Some large skull base tumours extend down the brainstem involving several of the lower cranial nerves. The Neurosign 800 can monitor these motor nerves. Use 4 channels for the VIIth nerve.
Use 1699-00 electrodes inserted when the patient is in the anaesthetic room, since there is more time available and the involvement of the anaesthetist avoids the need to remove or replace surgical tape for insertion. Insert the electrodes so that the leads face away from the surgery. Tape all electrodes so they are flat. Keep the electrode leads separate so they can be identified.
Use the laryngeal electrode (400X-00) to monitor the Vagus, using the 2 red connections only.
A single 1705-00 reference electrode should be inserted into the cheek on the contralateral side and taped down.
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, stimulating pod and the mute sensor. Enable the appropriate channels in either bargraph or waveform mode.
Observe the bargraph or waveform when the channels are switched on. If there are no segments lit, or only the bottom one or two are flickering, then impedance is good. If in waveform mode, ensure the screen is not updating. If several segments are permanently lit or the screen is continuously updating, impedance is poor and the electrodes should be re-inserted.
Use the concentric probe to stimulate the nerves directly, or the Kartush Stimulating Instruments.
Adjust the stimulator output to 0.05mA. The stimulator probe should be connected when required rather than at the start of the surgery.
The lower cranial nerves may be involved in large vestibular schwannomas, glomus jugulare tumours, meningiomas, or schwannomas of the IXth, Xth or XIth nerves.
Where the nerves are not myelinated, a lower level of stimulating current is necessary to stimulate them. However, if the surgery involves manipulation of the nerves, they may require more current. Ideally, 0.05mA should be sufficient, but 0.2mA may safely be used. If there is any sign of pre-operative weakness, a higher current will be required. Do not exceed 1mA.
Ideally, the same current used distally and proximally will elicit the same amplitude of response, both before and after tumour removal. An increased level of required current may indicate a post-operative weakness.
Be especially aware of train responses, as these indicate trauma to the nerve. It may be advisable to wait until the response has settled down before continuing. The exception to this is that cold irrigation will elicit a similar train response which may be ignored (other than to warm up the saline!)
Take special care when stimulating the Vagus to use the minimum current required, as the anaesthetist may have a particular interest!
If the channel order in the table is followed then it is simple to identify which nerve is which, especially where the tumour surrounds several nerves. It is recommended that the channel order as shown is followed. If the hypoglossal is to be monitored, then use 3 channels for the facial nerve and use Channel 4 with electrodes in the lateral aspect of the tongue.
Products & consumables able to be used for this procedure include: