Surgery is an alternative for patients whose seizures cannot be controlled by medications. Epilepsy surgerycan be especially beneficial to patients who have seizures associated with structural brain abnormalities, such as benign brain tumors, malformations of blood vessels (e.g. arteriovenous malformations, venous angiomas, and cavernous angiomas) and strokes. The operations generally involve removal of epileptogenic tissue from the area where seizures arise or interruption of nerve pathways along which seizure impulses spread.
Prior to surgery, a prospective patient undergoes a thorough evaluation consisting of complete physical and neurological examination and an assemblage of studies assess if a patient will ultimately benefit from epilepsy surgery.
Initially, the patient’s own testimony will be elicited for a medical history. However, depending on age, and also because the patient is often unaware, a witness(s) to the seizures can adequately describe the seizure characteristics and patterns. Thus, in keeping with our family-centered and sensitive approach, we engage family and caretakers as an important part of the evaluation process.
Evaluation prior to surgery for epilepsy has changed radically in the past few decades, most notably since the advent of long-term video-electroencephalography (EEG) monitoring in the late 1970s, and advanced neuroimaging, specifically with additional use of MRI.
Electroencephalography (EEG) is the fundamental measure of identifying seizure focus. The epileptologists on our team are adept at reading the spikes and sharp waves that constitute a spectrum of shifting seizure focus. Most patients undergoing evaluation for surgery will require EEG with video telemetry to document the seizure pattern.
However, an appropriate focus goes beyond single readings. Our experts evaluate EEG monitoring over a period of time. This is a critical measure of determine a clear onset and location of seizure activity.
While computerized tomography (CT) scan can provide some useful information about lesions which may be causing seizures, the “gold standard” for locating and evaluating brain lesions is magnetic resonance imaging (MRI). The greatest advance of MRI technology for epilepsy is the incredible clarity of both the normal and pathological anatomy of the brain. Since the mid-1980s, this technology has allowed us to routinely identify lesions that were previously undetectable.
fMRI scans use the same basic technological principles as MRI scans, but whereas MRI scans image anatomical structure, FMRI images chart metabolic function. Thus, functional MRI (fMRI) provides a non-invasive means of obtaining a ‘road map’ for the function of the brain. It is used to localize sensory, motor or cognitive function in the brain via changes in blood flow and hemoglobin oxygenation. Important information is obtained by integrating the results of fMRI with EEG.
Magneto-encephalography (MEG) is performed using a machine that consists of a variety of highly sensitive magnetic sensors. These magnets are positioned around the scalp in a helmet-shaped container called a Dewar. They are attached to the scalp to detect small magnetic changes that arise from normal and abnormal electrical brain activity. This technology is a valuable tool because it can be used to localize the source of epileptic activity and normal functional areas of the brain for surgical planning in patients with epilepsy.
Nuclear medicine, which deals with imaging the body for both diagnostic and treatment purposes, plays an important role in the pre-surgical assessment of patients with refractory epilepsy. This is evident in the case of single-photon emission computed tomography (SPECT) which is used to determine the seizure onset zone.
SPECT (single proton emission computed tomography) has an important role in the investigation of surgical candidates. Ictal (meaning taking place during a seizure) SPECT has also been useful to study seizure-spread patterns. Blood flow increases in the brain area in which the seizure originates and the blood flow can become less than normal in that same area during non-seizure states. By injecting a small and safe amount of a radioactive substance into a patient’s blood stream via IV we can evaluate the blood flow in the brain at a given time. Changes in blood flow during and after a seizure can be helpful in localizing seizure focus and spread patterns.
Intracarotid memory and speech evaluation, also referred to as the Wada test (named after the Japanese neurologist who first performed it), evaluates cerebral language and memory in both hemispheres of the brain. It is performed using a catheter placed in the main artery suppling blood to each brain hemisphere. Anesthetics are injected (to disable a hemisphere) while patients are asked to perform certain tasks testing language and memory. If the patient’s ability to perform these tasks is impaired by disabling a particular hemisphere this indicates the location of the function. In this way we can evaluate which side of the brain is most important for language and memory function.
Neuropsychology testing is an important part of evaluation for epilepsy surgery. Conducted by an expert neuropsychologist, it is a formal assessment of cognition such as language, memory, attention and problem-solving. The neuropsychologist is able to determine how various parts of the brain are related to each of these tasks. This testing should be able to distinguish frontal from temporal brain dysfunction. This evaluation is also compared pre- and post-surgery to asses the effects of the surgery.
The psychiatrist is a vital member of a quality epilepsy team. Those being monitored for epilepsy surgery are at particularly high risk for psychiatric conditions due to the burden of their disease. Among the conditions present in this population is depression, which may include suicidal characteristics. Depression is frequent in patients with temporal lobe epilepsy and surprisingly, may continue even after successful surgery and freedom from seizures. Although acute depression following surgery is often temporary, its severity should be monitored by a psychiatrist. The same holds true for delusions and hallucinations, which can be part of the seizure disorder. Some of this condition is directly related to seizures and is cured with surgery, while some delusions and hallucinations occur more generally, despite the seizures. In either case, a psychiatrist may be necessary for interventions. In addition, a psychiatrist may be required to treat resulting disorders related to stress, panic, and phobias, which are also often identified within the epilepsy population.
Electrocorticography (ECoG), or intracranial EEG (iEEG), is the direct recording of epileptic activity during craniotomy. It uses electrodes placed directly on the surface of the brain to gather a record of electrical activity from the cerebral cortex.
There are two types of intracranial EEG. One is done during surgery to remove a lesion causing epilepsy and electrodes are removed at the end of the surgery. In the other, extraoperative monitoring, the electrodes are placed in or over the brain where the epileptic focus is thought to reside and are left in at the conclusion of the surgery. Recordings are then made from the intracranial electrodes both while the patient is awake and alert, and during seizures. Usually in about five days, the necessary data is collected and the electrodes are removed.