What are invasive EEG recordings and when are they needed?
Invasive EEG recordings are those recordings that are made with electrodes that have been surgically implanted on the surface or within the depth of the brain. Occasionally, the patient’s epilepsy syndrome may require the use of invasive EEG recordings before epilepsy surgery can be considered.
- Subdural EEG electrodes are those electrodes which sit over the surface of the brain.
- Depth EEG electrodes are those electrodes which are placed within the substance of the brain.
These recordings are done to better localize the region of the brain from which the epilepsy is arising from. The placement of these electrodes are often confirmed with co-registration on an MRI san image.
In addition to the standard video and EEG monitoring that is done in the Invasive evaluation, a second phase of the evaluation entails the use of various “functional localization techniques.” The two commonly used functional localization techniques that are done during the latter half of the invasive evaluation to epilepsy surgery are cortical stimulation and mapping the central sulcus with cortical SSEP mapping.
Both of these functional localization techniques may be done separately during the operation as a part of the intraoperative neurophysiological monitoring.
Subdural EEG Electrodes
Subdural EEG electrodes are those electrodes which sit over the surface of the brain. The placement of these electrodes are often confirmed with co-registration on an MRI san image.
Depth EEG Electrodes
Depth EEG electrodes are those electrodes which are placed within the substance of the brain. The placement of these electrodes is often confirmed with co-registration on an MRI scan image.
The figure to the left shows an MRI image of the brain showing the position of electrodes which were placed into the substance of the brain into regions called the temporal lobes and specifically into a structure called the amygdala and hippocampus.
These structures are frequently involved in the generation of seizures experienced in patients with temporal lobe epilepsy. These electrodes are called “depth electrodes.”
Cortical stimulation entails delivering a short duration of electrical current to the brain via the invasive electrodes. This is done to determine what function that region of the brain subserves. Patients are often asked to perform a task during cortical stimulation, such as speaking, to determine if stimulation impairs the performance of that said task.
Functions that can be tested using this technique include language, vision, movement, and sensation.
This image shows a subdural electrode recording of waveforms emanating from the brain. These waveforms were derived from the brain following the stimulation of a nerve located in the hand called the “median nerve.” These responses give important information regarding the regions of the brain controlling hand function.
Cortical somatosensory evoked potentials are used to locate an important structure in the brain, called the central sulcus.
The brain’s surface is formed by ridges and valleys, the valleys are called ‘sulci’ or ‘sulcus.’ One important such valley lies between the regions of the brain which control movement and sensation.
By stimulating a nerve on the arm, face or leg and recording brain waves in response to the stimulation with the aid of a computer, location of the sulcus can be performed.
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