This procedure describes subdural and depth electrode implantation for long-term invasive recording of electroencephalography (EEG).
Indications
- •
Invasive monitoring of EEG in patients with medically refractory, focal-onset epilepsy can provide valuable information regarding an epileptogenic zone that is not clearly correlated with seizure symptoms and noninvasive studies, including scalp EEG, magnetic resonance imaging (MRI) of the brain, nuclear medicine studies, and magnetoencephalography (MEG).
- •
Invasive monitoring is indicated for the more precise identification of the epileptogenic zone in cases of “nonlesional” epilepsy, dual pathology, or discordant noninvasive data.
- •
Invasive electrodes can also provide important functional information about the underlying cortex and its relationship to the surrounding epileptogenic zone.
- •
Depth electrode recordings can provide valuable information regarding electrical activity in regions that are not easily or safely covered by subdural grid electrodes, including amygdala, various portions of the hippocampus, cingulate gyrus and mesial frontoparietal regions, and insula.
- •
Depth electrode recordings are indicated for the evaluation of temporal lobe epilepsy to identify a unilateral ictal onset within a mesial temporal lobe region.
Contraindications
- •
Patients with extensive dural scarring, prior cranial infections, increased intracranial pressure, or other space-occupying lesions generally are not considered for implantation of subdural electrodes.
- •
Patients with significant medical or psychiatric comorbidities that would prevent them from safely undergoing surgery are ineligible for invasive monitoring.
- •
Invasive recordings are best avoided in young patients, less mature patients, and patients with violent seizure activity who are at risk for cranial injury during the monitoring period.
Planning and positioning
- •
All patients who are offered subdural grid electrode (SDE) implantation for monitoring have previously undergone the standard preoperative evaluation, which includes not only imaging studies but also neuropsychiatric testing. The decision regarding invasive monitoring is made during a multidisciplinary meeting including neurologists, neurosurgeons, neuroradiologists, and behavioral health specialists.
- •
Areas of coverage are determined based on preoperative noninvasive studies and seizure symptoms. Preoperative noninvasive studies include video scalp EEG monitoring, subtraction difference single photon emission computed tomography (SPECT) blood flow studies, interictal positron emission tomography (PET) using 18-FDG radionuclide, and MEG. All patients receive neuropsychologic testing, and some receive functional MRI or amobarbital sodium (Amytal Sodium) intracarotid testing for identification of language and memory function.
- •
Incision and craniotomy are individualized to provide the desired coverage according to the preoperative hypothesis. The exposure should allow for placement of electrodes in addition to access to the anticipated area of resection.
- •
Positioning of the patient should allow for stereotactic guidance in the event that depth electrodes are to be placed during the same operation.
Figure 38-1::
All patients have the hair fully clipped, with care taken not to disturb the fiducial markers for stereotactic guidance. Pinning of the patient’s head within the head holder clamp is done such that a large incision and craniotomy can be turned; this usually involves fixing a single pin in the forehead, skewed to the contralateral side, and two pins in the occiput.
Figure 38-2::
The patient is placed in the supine position, with a shoulder roll positioned under the ipsilateral shoulder and the head turned 30 to 45 degrees toward the contralateral side, depending on the amount of parietal and occipital coverage needed. If mesial coverage is desired, the vertex is tipped superiorly. The frame is locked into place.
Figure 38-3:
When the patient is secured, the reference arm for the stereotactic guidance system is affixed to the head holder. Registration of the patient is completed and checked for accuracy. Planning of targets is performed to allow placement of depth electrodes and subdural electrodes during the same procedure.
Procedure
