What Type of Testing Is Used During Surgery?
Intraoperative Electrocorticography
This localizes precisely the region of seizure focus within the brain. Electrodes either on the surface of the brain or in its depths are used to pick up and record EEG. After resection of the focus is completed, a second recording is done to ensure adequacy. These tests can be done on either an awake or anesthetized child.
Intraoperative Somatosensory Evoked Potentials
These localize the primary areas of the brain responsible for sensing touch (the postcentral gyrus) and thereby the adjacent region responsible for movement of the body (the precentral gyrus). This test may also be done on an awake or anesthetized child. It is useful not only in identifying motor cortex, but also in establishing landmarks to be used for avoiding injury to cortex important in language processing.
Intraoperative Cortical Stimulation for Brain Mapping
This is the type of monitoring typically associated with epilepsy surgery. It may be either gross stimulation of the motor cortex to confirm its identity after intraoperative somatosensory recording, or stimulation of the cortex to delineate regions important in higher cortical function (e.g., speech centers). While stimulation of the motor cortex for purposes of its identification can be done on an anesthetized child, more sophisticated mapping requires an awake, cooperative patient who can communicate with the operative team. For that reason, mapping of the functionally important regions of the brain in children is frequently done outside of the operating room using implanted sheets of electrodes lying on the surface of the brain.
Implantation of Electrodes for Localization of Seizure Focus and Brain Mapping
Children frequently require electrode implantation to both precisely localize the focus triggering the seizures and to map regions of the brain important to movement of the body and to speech. Information gained from previous, noninvasive video-EEG monitoring is used to determine what surfaces of the brain must be covered with the electrode sheets. Rarely, a preliminary surgery is required to better define which regions of the brain are responsible for the seizures. In these cases, strips of electrodes are fed through small holes in the skull to accomplish broad coverage over the brain's surfaces. Coverage to define the focus of the seizure is broad so that all borders of the focus can be delineated (typical electrode coverage). Once the site of origin of the seizures has been determined, the electrode sheet is used to map brain function. A surgical plan is then devised using both the map of brain function and the map of site of origin of seizure activity.
What will be done during surgery?
Temporal Lobectomy
The anterior temporal lobe is the most common seizure focus site in patients with partial seizures. Thus temporal lobectomy is the most widely used surgical procedure for epilepsy, and has the greatest rate of success. Approximately 60 percent of patients become seizure-free, and 85 percent enjoy a marked reduction of seizures. Behavioral disorders like aggressivity and hyperactivity are often improved.
Extratemporal Resections
Seizure foci may be located in the frontal, parietal or occipital lobes of the brain. Surgical therapy can also be effective for these patients, but the extent of resection is often limited because of important adjacent functional areas. The success rate with extratemporal resections is consequently lower than with temporal ones.
Multiple Subpial Transections
This refers to vertical cuts being made through a region of the brain with a seizure focus. Approximately 3/8" of brain must simultaneously discharge to generate a seizure. Nerve fibers that run horizontally through the brain's surface connect this critical mass of brain tissue responsible for initiating the seizure. Nerve fibers that connect the brain with the body run in a vertical direction. Thus, vertical cuts through the brain will transect the fibers important for generating a seizure without disrupting many of the nerve fibers important to the brain's function because these fibers run parallel to the direction of the cut. Cuts are made every 1/4" so that not enough brain tissue is interconnected to generate a seizure.
Corpus Callosotomy
This involves the dividing the two major fiber bundles (the corpus callosum and hippocampal commissure) that connect the right and left sides of the brain. The corpus callosum is usually sectioned in two stages: first the anterior two-thirds, and, if seizures persist, the remaining posterior one-third. Candidates for callosotomy are often those who were evaluated for focal resection but have an extensive area, or multifocal areas, of the brain causing the child's seizures. In general, corpus callosotomy is palliative and not curative. It reduces the number of severe seizures and prevents secondary generalization (spread of the seizure activity to involve both sides of the brain with a resultant loss of consciousness) of seizure activity. Thus it reduces the number of injurious falls that occur with the loss of consciousness.
Hemispherectomy
Subtotal hemispherectomy (either the formal removal of one of the brain's hemispheres or the functional equivalent) is now used to treat medically refractory seizures in patients with pre-existing diffuse hemispheric injury and dysfunction (e.g., infantile spasms and Sturge-Weber). Most of these children have frequent, severe seizures arising from a hemisphere with little or no function, so that the operation does not increase pre-operative deficits.