Intraoperative Nerve Monitoring

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Intraoperative Nerve Monitoring

Otology/Audiology
Author Credentials 

Matthew Kircher, MD 
Associate Professor 
Otology, Neurotology and Skull Base Surgery 
Department of Otolaryngology – Head & Neck Surgery 
Loyola University Medical Center Maywood, IL 60153 
mkircher@lumc.edu 

 

Module Summary

Intraoperative neurophysiologic monitoring is a valuable tool in otologic and neurotologic procedures. While intraoperative monitoring is not a substitute for good surgical technique, monitoring can provide valuable real time neural integrity feedback allowing for awareness and opportunity to change surgical maneuver. Current opinion holds that facial nerve monitoring is cost effective and beneficial for otologic and neurotologic surgery, and its use, at the discretion of the operating surgeon, can reduce the risk of iatrogenic facial nerve injury. 

Module Learning Objectives 
  1. Review the technical and interpretive aspects of intraoperative otologic and neurotologic nerve monitoring. 
  2. Describe the relative indications for surgical nerve monitoring. 
  3. Explain the proper setup for intraoperative nerve monitoring systems and understand that inadequate fundamental knowledge of neurophysiologic monitoring may lead to misinterpretations and an inability to troubleshoot system errors. 
  4. Discuss that intraoperative facial nerve monitoring can improve patient outcomes. 

  

 

Background/Anatomy
  1. Recognize that neurophysiologic monitoring in the context of otologic and neurotologic procedures most commonly deals with monitoring cranial nerves: a. VII - facial nerve 
  • VIII - vestibulocochlear nerve 
  • IX, X, XII - lower cranial nerves 
  1. Cite that the inherent complexity of facial nerve anatomy, combined with tumor distortion or anatomic variability of the nerve and adjacent structures, has prompted methods to minimize intraoperative facial nerve injury 
  2. Recognize that facial nerve monitoring is helpful in localizing the nerve within a tumor distorted surgical field. 
  3. Explain that while numerous modalities are available, including auditory brainstem recording, direct eighth nerve monitoring, motor and sensory evoked potentials, and vagus nerve monitoring via the recurrent laryngeal nerve (RLN), it is intraoperative facial nerve monitoring (IOFNM) that is the simplest and most effective modality to improve outcomes following otologic and neurotologic surgery. 

 

References

  1. Heman-Ackah, SE, Gupta, S, Lalwani, AK. Is facial nerve integrity monitoring of value in chronic ear surgery? Laryngoscope. 2013;123:2-3.  
  2. Tawfik KO, Walters ZA, Kohlberg GD, Lipschitz N, Breen JT, O'Neal K, Zuccarello M, Samy R. Impact of Motor-Evoked Potential Monitoring on Facial Nerve Outcomes after Vestibular Schwannoma Resection. Ann Otol Rhinol Laryngol. 2019 Jan;128(1):56-61. 
  3. Bhimrao SK, Le TN, Dong CC, Makarenko S, Wongprasartsuk S, Westerberg BD, Akagami R. Role of Facial Nerve Motor-Evoked Potential Radio in Predicting Facial Nerve Function in Vestibular Schwannoma Surgery Both Immediate and at 1 year. Otol Neurotol. 2016 Sep;37(8):1162-7. 
  4. Kircher, ML, Kartush, JM. Pitfalls in intraoperative nerve monitoring during vestibular schwannoma surgery. Neurosurg Focus. 2012;33:E5. 
  5. Prasad SC, Russo A, Taibah A, Pasanisi E, Galletti F, Sanna M. Management of the Facial Nerve During Vestibular Schwannoma Microsurgery. “Part VI Management: Microsurgery.” In: Carlson ML, editors. Comprehensive Management of Vestibular 
    Schwannoma. New York: Thieme Medical Publishers; 2019. P.219-223. 

 

 

Indications
  1. Realize that identification of the facial nerve is a critical step in temporal bone procedures, and that facial nerve injury is a potentially devastating complication.  
  2. Explain the relative and absolute indications for intraoperative neurophysiologic monitoring in otologic and neurotologic surgery. 
  • Posterior fossa tumor surgery is an indication for monitoring involved cranial nerves. 
  • Otologic surgery is a relative indication for facial nerve monitoring. 

 
References

  1. Sampath, P, Holliday, MJ, Brem, H, Niparko, JK, Long, DM. Facial nerve injury in acoustic neuroma (vestibular schwannoma) surgery: etiology and prevention. J Neurosurg. 1997;87:60-66.  
  2. Lalwani, AK, Butt, FY-S, Jackler, RK, Pitts, LH, Yingling, CD. Facial nerve outcome after acoustic neuroma surgery: a study from the era of cranial nerve monitoring. Otolaryngol Head Neck Surg. 1994;111:561-570. 
  3. Eshraghi AA, Connell SS, Chang RC, Telischi FF. Intraoperative Neurophysiologic Monitoring. In: Brachmann D, Shelton C, Arriaga M, eds. Otologic Surgery. 3rd ed. Philadelphia: Saunders Elsevier; 2010. p. 773-784. 

 

Incidence/Usage
  1. Realize that intraoperative neurophysiological monitoring during skull base surgery has become common practice among skull base surgical teams. 
  2. Cite that there is a growing trend among otolaryngologists in recent years to utilize IOFNM in chronic ear surgery. 
  3. Recognize that intraoperative eighth nerve monitoring is commonly employed during hearing preservation skull base procedures. 

 
References

  1. Gidley PW, Maw J, Gantz B, Kaylie D, Lambert P, Malekzadeh S, Chandrasekhar S S. Contemporary Opinions on Intraoperative Facial Nerve Monitoring. OTO Open. 2018. Available: https://journals.sagepub.com/doi/full/10.1177/2473974X18791803
  2. Roche JP, Hansen MR. Intraoperative Eight Nerve Monitoring During Vestibular Schwannoma Microsurgery. “Part VI Management: Microsurgery.” Comprehensive Management of Vestibular Schwannoma.” In: Carlson ML, editors. Comprehensive Management of Vestibular Schwannoma. New York: Thieme Medical Publishers; 2019. p.173-184. 
  3. Friedmann DR, McMenomey SO, Roland JT. Intraoperative Facial Nerve Monitoring During Vestibular Schwannoma Microsurgery. “Part VI Management: Microsurgery.” In: Carlson ML, editors. Comprehensive Management of Vestibular Schwannoma. New York: Thieme Medical Publishers; 2019. p. 2019:164-172.  
  4. Eshraghi AA, Connell SS, Chang RC, Telischi FF. Intraoperative Neurophysiologic Monitoring. In: Brachmann D, Shelton C, Arriaga M, eds. Otologic Surgery. 3rd ed. Philadelphia: Saunders Elsevier; 2010:773-784. 
Techniques

Monitoring Neuromuscular Pathways 

  1. Recognize that current IOFNM techniques include direct electrical stimulation, free-running electromyography, and facial motor evoked potential. 
  • Subdermal needle electrodes at ipsilateral orbicularis oculi and orbicularis oris 
  1. Explain that monitoring other cranial nerves during skull base surgery may also be done using subdermal needle or surface electrode placement and can include: 
  • CN III, IV, VI in extraocular muscles 
  • Motor portion of CN V in temporalis or masseter muscle 
  • CN IX, X, XI, and XII from soft palate (CN IX), laryngeal muscle (CN X via endotracheal tube contact electrodes), sternocleidomastoid (CN XI) or tongue (CN XII) 
  1. Discuss that systemic neuromuscular blockade must be avoided to permit muscle stimulation and neural monitoring, and this fact must be communicated to the anesthesiologist. 
  2. Recognize that local anesthetic (especially in the middle ear with an exposed facial nerve) may also cause neuromuscular blockade and interfere with ability to monitor. 

 
Monitoring Auditory Nerve Pathway 

  1. Recognize that brainstem auditory evoked response (BAER) is a far-field response representing electric activity of auditory nerve, nuclei, and fiber tracts of the ascending auditory pathways 
  • BAER monitoring setup includes scalp electrodes and insert ear plug. 
  • Signal processing software employed to enhance small amplitude neural responses to improve signal-to-noise ratio and obtain reliable BAER waveform. 
  • Delay in intraoperative neural integrity feedback. 
  1. Explain that direct CN VIII monitoring or cochlear nerve action potential (CNAP) is a direct electrode placement on cochlear nerve 
  • Provides near-field large amplitude neural responses  
  • Resulting in near real-time intraoperative neural integrity feedback 

 

References

  1. Acioly MA, Liebsch M, de Aguiar PH, Tatagiba M. Facial nerve monitoring during cerebellopontine angle and skull base tumor surgery: a systematic review from description to current success on function prediction. World Neurosurg. 2013 Dec;80(6):e271-300. 
  2. Eshraghi AA, Connell SS, Chang RC, Telischi FF. Intraoperative Neurophysiologic Monitoring. In: Brachmann D, Shelton C, Arriaga M, eds. Otologic Surgery. 3rd ed. Philadelphia: Saunders Elsevier; 2010:773-784. 
  3. Kartush JM, Benscoter BJ. Intraoperative Facial Nerve Monitoring. In: Guntinas-Lichius O, Schaitkin BM, editors. Facial Nerve Disorders and Diseases: Diagnosis and Management. Stuttgart: Thieme; 2016. P. 200-212.
  4. Moller AR, Jannetta PJ. Auditory evoked potentials recorded from the cochlear nucleus and its vicinity in man. J Neurosurg 1983 Dec;59(6):1013-8. 
  5. Ruckenstein MJ, Cueva RA, Prioleau GR. Advantages of a new, atraumatic, self-retaining electrode for direct cochlear nerve monitoring. Skull Base Surg. 1997;7(2):69-75. 

 

Technical/Interpretation
  1. Recognize that a thorough understanding of monitoring technology is vital to avoid medical and legal pitfalls and provides real time neurophysiological status feedback, allowing the surgeon to detect and avoid stretch or ischemic nerve injury that may not otherwise be apparent with visualization only. 
  2. Identify that intraoperative prognostics are hampered by false-positive results. 
  3. Explain techniques for troubleshooting monitoring problems. 
  • Verify monitor set and working correctly by stimulating nerve of interest at first chance, if possible, prior to any significant dissection or manipulation. 
  • Direct communication with anesthesiologist to confirm no neuromuscular blocking agents administered. 
  • Direct communication with monitoring staff to check electric circuit, electrode placement and connections to monitor and probe. 

Monitoring Neuromuscular Pathways 

  1. Realize that train potentials in IOFNM represent repetitive EMG activity occurring seconds to minutes after stimulus from prolonged depolarization of the nerve beyond threshold and may be caused by: 
  • Direct or traction nerve injury 
  • Heat injury - suspect after laser application or cautery 
  • Changes in local temperature near the nerve (irrigation) - subsides after warming 
  1. Review artifact response (monitor activity not due to facial nerve stimulation) 
  • Common and may cause confusion. 
  • Inadequate anesthetic plane. 
  • Electrocautery saturates monitor with electric noise obscuring neural responses. 

Monitoring Auditory Nerve Pathway 

  1. BAER  
  • Disadvantages include delayed feedback with changes in neural integrity 
  1. CNAP 
  • Requires exposure of brainstem root entry zone in retrosigmoid procedures 
    • Tumor debulking to identify and preserve cochlear nerve 
  • Challenges with electrode displacement during dissection and/or tumor distortion of vestibulocochlear nerve 

 

References

  1. Eshraghi AA, Connell SS, Chang RC, Telischi FF. Intraoperative Neurophysiologic Monitoring. In: Brachmann D, Shelton C, Arriaga M, eds. Otologic Surgery. 3rd ed. Philadelphia: Saunders Elsevier; 2010:773-784. 
  2. Kartush JM, Benscoter BJ. Intraoperative Facial Nerve Monitoring. In: Guntinas-Lichius O, Schaitkin BM, editors. Facial Nerve Disorders and Diseases: Diagnosis and Management. Stuttgart: Thieme; 2016. P. 200-212.
  3. Roche JP, Hansen MR. Intraoperative Eight Nerve Monitoring During Vestibular Schwannoma Microsurgery. “Part VI Management: Microsurgery.” Comprehensive Management of Vestibular Schwannoma.” In: Carlson ML, editors. Comprehensive Management of Vestibular Schwannoma. New York: Thieme Medical Publishers; 2019. p.173-184. 
Patient Outcomes
  1. Recognize that IOFNM has been shown to decrease facial nerve morbidity and improve outcomes with skull base surgery and is commonly employed in this clinical situation. 
  2. Realize that routine use of IOFNM in otologic cases has been considered controversial, but more recent trends indicate increased usage in otologic surgery. 
  3. Realize that in lawsuits involving otologic surgery, facial nerve injury was the second most common alleged injury reported behind hearing loss. 

 
Medications

  1. Recognize that perioperative administration of hydroxyethyl starch and nimodipine may improve vestibular schwannoma functional outcome. 

 
Health Care Cost

  1. Cite that using a model of otologic surgery, intraoperative monitoring was found to be cost-effective when weighed against the significant health care cost burden created by a facial nerve injury. 

 
References

  1. Silverstein, H, Rosenberg, SI, Flanzer, J, Seidman, MD. Intraoperative facial nerve monitoring in acoustic neuroma surgery. Am J Otol. 1993;14:524-532.  
  2. Kartush, JM, Lundy, LB. Facial nerve outcome in acoustic neuroma surgery. Otolaryngol Clin North Am. 1992;25:623-647.  
  3. Ruhl DS, Hong SS, Littlefield PD. Lessons learned in otologic surgery: 30 years of malpractice cases in the United States. Otol Neurotol. 2013 Sep;34(7):1173-9.
  4. Scheller C, Richter HP, Engelhardt M, Köenigm R, Antoniadis G. The Influence of Prophylactic Vasoactive Treatment on Cochlear and Facial Nerve Functions after Vestibular Schwannoma Surgery: A Prospective and Open-Label Randomized Pilot Study,  Neurosurgery. 2007 Jul;61(1):92-7.
  5. Wilson, L, Lin, E, Lalwani, A. Cost-effectiveness of intraoperative facial nerve monitoring in middle ear or mastoid surgery. Laryngoscope. 2003;113:1736-1745. 

 

Review Questions
  1. What instructions should be communicated to your anesthesia team prior to induction when intraoperative evoked facial nerve monitoring is planned? 
  2. What is the appropriate strategy to employ when train potentials are noted on EMG after irrigating an exposed facial nerve? 
  3. What are some causes for artifact response on the intraoperative facial nerve monitor? 

 

 

Resources 

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