Epilepsy Surgery: The Presurgical Evaluation

by Liu Lin Thio, MD, PhD
Dr. Thio is an Assistant Professor of Neurology at Washington University School of Medicine and an epileptologist in the Pediatric Epilepsy Center at St. Louis Children’s Hospital.

Introduction

Surgery is one of several treatments available for treating epilepsy. In most cases, your child’s neurologist or epileptologist (neurologist who specializes in treating epilepsy) will recommend an antiepileptic medication first. Most children with epilepsy will become seizure-free when treated with medication. However, 25-30% of children with epilepsy continue to have seizures despite multiple medication trials. For these children, surgery may be the best option. Other treatment options for children whose seizures are not controlled by medication include the vagal nerve stimulator and the ketogenic diet.

Generally, the goal of epilepsy surgery is to cure or eliminate all seizures. For some children, the goal of epilepsy surgery may be to prevent a particular seizure type or reduce seizure severity.
Is your child a candidate for epilepsy surgery?

Your child is a candidate for epilepsy surgery if he/she meets the following criteria: 1) his/her epilepsy is medically intractable, 2) surgery can help his/her type of epilepsy, 3) he/she is medically and psychologically stable, and 4) your family is psychosocially stable. While the first two are required, your child may not have to meet the last two criteria to be a surgical candidate.

Medical intractability. Your child’s epilepsy is medically intractable if medicine does not control his/her seizures. Medical intractability is difficult to define precisely and may vary for each child. However, many epileptologists define it as a failure of 2-3 antiepileptic drugs to control your child’s seizures.

Epilepsy that can be helped by surgery. The presurgical evaluation, described in more detail below, will determine if surgery can help your child.

Medically and psychological stability. Medically stable means that your child must be healthy except for his/her seizures. He/she cannot have a condition that would make surgery dangerous. For example, epilepsy surgery is not an option if his/her heart is not working well. He/she also cannot have a neurodegenerative disease. Psychological stability means that your child must be psychologically healthy. For example, depression may prevent your child from being a candidate for epilepsy surgery. However, once any medical or psychological condition is treated your child may be a surgical candidate.

Psychosocial stability. Psychosocial stability means that you and your family are prepared to invest the time and effort required for a surgical evaluation and surgery. As described below, the presurgical evaluation can involve many appointments, tests, and hospitalizations requiring you to miss work and spend time away from home. In addition, you must be prepared to accept the recommendations, the risks, and the outcome of the presurgical evaluation and the surgery.

What kinds of epilepsy surgery are done?

Before discussing the presurgical evaluation further, it is helpful to know what types of epilepsy surgery exist. Usually, epilepsy surgery involves the removal of a part of your child’s brain by a neurosurgeon (brain surgeon). The part removed may be abnormal such as a tumor, a blood vessel abnormality, or a part of the brain that did not form normally. This is a lesionectomy. In some instances, the part removed may be the size of a grape. In other cases, the neurosurgeon may remove an entire lobe (e.g. temporal lobe) or lobes. This is a lobectomy. Alternatively, the neurosurgeon may remove a small part of the brain that appears to be normal. This is a corticectomy. Epilepsy surgery can involve the removal of one hemisphere or nearly one-half of the brain. This is a hemispherectomy. Some neurosurgeons only remove a portion of the hemisphere but disconnect the entire hemisphere from the rest of the brain. This is a functional hemispherectomy or hemispherotomy. Children with Sturge-Weber, hemimegalencephaly, or Rasmussen encephalitis may undergo a hemispherectomy. Some children have a corpus callosotomy. This is a cutting of the corpus callosum, the bridge connecting the right and left hemispheres. Children who injure themselves in drop attacks as seen in Lennox-Gastaut syndrome are candidates for this procedure. Finally, multiple subpial transections (MST) provide an option for children whose seizures come from a part of the brain involved with an important function. If removal of the area will cause a major neurological deficit such as paralysis of one side of the body or an inability to speak or understand speech, MST is an option. MST is like raking the surface of the brain. In theory, the seizures will stop while preserving critical neurological functions. In summary, your child’s epilepsy must be a type that one of these surgeries can improve.

Purpose of the presurgical evaluation

The primary purpose of the presurgical evaluation is to determine if your child meets the criteria for epilepsy surgery listed above (see “Is your child a candidate for epilepsy surgery?”). Your child’s neurologist can determine if your child has medically intractable seizures, if he/she is medically and psychologically stable, and if your family is prepared to undergo a presurgical evaluation. To determine if your child’s epilepsy can be helped by surgery, your child’s neurologist will likely refer you to an epileptologist at a specialized epilepsy center for a formal evaluation.

An epileptologist will determine if surgery can cure or improve your child’s seizures. This requires two pieces of information. First, your child’s epileptogenic zone must be located. The epileptogenic zone is the part of the brain the seizures start. Complete removal of this area will result in seizure freedom. Second, the relationship of the epileptogenic zone to important functions such as talking and walking must be determined. Obtaining this information requires the skills of many professionals including epileptologists/neurophysiologists, neuropsychologists, neurosurgeons, nurses, pharmacists, and social workers. Thus, a presurgical evaluation requires a team approach that is only available at specialized epilepsy centers.

Locating the epileptogenic zone or determining where the seizures start.

History and physical. This is an important part of any presurgical evaluation. Your child’s epileptologist will review your child’s medical history in detail. Specifically, he/she will need to know what your child’s seizures look like, when they started, and how often they occur. In addition, he/she will review any previous test results, and the seizure medications your child has taken. He/she will review your child’s development and past medical history. Your child’s epileptologist will perform a complete general and neurological exam. He/she will review any records and test results that you may have.

The information obtained from a history and physical is important in determining whether your child is a candidate for epilepsy surgery and where your child’s epileptogenic zone may be. For example, the history and exam may indicate that your child has a neurodegenerative disease excluding him/her from further consideration for surgery. Your description of your child’s seizures may help your child’s epileptologist determine a potential location for the epileptogenic zone.

Continuous video electroencephalogram (EEG) or closed circuit television (CCTV) EEG. An epileptologist will study your child’s seizures using continuous video EEG monitoring, sometimes called long term monitoring. During this test, your child simultaneously has an EEG and is video taped. The video allows an epileptologist to study what your child’s seizures look like. Since seizures look different depending on what part of the brain they start in, an epileptologist might be able to tell where they start just by studying videos of your child’s seizures. The EEG allows an epileptologist to study your child’s seizures electrically. An EEG measures the electrical activity of the brain. Since seizures are abnormal electrical discharges from the brain, a seizure produces an abnormal electrical pattern on an EEG. By carefully studying the EEG at the beginning of a seizure, an epileptologist may be able to determine where it starts. Abnormalities in your child’s EEG when he/she is not having a seizure may also help identify where the seizures start. For example, abnormalities called spikes (they look like spikes) identify potential areas where seizures may begin.

A continuous video EEG study typically requires admitting your child to a specially equipped hospital room. An EEG technician attaches EEG leads to your child’s scalp as they are for a routine EEG. The room is usually equipped with infrared cameras and a microphone to record the behaviors and sounds associated with all seizures, even those occurring during sleep. The length of the hospitalization depends on how often he/she has seizures but typically requires 3-5 days.

The study is a noninvasive and safe. Since one goal is to record seizures, reducing or completely discontinuing your child’s seizure medications may be necessary. If your child has less than one or two seizures a week, your child’s epileptologist will likely reduce his/her medications. The risk of reducing seizure medications is that your child may have status epilepticus or a prolonged seizure. Your child’s nurse will place an intravenous line (IV) for giving medications should your child have a long seizure or very frequent seizures.
The importance of the information obtained from a continuous video EEG study and its relative safety make it a component of nearly every presurgical evaluation.

Head magnetic resonance imaging (head MRI). Along with the history, physical examination, and continuous video EEG, an imaging study of your child’s brain is an integral component of any presurgical evaluation. The imaging study of choice to assess the anatomy of your child’s brain is a head MRI. In some cases, a head computed tomography (head CT) might also be helpful. A head CT uses x-rays to construct an image of your child’s brain. An MRI uses magnetic fields to construct detailed images of your child’s brain. These images may reveal anatomical abnormalities such as tumors, vascular abnormalities, strokes, or cortical developmental malformations (Figure 1). All of these abnormalities can serve as the starting point for seizures. Virtually all presurgical evaluations will include a head MRI. Obtaining an MRI is a safe, noninvasive procedure. The MRI machine is a long, noisy tube. The most significant risk comes from the need to sedate children who cannot lie still.

Figure 1. MRI scans can identify structural abnormalities that may be the starting point for a seizure. Axial FLAIR image from a 7 year old child with tuberous sclerosis showing cortical tubers (arrows). The tubers are structurally abnormal areas found in the brains of patients with tuberous sclerosis. Seizures may begin in any one of these tubers.

Positron emission tomography (PET) scan. A PET scan is another imaging study that can identify potential starting points for your child’s seizures. Places where seizures start can be hypometabolic (Figure 2). This means that the area uses less glucose than normal. These areas are detected by injecting a glucose analogue, usually 2-[18F]fluoro-deoxyglucose (FDG). The more glucose an area of the brain uses, the more FDG it will take up. Other radiopharmaceuticals such as [11C]flumazenil can also serve as tracers.

Although PET studies depend on the injection of a radioactive molecule into your child, it is a noninvasive and safe test. The radionucleotide injected is a positron emitting isotope. The positrons decay to photons, which an array of cameras detects. Thus, the difference between a PET scan and a head CT is the source of the radiation. In a PET scan, it comes from the radiopharmaceutical injected. For a head CT, the radiation comes from the machine. Since the radionucleotide is short lived and the amount of radioactivity used is small, this is a safe test. The most significant risk is from sedating children who cannot lie still. An EEG recorded during the injection with FDG will determine if your child had a seizure during the test. PET scans are not part of every presurgical evaluation.

Figure 2. FDG PET scans can identify where seizures begin by indicating a hypometabolic area of the brain. Left, A coronal view showing a hypometabolic area (arrow) in the right midline parietal area. Right, A sagittal view showing the same hypometabolic area (arrow).

Single photon emission computed tomography (SPECT). A SPECT scan is another method for determining where seizures start. The principle behind a SPECT scan is that blood flow transiently increases in the area where a seizure starts. Since SPECT scans measure blood flow, the seizure onset zone is the area with increased blood flow. Your child’s epileptologist will decide if this test could be helpful in your child’s presurgical evaluation.
Cerebral (brain) blood flow is determined by injecting a radiopharmaceutical that distributes throughout the brain in proportion to blood flow. In other words, the greater the blood flow to an area, the more it picks up. The tracer generally used is 99mtechnetium-ethyl cysteinate dimer (ECD) or 99mtechnetium-hexamethyl-propyleneamine-oxime (HMPAO).

A SPECT study consists of two scans-an interictal and an ictal scan (Figure 3). Your child will receive two tracer injections. One injection will occur while your child is not seizing. The scan done within a few hours of this injection is the interictal scan. A second injection will occur during a seizure and on a different day than the first. The scan done after this injection is the ictal scan. A neuroradiologist will compare the two scans and determine if an area of the brain had increased blood flow during the seizure. If your child has a SPECT study, your child will receive the injections during a continuous video EEG study.

Figure 3. SPECT scans identify where seizures begin by indicating an area of increased blood flow during a seizure. Left, A coronal view of an ictal scan showing an area of increased blood flow during a seizure in the right midline parietal area (arrow). Right, The interictal scan shows no significant increase in blood flow in the right midline parietal area when there is no seizure (arrow).

Magnetoencephalography (MEG). When a continuous video EEG, PET scan, or SPECT scan does not clearly identify where your child’s seizures start, your child’s epileptologist may consider a MEG. Unlike EEG, which measures electrical activity, MEG measures magnetic fields. The advantage of MEG is that it has better spatial resolution, meaning that it is easier to determine where a spike is located. However, MEG requires a magnetically isolated room, no movement, and frequent spikes. These requirements make it difficult to record seizures during a MEG study. Furthermore, few centers offer MEG. Although MEG can provide useful information in some situations, it generally is not a standard component of the presurgical evaluation.

Like EEG, MEG is a noninvasive and safe test that does not require hospital admission. Some but not all MEG machines look like a hair dryer in a beauty salon (Figure 4). The most significant risk is from the need to sedate your child if he/she cannot lie still. The test usually takes a few hours to complete.

Figure 4. MEG machine. (Adapted from Baumgartner et al. (2000)
with permission from Lippincott Williams & Wilkins).

Magnetic resonance spectroscopy (MRS). MRS measures chemical levels in the brain. It can identify chemically abnormal areas in the brain that do not appear abnormal structurally, i.e. on a routine head MRI. These abnormal areas may be areas where the seizures start. Having MRS is just like getting a head MRI. However, this technique is experimental and is not yet a routine part of the presurgical evaluation.

Do the seizures start in an area having an important neurological function?

A second set of tests determines whether epilepsy surgery will result in any neurological deficits such as a hemiparesis (weakness on one side of the body), hemianopia (loss of vision on the right or left side of the body in both eyes), or aphasia (loss of language). These tests include a neuropsychological evaluation, a Wada test, functional MRI, and visual field testing. Your child’s age, seizure type, head MRI, and continuous video EEG findings will determine whether any of these studies will be helpful in your child’s presurgical evaluation.

Neuropsychological evaluation. A neuropsychologist can determine if your child is at risk for a neurological deficit after epilepsy surgery by using a variety of psychological tests. The tests measure your child’s general intelligence or IQ, language abilities, ability to visualize objects, memory, fine motor ability, ability to pay attention, academic achievement, and behavior. The results of these tests will identify strengths and weaknesses in your child’s cognitive abilities. The pattern of strengths and weaknesses may indicate that a particular area of your child’s brain does not work as well. If this area is the same as the area where the seizures start, your child is not likely to have a neurological deficit after surgery. Even if your child does not have epilepsy surgery, this information is useful in developing an educational program for your child (e.g. Individualized Educational Program). This testing is safe and usually is part of a presurgical evaluation. The extent of testing will depend on your child’s age.

Wada test. The intracarotid amobarbital procedure or Wada test, named for Dr. Juhn Wada, has two functions. It can determine whether the left or right side of your child’s brain controls his/her ability to speak and understand language. It also can determine how important the left and right sides of the brain are for memory. Since the frontal lobe controls speech, the temporal lobe processes language, and the hippocampus plays a role in memory, these are the areas of the brain tested during a Wada test. If your child’s seizures start in one of these areas, he/she may have a Wada test because these are important functions to preserve.

The test involves putting one side of the brain to sleep at a time with a medication called sodium amobarbital. A neuroradiologist will inject one internal carotid artery, the major blood vessel feeding each side of the brain, at a time. After each injection, a neuropsychologist will then test your child’s ability to speak, understand speech, and remember. If these functions are impaired, then the side of the brain that is asleep must be important for these functions. Your child’s neuropsychologist may have your child go through a “dry run” before the actual test. This will help familiarize him/her with the test.

The Wada test is an invasive test having potentially serious but rare risks. Typically, the radiologist places a catheter into the femoral artery, a large artery in the leg, by making an incision in the groin. Then, the radiologist maneuvers the catheter into the internal carotid artery via major blood vessels. The risks of the study include stroke and an allergic reaction to the material injected. An EEG recorded during the study helps your child’s neuropsychologist determine if the injection worked. Only children able to cooperate fully are eligible for the study.

Functional MRI (fMRI). fMRI can help identify areas of critical neurological function. The underlying principle is that active brain areas receive increased blood flow. Since fMRI identifies areas of increased blood flow, it can identify areas activated during various tasks designed to identify language, visual, motor, or sensory areas. You and your child’s epileptologist may be reluctant to sacrifice any area associated with one of these functions.
fMRI is a noninvasive and safe test done in the same machine as a head MRI, described above. Like the Wada test, your child must cooperate fully and willingly perform any task required. For example, your child may need to move a finger at a particular to time to identify the area of the brain controlling finger movement. Other tasks can identify other functional areas. fMRI is not included in every presurgical evaluation.

In the future, fMRI used in conjunction with EEG may help to identify where seizures start.

Visual field testing. Visual field testing tests the brain’s ability to send visual information from the eyes to the occipital lobes where visual information is processed. The occipital lobes are located in the back of the brain. If the test is abnormal, the pattern of the abnormality tells your child’s epileptologist where in the brain the problem is. Your child may have visual field testing if epilepsy surgery may involve a part of the pathway transmitting visual information from the eyes to the occipital lobe. However, your child must be able to cooperate with the testing.
Who remains a candidate for epilepsy surgery?

All of the tests reviewed above serve two functions. First, they help identify where your child’s seizures start. Second, they help determine if the area where the seizures start overlaps with any area involved in a critical neurological function.

Your child’s epileptologist will order some but not all of the tests listed above to evaluate your child’s seizures. After the tests are completed, the members of the epilepsy surgery team will discuss the results and give you a recommendation. They will determine where the seizures start. They will also determine if a structural lesion exists such as a tumor, blood vessel abnormality, or abnormally formed area. The best surgical candidates are children whose test results all point to a single area of the brain as being the site where the seizures start. The ideal candidate will have a structural lesion on an anatomical head MRI in the same area. For example, a continuous video EEG study in an ideal candidate may indicate that the seizures start in the right frontal lobe in an area that shows a structural abnormality on a head MRI, is hypometabolic on a PET scan, and shows increased blood flow on a SPECT scan. In these cases, the epilepsy surgery team may recommend that your child undergo epilepsy surgery without any further testing. The surgery would consist of removal of the lesion. In practice, this scenario generally is limited to patients with temporal lobe seizures.

After reviewing the test results, the epilepsy surgery team may decide that your child has a diffuse or has more than one epileptogenic zone. For example, your child’s seizures may start all over his/her brain at once. Alternatively, your child may have some seizures starting in the left temporal lobe and others starting in the right frontal lobe. In these cases, the epilepsy surgery team may recommend against epilepsy surgery for your child.

Finally, the epilepsy surgery team may not know exactly where your child’s seizures start after reviewing the test results. This may be because the tests are contradictory. For example, the continuous video EEG may indicate that the seizures come from the temporal lobe, but the PET scan may show hypometabolism in the frontal lobe. Alternatively, some tests may identify one area as the place where the seizures start while others show no abnormality.

For example, continuous video EEG may indicate that the seizures start in the frontal lobe, but the MRI shows no structural abnormality. In these cases, your child may remain a candidate for epilepsy surgery, but the epilepsy surgery team may recommend additional testing. The epilepsy surgery team will also recommend more testing if all the tests identify the same area as the site of seizure onset, but the area has a critical neurological function. For example, a child’s seizures may start in the frontal lobe in an area important for speech as identified by fMRI. The additional testing in both of these cases will likely be continuous video EEG monitoring using intracranial electrodes.

What is continuous video EEG monitoring using intracranial electrodes?

Continuous video EEG monitoring using intracranial electrodes is identical to continuous video EEG monitoring using scalp electrodes except that the electrodes lie directly on the brain surface. Epileptologists hope to achieve two goals with intracranial electrodes. One goal is to determine exactly where the seizures start. The area where the seizures start can be identified more precisely than with scalp electrodes because the electrodes lie directly on the brain surface. The second goal is to map cortical function. Cortical mapping consists of stimulating the brain tissue under the electrodes to determine its function. Stimulating the area of the brain controlling hand movement will cause the hand to move. Stimulating the area controlling speech will disrupt speech. Thus, an epileptologist can determine the function of the brain underneath the electrodes by stimulating the electrodes and observing your child’s behavior. In some cases, your child will need to perform a task while in others he/she will only have to lie still. Usually, the aim of cortical mapping is to identify areas critical for motor and/or language function, though the same procedure can identify areas critical for other functions such as sensation. Cortical mapping usually requires 1-2 hours. Your child’s epileptologist will perform the procedure while your child is lying comfortably in bed. Cooperative teenagers may be candidates for intraoperative cortical mapping or mapping during the middle of an operation.

Your child’s epileptologist may also obtain evoked potentials while your child has intracranial electrodes. Evoked potentials can provide information that is complementary to the cortical mapping. Although several different types of evoked potentials exist, somatosensory evoked potentials are the most commonly used. Somatosensory evoked potentials help identify the part of the brain involved in processing sensation such as touch and pain. Your brain knows when something touches your arm because it receives an electrical signal from the nerve in your arm. Your child’s epileptologist can record this electrical signal from the intracranial electrodes by stimulating a nerve in the arm.

Placing intracranial electrodes requires surgery. Intracranial electrodes come as electrode strips, electrode grids, and depth electrodes. Electrode strips are flexible strips of Silastic or Teflon containing up to 10 contacts separated by 0.5-1 cm arranged in one row (Figure 5). A neurosurgeon inserts the strips through burr holes about 1 cm in diameter drilled in your child’s skull. Electrode grids are similar to the strips, but they have more than one row of contacts (Figure 5). They may have as many as 64 contacts arranged in 8 rows containing 8 contacts each.

Figure 5. Examples of intracranial strips (white arrows) and grids (black arrows). Reproduced and modified with permission from Ad-Tech Medical Instrument Corporation.

To place a grid, a neurosurgeon does a craniotomy, a procedure removing part of your child’s skull (Figure 6). After positioning the grid, the surgeon replaces the skull. Depth electrodes are wires with multiple contacts along their length. Although used less frequently than strips and grids at many epilepsy centers, they are useful for targeting an area deep in the brain. A neurosurgeon usually inserts these through burr holes. Some children may have a combination of some or all of these electrodes. Your child may stay in the intensive care unit briefly after intracranial electrode placement, but he/she will eventually stay in the same type of room used for the continuous video EEG monitoring with scalp electrodes.

Figure 6. Plain skull film showing position of intracranial electrodes.

After recording several seizures, the epilepsy surgery team will again determine where the seizures start. If they start from more than one place, especially if one is on the left and the other is on the right, they will likely recommend against surgery. The neurosurgeon will then remove the intracranial electrodes. If they cannot identify exactly where the seizures start, they may recommend the placement of additional intracranial electrodes. In this case, your child will undergo further continuous video EEG recording after placement of the additional intracranial electrodes. After recording more seizures, the epilepsy surgery team will attempt to define where the seizures start, and they will repeat the steps outlined in this paragraph. If the area where the seizures start is clear and it is far away from areas involved with critical functions such as language, vision, or movement, the team will likely recommend removal of this area. If the area is likely to overlap with a critical function, they will probably recommend cortical mapping. Then, the team will compare the cortical mapping results with the area where the seizures start. If the area where the seizures start does not overlap with an area responsible for an important function, they will likely recommend removal of the area where the seizures start. If the area where the seizures start overlaps with an area controlling an important function, the epilepsy surgery team may recommend multiple subpial transactions, a combination of multiple subpial transactions and removal of brain tissue, or against surgery.

Summary

If your child has medically intractable seizures, your child’s neurologist may recommend epilepsy surgery as a treatment option. Depending on your interest, your child’s neurologist may refer you to an epilepsy center specializing in epilepsy surgery. You will then have the opportunity to meet an epileptologist for an initial evaluation. Together, you and your child’s epileptologist can decide whether epilepsy surgery is a reasonable option for your child. If so, you may then decide to pursue a presurgical evaluation, a complex process involving the expertise of many professionals.

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      © 2002 by K. Liu Lin Thio.