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When the onset of a seizure cannot be determined by surface recordings (phase I), then invasive recordings or a phase II approach is recommended. As in phase I, the primary objective is to identify the region most responsible for generating a patient's habitual or typical seizure. Usually resection of this area is sufficient to stop the seizures even though interictal spikes and auras may persist. Phase II recordings are also necessary to demonstrate that this can be accomplished without producing neurological deficits. For this reason, it is important to understand the nature of the "epileptogenic region" or area that is responsible for the seizures.
The Nature of the Epileptogenic Region
An epileptic focus (where the seizure starts) is an electrophysiological concept defined on the basis of abnormal waveforms such as "spikes" seen between seizures. The epileptogenic zone can be best identified from a variety of tests of both function (EEG, MEG and PET scans) and structure (MRI and CT scans). This zone includes the precise site of seizure onset, but may also include all tissue necessary for seizure generation. There is some controversy among experts with respect to the nature of the epileptogenic zone, the extent of surgical resection required to treat seizures, and the reliability of diagnostic testing procedures. For these reasons, the approach to a phase II pre-surgical evaluation is carried out methodically, systematically and thoroughly at Cedars-Sinai by a multi-disciplinary team of experts.
Depth or Grid-Implanted Electrodes
Surgery is required to implant grid (a matrix of contacts imbedded in a thin sheet of silicone) or depth electrodes (thin cylindrical tubes with eight contact points spaced along the length of the tube). Grid electrodes are placed on the brain surfaces and require opening of the skull (craniotomy). Depth electrodes are placed using small holes through the skull, and penetrate the surface of the brain to allow recording from structures below the brain surface.
Electrocortocography (ECOG) is a technique comparable to having a routine electroencephalogram (EEG), although the recordings are taken from electrodes that are placed directly on the exposed cortex (surface of the brain). This type of recording is performed after the grids or depth electrodes have been placed.
Patients spend the first night in the Intensive Care Unit and if grid electrodes are placed may stay up to a few days in the Neuro-critical Care Unit. Once stable, patients are transferred to the Epilepsy Monitoring Unit (EMU) and an X-ray is taken to confirm the placement of the grid electrode matrix as shown in the image. Patients with depth electrodes may even go to the EMU a few hours after the procedure, while patients with grid electrode studies may take longer. Neocortical epilepsy may not be as clear cut as mesial or temporal lobe epilepsy and usually covers a wider area of the brain.
After the recordings are completed, the grid or depth electrodes are removed. At this time, a resection of the seizure zone is usually performed if it has been localized with the electrodes. In contrast, once depth electrodes are removed, most patients wait 4-6 weeks before returning to the hospital for the removal of the seizure zone.
The Epilepsy Monitoring Unit at Cedars-Sinai has state-of-the-art equipment capable of real-time networking of amplifiers with 128 multi-channel recordings, which also employ digital video, spike and seizure detection software, and nurse alarm systems to announce events to increase safety and to improve testing. Many studies have demonstrated the potential importance of very fast brainwave components for determining where seizures originate. Our new, high-end technology offers high sampling rates and storage systems that can store large quantities of digital video at high sampling rates. The patient's data is of the utmost importance and is safeguarded following strict HIPAA regulations.
Our team is prepared to discuss the risks and benefits of phase II procedures with you and is available during your hospital stay to answer any questions. We emphasize patient education and strive to ensure patients understand the risks and benefits so that they can make informed decisions.
Risks involved with Phase II monitoring include infection, hemorrhage, stroke and increased intracranial pressure (ICP). Please be sure to discuss these risks with your physician before undergoing phase II monitoring.
For additional information, a second opinion, to make an appointment or refer a patient, please call 310-423-7420 or email us at firstname.lastname@example.org.