gms | German Medical Science

Objective: Intra-operative EMG monitoring is a helpful tool for observing motor cranial nerve function during surgery of CPA tumors, but it is only feasible after nerve exposure. In order to improve intra-operative monitoring and obtain information on nerve function throughout surgery, motor evoked potential techniques were applied and combined with conventional EMG monitoring.

Methods: In a prospective on-going clinical study, patients with vestibular schwannomas or cpa meningiomas are being investigated by multipulse (3 to 4 pulses) transcranial electrical activation elicited either by constant voltage (250 to 400 V) or constant current (60 to 190 mA) stimulators in an anodal mode (M3_M4 versus Mz). Recording is performed using conventional EMG electrodes for orbicularis oculi and orbicularis oris muscles (the latter on both sides). Single pulse stimulation is applied before the start of surgery in order to identify any direct peripheral nerve stimulation. The surgeon is notified of the onset of intermittent stimulation. Latency and amplitude of the MEP are registered and correlated with the immediate surgical findings and with video tapes of early and long-term clinical results.

Results: In 54 patients MEPs of the trigeminal and facial nerves were carried out, including 5 patients with trigeminal neuralgia serving as controls. In 43 patients, the facial MEP could be identified at a latency of 12 msec to 18 msec. In 11 patients marked artefact formation prevented reliable identification of facial MEPs on the tumor side, but MEPs of the healthy side could be recorded more easily. In 4 patients nerve discontinuity correlated with MEP loss. A significant amplitude reduction by 35% or 50% predicted postoperative partial nerve palsy; However, some preserved MEPs predicted fast nerve recovery within a few weeks.

Conclusions: Routine application of cranial nerve MEPs is still limited due to serious artefact contamination in about 20%. Further techniques for improvement will be discussed. In about 80% of the patients, MEP can be obtained and are characterized by three advantages: Stimulation conditions are stable throughout surgery and may be carried out even without nerve exposure. Preserved MEPs are a reliable sign of functional nerve continuity. In case of reduced or lost EMG response, preserved MEPs are a favourable indicator of recovery.