Implanted EEG electrodes are often necessary to pinpoint the origin of seizure activity. They are particularly helpful for more precisely localizing seizure activity that has been identified with scalp EEG recordings. For example, the scalp recordings may determine that seizures arise from the right hemisphere. Implanted EEG electrodes can then reveal whether the epileptogenic activity arises specifically from the frontal lobe, rather than the temporal, parietal, or occipital lobes. These invasive electrodes allow EEG recording directly from the surface of the brain or from deeper cortical structures.
Implanted EEG electrodes also can be used to stimulate the brain and map cortical and subcortical neurologic functions, such as motor or language function, in preparation for epilepsy surgery. This information is then used in conjunction with the seizure data to determine the risk-benefit profile of the surgery. In some cases, the stimulation of electrodes may trigger auras or seizures during this functional mapping session.
There are two main categories of implanted electrodes: subdural electrodes and stereotaxic depth electrodes.
Subdural electrodes consist of a series of discs mounted in thin plastic and designed to lie on the surface of the cortex. These discs are often configured as linear strips consisting of a selected number of electrode contact points. These strips are inserted into the intracranial cavity through small burr hole openings and are placed under the temporal or frontal lobes or along the medial surfaces of the hemispheres.
Another arrangement of subdural electrodes is a square or rectangular grid, intended to cover large surface areas and placed over the cortical convexity. (See photos.) Many centers use grid electrodes in combination with strip electrodes. They are generally used when there is a question about the specific site or lobe of origin of the seizures. Grid electrodes were once placed on both sides of the brain in some instances, but the need for bilateral craniotomies was occasionally associated with unacceptable morbidity. Now, if bilateral sampling is needed, bilateral burr holes are typically used for implantation of strip electrodes or depth electrodes.
Stereotaxic depth electrodes
Stereotaxic depth electrodes are fine, flexible plastic electrodes attached to wires that carry currents from deep and superficial brain structures. These currents are recorded through contact points mounted in the walls of the electrodes. Fine wires extending through the bores of the plastic electrodes are inserted with stylets placed in the bores. Stereotaxic depth electrodes are particularly helpful in determining the side of origin in temporal lobe epilepsy or, more commonly, in frontal lobe epilepsy in which the spread of abnormal discharges from one frontal lobe to the other is so rapid that the site or side of origin is difficult to ascertain. Targets for the deepest contact points commonly include:
- the cingulate gyrus
- the subfrontal region
- the amygdala
- several sites within the hippocampus
Depth electrodes may be placed from the convexity in angled trajectories from the surface of the skull, or they may be placed along the side of the head in trajectories that are all parallel if they are truly horizontal or perpendicular to the side of the skull. At some centers, a local anesthetic plus a sedative is administered when the electrodes are placed. Other centers use a general anesthetic.
Generally, a stereotactic headframe is affixed to the patient's skull with pins, and then CT or MRI scanning of the head is performed. Target sites for placing the electrodes are selected using the stereotactic imaging studies. This technique is extremely precise in localization. Some centers use arteriography to visualize the blood vessels of the brain in conjunction with the stereotactic headframe, to avoid injury to critical vascular structures. More recently, computer-assisted "frameless stereotaxy" has been used to place electrodes, avoiding the need for stereotactic headframe placement.
Once the electrodes are in place, they may be left there for a week or two, with the wires tunneled through the skin and connected to the EEG. During that time, continuous telemetry is performed to record the onset of seizure activity. Typically, antiepileptic medications are tapered to facilitate the capture of seizure activity.
Risks of implanted electrodes
Infection is considered the major risk of implanted EEG electrodes. Recent series report an infection rate of about 2-3%. Meticulous surgical technique, to prevent cerebrospinal fluid leakage, keeps the risk of infection low.
Another risk of electrode placement is hemorrhage. Significant intracerebral hemorrhages have been reported, but the incidence is 1% or less. Direct brain injury due to the passing of depth electrodes has not been demonstrated because the electrodes are so fine that they normally dissect through neural tissue without direct brain injury.
Adapted from: Blume H. The surgical treatment of epilepsy. In: Schachter SC, Schomer DL, eds. The comprehensive evaluation and treatment of epilepsy. San Diego, CA: Academic Press; 1997. p. 197-206. With permission from Elsevier (www.elsevier.com).