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This young man in his 40s has a large clival meningioma which has been monitored for some years but has recently grown significantly. He has ataxia, facial pain, difficulty swallowing and loss of balance.
The brainstem is significantly compressed. Whilst the VITH cranial nerve is likely to be involved, and the trigeminal is compressed (leading to his facial pain), I have been asked to provide brainstem monitoring.
The brainstem is the most primitive part of the brain, but controls respiration and heartbeat. The tumour has been compressing the brainstem for many months, if not years, and suddenly removing the pressure can lead to problems of the brainstem and changes in brainstem function will change the postoperative care.
I am using SSEPS and MEPs; the SSEPS are from the tibial nerve at the ankle, collected from the scalp over the sensory cortex, referenced to fz, with a ground in the hairline; the MEPS are elicited from the motor cortex and collected from the hands and the legs.
After wiring up the patient and with the patient in a Mayfield frame, I have established excellent SSEPS and MEPS. The MEPS are present at 300V.
At present, the surgeon is drilling bone to create access through the nose and sinus. There are dangerous structures to avoid and gaining access may well be the longest part of the operation. No change in baseline in either SSEPS or MEPS.
There are a number of high-rise structures which must not be damaged; the internal carotid arteries, the basilar artery and the circle of Willis. The optic chiasm and IIIrd cranial nerves are also nearby.
Dura has now been opened and surgeon is using CUSA to debulk tumour. SSEPS on right remain as per baseline, those on the left are at about 80% of baseline, but as the surgeon has only just started actually removing tumour, this is likely to be as a result of other, nonsurgical factors.
SSEPS remain at baseline on right, continue to deteriorate on left, now 60% of baseline. I can’t do further MEPS at present as endoscope is permanently in place. Anaesthetist wants to reduce the proposal infusion as patient is beginning to show adverse signs, changing to ciboflurane. This will probably eliminate MEPS, although I probably won’t find out till later. This should not affect the SSEPS.