ECoG
ECoG Setup
EEG Setup
EMG
What to look for
- Frequency 60 Hz
- 2000 uv/div per trace
- 1000 ms/div per grid
- E1-Ref - E16-Ref
EEG Setup
- CP3-Fpz
- T7 - FPz
- CPZ - FPz
EMG
- Orbicularis Oris
What to look for
- epileptic discharges will resonate at a specific number and can let the surgeon know the channel and when it quiets, he will likely apply cool saline over the seizing cortex
- the surgeon will change values
- the surgeon will be looking for errors for the slide show
- the most distal part of the strip is contact 1
- PROCEDURE FOR PHASE REVERSAL AND DCMEP: Establish SEP, tcMEP, and EEG baselines before craniotomy. After dural opening, connect 4-contact strip electrode to amplifier inputs, have surgeon place electrode as close as possible to presumed hand area of central sulcus, oriented perpendicular to sulcus (roughly A/P). Check impedance to confirm all contacts are making good contact with the brain (placing a 1 X 5 cm pattie over the electrode may help). Record both scalp SEP and cortical MEP from strip (vs common reference) to stimulation of median or ulnar nerve contralateral to craniotomy. Note that the most distal contact of the strip electrode is #1, which may be the most anterior or most posterior depending on exact location of craniotomy in relation to central sulcus. Look for major deflection at same latency as scalp N20. If all 4 contacts show a negativity, the electrode is too posterior. If all 4 show positivity, the electrode is too anterior. (Note that the central sulcus is often not the one initially hypothesized). If no contacts show a response, ask surgeon to reposition electrode more medially or laterally until hand area is located. When location is ideal, a polarity reversal will be seen, with posterior contacts showing N20 negativity, and anterior contacts showing a positivity at a similar latency. Often the contact directly over the precental motor gyrus will show a positivity at a slightly longer (~2 ms) latency, due to activation of a second dipole. Once the central sulcus is identified, replug the strip electrode into the stimulator box (begin with 1-4), and ask surgeon to rotate electrode if possible so that all 4 contacts are over precentral motor cortex. Starting at 5 mA, deliver single pulse trains (biphasic, 300 ms pw, 5@2 ms ISI) and adjust intensity until consistent dcMEPs are obtained, monitoring EEG for any seizure activity or afterdischarges, treat w cold irrigation or anesthetic intervention if necessary. With ideal placement, dcMEP response may be seen in both upper and lower extremities with biphasic stimulation. Reorient electrode and/or change contacts as required to obtain stable dcMEP recordings. Have surgeon secure electrode as well as is possible while keeping lead out of the way. Note that suturing the electrode lead to the dura may result in a lost connection if the brain falls away from the dura with removal of tumor. Once everything is stable, continue monitoring SEP continuously, and trigger dcMEP responses frequently as long as no seizures are elicited. Often it is possible to set the stimulator to automatically deliver pulse trains at its slowest rate (0.5 Hz, or one per 2 seconds) provided that EEG remains unaffected. If response are lost, increase stimulus slightly and if no recovery, alert surgeon. Often the cause is movement of the strip electrode and the surgeon can reposition. If this does not work, suggest subcortical stimulation at the site of recent dissection. Use either ball tip probe or electrified Frazier suction, monopolar cathodal stimulation, pulse train parameters similar to dcMEP but start at 1 mA and ramp up until responses obtained, or no response obtained at ~7 mA. If surgeon wishes to use Frazier during resection, leave it continuously active at 7 mA, alert surgeon if responses obtained and determine threshold. According to several studies, distance to CST is roughly same as threshold in mA, so can continue resection until threshold is < 2 mA with good outcomes. When resection completed, surgeon will remove strip electrode to close dura, switch back to tcMEP and record final SEP and tcMEP responses after dural closure.
NUWER
Intraoperative electroencephalography (EEG) recorded directly from the human brain commonly called intraoperative electrocorticography (ECoG). The technical aspects of modern ECoG. The presence of low frequency oscillations from 0.5Hz down to ~0.01Hz as well as spontaneous direct current fluctuations in human EEG recordings are well established and detectable from both scalp and intracranial EEG recordings. Similarly, the high frequency end of the EEG spectrum ~40Hz up to 600 Hz is an active area of research and clinical interest. Nevertheless, the majority of huan EEG recordings from intracranial subdural and/or depth electrodes report a limited dynamic range (~0.5-70Hz).
The primary role of ECoG in most centers is currently as part of epilepsy surgery and stimulation mapping to localize eloquent cortex. The practice at many centers is for patients with intractable and poorly localized partial epilepsy to undergo long-term intracranial monitoring in an effort to localize the region of seizure onset. In these patients, the role of intraoperative ECoG recordings at the time of subdural and depth electrode placement. Often the placemen of the recording elecctrode is performed using stereotactic imaging techniques to guide the placement electrodes to the hypotesized region of epeleptogenic brain. The ECoG recording at the time of electrode implantation provides assurance that the electrodes are recording at the time of surgery and will often show marked epileptiform abnormalities. Occasionally, the subdural electrodes may be re positioned or additional electrodes added because of the presence of epileptiform activity at the margin of the recording electrodes.
It is important to realize the significant limitations generally associated with ECoG. The sulcated structure of human brain leaves ~2/3 of the cortical mantle inaccessible to subdural electrodes. Subdural electrodes are only in direct contact with the crowns of gyri, and primarily record local activity. Cortical generators that are deep within the sulci are difficult to access. In addition to being farther away from the subdural electrodes placed over the cortex, a generator within a corical sulcus produces a dipole and potential field that is likely oriented parallel to the subdural surface, thus yielding a much smaller contribution to the local potential field. In some cases, it is possible to record from the gray matter within a sulcus by careful placement of depth or strip electrodes.
ECoG Recording Methods
Grid and strip electrodes are placed through a craniotomy by direct visualization and stereotactic imaging based on the area of brain region of interest. Strip electrodes can be placed through cranial burr holes and slipped into the subdural space. However, given the strip is advanced through the burr hole without visual guidance, on occasion the postoperative CT scan may show that the strip is off target. Depth electrodes are most commonly used. The structures routinely recorded from include temporal, frontal, parietal and occiptial neocortex, interhemispheric, subfrontal, and inferior mesial temporal sites using subdural strips and grid electrodes.
It remains a challenge to record directly from cortex lying within sulci. In some situations, it is possible to place a depth electrode within gray matter lying within a sulcus. However, the amount of gray matter sampled is limited due to the thickness of the cortical mantle, generally 3-5mm.
Digital EEG acquisition system
To obtain the ECoG activity recorded from human brain within the commonly used clinical 0.5-70Hz bandwidth, a sampling rate of at least 250 Hz
Reference and ground electrodes
Intraoperative electroencephalography (EEG) recorded directly from the human brain commonly called intraoperative electrocorticography (ECoG). The technical aspects of modern ECoG. The presence of low frequency oscillations from 0.5Hz down to ~0.01Hz as well as spontaneous direct current fluctuations in human EEG recordings are well established and detectable from both scalp and intracranial EEG recordings. Similarly, the high frequency end of the EEG spectrum ~40Hz up to 600 Hz is an active area of research and clinical interest. Nevertheless, the majority of huan EEG recordings from intracranial subdural and/or depth electrodes report a limited dynamic range (~0.5-70Hz).
The primary role of ECoG in most centers is currently as part of epilepsy surgery and stimulation mapping to localize eloquent cortex. The practice at many centers is for patients with intractable and poorly localized partial epilepsy to undergo long-term intracranial monitoring in an effort to localize the region of seizure onset. In these patients, the role of intraoperative ECoG recordings at the time of subdural and depth electrode placement. Often the placemen of the recording elecctrode is performed using stereotactic imaging techniques to guide the placement electrodes to the hypotesized region of epeleptogenic brain. The ECoG recording at the time of electrode implantation provides assurance that the electrodes are recording at the time of surgery and will often show marked epileptiform abnormalities. Occasionally, the subdural electrodes may be re positioned or additional electrodes added because of the presence of epileptiform activity at the margin of the recording electrodes.
It is important to realize the significant limitations generally associated with ECoG. The sulcated structure of human brain leaves ~2/3 of the cortical mantle inaccessible to subdural electrodes. Subdural electrodes are only in direct contact with the crowns of gyri, and primarily record local activity. Cortical generators that are deep within the sulci are difficult to access. In addition to being farther away from the subdural electrodes placed over the cortex, a generator within a corical sulcus produces a dipole and potential field that is likely oriented parallel to the subdural surface, thus yielding a much smaller contribution to the local potential field. In some cases, it is possible to record from the gray matter within a sulcus by careful placement of depth or strip electrodes.
ECoG Recording Methods
Grid and strip electrodes are placed through a craniotomy by direct visualization and stereotactic imaging based on the area of brain region of interest. Strip electrodes can be placed through cranial burr holes and slipped into the subdural space. However, given the strip is advanced through the burr hole without visual guidance, on occasion the postoperative CT scan may show that the strip is off target. Depth electrodes are most commonly used. The structures routinely recorded from include temporal, frontal, parietal and occiptial neocortex, interhemispheric, subfrontal, and inferior mesial temporal sites using subdural strips and grid electrodes.
It remains a challenge to record directly from cortex lying within sulci. In some situations, it is possible to place a depth electrode within gray matter lying within a sulcus. However, the amount of gray matter sampled is limited due to the thickness of the cortical mantle, generally 3-5mm.
Digital EEG acquisition system
To obtain the ECoG activity recorded from human brain within the commonly used clinical 0.5-70Hz bandwidth, a sampling rate of at least 250 Hz
Reference and ground electrodes
- There are no truly inactive references for EEG and we routinely use cranial ground and reference electrode sites and less commonly have employed intracranial epidural reference electrodes. The choice of an extreacranial reference, such as mastoid, provides a quiet reference given that voltage of ECoG activity is 4-5 times the amplitude of scalp-recorded actiity. Alternative reference choices include average reference from the intracranial electrodes or epidural strip electrodes, although this is not routinely available in hardware form and so when used is constructed from the composite digital signas, which temselves were recorded against a singel fixed reference. our current practice is to use either mastoid ground and reference eletrodes or suture wires placed within scalp if the patient is to subsequently undergo chronic intracranial monitoring to record their habitual seizures. The scalp reference can introduce additional artifact such as movement and muscle.
- It is often useful to have additional ground and reference elevtrodes in place to allow for rapid troubleshooting, for example an additional 2-3 mastoid electrodes placed prior to ECoG can be very useful when troupleshooting a flwawed intraoperative ECoG recording, allowing for quick changes in reference and ground electrodes
- An awake craniotomy is performed for funcitonal cortical mapping for lesions close to eloquent cortex used for motor speech
- for localization of epileptic foci, during intraoperative electrocorticogram (ECoG)
- for epilepsy surgery
- excision of lesions adjacent to eloquent areas of the cortex in the dominant hemisphere
- steriotactic surgery
- **Anesthesia** safe airway and adequate ventilation, hemodynamic stability, normal intracranial pressure
- numerous technicques have evolved along with surgical indications
- MAC Monitored Anesthesia Care
- According to the ASA, MAC is a specific anesthetic protocol that includes careful monitoring and support of vital functions
- the anesthesiologist admnisters sedatives, analgesics, and hypnotics, adresses any clinical problems and provides the patient with psychological support during diagnostic and therapeutic procedures
- The ASA recommends that the provider of MAC must be repared and qualified to convert to general anesthesia, if necessry
- AAA Asleep Awake Asleep
- general anesthesia before and after brain mapping
- The most important for epilepsy surgery is limited interference with electrophysiological recordings
- INTRAOPERATIVE MONITORING
- Originally used for epileptic surgery, is now utilized for tumor resection
- more widely used within the last two decades
- Identifies: Regions of language representation (dominant cerebral hemisphere)
- Motor cortex (either hemisphere)
- Intraoperative mapping helps distinguish between eloquent cortex and tumor tissue, which facilitates: accessing the tumor from safest transcortical route, aggressive tumor resection while preserving functional tissue
- Indicated if the surgical site is near language associated cortical sites or "speech areas"
- Broca's (expressive speech) posterior/inferior/frontal lobe of the dominant hemisphere
- Wernicke's (comprehensive speech) posterior/temporal lobe of the dominant hemisphere
- direct electrical stimulation of the cortex during language tasks while observing for speech hesitation, arrest or dysnomia (difficulty with, or inability to retrieve the correct word from memory when needed)
- a grid of electrodes is placed on the brain surface to identify a phase reversal of SSEPs recorded over the posterior sensory cortex and precentral motor gyrus.
- Direct electrical stimulation of the cortex to elicit motor movement
- MEPs, more recently used to map and monitor subcortical motor pathways
- Obtaining the patient's confidence and agreement to cooperate during the surgery is key
- developing good rapport with the patient and their family is crucial
- Inform the patient of our expectations of them during the awake phas and what they can expect form us; committment, safety, comfort
- Aspects to be considered in preoperative evaluation; upper airways; Epilpsy; type and frequency of seizure
- A visit to the operating room before surgery is a good idea in order to familiarize the patient with the sounds and equipment in the operating room
- Conclusion: Awake craniotomy for tumor resection involving functional areas is a surgical approach that offers great advantages with respect to patient outcomes. This is a complex technique that requires great patient and equipment engagement. Personal experience, careful planning and attention are the basis for obtaining good results.
- Visual Object naming is the task most commonly used for mapping the dominant hemisphere, and the procedure described by ojemann is the most widely reported on in the contex of awake surgery.
- Prior to language mapping, the rolandic cortex is identified by simulation. The sensorimotor cortex is identified by stimulation. The sensorimotor cortex is identified by associated evoked motor and sensory responses to stimulation of the tongue, teeth, throat or face.
- Language mapping uses the highest current that does not produce an afterdischarge on electrocorticography.
- A current range of 1.5-10mA is delivered by a constant-current stimulator in 4 second trains at 60 Hz across 1-mm bipolar electrodes separated by a 5mm gap
- The sites selected for stimulation mapping are identified by small tags randomly applied to the cortex (10-20 sites per subject)
- Identify broca's area by asking patients to count and also by electromyography (three electrodes placed on the upper lip, lower lip and cheek).
- Step by Step procedure: language mapping: 1. language function is measured by showing the patient a set of blac and white slides with a line drawing of a common object such as a bell, a hand, a car or a ring, prompted by a carrier phrase, "That is a ..." or "This is a ..." The slides are displayed for 4 seconds each, and the patient is instructed to name each slide as it appears. A current is applied as the slide appears on screen and continued until the next slide appears or until the patient shows disturbances. The sequence proceeds with each slide associated with stimulation followed by at least one "stimulation-free" slide. At least three stimulations are delivered for each small selected cortical site. As regards the distance between tags, it has been shown that the effects of stimulation on nameing may be quite localized, changing within a centimeter, even on the same gyrus. Responses are recorded by manual scoring for immediate feedback to the surgeon and by audiotape for further analysis
- Operating room Organization
Protocol Settings
Ojemann Cortical Stimulator Settings
- Volume all the way up
- Pulse duration: 1
- Polarity: +
- mA Peak 1: 0.8 - 0.9
- Range: Full
- mA Peak 2: ~ 1.5
- Pulse Rate: 60
- Stimulator cables attached to the output
- Power: On
