Method

Vestibular schwannomas

As an example for neurosurgery, the neuromonitoring method during resection of a vestibular schwannoma is described below. In addition to vestibular schwannomas, neuromonitoring is also recommended when resecting most other brain tumours, see intraoperative neuromonitoring (IONM) in neurosurgery.

Resection of vestibular schwannomas

Vestibular schwannomas usually grow, as the name tells, from the Schwann cells of the vestibular portion of the vestibulocochlear nerve into the cerebellopontine angle towards the brainstem. Vestibular schwannomas are the most common cerebellopontine angle tumours. Due to their location, they put especially the VIIth and VIIIth cranial nerve (facial nerve and vestibulocochlear nerve) at risk.

IONM has become one of the essential parts of vestibular schwannoma treatment and can be used during any of the common resection methods. Please note that the following recommendations on neuromonitoring during resection of vestibular schwannomas are of a general nature. The patient’s individual vital parameters, the existing surgical situation as well as the technology used should always be considered.

Neurostimulation and recording using EMG (electromyography) and AEPs (auditory evoked potentials) are used to monitor the nerve structures at risk. In this context, it is important to ensure correct anaesthesia, because although the recording of AEPs is not affected by muscle relaxants, they may significantly reduce the possibilities of EMG recording, an important method to intraoperatively determine the facial nerve function.

EMG can be useful to monitor the facial nerve and to test it after resection. A good prognosis is provided by an equally low intensity of proximal and distal muscle responses. Studies recommend a maximum stimulation value of up to 0.05 mA with recorded amplitudes of >240 µV to determine an intact facial nerve.

AEP recording is used to monitor the acoustic part of the vestibulocochlear nerve during resection of a vestibular schwannoma. The stimulation is carried out via plug-in earphones. The recording takes place from the vertex towards the earlobe or mastoid. Here, not only the integrity of the nerve endangered by mechanical manipulation is monitored, but also changes in the brainstem are detected before cardiovascular functions are affected. An increase in inter-wave latency or reduction in amplitude of the waves I, III and V of the AEP can precede damage of the vestibulocochlear nerve.