Epilepsy does not represent a single disease entity, but a spectrum of disorders. It includes many different conditions with a wide range of underlying causes, all sharing the common characteristic of recurrent, unprovoked seizures. Seizure is the actual event when disturbances in electrical activity of the brain occur and it manifests by a variety of symptoms depending upon the type of seizure and which part of the brain is involved in the seizure. In many patients with epilepsy, seizures are only one of the problems and they suffer from other neurological, physical or psychological symptoms.
The evaluation of patients with epilepsy is complex and best achieved through a systematic approach involving three phases of evaluation. Selective neuroimaging and other advanced diagnostic techniques are used during different phases of evaluation.
During initial evaluation of a patient with epilepsy, a detailed history and physical examination is crucial in establishing the diagnosis and gaining insight into understanding the underlying cause. Even under the best of circumstances, a clinic visit and detailed interview may be insufficient to make an accurate diagnosis. As a result, additional investigations should be performed after an initial interview. This is commonly referred to as Phase I evaluation.
Modern neuroimaging techniques such as CT and MRI scans have revolutionized our understanding of the brain. MRI has replaced CT scanning as the preferred mode of imaging for patients with epilepsy because it provides superior anatomical details of the brain. The other test often used during Phase I evaluation is an electroencephalogram or EEG. An EEG is an amplified recording of electrical activity generated by brain cells. The EEG is obtained by applying a set of metal electrodes over the scalp.
The main goals of epilepsy treatment are control of seizures with minimal or no treatment-related side effects. In many patients, it is possible to achieve these objectives with seizure medications. In cases, where two or more seizure medications fail to stop seizures completely or seizures are not brought under control after a year of treatment, a referral to a regional epilepsy center for a more comprehensive assessment is warranted. The next step of a more detailed investigation is commonly referred to as a Phase II evaluation.
The Phase II evaluation is carried out by a neurologist with special fellowship training in epilepsy. This specialist is also known as an epileptologist. The first goal of this evaluation is to establish a diagnosis of epilepsy or provide an alternative explanation for events that may mimic seizures. The other aim is to make a diagnosis of the specific epilepsy syndrome, when possible, and to identify the seizure onset zone in cases of focal epilepsy. To achieve these goals, a complete seizure evaluation is performed and typically includes a more detailed epilepsy focused interview and exam as well as video-EEG or other long-term EEG monitoring in our Epilepsy Monitoring Unit (EMU) and a high-resolution MRI scan of the brain. It may require more specialized neuroimaging tests, such as positron emission tomography (PET) scanning, single photon emission computed tomography (SPECT) scanning, functional MRI (fMRI), MR spectroscopy (MRS), and formal neuropsychological testing. In most instances, the treatment plan is altered to provide best possible interventions during and following Phase II evaluation as more information is gathered. In case seizure medications fail to control seizures and it is determined that a particular type of surgical intervention may provide benefit or elimination of seizures, a surgical procedure may be recommended at this stage.
In case of localization-related epilepsy where seizures are originating from a single focus in the brain, but the precise location cannot be determined during the Phase II evaluation, a more advanced evaluation with monitoring of EEG using intracranially implanted electrodes may be necessary. This is at times known as Phase III evaluation or invasive EEG monitoring.
Video-EEG monitoring with intracranial electrodes (electrodes placed on the surface of the brain or deep inside the brain) are necessary when scalp EEG recordings of seizures do not localize the epileptic seizure focus precisely. This phase of evaluation is needed in a minority of patients; not every patient undergoing epilepsy surgery requires it. When EEG recording with intracranial electrodes is desired, electrodes are placed where seizures are suspected to begin. The number, type and placement of the electrodes are decided at the multidisciplinary team meeting based on all the previously gathered information regarding the patient’s seizures — including detailed clinical interviews, watching video of seizures along with simultaneous EEG recordings, MRI scans, PET scans, other advanced neuroimaging studies and neuropsychological studies.
Several different types of intracranial electrodes may be used.
- Depth Electrodes: These are very fine electrodes (wires) that are placed inside the brain to record electrical activity produced by the brain.
- Subdural Strips: These are strips of four to eight contact points from which electrical activity from the brain is recorded. They are placed on the surface of the brain.
- Subdural Grids: Similar to subdural strip electrodes, these are an array of up to 64 contact points from which electrical activity from the brain surface is recorded.
In most instances, a combination of depth, subdural strip and grid electrodes are used in various configurations to achieve the best possible coverage. With subdural grid/strip electrodes placed, patients often undergo cortical mapping to determine what part of the brain has control of important functions, such as motor control and speech. This is performed at the bedside in the Epilepsy Monitoring Unit (EMU) by stimulating the intracranial electrodes on the surface of the brain with brief electrical current and observing the effects of the stimulation. For example, when the electrodes over the motor cortex for fingers are stimulated, the patient’s fingers may twitch. If the electrodes over the area of speech control are stimulated, it may have an effect on the patient’s speech. This mapping procedure may last one to two hours and will help to localize important brain functions — enabling neurosurgeons to avoid these areas during epilepsy surgery.