MONITORING OPERATIONS FOR VESTIBULAR

SCHWANNOMA

CHAPTER III

Monitoring of the facial nerve is a model for monitoring other

cranial nerves

How to activate the motor system?

• Electrical stimulation of motor nerves• Magnetic stimulation of motor nerves• Electrical stimulation of the motor cortex• Magnetic stimulation of the motor cortex

How to record the response?

• Recording of electromyographic (EMG) potentials

• Mechanical recordings of muscle contractions

• Recording of motor nerve CAP

Recording of EMG potentials

Recording muscle responses

• Muscle relaxants cannot be used

Monitoring of motor systems

The facial nerve

Preservation of the facial nerve in operations for vestibular schwannoma

• Identification regions of the tumor where there is no part of the facial nerve present

• Identification of all parts of the facial nerve• Monitoring of mechanical induced facial

nerve stimulation• Monitoring of injury induced facial nerve

activation

Preservation of the facial nerve in vestibular

schwannoma operations

• Monopolar, constant voltage stimulation

• Facial EMG made audible

Finding where the facial nerve is

and where it is not

Find the location of the facial nerve

• Vary the strength of the stimulation to obtain less than maximal response

• Note change in amplitude as stimulating electrode is moved

• Increased amplitude of EMG means that the electrode was moved towards the facial nerve

• Decreased amplitude of EMG means that the electrode is moved away from the facial nerve

Absence of mechanically induced EMG activity does not guarantee

that injury has not occurred !

Always use electrical stimulation to verify the location of the facial nerve and its

integrity

The likelihood of postoperative facial weakness

• Increases with the number of occurrences spontaneous EMG

• The duration of the activity is important

If the facial nerve is injured in the beginning of the operation

(neurapraxia)

It will not be possible to monitor the facial nerve during the

remaining part of the operation

Use of partial muscle relaxation

• Difficult to keep constant level of muscle relaxation

• Prevent repetitive muscle contractions

• Questionable whether partial muscle relaxation offers any protection of the patient from moving

Testing the function of the facial nerve

For prediction of post operative facial function

Always use electrical stimulation

Use of EMG for decision making regarding grafting?

Electrophysiologic methods cannot distinguish between neurapraxia,

axonotmesis or neurotmesis

Monitoring the trigeminal motor nerve (portio minor)

Recording EMG from the masseter muscle (CNV) together with recording of facial EMG

Difference in EMG response to stimulation of the facial and the trigeminal nerves

The goals to reduce complications have been accomplished

This lady could have hadfacial palsy on one sideafter an operation for a vestibular schwannoma

Auditory neuromonitoring

Recording of auditory evoked potentials in operations in the

posterior fossa

Auditory monitoring for preservation of the function of

the auditory nerve

Recording of auditory evoked potentials in operations in the

posterior fossa

Monitoring of ABR can detect manipulations of the

brainstem before cardiovascular signs change

Use of ABR to detect manipulations of the

brainstem in operations for large acoustic tumors

 ABR evoked from the contralateral ear has

advantages over cardiovascular signs

Recording of ABR elicited from the contra-lateral ear monitor the brainstem

Recording of ABR elicited from the contra-lateral ear monitor the brainstem

Amplitude of peak V decreases

BEFORE BLOOD PRESSURE

AFTER

Effect of brainstem manipulation

SAMETIME

BEFORE HEART RATE

AFTER

SAMETIME

Latency of peak V increases

BEFOREBLOODPRESSURE

BEFOREHEART RATE

SAMETIME

SAMETIME

Effect of brainstem manipulation

Waveform analysis of the ABR provides information about the

anatomical location of an injury

Different conventions for display of BAEP

NEURAL GENERATORS OF THE ABR:

• Peak I: distal auditory nerve• Peak II: central auditory nerve• Peak III: mainly cochlear nucleus• Peak IV: unknown• Peak V: termination of the lateral lemniscus in the contralateral inferior colliculus

Waveform analysis of the BAEP provides information about the anatomical location of an injury

PEAK III

PEAK VIpsilateralstimulation

Contralateral stimulation

Waveform analysis of the BAEP provides information about the anatomical location of an injury

PEAK III

PEAK V