Intraoperative identification of brain regions that subserve speech & language functions is important during surgical procedures that place these regions at risk for iatrogenic injury. Commonly, the surgeon endeavors to extract a tumor, or abnormal brain tissue that causes intractable seizures, while also preserving eloquent cortex. Damage to language centers can result in permanent disability.
For example, receptive aphasia describes a condition in which a patient is unable to understand language in its written or spoken form, but they can usually speak normally. Expressive aphasia, on the other hand, describes a condition in which a patient is unable to produce language in its written or spoken form, but they can usually understand other’s speech.
Here’s a quick overview of surgery: the patient is usually heavily sedated, but not intubated, and local anesthetics are administered to block nerve conduction and eliminate the sensation of pain. The surgeon will then perform a craniotomy to expose the area of the brain where surgery will be performed. Next, the sedation is allowed to wear off, and the patient is fully awakened. Remember, she can’t feel pain. With the patient awake, a neuropsychologist or speech pathologist performs tests to assess the patient’s language function in real time – most commonly picture naming or countring. With each test, electrical stimuli are delivered to different regions of the brain, and the neuropsychologist looks for speech arrest, a sign that the stimulated area of brain is actually involved in language. The surgeon marks these regions for preservation. There are no pain receptors in the brain, so mapping may continue as brain/tumor tissue is removed through the duration of the procedure. The patient is usually heavily sedated again for closing.
You can probably already tell that “language mapping” is optimally performed after significant planning, and with careful communication and cooperation among all members of the surgical team. Of course, this doesn’t always happen…
On more than one occasion, I have been called last minute to a surgery described only as “left crani for tumor” only to learn that we’ll need to map both motor and language function. As you will see below, the planning required for language mapping is extensive.
Another common problem – luckily one that I have never had to deal with, but a problem that is absolutely prevalent in the field – is that surgeons want to perform language mapping and they have no freaking idea what they are doing…but you know they often think they’re experts in neurophysiology. Surgeons want to control all of the mapping, don’t want to do it correctly, and quickly blame the neurophysiologist or neuropsychologist when the patient wakes with deficits. If you want to see a fantastic example of how NOT to do language mapping, google Language Mapping and find the video from CNN.
Below you will find something akin to a protocol for language mapping, or, at least it’s what I have used many times successfully. I would enjoy your feedback. As always, do not perform these methods without proper training.
- The surgeon and neurophysiologist will work closely during the mapping phase of the surgical procedure.
- The neurophysiologist should meet with the surgeon to discuss:
- The surgical plan and risks in the context of available radiographic imaging.
- An appropriate neurophysiological testing plan in the context of risk.
- Protocols for sensorimotor and cognitive mapping (speech/language).
- Order of operations.
- Benefits, limitations & alternative strategies.
- Preferred and required equipment for neurophysiology (probes, stimulating devices, etc.).
- Contingency plans for unanticipated events.
Cognitive Assessment Team
- The Cognitive Assessment Team (CAT) usually consists of a neuropsychologist or speech pathologist who will perform the language tests during surgery.
- Cognitive and electrical stimuli must be delivered precisely and contemporaneously during surgery.
- This requirement necessitates a pre-operative planning meeting between the CAT, the neurophysiologist and, preferably, the surgeon. The team should discuss the following:
- The presentation of electrical stimulation must be known to the surgical team and unanticipated by the patient (single blinded).
- Specific cognitive task(s) to be used (e.g., Boston Naming Test, counting), measures to be quantified (e.g., anomia, speech arrest) and how the timing of stimulus delivery will be appropriately matched to stimulus presentation.
- Contingency plans for unanticipated events (uncooperative patient, failure of specific task, failure of stimulus mapping, etc.).
- Various anesthetics can disrupt cognitive mapping and this should be discussed with the attending neuroanesthesiologist.
- Volatile anesthetics and gasses (e.g., NO2) should be avoided because they:
- Increase intracranial pressure (ICP) secondary to vasodilation.
- Distort EEG/ECoG recordings in a dose-dependent manner.
- If sensorimotor mapping is to be performed, the patient must receive total intravenous anesthesia (TIVA) from induction to the end of mapping, with the following caveats:
- Dexmedetomidine (Precedex/ Dexdor/Dexdomitor) must be avoided because it has been shown to negatively impact acquisition of motor evoked potentials (MEPs) at clinically-relevant target plasma concentrations (0.6–0.8 ng/ml).
- Antiepileptic drugs are known to increase thresholds in electrocortical mapping and should be discontinued accordingly.
- With regard to motor mapping, patients receiving Lamotrigine (Lamictal) should be discontinued or switched to an alternative medication approximately 1 week prior to surgery. While there is not presently evidence in the literature to support the notion that this drug negatively impacts acquisition of MEPs, I have personally observed this phenomenon in a series of approximately 6-8 children who underwent cranial surgery with motor mapping.
- Propofol administration should be completely discontinued 15-20 minutes before ECoG.
In the coming months, I plan to publish a post about sensorimotor mapping. Until then, please simply note the following idiosyncrasies related to awake surgery:
- To the extent possible, all sensorimotor mapping should be performed before language mapping and, importantly, before waking the patient.
- Phase reversal is used to identify central sulcus.
- The precentral and inferior frontal gyri are mapped for eloquent motor function using the monopolar pulse-train stimulation technique (see upcoming Sensorimotor Mapping Protocol).
- Used to detect electrocortical seizures and after-discharges that result from cortical & subcortical electrical stimulation.
- A cortical strip/grid electrode is placed over the frontoparietal convexity. A 1×6 grid is common, but the size of the grid may vary, depending on the available region of exposed cortex.
- The recording montage may be bipolar or referential. A sample montage is presented below assuming a 1×6 cortical strip electrode:
|Bipolar Example||Referential Example|
|Ac = contralateral ear|
- Baseline cortical activity is characterized.
- After each test (see Electrical Stimulation), an ECoG reading is performed to detect afterdischarges and electrocortical seizures, which are then reported to the surgeon.
- Based on ECoG recordings, stimulation parameters may be altered for subsequent tests (see Language Mapping).
Electrical Stimulation for Language Mapping
- While an Ojemann stimulation apparatus can be used, it is recommended to use equipment that is capable of concurrent electrical stimulation and neurophysiological recording. All modern multimodality intraoperative monitoring systems have a wide-range of capabilities that make them far superior to the Ojemann apparatus. This equipment is preferred.
- A hand-held probe with the following characteristics is used to localize language functions (not for use in motor testing):
- Tip Orientation: side-by-side bipolar.
- Tip Characteristics: Stainless steel ball-tip or semi-flush tip (1 mm diameter).
- Tip Spacing: 2-10 mm (5 mm best).
- Pulse: constant current biphasic rectangular pulse (-/+ or +/-).
- Duration: 1 msec
- Frequency: 50-60 Hz (may use 5-10 Hz if you observe after-discharges).
- Range: 0-15 mA.
- Note: it is generally suggested that the safety limit for brain stimulation is 40 µC/cm^2/phase.
- Technical steps to be performed during all stimulation:
- Surgeon ensures that probe tips are both in contact with cortical surface, and not making contact with each other.
- Neurophysiologist ensures that current is returned, demonstrating closed circuit.
Cortical Language Mapping Protocol:
- Chosen sites for mapping stimulation are identified with marking labels applied to the cortex by the surgeon (~10-20 sites/patient).
- Electrical stimulation is delivered concurrently with cognitive stimulus, usually beginning just before stimulus presentation.
- Note: I usually have the CAT say to the patient, “Tell what this is NOW.” The present the image to the patient at the precise moment that they say the word, “NOW”, and I use that verbal cue to present the electrical stimulus to the brain.
- Continuous stimulation is delivered with parameters above, starting at 2.0 mA and increased to a maximum of 15 mA over the course of a maximum of 10 seconds (longer may provoke epileptiform activity).
- If a given stimulation intensity consistently disrupts language function, we may elect to proceed with this intensity.
- If an after-discharge is noted on ECoG after a given stimulation:
- The intensity is reduced by 10% for subsequent tests.
- Alternatively, the frequency can be decreased to 5-10 Hz.
- Each stimulation site, positive or negative, is tested 3 times.
- A positive essential site can be operationally defined as an inability to name objects or read words in 66% or more of the testing per site. So, speech arrest in at least 2/3 tests.
- The results of each stimulation test are recorded for reference and archiving (see Scoring Sheet below).
- A 1-cm margin of tissue is measured and preserved around each positive language site to protect functional tissue from the resection.
Subcortical Language Mapping Protocol:
- Methods are identical to those employed in cortical mapping of language function, although we may start with higher intensity stimulus due to higher resistance of white matter.
Vogel Language Mapping Scoring Sheet
|Test 2||Test 3||Test 4|
|+||Positive for language|
|–||Negative for language|