The use of brainstem auditory evoked potentials during posterior fossa surgery as a monitor of brainstem function

Summary

Nine patients with unilateral ponto-cerebellar tumour, one with epidermoid, eight with acoustic neuroma had intraoperative monitoring of the brainstem auditory evoked potentials (BAEP) using bilateral scalp recording (Cz-A₁ and Cz-A₂) and binaural stimulation in order to monitor brainstem function during the surgery. Results were correlated with preoperative and postoperative BAEP findings. There was some intraoperative BAEP change in all the patients.Halothane anaesthesia caused bilateral wave V. prolongation in six patients.Lumbar drainage produced ipsilateral or bilateral wave V. prolongation in three of four patients in whom lumbar drainage was established.Opening the dura produced ipsilateral or bilateral wave V. depression or bilateral prolongation in four patients.Tumour dissection caused BAEP changes in all the patients: bilateral prolongation or depression in five patients, and ipsilateral prolongation and/or depression in six patients, three of them together with bilateral changes. The only patient who had no changes in wave I–IV. but in whom a big wave VI. appeared at the tumour side during removal, had epidermoid tumour.At the end of surgery there was some improvement in all patients, but in two the improvement was not striking: in one BAEP remained flat ipsilaterally, and in other, latency did not improve. All the patients had a good clinical recovery a few days later, but the control BAEP performed from the 3rd–7th postoperative day was not yet completely recovered in four patients.

Intraoperative BAEP monitoring can be a useful technique to monitor brainstem function. It seems to us that the sensitivity of the method is perhaps unduly high for routine clinical purposes, the potentials being easily disturbed by relatively insignificant brainstem manipulation. However, it is clear that BAEP constitute a most sophisticated early warning system in posterior fossa surgery, and their use will clearly continue on a research basis.