Intraoperative NeurophysiologicMonitoring in otolaryngology

BYDR . SANDEEP CHANDRA

PGT (M.S.) ENTNorth Bengal Medical College & Hospital

Intraoperative neuromonitoring (IONM) is a relatively recent advance in electromyography (EMG) applied to otolaryngology-head and neck surgery.

Its purpose is to allow real-time identification and functional assessment of vulnerable nerves during surgery.

The nerves most often monitored in head and neck surgery are the motor branch of the facial nerve (VII), the recurrent or inferior laryngeal nerves (X), the vagus nerve (X), and the spinal accessory nerve (XI), with other cranial lower nerves monitored less frequently.

Morbidity from trauma to these nerves is significant and obvious, such as unilateral facial paresis.

INTRODUCTION

Avoidance of intraoperative nerve injury is of para- mount importance in order to reduce patient morbidity. In addition, both RLN and facial nerve paralysis are common reasons for litigation following otolaryngology surgery.

Krause first described facial nerve monitoring in 1912 using a faradic stimulation during cochlear nerve section for tinnitus. Twitching of the ipsilateral facial muscles

during stimulation helped him preserve the facial nerve, and the patient had transient facial weakness postoperatively. In the 1960s, dedicated facial nerve monitoring systems were developed. The Hilger stimulator was used principally in the assessment of facial paralysis, but was also used during surgery. Further developments in facial nerve monitoring occurred in

the 1970s and 1980s. Delgado and colleagues described the use of electromyography (EMG) monitoring in cerebellopontine angle (CPA) surgery.

History

Moller and Jannetta combined the specificity of EMG recording with the advantage of acoustic feedback to the surgeon.

The underlying principle for intraoperative monitoring is that some types of injury can be reversed. Facial nerve monitoring can be useful to identify the facial nerve when it is not clearly visible in the surgical field. Localizing distal nerve fragments in trauma cases, and

identifying sites of nerve compression, as long as wallerian nerve degeneration has not completely occurred.

Otologic procedures in which the nerve is at risk include cochlear implantation, revision tympanomastoidectomy, and repair of external auditory canal bony stenosis.

MONITORING FACIAL NERVE

Monitoring may augment safety in cases where the anatomy is altered by infection, trauma, or congenital malformation.

It may also be beneficial in training centers where some portions of operations are performed by less experienced surgeons.

Many facial nerve monitoring systems are available commercially, including the Nerve Integrity Monitor (Xomed, Inc., Jacksonville, FL), Neurosign 100 (Smith & Nephew Richards, Inc., Memphis, TN), Brackmann II (WR Medical Electronics Co., Stillwater, MN), and NEI (Grass Instrument Co., Quincy, MA). All of these devices use EMG. The Silverstein Facial Nerve Monitor (WR Medical Electronics Co.,) is an example of a motion detector device. Some systems, such as the Silverstein Monitor, include the

ability to electrify instruments to aid with monitoring.

NERVE MONITORING SYSTEMS

The electrodes have to be placed in the muscles supplied by the facial nerve.

Four electrodes are placed in frontalis, orbicularis oculi, orbicularis oris and mentalis muscles.

The ground electrodes are placed over sternum. There are stimulators which can stimulate the nerve or could

map the course of the nerve. One could adjust the strength of the current by which nerve could be stimulated.

A low current is used when nerve is stimulated directly so there is no damage to the nerve.

Electrode placement

Once before surgery starts electrodes are tapped, which produces sound as well as an impulse onthe monitor, indicating proper fixation and connections of the electrodes.

During surgery as one reaches close proximity of the facial nerve, the mechanical pull also produces a sound, warning surgeon that he is in close vicinity of the nerve, preventing any accidental damage.

Once the nerve is exposed, it is stimulated directly to identify it, which produces a different type of sound and also produces an impulse [EMG] on the monitor.

Nerve and branches are stimulated as the surgery proceeds. When the main trunk is stimulated all the electrodes inserted in different muscles get stimulated and produce an impulse on the monitor indicating that main nerve as well as the branches are intact.

While performing a parotid surgery, one comes accross many fibrous strands which appears similar to nerve. With use of nerve stimulator, this situation is avoided and precise identification of nerve is possible and surgical time is saved.

The nerve integrity monitor (NIM-Response 3.0 System, Medtronic Xomed, Jacksonville, Florida) is the most widely used device for laryngeal nerve monitoring providing both audio and visual evoked waveform information when either the recurrent laryngeal or vagus nerve is stimulated.

This nerve monitoring device transforms laryngeal muscle activity into audible and visual electromyographic (EMG) signals whenever the RLN or vagus nerve is stimulated intraoperatively.

Recurrent laryngeal nerve monitoring

During IONM, a low pressure-cuffed silicone endotracheal tube (NIM Standard EMG Reinforced Tube, (Medronic Xomed) is used.

This tube is similar to a standard endotracheal tube but in addition, it has two integrated stainless steel contact electrodes on each side of the tube that monitor vocal cord EMG activity.

These endotracheal surface electrodes make good contact with the luminal surface of the true vocal cords.

Following correct positioning of the endotracheal tube, the vocal cord electrodes and ground wires are connected to the NIM 3 monitor via a connector box.

Both the recording electrodes and nerve stimulator probe require grounding electrodes (adhessive or subcutaneous) usually placed on the patient’s shoulder or sternum region closest to the monitor unit.

NIM 3 monitor has a pulse generator which is connected to the stimulating probe.

Neural stimulation occurs via a sterile, handheld probe (Prass Standard Flush-Tip Probe, Medtronic Xomed).

Stimulating probes can be monopolar or bipolar.

Neuromuscular blockade interferes with monitoring as it reduces the EMG amplitude and the optimal laryngeal response.

Thus after induction with a short acting neuromuscular blocking agents, neuromuscular blocking agents should be avoided for the rest of the case.

Aneasthesia

Skull base tumours may involve many cranial nerves. CN III, IV and VI which control extra ocular muscles, are at

risk in tumours of cavernous sinus. Intraoperative monitoring of CN VIII has become the standard

of care for skull base and CPA surgery. Motor portions of trigeminal nerve may be involved and CN

IX, X, XI and XII can be involved in large skull base lesions. Nerves are localized using the same technique as used for the

facial nerve electrode, employing subdermal needle electrodes placed with great care.

Monitoring of other cranial nerves

Subdermal needle electrodes can be placed percutaneously in extraocular muscles that are innervated by a respective cranial nerve.

The opposite forehead serves as a good location for reference electrode to serve to avoid contamination with EMG potentials from ipsilateral facial muscles.

EMG from soft palate (CN IX), sternocleidomastoid or trapezius muscle (CN XI), or lateral tongue (CN XII) can be used for the lower cranial nerves.

CN XII monitoring can be done by placing recording electrodes spaced 1cm apart in the lateral tongue.

Consultant surgeons find the technology helpful. It is therefore hoped that increased use of intraoperative nerve monitoring will be associated with improved patient outcomes.

Innovative methods of cranial nerve monitoring using techniques such as intra operative F wave and transcranial electric motor evoked potential mesurement are being investigated, but have yet to be widely adopted in otolaryngology.

As a cautionary note, it must be emphasised that these devices do not compensate for poor surgical technique. Visual nerve identification will continue to be the gold standard for preventing intraoperative nerve damage.

Conclusion

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