Continuous dynamic mapping during surgery of large vestibular schwannoma

Abbreviations

23.1

Introduction

In vestibular schwannoma (VS) surgery, postoperative facial nerve (CN VII) palsy is an important neurological deficit and a stigma that significantly reduces the quality of life. In 25%–70% of patients who have undergone complete removal or gross total removal of large tumors (Koos IV or >3 cm diameter), permanent moderate or severe CN VII deficits of House–Brackmann score (HBS) 3 or worse are reported . The corresponding rates after near-total removal are 15%–51% and 0%–53% after subtotal removal . Because facial nerve function is better following less than total or gross total resection, a “nerve-centered” approach using planned subtotal or near-total resection with immediate or delayed radiosurgery on growing tumor remnants is becoming increasingly popular . In addition to the manipulation of the visible nerve, accidental injury may also contribute to CN VII deficits. Particularly in large VSs, the anatomical course of CN VII is usually distorted. There may be moments during the resection when the surgeon is unaware of the adjacent, but hidden, stretched, and dispersed, nerve and injury may occur.

The classical intraoperative neurophysiological methods used for CN VII monitoring/mapping during VS surgery comprise free-running electromyography (EMG) and categorization for different train types , monitoring of corticobulbar motor-evoked potential (MEP) , or mapping the cranial nerves with different stimulation probes [compound muscle action potentials (CMAPs)] . The drawback of classical stimulation is that it is only intermittent in space and time when the surgeon stops the resection, switches to the mapping probe, and touches the tissue where the CN VII is suspected. We have recently introduced a new mapping method for continuous monopolar stimulation of the corticospinal tract during brain tumor surgery using a standard but electrified suction probe . The objective of a recent study was to adapt this technique of continuous dynamic mapping to VS surgery. They investigated whether the method could reliably warn the surgeon about adjacent hidden nerve fibers and whether it helps to avoid inadvertent injury of CN VII during surgery of large VSs . Further details will be discussed in the following chapter.

23.2

Neurophysiological setup

23.2.1

Continuous dynamic mapping suction probe

The introduced device used consists of a combination of an electrically isolated standard suction device with a monopolar mapping capability ( Fig. 23.1 ) . Connected to the intraoperative neuromonitoring machine, it provides continuous stimulation at the spot where the tip of the suction device is placed. The neurophysiological stimulation parameters may be identical to the parameters of classical mapping with a monopolar or bipolar (concentric) probe. The surface of the suction probe is isolated to limit the electrical contact to the tip of the device. With the term “dynamic” we refer to the quickly changing location of the tip of the suction probe according to the “flow” of surgery during dissection and tissue removal .

CMAPs and MEP are recorded by pairs of needle electrodes inserted in standardized target muscles for the cranial nerves of interest including orbicularis oculi, levator labialis, orbicularis oris, and mentalis muscle. For cranial nerve mapping, we apply monopolar (referential) cathodal stimulation with 0.3 ms pulse duration, a frequency of 2 Hz, and a stimulation intensity ranging from 0.05 to 2 mA. In addition to the dynamic mapping method, we use free-running EMG, monitoring of corticobulbar MEP, at least in the facial nerve innervated muscles, and, if possible, auditory-evoked potential recordings. More details about our neurophysiological setup and the total intravenous anesthesia protocol are published elsewhere .

23.2.2

Software adaptations

For intraoperative neurophysiological monitoring and mapping, the ISIS system (Inomed Co. Teningen, Germany) equipped with a constant current stimulator (OSIRIS, Inomed Co. Teningen, Germany) is used. Two sounds are used for acoustic feedback to guide the surgeon. Both sounds are easy to differentiate even in a noisy environment. The first sound (high pitch) is emitted with every single stimulation pulse to confirm that adequate current is delivered to the tissue. The second sound (low pitch) is only emitted if the amplitude of a CMAP response in the muscles being monitored reaches a value above 300 µV ( Fig. 23.2 ). At the same time, the responses are observed on both the free-running and triggered EMG screens.