MONITORING OPERATIONS FOR VESTIBULAR SCHWANNOMA CHAPTER III.
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Transcript of MONITORING OPERATIONS FOR VESTIBULAR SCHWANNOMA CHAPTER III.
![Page 1: MONITORING OPERATIONS FOR VESTIBULAR SCHWANNOMA CHAPTER III.](https://reader036.fdocuments.net/reader036/viewer/2022062500/56649e675503460f94b62e4e/html5/thumbnails/1.jpg)
MONITORING OPERATIONS FOR VESTIBULAR
SCHWANNOMA
CHAPTER III
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Monitoring of the facial nerve is a model for monitoring other
cranial nerves
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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
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How to record the response?
• Recording of electromyographic (EMG) potentials
• Mechanical recordings of muscle contractions
• Recording of motor nerve CAP
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Recording of EMG potentials
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Recording muscle responses
• Muscle relaxants cannot be used
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Monitoring of motor systems
The facial nerve
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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
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Preservation of the facial nerve in vestibular
schwannoma operations
• Monopolar, constant voltage stimulation
• Facial EMG made audible
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Finding where the facial nerve is
and where it is not
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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
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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
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The likelihood of postoperative facial weakness
• Increases with the number of occurrences spontaneous EMG
• The duration of the activity is important
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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
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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
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Testing the function of the facial nerve
For prediction of post operative facial function
Always use electrical stimulation
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Use of EMG for decision making regarding grafting?
Electrophysiologic methods cannot distinguish between neurapraxia,
axonotmesis or neurotmesis
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Monitoring the trigeminal motor nerve (portio minor)
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Recording EMG from the masseter muscle (CNV) together with recording of facial EMG
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Difference in EMG response to stimulation of the facial and the trigeminal nerves
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The goals to reduce complications have been accomplished
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This lady could have hadfacial palsy on one sideafter an operation for a vestibular schwannoma
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Auditory neuromonitoring
Recording of auditory evoked potentials in operations in the
posterior fossa
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Auditory monitoring for preservation of the function of
the auditory nerve
Recording of auditory evoked potentials in operations in the
posterior fossa
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Monitoring of ABR can detect manipulations of the
brainstem before cardiovascular signs change
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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
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Recording of ABR elicited from the contra-lateral ear monitor the brainstem
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Recording of ABR elicited from the contra-lateral ear monitor the brainstem
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Amplitude of peak V decreases
BEFORE BLOOD PRESSURE
AFTER
Effect of brainstem manipulation
SAMETIME
BEFORE HEART RATE
AFTER
SAMETIME
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Latency of peak V increases
BEFOREBLOODPRESSURE
BEFOREHEART RATE
SAMETIME
SAMETIME
Effect of brainstem manipulation
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Waveform analysis of the ABR provides information about the
anatomical location of an injury
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Different conventions for display of BAEP
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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
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Waveform analysis of the BAEP provides information about the anatomical location of an injury
PEAK III
PEAK VIpsilateralstimulation
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Contralateral stimulation
Waveform analysis of the BAEP provides information about the anatomical location of an injury
PEAK III
PEAK V