Look at the right hand diagram to see where the facial nerve lies anatomically, and then at the left hand diagram to see the muscles which are used to monitor the various branches.
Note the names of the muscles and their relationship to the nerve branches which control them.
Ensure that that the electrodes are inserted into the muscles as shown with the wires leading away from the surgical site. The needles must be in the muscle proper and not simply under the skin.
Secure the electrodes with tape, allow 5cm of free cable for movement and then secure again with tape before leading the electrode wires to the preamplifier pod.
To check that electrode impedance is OK, switch on both channels and check that the bargraph does not show any lit segments. The bottom segment may flicker – this is a normal muscle reaction to the needle and will die down in a few minutes. Tap the face to demonstrate that the electrodes and monitor is connected.
Set the stimulator to 30Hz and 0.5mA. A higher current may well be necessary because the surgeon will be stimulating through bone; 0.5mA is a safe value to start. If there is no response at 0.5mA, increase the current until a response is heard. If the anatomy is normal, it may require 5mA to stimulate through the bone. If the nerve is dehiscent, only 0.2mA will be necessary – hence the starting value of 0.5mA.
The precision bipolar probe P/N 4604-00 is recommended. It is accurate, and the 2 electrodes are identified. The white is the return, the blue is the active stimulator. Place the blue electrode over the nerve.
The probe can give an indication of depth of bone between probe and nerve. Bearing in mind that stimulation at 0.2mA represents exposed nerve, and 5mA denotes normal anatomy, as the drill is used the stimulation current can be reduced and early warning given when there is very little bone remaining.
Surgical Procedure Information
The risk of injury to the facial nerve during stapedectomy and other middle ear procedures is small but usually occurs because of a dehiscence in the fallopian canal above the oval window, which allows the facial nerve to lie unprotected outside its normal anatomy. The use of the monitor allows this condition to be verified at an early stage.
If the nerve is not visible under the microscope, the surgeon can use the precision bipolar probe to stimulate the canal and gain a response, verifying that the nerve is in its normal position. The nerve is dehiscent in about 50% of cases presenting for surgery; the current should be set to 0.2mA if the nerve is dehiscent, and in the range 0.5mA – 5mA depending on the thickness of intervening bone if it is not dehiscent. Starting at 0.5mA and then increasing or decreasing the current is recommended.
As the surgeon drills, you may hear a high pitched whining which changes frequency with the drill. This is caused by one of two factors; either the drill is vibrating the canal and the nerve is reacting, or the electrodes in frontalis are resting against the bone and the vibration in transmitted directly to them. This is not a fault and no action need be taken. However, if the response continues once the drill has stopped, this is an indication of heating of the nerve and irrigation must be used to cool both the nerve and the drill.
Once the nerve has been identified, the monitor is not likely to be used further. Its purpose in this surgery is primarily to guard against the risk to the nerve before it has been located, and so prevent accidental injury.
Products & consumables able to be used for this procedure include: