. The tracer binds to glucose molecules, which are indicators of metabolism. After the injection, images are taken with a special scanning camera that measures the amount of glucose used by the brain. Areas of decreased usage may point to the seizure focus.
Single Photon Emission Computed Tomography (SPECT) -- SPECT is a test that shows blood flow in the brain. A small amount of a radioactive substance is injected into the patient's bloodstream through a vein in the patient's arm. After several hours, scanning is done to see how the brain has absorbed the material. This provides a picture of how blood flows through the brain.
Visual field evaluation -- This is done to measure the person's peripheral (sideways) field of vision. The person holds his or her head still while looking at an object in front of him or her or beside him or her.
EEG-video monitoring -- Video cameras are used to record seizures as they occur, while EEG electrodes on the scalp monitor the brain's activity. The characteristics of the person's behavior during a seizure can help to identify the seizure focus. This is generally done over several days in a special monitoring room.
Invasive monitoring -- Also called intracranial EEG, this technique involves surgically placing electrodes inside the skull directly in or over a specific area of the brain to record electrical activity. Invasive monitoring also may be used to stimulate areas of the brain to help determine which areas are associated with critical functions such as memory, movement, and language.
Wada Test -- This test is done to determine which hemisphere (side of the brain) is dominant, or most responsible, for critical functions such as speech and memory. If the seizure focus and speech or memory center are on the same side, the surgery may be slightly altered to avoid damaging or removing the speech/memory area of the brain. During this test, each hemisphere is alternately injected with a medication to "put it to sleep." While one side is asleep, the awake side is tested for memory, speech, and ability to understanding speech. The patient may need to stay in the hospital overnight.
Neuropsychological testing -- This include tests that evaluate memory, language, personality, and thinking. They provide baseline information and are then repeated after surgery to see if there is any change in mental functioning.
Psychiatric evaluation -- Undergoing surgery for epilepsy is a long and difficult process. A psychiatric evaluation helps the person develop reasonable goals and expectations and to prepare for the surgery and recovery phases.