The main goal in performing cortical excisions is to remove as much of the epileptogenic tissue as possible without disturbing normal brain function. To achieve this goal, the surgeon must greatly restrict the total amount of tissue removed. Various techniques may be used to tailor resection. When performing temporal lobectomies, some neurosurgeons may choose to remove mainly medial temporal structures, such as the lower amygdala and hippocampus, while leaving the lateral temporal lobe intact. This procedure can be accomplished either by making a transcortical incision down into the temporal horn of the lateral ventricle or by splitting the sylvian fissure.
The basic technique for excision of epileptogenic tissue is to coagulate the surface or pial vessels, sharply incise the pial membrane covering the cortex, and use fine suction or the Cavitron (CUSA) to cut through the tissue and define a resection line. The surgeon accomplishes this procedure by suctioning the cortex and white matter along the inner surface of the pial lining of the gyrus, after making a sharp incision in the surface pia over a gyrus.
In the frontal and temporal regions, it is important to keep the “pial barrier” as a protection over the main middle cerebral artery branches that course over the surface of the insula. Injury to or spasm of these vessels could lead to infarction, or stroke, of the frontoparietal cortex. The cranial nerves, the brain stem itself, and major arteries of the circle of Willis are likewise protected from injury by leaving the medial pia as a protective structure and boundary.
Most data suggest that leaving medial temporal structures while removing the lateral temporal lobe does not have as good an outcome with respect to seizure control as resections including the bulk of the hippocampus, even when specific lesions appear to involve only lateral temporal areas.
Some neurosurgeons carry out the procedures with the patient under sedation but able to be easily awakened, using information gained from cortical mapping and intraoperative EEG recordings (electrocorticography, or EcoG) to decide precisely which areas to remove. Recordings are sometimes made after initial removal of cortical tissue. A postexcision EcoG recording that finds active epileptiform activity in areas of the brain not operated on is less favorable than one free from epileptic discharges.
Other neurosurgeons carry out a standard lobectomy, such as an anteromedial temporal lobectomy, with the patient under general anesthesia. They excise a defined amount of tissue (based on preoperative evaluation and imaging studies) without using intraoperative recordings.
In both frontal and temporal brain tissue removals, the preservation of speech function often restricts the extent of removal, especially from the speech-dominant hemisphere, which is typically the left hemisphere. The area of speech function nearly always lies behind the rolandic fissure. In general, a safe margin of resection in temporal tissue removal is half a gyrus behind the rolandic fissure. The exact dimensions vary with each patient, because many epilepsy patients have fairly small anterior temporal lobes on the affected side. The rolandic fissure usually lies about 5 centimeters from the anterior temporal tip, so that approximately 2 inches, or 5 centimeters, of tissue is removed in a standard resection.
Speech areas may be mapped out more fully with cortical stimulation and functional MRI. The frontal speech area, or Broca’s area, is somewhat variable in size and shape but generally is a triangular-shaped gyrus along the base of the skull several gyri anterior to the rolandic fissure. This area, too, can often be mapped out by functional MRI or stimulation of the cortex during surgery.
Functional mapping may also be carried out extra-operatively, by utilizing a stimulation technique once a grid electrode has been implanted surgically. Language, motor, sensory, and visual functions may be mapped in this fashion, generating a functional “map” that may be co-registered with the seizure map obtained with the grid. This information is critical in preoperative decision making, and helps determine the extent of resection that is safe.
The supplementary motor speech area just anterior to the rolandic fissure in the most medial portion of the frontal lobe along the midline can be removed, but this procedure often causes temporary speech impairment. This supplementary motor speech area is also somewhat variable in its location. Frontal resections, then, must skirt around Broca’s area and the supplementary motor speech area if at all possible, sparing connecting white matter pathways to these regions as well as the surface cortex of these areas.
It is important to spare the uppermost part of the amygdala in the medial portion of a temporal removal, to avoid injury to the optic tract on that side. In that way, a contralateral complete homonymous field cut may be avoided. Generally, only the lower two-thirds of the amygdala is removed, although some surgeons have reported removing as much as four-fifths of the amygdala with careful attention to microdissection.
Another type of visual dysfunction may arise if removal of superficial white matter of the temporal lobe extends back as far as 7 centimeters. This extent of excision causes an interruption in the optic radiations of the temporal lobe, which sweep around the temporal horn en route to the lower occiput below the calcarine fissure. An interruption causes an upper quadrant visual field cut on the side opposite the surgical site. The farther back the extent of the removal, the greater the degree of quadrant field cut into the more central portion of the visual field. Causing a slight visual field quadrant defect is often acceptable in removing a lesion or epileptogenic region that is posteriorly located in the lateral temporal region. When only a more medial excision is required, however, microsurgical techniques allow the surgeon to perform a very small anterior temporal lobectomy in which up to 3 centimeters of the hippocampus is removed medially, along with the lower portion of the amygdala, but not extensive lateral white matter. This procedure prevents visual field defects and can be accomplished by an angled anterior-to-posterior approach.
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