ApplicationsProceduresNeurosurgeryVestibular schwannomas

Neuromonitoring during resection of vestibular schwannomas

Surgical treatment of vestibular schwannomas is challenging for many reasons. Not only positioning, access route and wound closure require special care, but also the intraoperative protection of the surrounding cranial nerves and the brainstem is of the utmost importance. Neuromonitoring focuses in particular on the VII and VIII cranial nerves.

Introduction to neuromonitoring in the cerebellopontine angle

Technique

Preparation

Procedure

Monitoring
After the baseline measurements have been completed, the monitoring team should report back to the surgical team whether all planned modalities have produced reproducible measurements or whether pre-existing deficits are limiting the intraoperative measurements. In particular, the surgical strategy can be influenced by this with regard to hearing ability.

Once the access has been made and the tumour resection begins, it is important to keep an eye on the function of the auditory nerve while localising the facial nerve, which is often displaced or fanned out by the tumour. Functional mapping using stimulation has proven to be a reliable method for localisation.

To prepare the mapping, the stimulation probe or Mapping Suction Probe used must be ready on the instrument table. A reference electrode is placed and fixed at the edge of the surgical field, e.g. in the subcutaneous tissue. This electrode and the connection cable of the probe or suction device are then inserted into the appropriate channel of the monitoring system in the non-sterile area by the person responsible for monitoring.

The instruments used can now be tested by the surgeons making contact with the tissue near the reference electrode and the monitorists triggering the stimulation. A current should flow, which the monitoring device reports back.

Low currents are now used to stimulate different parts of the tumour. In the case of very large tumours, it may also be necessary to first perform a debulking and to stimulate during this procedure in order to be warned when the facial nerve is approached.

Seidel et al., 2020. Continuous dynamic mapping to avoid accidental injury of the facial nerve during surgery for large vestibular schwannomas

During resection, spontaneous activity in the EMG can be used to detect irritation of the cranial nerves, in particular the facial nerve. Clinically, the high-frequency A-trains are particularly relevant, as they indicate the threat of nerve damage. If A-trains occur more frequently, a change in the surgical strategy is recommended, including a break with warm irrigation if necessary.

If the AEP deteriorates intraoperatively, a surgical break and, if necessary, an improvement in the blood flow situation by raising blood pressure or flushing with vasoactive substances may also be helpful until the measurements normalise. Not to forget: A retractor can also exert tension on the nerves. Repositioning should therefore also be considered.

In this way, the tumour can be gently removed using monitoring. Cranial nerves can be directly stimulated at low intensity for identification and functional testing.

Product information

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