Intraoperative Cranial Nerve Monitoring During Posterior Skull Base Surgery

Intraoperative monitoring of neurophysiologic function is rapidly evolving as an important adjunct during skull base surgery to reduce the incidence of neurologic deficit. Facial nerve monitoring is an excellent model, since electrical and mechanical evoked potentials can be directly presented to the surgeon in real-time through an acoustic loudspeaker display. The lower cranial nerves may also be monitored using similar electromyographic techniques. Auditory system monitoring is more difficult due to the low amplitude response that requires averaging and filtering to extract the evoked potential. In conjunction with auditory monitoring, improved hearing preservation may be further enhanced by concomitant facial nerve monitoring, since the surgeon is alerted to traumatic manipulations that may affect both facial and cochlear nerves. Techniques and interpretative issues are presented to maximize the efficacy and safety of cranial nerve monitoring.     

Intraoperative monitoring of motor cranial nerves in skull base surgery

Intraoperative monitoring of cranial nerves is performed to minimize postoperative cranial nerve dysfunction. We performed electrophysiologic monitoring of motor cranial nerves with a NIM 2 unit from Xomed Treace and a patient multiplexer developed in our clinic. This multiplexer allows simultaneous monitoring of four cranial nerves and is additionally equipped with a bipolar stimulation mode. This intraoperative monitoring was used during 102 skull base operations. Of these, 44 operations were acoustic neuroma removals by translabyrinthine approach and 36 by a middle fossa approach. Various operations, including removal of tumors of the jugular foramen and the infratemporal fossa, were performed in the remaining 22 patients. The facial nerve, being the most frequently monitored nerve, was evaluated both preoperatively and intraoperatively. Electrophysiologic data were evaluated with respect to their predictive value for postoperative facial nerve function. The relative percent decrease in amplitude of the electromyogram after resection compared to that observed before resection seems to be of some predictive value for the postoperative facial nerve function. A 50 to 60% decrease or more is associated with an increase in the House classification. Intraoperative monitoring is a useful tool in skull base surgery, allowing for safer and faster identification of motor nerves in pathologic-anatomic conditions. It allows the surgeon a degree of comfort by providing immediate information regarding the status of the nerve. It may also improve postoperative nerve function and shorten operating time. Additionally, neuromonitoring provides some information about expected postoperative facial nerve function.     

Electrophysiological identification of the cochlear and vestibular nerves in the cerebellopontine angle: experimental study and clinical implication]

Electrical stimulation was applied on the surface of the eighth cranial nerve in the cerebellopontine angle of dogs and the evoked potentials were recorded with surface electrodes on the scalp. The shape of the evoked potentials recorded was different according to the portion of the nerve stimulated electrically. It was, therefore, possible to precisely identify the nerve stimulated from the surface recorded evoked potentials. This electrophysiological method may be helpful for the surgeon to precisely locate the cochlear and vestibular nerves in various operations in the cerebellopontine angle in human. Our preliminary experience of this monitoring method in acoustic neuroma excision was presented.     

Issues in the Optimal Selection of a Cranial Nerve Monitoring System

Intraoperative nerve monitoring (IONM) is a safe technique that is of clear clinical value in the preservation of cranial nerves in skull base surgery and is rapidly becoming the standard of care. Available nerve monitoring systems vary widely in capabilities and costs. A well-informed surgeon may best decide on monitoring needs based on surgical case selection, experience, operating room space, availability of monitoring personnel, and cost. Key system characteristics that should be reviewed in the decision-making process include the monitoring technique (electromyography, pressure transducer, direct nerve monitoring, brainstem auditory evoked potential) and the stimulus technique (stimulating parameters, probe selection). In the past, IONM has been primarily employed in posterior fossa and temporal bone surgery, but the value of IONM is being recognized in more skull base and head and neck surgeries. Suggested IONM strategies for specific surgeries are presented.     

Issues in the optimal selection of a cranial nerve monitoring system

Intraoperative nerve monitoring (IONM) is a safe technique that is of clear clinical value in the preservation of cranial nerves in skull base surgery and is rapidly becoming the standard of care. Available nerve monitoring systems vary widely in capabilities and costs. A well-informed surgeon may best decide on monitoring needs based on surgical case selection, experience, operating room space, availability of monitoring personnel, and cost. Key system characteristics that should be reviewed in the decision-making process include the monitoring technique (electromyography, pressure transducer, direct nerve monitoring, brainstem auditory evoked potential) and the stimulus technique (stimulating parameters, probe selection). In the past, IONM has been primarily employed in posterior fossa and temporal bone surgery, but the value of IONM is being recognized in more skull base and head and neck surgeries. Suggested IONM strategies for specific surgeries are presented.     

Technical aspects of intraoperative monitoring of lower cranial nerve function

The efficacy of monitoring facial nerve activity in decreasing long-term morbidity has promoted an interest in monitoring other at-risk cranial nerves during procedures that involve manipulation of the basal cranial nerves. This presentation details practical techniques for monitoring the lower cranial nerves, which have been experientially developed over the past 9 years. Emphasis is placed on the selection of electrodes and procedural changes required for reliable and safe stimulation of the basal cranial nerves. Either paired hook-wire or tethered needle electrodes can be used for monitoring glossopharyageal, accessory, and hypoglossal nerve function. Several options for monitoring vagus nerve function are discussed. Of these, the transoral placement of paired hook-wire electrodes remains the most reliable, cost-effective, and least morbid technique. Electrical stimulation of the glossopharyngeal and vagus nerves carries the risk of unanticipated, potentially irreversible disturbances in cardiovascular function. Guidelines for type and optimal placement of stimulating electrodes and recommended intensity levels to prevent unfavorable reactions are presented.     

Technical Aspects of Intraoperative Monitoring of Lower Cranial Nerve Function

The efficacy of monitoring facial nerve activity in decreasing long-term morbidity has promoted an interest in monitoring other at-risk cranial nerves during procedures that involve manipulation of the basal cranial nerves. This presentation details practical techniques for monitoring the lower cranial nerves, which have been experientially developed over the past 9 years. Emphasis is placed on the selection of electrodes and procedural changes required for reliable and safe stimulation of the basal cranial nerves. Either paired hook-wire or tethered needle electrodes can be used for monitoring glossopharyageal, accessory, and hypoglossal nerve function. Several options for monitoring vagus nerve function are discussed. Of these, the transoral placement of paired hook-wire electrodes remains the most reliable, cost-effective, and least morbid technique. Electrical stimulation of the glossopharyngeal and vagus nerves carries the risk of unanticipated, potentially irreversible disturbances in cardiovascular function. Guidelines for type and optimal placement of stimulating electrodes and recommended intensity levels to prevent unfavorable reactions are presented.     

Intraoperative recurrent laryngeal nerve monitoring

Intraoperative nerve monitoring has become common for surgical procedures in which cranial or peripheral nerves may be compromised. Intraoperative monitoring of recurrent laryngeal nerve function can be accomplished by recording electromyographic activity from fine-wire electrodes placed in the vocalis muscle. The technique and instrumentation for this are adapted from those used in intraoperative facial nerve monitoring for acoustic neuroma excision. Specifically, a commercially available instrument, the XOMED-NIM provides the capability for monitoring the vocalis muscle electromyogram by means of visual and auditory display. It also provides the capability for performing evoked electromyographic tests of nerve integrity. Intraoperative monitoring of the recurrent laryngeal nerve during thyroidectomy may assist in the more precise dissection of the nerve as well as in verification of nerve integrity during the operative procedure, thus reducing the risk of injury.     

Orthodromic (intra/extracranial) neurography to monitor facial nerve function intraoperatively

This report introduces the technique of orthodromic neurography for monitoring of facial nerve function during operation in the cerebellopontine angle. By stimulation of the intracisternal segment of the facial nerve, a compound nerve action potential with amplitudes of 15 to 480 microV can be recorded extracranially from the nerve near the stylomastoid foramen after 0.95 to 2.27 ms. Usually there is no need for signal averaging, and the method is independent of the effect of muscle relaxants. With the use of the same electrophysiological equipment as for evoked potential neuromonitoring, immediate and repeated localization of the facial nerve and its discrimination from the trigeminal and the lower cranial nerves during nerve preparation within the tumor capsule is possible.     

Évaluation électrophysiologique des nerfs crâniens moteurs (V, VII, IX, X, XI, XII)

In various neurosurgical operations, there is a risk of cranial nerve lesion that can be avoided or minimized with intraoperative electrophysiological monitoring. Regarding motor function of the cranial nerves, stimulodetection techniques are used, including electrical stimulation of nerve trunks and electromyographic recording of evoked motor responses. These techniques can be used for monitoring the trigeminal nerve (Vth cranial nerve), facial nerve (VIIth), glossopharyngeal nerve (IXth), pneumogastric nerve (Xth), spinal accessory nerve (XIth), and hypoglossal nerve (XIIth), in particular during surgical removal of tumors of the cerebellopontine angle or skull base. When beginning an operation, electrical stimulation is only used to identify the nerve structures. As removal of the tumor progresses, the goal is to verify that a surgical injury to the nerve is avoided by looking for the absence of any change regarding amplitude, morphology, and latency of motor responses. Intraoperative electromyographic monitoring can also be applied during the surgical treatment of primary hemifacial spasm by microvascular decompression. An effective decompression is usually associated with the disappearance of “lateral spread” motor responses to facial nerve branch stimulation. Therefore, the intraoperative disappearance of the lateral spread responses can be considered a predictive factor of good postoperative clinical outcome, even if this assertion remains a matter of debate.     

Occurrence of vestibular and facial nerve injury following cerebellopontine angle operations

Summary To elucidate how surgery in the cerebellopontine (CP) angle may cause vestibular and facial nerve injury, the 7th and 8th cranial nerves of dogs were manipulated as in human surgery along with monitoring of auditory evoked brain stem responses. Postoperatively, histological examinations were performed to investigate the effect of the surgical manipulations.We found that the occurrence of vestibular, facial and cochlear nerve injury was dependent on the direction of theexcessive movement of the nerves in the cerebellopontine (CP) angle. Caudal-to-rostral shift of the nerve trunk most effectively avulsed the vestibular nerve. Haemorrhages were revealed between the vestibular ganglion and the fundus of the internal auditory canal. This caudal-to-rostral retraction could also damage the facial nerve in its intrapetrous labyrinthine portion. This was likely to be one of the pathophysiological mechanisms responsible for postoperative facial nerve palsy occasionally observed in human cases.Rostral-to-caudal retraction of the cerebellum damaged the cochlear nerve selectively. Although caudal-to-rostral retraction, instead of lateral-to-medial one, has been recommended to protect the cochlear nerve, this retraction was shown to be dangerous to the vestibular nerve if excessive.The clinical significance of the fragility of the vestibular nerve was discussed and the importance of preserving the vestibular nerve function is stressed.     

Hamartomas of the internal auditory canal: report of two cases

OBJECTIVE AND IMPORTANCE: To highlight the clinical, radiological, and surgical findings and therapeutic options for this rare entity, which may mimic a purely intrameatal vestibular schwannoma, and to define the particular aspects of preoperative differential diagnosis and surgical management. CLINICAL PRESENTATION: Two patients presented with clinical findings typical of vestibular schwannomas, i.e., tinnitus, hearing loss of 30 dB, and an intrameatal contrast-enhancing lesion on magnetic resonance imaging studies. TECHNIQUE: The lesions were exposed via a suboccipital transmeatal approach, and tumor infiltration of the cochlear and/or facial cranial nerves was identified. In view of the unclear intraoperative histology, surgical management was based on criteria of cranial nerve function. In Patient 1, after nerve decompression by subtotal tumor removal, preserved auditory brainstem responses and facial nerve electromyography indicated functional nerve preservation and facilitated the decision for partial resection. In Patient 2, minimal tumor dissection resulted in complete loss of auditory brainstem response without reversibility. Therefore, a radical tumor removal was performed that sacrificed the cochlear but preserved the facial nerve. CONCLUSION: Symptoms and signs of internal auditory canal hamartomas are congruent with other typical pathological lesions of the internal auditory canal and cerebellopontine angle. Accurate preoperative diagnosis by radiological means is not possible, but careful evaluation of the different signal intensities on magnetic resonance imaging studies may indicate this rare pathological condition. Intraoperative surgical findings of tumor infiltration of the faciocochlear cranial nerve complex may support simple observation. In view of the nonneoplastic characteristic of these lesions, a more conservative approach is justified. The decision should be based on the functional status of the cranial nerves, for which reliable electrophysiological monitoring is indispensable.     

Continuous electromyography monitoring of motor cranial nerves during cerebellopontine angle surgery

OBJECT: Electromyography (EMG) monitoring is expected to reduce the incidence of motor cranial nerve deficits in cerebellopontine angle surgery. The aim of this study was to provide a detailed analysis of intraoperative EMG phenomena with respect to their surgical significance. METHODS: Using a system that continuously records facial and lower cranial nerve EMG signals during the entire operative procedure, the authors examined 30 patients undergoing surgery on acoustic neuroma (24 patients) or meningioma (six patients). Free-running EMG signals were recorded from muscles targeted by the facial, trigeminal, and lower cranial nerves, and were analyzed off-line with respect to waveform characteristics, frequencies, and amplitudes. Intraoperative measurements were correlated with typical surgical maneuvers and postoperative outcomes. Characteristic EMG discharges were obtained: spikes and bursts were recorded immediately following the direct manipulation of a dissecting instrument near the cranial nerve, but also during periods when the nerve had not yet been exposed. Bursts could be precisely attributed to contact activity. Three distinct types of trains were identified: A, B, and C trains. Whereas B and C trains are irrelevant with respect to postoperative outcome, the A train--a sinusoidal, symmetrical sequence of high-frequency and low-amplitude signals--was observed in 19 patients and could be well correlated with additional postoperative facial nerve paresis (in 18 patients). CONCLUSIONS: It could be demonstrated that the occurrence of A trains is a highly reliable predictor for postoperative facial palsy. Although some degree of functional worsening is to be expected postoperatively, there is a good chance of avoiding major deficits by warning the surgeon early. Continuous EMG monitoring is superior to electrical nerve stimulation or acoustic loudspeaker monitoring alone. The detailed analysis of EMG-waveform characteristics is able to provide more accurate warning criteria during surgery.     

Cochlear and vestibular gross and histologic anatomy (as seen from postauricular approach)

The otologic surgeon must have a clear understanding of the anatomy of the seventh and eighth cranial nerves from the labyrinth to the brain stem, as seen from the postauricular approach. The surgical anatomy of the seventh and eighth cranial nerves was studied in 64 transcochlear eighth-nerve sections and 33 retrolabyrinthine vestibular neurectomies. Analysis indicates the nerves rotate 90 degrees in their course from the ear to the brain. The key relationship is that the cochlear nerve is always the most inferior, rotating from anterior (medial) near the labyrinth to posterior (lateral) near the brain stem. The seventh (facial) nerve rotates from anterosuperior (medial superior) near the labyrinth to anteroinferior (medial inferior) near the brain stem. The seventh nerve is easily seen in the transcochlear approach and hidden from view in the retrolabyrinthine approach. Twenty-seven fixed nerve specimens were examined with an operating microscope before being prepared for sectioning. In 73% (19 of 26) a cleavage plane was seen on the lateral aspect of the eighth nerve (that portion of the nerve facing the surgeon in the retrolabyrinthine approach).     

Intraoperative monitoring of the facial nerve: anesthesia and neurophysiology considerations

Intraoperative monitoring of the motor component of the 7th cranial pair or any other nerve should be routine during any surgical procedure involving risk of neural damage, whether or not the skull is opened. This paper discusses the main indications for monitoring facial and acoustic nerves during ear and neurosurgery involving the pontocerebellar angle (for acoustic neuroma or in surgery on the 8th cranial pair). Intraoperative electromyographic monitoring of the facial nerve should be used routinely in acoustic neuroma surgery to reduce the degree of postoperative neurological impairment and avoid possible malpractice suits. Although such monitoring requires that the muscle remain unblocked, intraoperative neuromuscular relaxants can be used if doses are administered along with appropriate monitoring of the level of peripheral neuromuscular block.     

A newly designed nerve monitor for microneurosurgery: Bipolar constant current nerve stimulator and movement detector with a pressure sensor

Summary The authors describe a newly designed nerve monitor which is useful for numerous microneurosurgical procedures. Standard bipolar forceps are used to apply constant current stimulation. Muscle contraction evoked by the stimulation is detected by a small discshaped pressure sensor taped to the overlying skin. The responses are monitored both quantitatively on a liquid crystal display and qualitatively through an on-off auditory signal. Surgery can proceed without interruption. This apparatus can safely and reliably monitor the facial nerve, nerves involved in eye movements, lower cranial nerves and spinal nerves. This portable system weighs only 1.8 kg and can easily be used by a neurosurgeon.     

Effectiveness of C5 or C6-Cz assembly in predicting immediate post operative facial nerve deficit

Background

Intraoperative neurophysiology monitoring (IOM) is a valuable tool in cerebellopontine angle (CPA) surgeries posing risk to the cranial nerves. Transcranial electrical stimulation (TES) for cranial nerves has been performed in the last 7 years, for obtaining the facial nerve motor evoked potential (MEP), using either C3/C4-Cz or C3-C4 (or inverse) stimulating points, which have been correlated with facial nerve functional outcome.

Method

Intraoperative surgical and electrophysiological findings were documented prospectively. Patient files were reviewed for clinical data. We studied 23 patients undergoing CPA tumor resection using C5 or C6-Cz montage for TES, and were able to determine the correlation between facial nerve functional outcome and the amplitude drop of facial MEP above 50 %. Patients were evaluated for immediate facial nerve outcome and 6 months after the surgery. Follow-up was performed by structured telephone interviews with local physicians.

Results

The sensibility of the studied parameters was 92.8 % for amplitude drop of facial nerve MEP, with positive predictive value of 81.2 %. The absence of changes during IOM has shown a negative predictive value of 100 %.

Conclusion

In this series, the used montage was effective in predicting new facial deficit.     

Monitoring auditory nerve potentials during operations in the cerebellopontine angle

Direct monitoring of auditory nerve potentials was performed in 19 patients undergoing retromastoid craniectomy and microvascular decompression of cranial nerves. In addition, brain stem auditory evoked potentials (BSEPs) were monitored in these patients. No patient suffered significant hearing loss. Direct monitoring of auditory nerve potentials complements the recording of BSEPs because the auditory nerve potentials can be visualized without averaging many responses. Therefore the effect of any intraoperative manipulation that is harmful to the auditory nerve can be detected instantaneously.     

Recordings of evoked electromyographic responses from the extraocular muscles to monitor the oculomotor, trochlear, and abducens nerve function during skull base and orbital surgery

The methods to record evoked electromyographic responses by micro malleable clip electrodes applied directly to the extraocular muscles are described. This electrophysiological monitoring enables surgeons to localize the ocular motor nerves accurately in the skull base of the middle, posterior fossa, and orbit. In cavernous sinus surgery, electrical stimulation over the dura elicited vigorous responses from the extraocular muscles and subsequently it was possible for the surgeon to avoid severing the ocular motor nerves. In orbital surgery, distended and thinned extraocular muscles were precisely localized and preserved anatomically and functionally. These monitoring methods may play the same role as electrical stimulation to the facial nerves in acoustic neuroma surgery.