Select Mastoidectomy from setup list. Use Channels 1 and 4 only from diagram above
Stimulating Current: will vary between 0.2mA for a dehiscent nerve, to 4mA for a nerve covered with bone. Start with current set to 0.5mA, and increase in 1mA steps if no response, or reduce to 0.2mA if there is a response at 0.5mA. 0.2mA = exposed nerve, 1mA+ = nerve covered by bone, with the higher the current, the more bone
Stimulating probe: P/N 3604-00 precision bipolar probe recommended. The white is the return, the blue the active – place blue tip over nerve. Probe may be bent for use under microscope
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 mastoid process is a section of the skull behind the ear which is like a sponge in structure, being full of cavities. These cavities may become infected and the disease can destroy the bone, in particular the incus, malleus and stapes. The surgical technique is to drill away the bone until fresh, clean bone is exposed, and to seal the cavities to prevent air-borne infection from other tissue. At the same time, prostheses may be fitted to improve hearing and replace missing or destroyed bones.
The facial nerve runs in a canal on the edge of the mastoid process. It is important that the surgeon does not drill through the canal and the nerve! Sometimes disease may have attacked the canal and the nerve is dehiscent – then the canal will need to be opened in order to clear out disease inside the canal.
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.
The nerve is often left within the canal and so the monitor may not appear to be particularly useful. It is guarding against a small but catastrophic risk.
Products & consumables able to be used for this procedure include: