General
- Motor function monitoring via electrodes in larger facial muscles
- Most often orbicularis oculi and orbicularis oris
- Difference between EMG and CMAP/MEP
EMG | CMAP / MEP |
Neurotonic discharges | Nerve action potentials |
No surgical requirements | Require surgical stimulation |
Indicate mechanical nerve stimulation | Indicate nerve and neuromuscular junction integrity |
Limited utility | Technical flaws |
- If local anaesthetics such as lidocaine inadvertently contact the facial nerve, the nerve will be unresponsive for hours. Avoid local injection around the Stylomastoid foramen
Steps
- Consult with anesthesia to avoid long-acting muscle relaxants
- Avoid local anesthetic near the facial nerve
- Demonstrate absence of neuromuscular blockade via:
- Train-of-four,
- Transcutaneous facial nerve stimulation or,
- Facial nerve stimulation within the operative field
- Assure that the monitor’s loudspeaker is set at a volume sufficient to be heard over the operating room’s ambient noise
- Perform a tap test
- Check electrode impedance
- Check stimulus current flow through soft tissue
- Obtain a baseline facial nerve response to stimulus
- Electrically map the location of the nerve
- Selectively use monopolar, bipolar, or stimulating instruments
- Titrate stimulus current based on nerve location and surrounding tissue (bone, soft tissue, blood, spinal fluid)
- Obtain a final proximal facial nerve response to stimulus prior to closure
- Generate a brief written report (can be within operative report):
- Confirm adherence to protocol,
- Highlight key events
Vestibular schwannoma surgery
- A free running EMG can display activity in both visual and audible form so irritation to a nerve will alert the surgeon.
- For example, a pair of needles in a facial muscle will record a discharge when the facial nerve is irritated during acoustic neuroma resection.
- EMG can prove useful in theatre when a motor nerve that supplies an accessible muscle is at risk of injury.
- Can be assessed with
- Sounds (surgeon alone)
- Uses a higher current and low fq
- Can lead to electrical injury to nerve
- Neurophysiologist reading the EMG graph
- Uses a lower current
- Less likely electrical injury
- Abnormal compound muscle action potentials (CMAP)
- Burst potentials
- “Burst” potential, representing a single poly- phasic electromyography response due to activation of multiple motor units simultaneously.
- Occur with direct mechanical stimulation
- Have a characteristic visual single wave and an audible “click” or “beep”
- A Train potentials
- “Train” potential, representing asynchronous firing of multiple motor units.
- During VS resection the detection of the A train (a pattern of high frequency and homogenous appearance) in muscles supplied by the facial nerve, is a reliable indicator of facial nerve paresis postoperatively
- Last seconds to minutes depending on the severity of injury
- Show multiple irregular waves
- Prolonged audible sound similar to an airplane propeller
Hemifacial spasm
- Looking at lateral spread pre op and if successful will improve post op
- Lateral spread is when stimulation of one branch of the facial nerve, other branches will activate and causes facial contraction
Error and possible cause
Error | Possible cause |
False-positive response | Inadvertent stimulation of adjacent structures = “current jump” |
False-negative response | - Stimulating through blood or fluid, causing “current shunting” - Inadequate facial nerve stimulation - Severe injury precluding depolarization - Complete transection of the nerve - Technical problems |
Checklist for intraoperative facial monitoring problems
Problem | Possible solution/cause |
Current jump | Lower the current intensity or use bipolar stimulation |
Current shunting | Insulated stimulator, aspirate adjacent fluids |
Cautery artifact noise | Muting circuit |
Cautery precludes monitoring | Visualize or palpate the face |
Laser heating effect | Monitor baseline amplitude |
Saline cooling | Warm saline with “blood warmer” |
Stimulus artifact | Increase “stimulus ignore” time; exclude an overcharged capacitor |
Static discharge | Use insulated instruments |
No response to stimulation | - Power off - Current intensity too low - Current measured too low - Electrode impedance too high - Electrode disconnected - Current shunting - Threshold setting too high - Volume too low - Muscle relaxant used - Local anesthetic on nerve - “Stimulus ignore” too long - Nerve not contacted - Other cranial nerve/tissue - Nerve injured |
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