Deep lobe parotidectomies, or revision cases, can benefit from having the branches of the facial nerve monitored individually.
Use 1699-00 electrodes inserted into the muscles as shown. The needles must be located in the muscle proper and not simply under the skin.
Use 2 single 1705-00 reference electrodes inserted into the contralateral cheek.
Secure the electrodes with tape so that the needles lie flat.
Attach the pre-amplifier pod to the side rail of the operating table, and following the diagrams above, ensure that the electrodes are connected to the correct channel.
Switch the main unit on and connect the preamplifier and stimulating pods and the mute sensor. Enable channels 1-4 in either bargraph or waveform mode.
To check impedance, check the screen; in bargraph mode, no segments should be lit; in waveform mode, the screen should not be updating (although an artefact signal may be on screen, it should not update).
Use the concentric probe to stimulate. The concentric is very accurate and will not penetrate through tissue, but is excellent for mapping and stimulating the branches and small interconnecting fibres.
Adjust the stimulator output to 0.5mA to find the nerve, and reduce to 0.2mA once located. The stimulator probe should be connected when required rather than at the start of the surgery.
Surgical Procedure Information
A superficial parotidectomy involves locating the facial nerve, identifying the branches as it divides within the parotid gland, and removing the gland lying above the facial nerve; the facial nerve is left attached to the remainder of the parotid gland.
A deep lobe parotidectomy removes the tumour above and below the nerve. The end result is a nerve fully mobilised from surrounding tissue.
By monitoring each branch separately, the surgeon can quickly identify where each branch is going – important if there are loops or if the nerve is distorted by the tumour.
If set to bargraph mode and there is a response, check to see which channel was last indicated. Always keep to the same electrode and channel arrangement so that this is easy to do.
If used in waveform mode, a trained technician should monitor the display to relay which branch is stimulated.
A stimulator setting of 0.5mA should be used to locate the nerve; the current can then be reduced to 0.2mA which should be adequate to elicit a brisk response in the normal nerve. In some cases where there is already weakness, a higher level may be necessary, up to 0.5mA. It should not be necessary to exceed 1mA as greater levels may induce fatigue in the nerve.
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. This current is used to locate the nerve at its trunk near the stylomastoid foramen. Once located the current can be reduced to a minimum of 0.2mA.
The concentric probe P/N 4600-00 is recommended. It is very accurate, does not stimulate through tissue, and is excellent for stimulating the small interconnecting fibres between the branches.
Surgical Procedure Information
The operation is normally a superficial parotidectomy, where the tumour lies on top of the facial nerve. The tumour, together with a margin of healthy tissue, is removed leaving the facial nerve lying on top of the remaining gland.
Occasionally, the tumour extends below the facial nerve; this is termed a deep-lobe parotidectomy. The facial nerve needs to be mobilised so that the tumour can be removed from above and below the nerve. This is technically more difficult and the facial nerve is at greater risk.
The usual way to find the facial nerve is to follow the digastric groove and find the pes anserinus, the point at which the nerve trunk divides, and then to follow the branches until sufficient space has been created to provide access.
Use the concentric probe set to 0.5mA to find the nerve trunk; once located, reduce the current to 0.3mA and ultimately to 0.2mA once the branches are identified.
Note that because the Neurosign 100 only has 2 channels, and you are monitoring 4 branches, the monitor can only discriminate between upper and lower bifurcations, and not the individual branches. As each branch only has a single needle inserted into the relevant muscle, it is possible for the surgeon to stimulate fine interconnecting branches, for part of the face to twitch, but not to see any response from the monitor. This is simply a limitation of using 2 channels where 4 would be more appropriate. It is advised that the face is covered in OpSite or similar material, and left visible so that any twitches can be observed.
Products & consumables able to be used for this procedure include: