gms | German Medical Science

Objective: Recording of muscle action potentials (MAP) evoked upon electrical stimulation is the method of choice for intra-operative mapping of the cranial nerves in surgery for lesions around the brainstem and at the skull base. Waveforms and thresholds of these MAP vary depending on the nerve to be stimulated, the functional integrity of the stimulated nerve, and the setup for response recording.

We have measured and analysed these parameters during surgery of lesions located in the vicinity of the cranial nerves.

Methods: Intra-operative monitoring was used to measure MAP threshold and waveforms upon electrical stimulation during operations in 355 patients. Lesions included various pathologies adjacent to all cranial nerves carrying motor fibres. Stimulation by handheld probes with concentric electrodes (INOMED) started at 0.05 mA at the most proximal segment of the respective nerve. Intensity was continuously increased up to the threshold for the MAP of the muscle innervated by the stimulated nerve. Latency, amplitude and waveform of the MAPs were recorded, stored and analysed off-line. Intra-operative infrared angiography was employed to visualize the integrity of nerve-supplying vessels.

Results: Intact oculomotor nerves, the facial nerve and the caudal cranial nerves all showed a similar threshold at the lowest stimulation level (0.05 to 0.1 mA). Excitation of responses from the trigeminal nerve often required stimulation intensities of 0.2 mA and higher. When the recording setup was not varied, amplitudes and waveforms of responses were similar in repeated measures and interindividually were specific for the respective target muscles. Surgical manoeuvres, especially the separation of nerves from tumours compromising their vascular supply, altered the muscle responses and their threshold more markedly than in cases where the supplying vessels remained intact.

Conclusions: There is no physiological justification to apply stimulation intensities of more than 0.5 mA to intact cranial nerves at direct contact in mapping procedures during surgery around these nerves. The integrity of the nerve-supplying vessels may be more important for the functional outcome than was assumed previously.