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Localizing Brain Pathology
Neurologists use a range of tests to diagnose seizures, including:
- MRI/CT Scan
- Continuous Video-EEG, Long-Term Monitoring (LTM)
- ICTAL SPECT Study
- FDG PET Scan
- Neuropsychological Evaluation
- Intracarotid Sodium Amybarbital Test (WADA) for Memory and Speech Lateralization
- ICTAL and Interictal MEG Study
Not only is the EEG a relatively inexpensive and non-invasive procedure, it also is extremely sensitive to detecting epilepsy and related seizure disorders.
Neurosurgeon Walter Penfield, MD is credited with beginning the exciting field of epilepsy surgery at the Montreal Neurological Institute in 1954. He said that "in the study of any case of focal epilepsy the X-ray often provides the clinician with indispensable clues to localization." Since then, the field of imaging has evolved and given way to Magnetic Resonance Imaging (MRI). An MRI is a scan that creates images of the brain to help detect abnormalities. To the right is an MRI of the brain (a coronal oblique along the long axis of the hippocampus). Unlike CT or CAT scans, MRI scans use no X-rays.
There are a variety of MRI techniques that are used to diagnose epilepsy. They include, but are not limited to the MRI Epilepsy Protocol, which uses specialized techniques to detect lesions.
Below is an MRI showing right hippocampal atrophy, often found in patients with seizures arising from the temporal lobe.
Single Photon Emission Computerized Tomography (SPECT) is a nuclear medicine study using a radioactive tracer to map blood flow to the brain. Ictal means that the scan is performed just after a seizure. The study compares blood flow between seizures (ictal) and the blood flow when seizures are not occurring (inter-ictal). The test is performed during a hospital stay, since it is necessary to perform the scan during a seizure. When a seizure occurs, there is usually increased blood flow to the region where the seizure starts. The patient is injected with the tracer (neuro-lyte) as close as possible to when the seizure starts, usually within 30 seconds. The SPECT scan is then performed within 2-4 hours of the injection. The study will usually show the area of the brain with increased activity.
Usually a repeat study without a seizure (inter-ictal) is performed as a baseline study. Such a study is usually performed at least 24 hours after the seizure. The ictal study is then compared to the baseline study. This technique is called co-registration of the subtracted ictal SPECT from the inter-ictal SPECT. Seizure onset or the focal point where the seizures arise can be determined for 85 percent to 90 percent of patients who are injected early in their seizure. The purpose of the subtraction method is that it provides more sensitivity and specificity than side-by-side comparison of ictal and inter-ictal images. To the right is an image of an area of hyper-perfusion, which identifies the epileptogenic zone where seizures originate.
The positron emission tomography or PET scan is performed in the Nuclear Medicine Program. The study uses a very low an safe dose of an radioactive isotope to map the use of glucose in the brain. Areas of the brain associated with seizures sometimes use less glucose, as shown in the image below. Such areas are called hypo-metabolic areas.
Patients cannot eat for 6 hours before the study. An EEG is recorded throughout the study to assess the presence or absence of an electrographic seizure (seizure activity captured by the EEG).
A PET scan is used to identify unilateral temporal hypo metabolism in about 95 percent of temporal lobe epilepsy (TLE) cases. The test also identifies focal hypo-metabolic regions in half of extra-temporal cases. It provides supportive data when the MRI shows atrophy (shrinkage) of the hippocampus on one side.
A neuropsychological evaluation is used to assess cognitive function in patients who may be surgical candidates. Patients may have weaknesses in certain tasks that are typical for the right temporal lobe, left temporal lobe or frontal lobe. These weaknesses may help point out the areas affected by seizures and give clues as to where seizures are originating.
An Intracarotid Sodium Amybarbital, often referred to as a Wada test in honor of the neurologist Jun Wada, MD who first developed it, is an invasive procedure performed in the interventional radiology suite. It is similar to having an angiogram. The procedure is performed on an outpatient basis. After the procedure, the patient is sent to a recovery room for a limited time (under six hours) and then once the patient is stable, is discharged.
During the procedure, the EEG is recorded to assess the effects of sodium amytal (a sedative), which causes slowing of brainwave activity only over part of the brain. The sodium amytal is administered through a catheter in the femoral artery up to the carotid artery and then into one hemisphere of the brain to induce that side of the brain to sleep. The side involved in language and memory is usually put to sleep.
The test is especially important in patients who may require a temporal lobe resection. These resections often include the mesial (deep) temporal areas and this test helps assess the risk of memory impairment after surgery. Patients who pass this test usually demonstrate that the other side of their brain can sustain memory even when the side a surgeon proposes to take out is "asleep" for a few minutes. It is also used to determine which side of the brain is important for language function, as this information may limit or increase the amount of tissue that may be removed.
A magneto-encephalogram (MEG) is used to measure the magnetic field activity of the brain between seizures (inter-ictal), as well as in some cases during seizures (ictal). This is useful in the surgical treatment of epilepsy and helps to plan how most effectively to perform the surgery or electrode placement in neocortical epilepsies.
The brain and heart normally generate magnetic activity, but abnormal magnetic epileptic activity is generated by disturbed neurons. The source of the abnormal MEG is determined by using computer models of epilepsy.
For a MEG test, the patient is placed in a magnetic-shielded room and magnetic sensors are placed above their head. EEG activity is measured and the source of abnormal MEG is determined. The patient's head shape is mapped in a digital format with 3D coordinates. The coordinates or sources are co-registered on an MRI of the patient's brain. The abnormal epileptiform magnetic activity of the brain is displayed on the MRI.
This procedure is not only costly but require expensive equipment and the need for a magnetic shielded room. Not many centers have access to this type of testing. Our Medical Director of Epilepsy, Dawn Eliashiv, MD is an expert in this type of testing and works closely with the Scripps Clinic to conduct MEG testing.
Many studies support the usefulness of MEG. Some studies show that for 11 out of 12 patients with normal or ambiguous MRI, MEG was able to determine the origin of spike sources. Sixteen of 22 patients had inter-ictal epileptiform abnormalities. In five of six patients with a focal abnormality on the MRI and the MEG, the spike sources co-localized as in the case at right. The image is of an inter-ictal MEG (between seizures). The yellow triangles are the inter-ictal source localizations; the red is the area of ictal onset, where the seizure begins.
MEG is most useful in neo-cortical epilepsy (seizures that involve widespread areas of the surface of the temporal lobe). MEG recordings have an even superior localizing value when performed on patients during a seizure (ictal) compared to between seizures (inter-ictal). Even in patients who do not have a seizure during the recording period, useful information is obtained. MEG is especially effective in determining where to put electrodes in patients who need invasive recordings.
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