monitoring operations for vestibular schwannoma
DESCRIPTION
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 - PowerPoint PPT PresentationTRANSCRIPT
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
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
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
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
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
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
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