Disclaimer: The information contained within the Grand Rounds Archive is intended for use by doctors and other health care professionals. These documents were prepared by resident physicians for presentation and discussion at a conference held at Baylor College of Medicine in Houston, Texas. No guarantees are made with respect to accuracy or timeliness of this material. This material should not be used as a basis for treatment decisions, and is not a substitute for professional consultation and/or peer-reviewed medical literature. Facial Nerve Monitoring In 1965, Dr. Donaldson said: "Knowledge of the precise relationship of the facial nerve to other landmarks in the temporal bone is the basic requirement for safe, effective temporal bone surgery." At the same time, surgery causes deliberate exposure and trauma to the nerve. While the otologist may be as familiar with temporal bone anatomy as he is with the rooms in his own house, normal anatomy can be distorted by trauma or cases of anomalous development. Under these circumstances, the surgeon feels like someone has rearranged the furniture in his house. All the pieces may be present but he can’t find them where he looks for them. In these cases facial nerve monitoring provides feedback regarding location, extent and facial nerve function and is important for the prevention of iatrogenic injury in those that are high risk for injury. But it is only effective when the operator has the requisite knowledge and training. In 1898 Dr. Krause performed a cochlear nerve section for tinnitus and noted that unipolar irritation of the facial nerve trunk with the weakest possible current resulted in contractions of the orbicularis occuli as well as the branches supplying the nose and the mouth. In 1965, Jaco designed a photoelectric sensing device, which is placed inside the patient’s mouth and responded to light transmission through the cheek and activated an audible signal. Two years later, in 1966, Parsons used electrical stimulation during parotid surgery and in 1979, Del Gado et al reported the use of facial EMG in cerebellopontine angle surgery. The goals of facial nerve monitoring are early identification of the nerve, warning the surgeon of any unexpected stimulation, mapping the course of the nerve by using electrical stimulation, reducing trauma to the facial nerve during rerouting or dissection, and evaluation and prognosis of facial nerve function at the conclusion of surgery. There are basically two types of facial nerve monitoring. The first is EMG. Ipsilateral facial muscle electromyographic activity is used and is continuously displayed into the O.R. via loud speaker. Needle electrodes are placed into the muscle. Electrical activity is sent between a needle electrode pair, and this potential difference is displayed on an oscilloscope. Positive feedback occurs in relation to surgical events. There is a background baseline white noise and increased EMG activity with either nerve irritation or direct nerve stimulation. This can also be due to nerve identification or used as a predictor of prognosis. There are basically two broad categories of responses seen on EMG: repetitive and non-repetitive. Pulse and burst responses are non-repetitive single responses to a single stimulus, due to direct mechanical or electrical stimulation. A pulse occurs in response to an electrical stimulation and sounds like a precisely timed click or a machine gun type of sound. A burst is due to direct surgical manipulation. This sounds more like a few clicks and is synchronous with surgical manipulation. A train response is a repetitive response. These are multiple responses from one or more motor units to a single stimulus, and are a result of prolonged depolarization beyond threshold for development of an action potential. This can be seen with traction, pressure, or even caloric effects from heat, and are repetitive asynchronous clicks that sound like popping corn. Burst responses are due to direct contact. With nerve mechanical stimulation you see a single spike. The train response is asynchronous and is due to pressure or traction on the nerve. Prolonged train activity is associated with nerve irritation and can indicate impending damage. The other type of monitoring device is the pressure or strain gauge sensor. This is a mechanical pressure sensor. It is positioned in the corner of the patient’s mouth. It applies light pressure to the facial muscles of the cheek. Muscle contractions cause a change in the pressure applied to the sensor. This results in an audible alarm that alerts the surgeon. These are supra- threshold responses that detect actual movement and are less sensitive in the EMG systems. Facial nerve monitoring can be used in conjunction with the nerve stimulator. Stimulators come in two basic flavors. The first is a monopolar, when current disseminates in all directions from the tip. This is dependent on the distance of the probe to the nerve as well as the conductive properties of the intervening tissue. The strength of the stimulus necessary is related to the integrity of the nerve. This is very useful in nerve identification. A bipolar stimulator has two probes - the current flow is limited to the region between those two probes. There is more precision with a bipolar stimulator and it requires closer proximity in alignment to the nerves. While a mono-stimulator is more useful in nerve identification, the bipolar stimulator is more useful if the course of the nerve as evident. The EMG system is more invasive, but more sensitive, and can give information regarding the nerve status. However, it is affected by external electrical noise and electrocautery use that can cause outside artifacts. It also requires a higher level of expertise to operate. The pressure strain-gauge system is inexpensive, and is not invasive. There is artifact with head movement but there is no electrical interference. It detects true muscle contractions, so it will not detect low levels of nerve irritation. When working at the skull base you can get cross stimulation from the trigeminal nerve if you stimulate the muscles of mastication as well. There are several factors to consider when you are using nerve monitoring. The first is nerve irritation and trauma. Constant manipulation produces trains of EMG activity that can lead to conduction blockade and contractions will not occur, even at higher settings. A preoperative facial nerve paralysis or paresis can confound monitoring results. External mechanical noise artifacts can interfere and cause false artifacts with the EMG system. Paralyzing agents or local anesthesia can also affect nerve monitoring as can the level of anesthesia. If a patient is very light and has spontaneous contractions or if the patient awakens these will be detected by the monitoring system. Lastly, there is the effect of electrical shunting. With non-insulated instruments, there can be shunting of current to the surrounding tissue. Breaks in the insulation or blood or CSF around the stimulating probe can also cause a shunting of current giving a false sense that either the nerve is not exposed or not functioning. Facial nerve monitoring is useful in otologic or neuro-otologic cases where there is increased relative risk of facial nerve injury. High risk cases are congenital ear cases, acoustic resections, facial nerve rerouting. Median risk type cases are revision mastoids, repair of external auditory canal exostoses, vestibular nerve sections or cases of severe inflammation. Stapedectomy, tympanoplasty and primary mastoidectomy are considered more low risk cases. Numerous studies have shown the benefits of intraoperative facial nerve monitoring in improving facial nerve outcomes in acoustic neuroma resection. Surgical goals incude total tumor removal and absence of major neurologic deficits. These are no longer major issues with modern otomicrosurgical techniques and are more routinely achieved. The last two goals are preservation of facial nerve function and preservation of hearing when feasible. Although modern otomicrosurgical techniques have improved rates of facial nerve preservation, by the early 1990s, advantages of nerve monitoring were becoming clear. Dickens and Graham in 1990, looked at 188 consecutive cerebellopontine angle cases. The first 38 patients were done without any monitoring. The next 29 patients were done with a surface detector or strain gauge sensor and the lasts 41 patients were done with a needle-evoked EMG. In the unmonitored group, 37% had a poor facial nerve outcome defined as House-Brackmann five or six. This drops to 21% with a surface detector strain gauge sensor and to 4% with EMG. Similar results are seen with good functional outcomes defined as a House-Brackmann I or II. There is 39% in the unmonitored group, increasing to 55% with the Jako stimulator, and then 87% with the EMG group. Some confounding variables here include increasing surgical experience over 11 years. Also, several tumors in the unmonitored group were greater than 3 cm while tumors in the EMG group were much smaller, reflecting increased use of MRI. EMG has resulted in changes in surgical technique that reduce neural trauma as well. Lalwani et al, from 1991 reviewed 129 cases of monitoring over four year period. They reported nearly 100% nerve preservation with 90% House-Brackman I or II function at one-year postop. Their long-term function was inversely correlated with tumor size independent surgical approach. Stimulation thresholds at the conclusion of the procedure were predictive of long-term facial nerve outcomes. In acoustic neuroma surgery, nerve monitoring has improved technical skills although there continues to be poor results with larger tumors. This is due to several factors that increase neuropraxic injury. The nerve is stretched and thinned in a larger tumor. There are more adhesions and there is longer operative time, resulting in greater surgical manipulation. These improved results also reflect a subtle change in operative philosophies. When weighing total tumor removal versus the risk of facial nerve disruption, some surgeons may opt for a near total resection which would involve leaving a thin veil of tumor capsule to the most adherent portion of the nerve. Nerve monitoring can be used as a predictor of facial nerve function, and most studies will show an excellent prognosis at one year with EMG response at 0.2 milliamps or less. Silverstein from 1998, showed House-Brackman I function for 19 of 20 patients with an EMG response at 0.1 milliamps at the conclusion of surgery. It is important to find a nerve segment that is representative of the entire nerve, however. Excellent stimulation can be seeb with a markedly splayed nerve if there is a small sub-population of fibers that are intact. Conversely, there can be a non-uniform injury which may lead to elevated thresholds in a patient with good postoperative function. In chronic ear surgery, the normal mastoid architecture can be distorted by inflammation, purulence or cholesteatoma. The risk of facial nerve injury is less in chronic ear surgery and has also been decreased with modern photo-microsurgical techniques. Paralysis rates or injury rates are 1% in primary cases and as high as 4-10% in all revision cases. This is also the second most common reason for litigation in otologic surgery after poor hearing outcomes after stapedectomy. There have been very few objective studies that have looked at intraoperative facial nerve monitoring in chronic ear surgery. Leonetti et al from 1990 ooked at 129 major otologic operations over two years. They subjectively chose 16 to be performed with a monitor. These were chosen because they had a greater than normal risk to the nerve. They had no difference in facial nerve outcomes with that small sample of patients. Silverstein and Rosenberg use monitoring for all cases and have done so since 1985. In 1991 they reported no immediate facial nerve paralysis in 500 consecutive monitored cases. They further stated that a monitor helped prevent injury to an exposed nerve in 20 cases. Pensak et al in 1994 looked at monitoring in a residency training program. They reviewed 250 cases for chronic otitis media, with or without cholesteatoma. They found that the facial nerve was grossly identified in 100% of cases and this visualization was confirmed by nerve stimulator in 82%. About one-third of cases had a dehiscent nerve. The auditory signal alerted the surgeon to an exposed nerve in 93%, but it failed to identify an exposed nerve in 7%. The monitor failed to work overall in 6% of cases beyond that. Supervising surgeons felt that monitoring affected the outcomes in two cases overall. All studies stress that reliance on monitoring in lieu of good surgical technique is to be condemned. Congenital ear surgery is a high-risk procedure. The facial nerve follows an anomalous course here, and the normal landmarks can be useless. Tympanic bone is often hypoplastic or atretic, and the nerve takes a more acute angle at the second genu. There have been iatrogenic injuries by even the most respected otologists. Nerve monitoring in these cases is a very important adjunct. Stapedectomy is more of a low risk procedure. In a review from the House Group from the 1970's of over 2,000 stapedectomies, the facial nerve injury rate was 0.2%. The facial nerve may be dehiscent over the oval window in up to 50% of temporal bones, but is only clinically significant in about 10-15% of those. The facial nerve is not at risk in just otologic surgery. In parotid surgery, permanent facial paralysis rates are reported as low as 3-5% but paresis rates range anywhere from 8 to 60%. The reason for this range is that reporting is not standardized making it is difficult to assess from the literature. Monitoring may be useful for high risk cases, those cases with a fixed mass or deep lobe involvement or revision cases when the nerve may be encased in scar tissue. Paralysis rates in these high risk cases can be as high as 30%. Witt reviewed 69 of his own cases done over a 10-year period. He excluded 16 high risk cases. The first 33 were performed without a monitor and the second 20 with a monitor. The transient paralysis rates were about the same. There was no permanent facial nerve paralysis and he concluded the best way to reduce paralysis is a keen understanding of anatomy and gentle surgical technique. In a follow-up study from 1999, Dulguerov et al looked at 70 consecutive patients done with nerve monitoring. They reported a 27% temporary deficit and a 4% permanent deficit. The permanent deficits were only nerves sectioned due to malignancy. Their results were directly influenced by the extent of surgery and especially, the histopathology. For benign pathology, the transient paresis rate dropped to 20% with 0% permanent deficits. Terrell et al from 1997 reviewed 117 patients done with seven different surgeons. Fifty-six were done with continuous EMG monitoring, and 61 were done without any monitoring. They found a statistically significant decreased temporary paresis in the monitored group. This ranged from 57% to 33% with a monitor, but there was no difference in long-term facial nerve outcomes. They did find that the longer OR times were associated with decreased paresis rates. Monitoring does result in increased cost, estimated at $380 per case. It does require an observer for interpretation and does not prevent direct injury to the nerve from dissection. Monitoring is not a very useful adjunct when the nerve and its branches are easily identifiable. While facial nerve monitoring is a well-accepted adjunct for acoustic neuroma or cerebellopontine angle surgery, the role of facial nerve monitoring and chronic ear surgery has been less well defined. A panel discussion from COSM in 1993 with a panel of five respected otologists found there no consensus for use of nerve monitoring in chronic ear surgery. Two of the five used a monitor for all cases and three out of five used it only when the nerve was at increased risk. One panelist did stress it was very important for revision surgery and, one mentioned how dissecting an acoustic neuroma off the facial nerve is no different from dissecting cholesteatoma matrix off the facial nerve. They all agreed that monitoring is valuable during residency training but is not a substitute for knowledge of temporal bone anatomy. There have been a few surveys done about monitor use. One is the Ear Foundation Alumni survey of the Otology Group Fellowship in Nashville. 93% of respondents used monitoring. They had 100% use for posterior fossa surgery, 71% for congenital atresia cases, and 21% for routine chronic ear surgery. Of those that did chronic ear surgery, 40% had an incidence of a seventh nerve injury and 15% of these occurred despite nerve monitoring. Two-thirds felt that monitoring was not the standard of care in their communities, and about two-thirds felt that monitoring should be used for resident teaching. Similar results were seen in a survey of the Ear Research Foundation Alumni survey where monitoring has been used for all cases since 1985. They had 100% use for posterior fossa and chronic ear surgery. One-third of their respondents reported a post-operative weakness, and two-thirds said monitoring was not the standard of care in their communities. Roland and Meyerhoff surveyed all members of the American Otologic and Neuro-Otologic Society. 95% of respondents said that monitoring should be used for neuro-otologic procedures and tympmastoid surgery when the nerve was at increased risk while 4% said it should be used for all tympmastoid surgery. Case Presentation: GQ is a 61-year old Latin-American female with a several year history of predominantly left-sided hearing loss and tinnitus. She denies any vertigo, otalgia, otorrhea, or aural fullness. She has not had any history of trauma, prior otologic surgery, or recent episodes of otitis. Her past medical and surgical histories are unremarkable. She takes no medications and has no known drug allergies. She denies any tobacco or alcohol use. Physical examination reveals clear and intact tympanic membranes. The nose is clear. Oral cavity and oropharynx are also clear. True vocal cords are mobile bilaterally. There is no lymphadenopathy or masses in the neck. Cranial nerves are intact. Audiogram showed an asymmetric left-sided sensorineural hearing loss with poor discrimination. MRI revealed a 1.5 cm mass involving the left sided cerebellopontine angle. The patient subsequently was taken to the operating room where she underwent a left translabyrinthine excision of an acoustic neuroma with facial nerve monitoring. The tumor was removed in its entirety, and the facial nerve was preserved. She had no post-operative facial nerve deficit and was discharged home on post-operative day six. Bibliography: Axon PR, Ramsden RT. Intraoperative electromyography for predicting facial function in vestibular schwannona surgery. Laryngoscope 1999;109:922-926. Beck DL, Atkins JS Jr, Brackmann DE. Intraoperative facial nerve monitoring: Prognostic aspects during acoustic tumor removal. Otolaryngol Head Neck Surg 1991;104:780-782. Beck DL, Benecke JE Jr. Intraoperative facial nerve monitoring: Technical aspects. Otolaryngol Head Neck Surg 1990;102:270-272. Benecke JE Jr, Calder HB, Chadwick G. Facial nerve monitoring during acoustic neuroma removal. Laryngoscope 1987;97:697-232. Benet E, Rosenberg SI, Willcox TO, Gordon M, Silverstein H. Intraoperative facial nerve monitoring: A comparison between electromyography and mechanical-pressure monitoring techniques. Am J Otol 1999;20:793-799. Dew LA, Shelton C. Iatrogenic facial nerve injury: Prevalence and predisposing factors. 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Terrell JE, Kileny PR, Yian C, Esclamado RM, Bradford CR, Pillsbury MS, Wolf GT. Clinical outcome of continuous facial nerve monitoring during primary parotidectomy. Arch Otolaryngol Head Neck Surg 1997;123:1081-1087. Witt RL. Facial nerve monitoring in parotid surgery: The standard of care? Otolaryngol Head Neck Surg 1998;119:468-470. Yingling CD, Gardi JN. Intraoperative monitoring of facial and cochlear nerves during acoustic neuroma surgery. Otolaryngol Clin North Am 1992;25:413-449. Zeitouni AG, Hammerschlag PE, Cohen NL. Prognostic significance of intraoperative facial nerve stimulus thresholds. Am J Otol 1997;18:494-497. BCM Public | BCM Intranet | Privacy Notices | Contact BCM | BCM Site Map | ©2001-2005 Baylor College of Medicine
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