gms | German Medical Science

Objective: Identification and functional preservation of the vagus nerve is important to avoid devastating complications in skull base surgery but remains challenging especially if the nerve is displaced by tumor formations. In the present study we evaluated the technical feasibility of endotracheal tube surface electrodes for non-invasive monitoring of the vagus motor nerve during surgery in the cerebellomedullary cistern.

Method: Medical records of patients who underwent intra-operative direct stimulation of cranial nerves VII-XII during skull base tumor surgery (N=17: 12 neuromas, 3 meningiomas, 2 epidermoid cysts) and vascular decompression for trigeminal neuralgia (N=3) were systematically analysed. Intra-operative evoked motor potentials (EMP) of the vocal cord were analysed and correlated with EMPs of the other lower cranial nerves, the proximity of the vagus nerve to the tumor (according to MRI/ intra-operative findings), the extent of resection, and postoperative vagus motor function.

Results: Placement of endotracheal tube surface electrodes and analysis of vocal cord EMPs were feasible and safe in all patients. Anatomical and electrophysiological identification of the vagus nerve was possible in 8/20 patients. However, in 12 patients stimulation protocols caused simultaneous vagus and glossopharyngeal nerve EMPs limiting electrophysiological differentiation of both nerves. This co-activation of EMPs mainly occurred in case of considerable anatomical displacement of both cranial nerves by tumor formation (11/12 patients) but was rare in case of undisturbed anatomy (3/8) (p=0.018). Amplitude and latency of vocal cord EMPs did not differ between patients with displaced and undisplaced vagus nerve anatomy. However, one tumor patient with considerable structural displacement and longest latency of vocal cord EMPs developed transient hoarseness and dysphagia postoperatively. In 16/17 patients gross total tumor removal was achieved, in one patient function preservation of the vagus nerve required subtotal tumor resection.

Conclusions: Non-invasive placement of endotracheal tube surface electrodes is safe and enables intra-operative monitoring of the vagus motor nerve in skull base surgery. Anatomical displacements by tumor formations critically limit electrophysiological differentiation between the vagus and glossopharyngeal nerves by the aid of EMP.