1. MONITORING OPERATIONS FOR VESTIBULAR SCHWANNOMA CHAPTER III

2. Monitoring of the facial nerve is a model for monitoring other cranial nerves

3. 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

4. How to record the response? Recording of electromyographic (EMG) potentials Mechanical recordings of muscle contractions Recording of motor nerve CAP

5. Recording of EMG potentials

6. Recording muscle responses Muscle relaxants cannot be used

7. Monitoring of motor systems The facial nerve

8. 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

9. Preservation of the facial nerve in vestibular schwannoma operations Monopolar, constant voltage stimulation Facial EMG made audible

11. Finding where the facial nerve is and where it is not

12. 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

13. 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

14. The likelihood of postoperative facial weakness Increases with the number of occurrences spontaneous EMG The duration of the activity is important

15. 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

16. 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

17. Testing the function of the facial nerve For prediction of post operative facial function Always use electrical stimulation

18. Use of EMG for decision making regarding grafting? Electrophysiologic methods cannot distinguish between neurapraxia, axonotmesis or neurotmesis

19. Monitoring the trigeminal motor nerve (portio minor)

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

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

22. The goals to reduce complications have been accomplished

23. This lady could have had facial palsy on one side after an operation for a vestibular schwannoma

24. Auditory neuromonitoring Recording of auditory evoked potentials in operations in the posterior fossa

25. Auditory monitoring for preservation of the function of the auditory nerve Recording of auditory evoked potentials in operations in the posterior fossa

26. Monitoring of ABR can detect manipulations of the brainstem before cardiovascular signs change

27. 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

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

30. Amplitude of peak V decreases BEFORE BLOOD PRESSURE AFTER Effect of brainstem manipulation SAME TIME BEFORE HEART RATE AFTER SAME TIME

31. Latency of peak V increases BEFORE BLOOD PRESSURE BEFORE HEART RATE SAME TIME SAME TIME Effect of brainstem manipulation

32. Waveform analysis of the ABR provides information about the anatomical location of an injury

33. Different conventions for display of BAEP

34. 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

35. Waveform analysis of the BAEP provides information about the anatomical location of an injury PEAK III PEAK V Ipsilateral stimulation

36. Contralateral stimulation Waveform analysis of the BAEP provides information about the anatomical location of an injury PEAK III PEAK V