Depending on the information the doctor is trying to obtain, additional electrodes may be needed. These include:
Like many epilepsy centers, NYU Comprehensive Epilepsy Center routinely records the EEG with sphenoidal electrodes. The electrodes are inserted into the cheeks with a needle to record brain electrical activity from regions deep within the temporal and frontal lobes. Many doctors apply a topical anesthetic to the skin before inserting the electrodes. The needle is immediately withdrawn after insertion, leaving in place a thin wire that is bare at the tip. The patient feels some discomfort during the insertion and for several hours afterward, particularly when yawning or chewing.
The risks of using sphenoidal electrodes are rare and almost always minor. A small amount of bleeding may occur during the needle insertion, but it is rarely a problem. Other risks include the possibility of infection or of a tiny piece of the bare wire remaining in the cheek.
Subdural and depth electrodes are used to record electrical activity directly from the brain, and they are often used to map precisely the area from which seizures arise.
Whether subdural or depth electrodes must be used depends on the findings from the noninvasive studies and another test called the intracarotid sodium amobarbital test, which we’ll describe later. For example, if the routine scalp-recorded EEG, video-EEG recording, neuropsychological testing, PET scan, and amobarbital test all point to the same area of the brain as the focus of the seizures, most epilepsy centers will proceed without using invasive electrodes. If the information is inconsistent or indefinite, however, subdural or depth electrodes, which are invasive, are often used. For example, the MRI may appear normal, neuropsychological tests and the PET scan may suggest an abnormality in the left temporal lobe, but the video-EEG may suggest that seizures begin in the right temporal area.
With the use of subdural electrodes, the brain can be stimulated electrically for mapping of brain areas involved in language, movement, and other important functions. Seizures can occur with the electrodes in place, and care must be used to protect the patient, and therefore the electrodes, during and after the seizures. In many centers, invasive electrodes are used in an intensive care unit or similar setting. The electrodes may be left in place for several days to weeks, depending on the specific case and how quickly seizures occur after they are placed.
Subdural electrodes consist of a series of metal electrodes embedded in plastic and arranged as a strip or a large grid. They cover a large area and record directly from the brain, without interference from the scalp and skull. An operation is required for placement of these electrodes. The dura mater is one of the layers of tissue covering the brain, and the word subdural means that the electrodes are placed on the brain underneath the dura mater, but they do not penetrate the brain. In some cases, several strips of electrodes can be inserted through a small hole drilled in the skull, called a “burr hole.” In other cases, a section of the skull is removed, the electrodes are put in place, and the skull is replaced. If the skull section is not immediately replaced, it is kept sterile and frozen, and the electrodes are covered with the dura mater, the scalp, and a surgical dressing. After the testing is completed, the piece of the skull that was removed is then replaced. There is a moderate amount of discomfort for several days after the subdural electrodes are placed on the brain. In general, the greater the number of electrodes that are used (especially the grids), the greater the headache. Medicine can be given for pain relief.
The mapping procedures performed with subdural electrodes involve stimulation of the brain with mild electrical currents to temporarily activate or shut down certain brain areas. For example, activating the left motor cortex controlling movement in the right thumb can cause a series of jerks in this finger, or stimulating language areas in the temporal or frontal lobes can cause a person who is counting to suddenly stop speaking. The mapping procedure is almost always painless. If pain occurs, it is momentary and caused by the electrical stimulation, which can be stopped immediately. The major risks of subdural electrodes are infection (which increases during prolonged use, especially after 6 to 8 days), bleeding, and brain swelling.
Depth electrodes are thin, wirelike plastic tubes with metal contact points spread out along their length. Unlike subdural and other invasive electrodes, depth electrodes are placed directly into the brain, but they do not require that a large opening be made in the skull, as is need to place a grid of subdural electrodes. Depth electrodes are inserted through burr holes drilled in the skull. The patient is usually awake while the electrodes are being placed, but may be sleeping.
The placement of depth electrodes can be painful, depending on the exact procedure that is used. In some centers, the electrodes are placed using a frame that attaches to the skull and allows a computer to assist in calculating the exact course of the electrodes in the brain. Attaching the frame can be painful. The pain associated with depth electrodes is usually mild or moderate, however, and lasts only hours, or occasionally, several days. Medication can reduce the discomfort.
Depth electrodes provide the best recordings of seizures arising in areas deep in the brain, but they also carry some additional risks, especially bleeding within the brain. They are less likely than subdural electrodes to cause infection or brain swelling.
The foramen ovale is an opening in the skull near the temporal lobe. Electrodes can be inserted into this opening to provide recordings of electrical activity of the lower and middle portions of the temporal lobe, an area from which seizures often arise. These electrodes are intermediate between sphenoidal and subdural or depth electrodes in the information they provide, their invasiveness, and their risk of complications.
Overall, foramen ovale electrodes are well tolerated, and in selected cases they can provide important information about the origin of the seizures. One of the problems with them is that they record information from a very limited area of the brain. Therefore, the actual area from which the seizures arise may be missed.
Nasopharyngeal electrodes are used occasionally to record deep brain electrical activity. These electrodes are plastic tubes with a wire inside, ending as a blunt metal tip. The electrodes are inserted through the nose (usually by the EEG technologist) and the metal tip is situated in the upper back part of the nose (the nasopharynx). There may be some discomfort while the electrodes are inserted and less discomfort while they are left in place for approximately 20 to 30 minutes during the study. Nasopharyngeal electrodes have been used less often during the past few decades, because regular electrodes placed in front of and slightly above the ears can often provide the same information with no discomfort to the patient.