1. Ensure anesthesia avoids long-acting muscle relaxants. If in doubt, check train-of-four electromyography (EMG) response.
Explore this issue:
May 20142. Be wary of local anesthetics (lidocaine or bupivacaine), which can chemically induce a temporary facial paresis, rendering monitoring useless.
3. Place bipolar electrodes in ipsilateral orbicularis oculi and oris. Use uniform color coding for electrodes = “blue eyes, red lips.” The electrode pair should be close to, but not touching, each other.
4. Place a green ground electrode in sternum above a white anode electrode.
5. Plug the electrodes into proper channels on the patient interface box, preferably color coded to match the above protocol.
6. Ensure that the electrode impedance is less than 5 kOhm and that the interelectrode impedance is less than 1 kOhm for each electrode pair.
7. Perform a tap test on the skin surface above the electrodes to elicit an artifactual response (which only tests the recording portion of the system).
8. Adjust the volume of the nerve monitor so that it can be heard above the ambient noise of the OR.
9. Once the procedure has begun, always confirm that current flow is present by using a stimulating probe or Kartush surgical instrument to touch muscle, soft tissue, or wet bone. These tissues conduct nearly 100% of the current due to their low impedance and therefore require less current to elicit a confirmatory. If the monitor confirms proper current flow (visually or audibly), then the stimulating portion of the system has been appropriately confirmed.
10. Obtain a baseline response to stimulation at an early point in surgery, before any significant nerve manipulation is performed. If the nerve is visible, initial stim levels of 0.5mA to 0.8mA are appropriate. If the nerve is covered in bone, granulation tissue, cholesteatoma, or tumor, progressively increase the initial stim up to 2mA. Once an initial baseline response has been recorded, the integrity of the entire stim and recording system has been properly confirmed.
11. Stim during surgical dissection. If surgical dissection is required along the facial nerve, titrate the optimal stim levels based on nerve proximity and intervening tissue. Higher stim levels should be used to “map” the general location of the nerve and then lowered once it is found.
Stim levels as low as 0.1mA are often sufficient when dissecting along the bare nerve in the cerebellopontine angle. During cholesteatoma dissection along a slightly dehiscent tympanic facial nerve, 0.5 – 0.8mA may be more appropriate to alert the surgeon when the dehiscence is below the stim probe or stimulating dissection instrument.