Electrophysiological monitoring during acoustic neuroma and other posterior fossa surgery

The brainstem and its neurosurgical history

Brainstem is one of the most complex structures of the human body, and has the most complex intracranial anatomy, which makes surgery at this level the most difficult. Due to its hidden position, the brainstem became known later by anatomists, and moreover, brainstem surgery cannot be understood without knowing the evolution of ideas in neuroanatomy, neuropathology, and neuroscience. Starting from the first attempts at identifying brainstem anatomy in prehistory and antiquity, the history of brainstem discoveries and approach may be divided into four periods: macroscopic anatomy, microscopic anatomy and neurophysiology, posterior fossa surgery, and brainstem surgery. From the first trepanning of the posterior fossa and later finger surgery, to the occurrence of safe entry zones, this paper aims to review how neuroanatomy and brainstem surgery were understood historically, and how the surgical technique evolved from Galen of Pergamon up to the twenty-first century.

Cochlear nerve action potential monitoring with the microdissector in vestibular schwannoma surgery

We developed a cochlear nerve action potential (CNAP) monitoring technique using a microdissector and compared the results of CNAP and auditory brainstem response (ABR) monitoring. Thirty-six patients underwent vestibular schwannoma resection via the retrosigmoid approach to preserve hearing. Both CNAP with the microdissector and surface ABR were recorded during the operation. We used the microdissector as an intracranial electrode for CNAP monitoring. The CNAP waveform was classified into four types: triphasic, biphasic, positive, and flat. At the completion of the tumor resection, the triphasic waveform was observed in 11 patients and the biphasic waveform was observed in 11 patients. Hearing function was preserved in all of them, although it was preserved in only two patients with other CNAP waveform types. The prognostic value of CNAP is significantly higher than that of ABR. We found that although CNAP with a microdissector does not provide real-time monitoring, with the classification of waveforms it can be used as predictable tool for postoperative hearing more accurately than ABR.

Intraoperative cranial nerve monitoring

The purpose of intraoperative monitoring is to preserve function and prevent injury to the nervous system at a time when clinical examination is not possible. Cranial nerves are delicate structures and are susceptible to damage by mechanical trauma or ischemia during intracranial and extracranial surgery. A number of reliable electrodiagnostic techniques, including nerve conduction studies, electromyography, and the recording of evoked potentials have been adapted to the study of cranial nerve function during surgery. A growing body of evidence supports the utility of intraoperative monitoring of cranial nerve nerves during selected surgical procedures.

Intra-operative electromyographic monitoring of the lower cranial motor nerves (LCN IX-XII) in skull base surgery

The functional preservation of lower (motor) cranial nerves (LCN) is endangered during skull base surgery. Intra-operative EMG monitoring of the LCN IX-XII was investigated in 78 patients undergoing 80 operations on various skull base tumors with regard to technical feasibility and clinical efficacy. Ongoing 'spontaneous muscle activity' (SMA) and 'compound muscle action potentials' (CMAP) following supramaximal bipolar stimulation were intra-operatively recorded applying needle electrodes into the soft palate (CN IX: n=76), the vocal cord (CN X: n=72), the trapezius muscle (CN XI: n=18), and the tongue (CN XII: n=71). From 24/22/8 cases with LCN IX/X/XII deficits (despite monitoring) only 5/6/4 remained unchanged (3-6 months postoperative). An irreversible plegia of the LCN IX/X/XII occurred in three (1/1/1) patients. In 7/6/1 patients postoperative (3-6 months) LCN IX/X/XII function was better than preoperatively. In all patients accessory nerve function remained unchanged. 'Pathological' SMA of the LCN IX/X/XII occurred in 12/16/8 cases, but in only 6/5/3 cases corresponded to postoperative LCN deficits. Corresponding 'pathological' SMA patterns were found in 18/17/5 out of 24/22/8 cases with postoperative LCN IX/X/XII dysfunction. Reproducible CMAP of LCN IX/X/XI/XII could be recorded in 59/56/11/32 patients. Approximate 'normal' values were calculated and compared to (very few) data so far given in the literature. Electromyographic monitoring proved to be a safe tool for the intra-operative identification and localization of the LCN contributing to their anatomical and functional preservation. The predictive value of standard neurophysiological parameters for functional outcome, however, is limited.

Acoustic neuroma surgery: absent auditory brainstem response does not contraindicate attempted hearing preservation

OBJECTIVE

Absence of auditory brainstem response (ABR) waveforms has been associated with a poor likelihood of hearing preservation following resection of acoustic neuromas. Our experience is reviewed for patients with absent preoperative ABR regarding hearing preservation, hearing improvement, and return of ABR.

STUDY DESIGN

Retrospective review of 22 cases of acoustic neuroma resection. Nine patients with absent preoperative ABR were identified. All underwent tumor resection utilizing intraoperative cochlear nerve action potential (CNAP) monitoring. Postoperative hearing results and ABR waveforms were examined.

METHODS

Charts were reviewed and tabulated for age, sex, tumor side, tumor size, preoperative and postoperative audiometric and ABR results, intraoperative monitoring results by ABR and CNAP, and surgical complications.

RESULTS

Hearing preservation was achieved in seven of nine patients (78%) with absent preoperative ABR, as well as six of seven patients (86%) with tumors less than or equal to 20 mm in greatest dimension. Although intraoperative ABR monitoring was not possible in any of these patients, CNAP monitoring was successful in all. Return of ABR waveforms was observed in four of the six patients (67%) tested from 3 to 22 months postoperatively. Four of the seven patients (57%) enjoyed improvement in hearing class as defined by the guidelines of the American Academy of Otolaryngology-Head and Neck Surgery.

CONCLUSIONS

Absent ABR waveforms have not been a negative prognostic sign regarding hearing preservation. CNAP monitoring is possible in these patients and likely helps to minimize iatrogenic cochlear nerve trauma. Patients with no ABR waveforms have hope of hearing preservation and even improvement following acoustic neuroma resection performed utilizing CNAP monitoring and hearing preservation surgical techniques.

Neurotopographic considerations in the microsurgical treatment of small acoustic neurinomas

OBJECT

The authors studied the relationships between tumor size, location, and topographic position relative to the intact facial nerve bundles in acoustic neurinomas to determine the influence of these factors on hearing preservation postoperatively. Consistent topographic relationships were found.

METHODS

Four hundred fifty-two patients with acoustic neurinoma treated via a retrosigmoid approach were analyzed with respect to hearing preservation and facial nerve function. One hundred fifteen tumors were identified as small and were categorized as Grades I and II. Patients with Grade I tumors, that is, purely intracanalicular lesions, all had good hearing preoperatively, defined by a less than 50-dB pure tone average and 50% speech discrimination score. All 14 Grade I tumors were removed, resulting in preservation of the patient's hearing by these criteria. There were no particular topographic anatomical relationships associated with these tumors that affected hearing preservation. Grade II tumors, defined as those protruding into the cerebellopontine angle without contacting the brainstem, were found in 101 patients and were divided by size into two grades: IIA (< 1 cm) and IIB (1-1.8 cm). In 90 patients with Grade IIA tumors, 72 (89%) of 81 who had preserved hearing preoperatively maintained it postoperatively, and in the 11 patients with Grade IIB tumors, six of whom had good hearing preoperatively, four (67%) had preserved hearing postoperatively. Six morphological types were identified based on their neurotopographic relationships to the elements of the vestibulocochlear nerve.

CONCLUSIONS

Hearing preservation postsurgery by tumor type was as follows: 1A, 92%; 1B, 88%; 1C, 100%; 2A, 83%; 2B, 92%; and 3, 57%. Combined, this represents a hearing preservation rate of 87% after surgical treatment of Grade II acoustic neurinomas. Full nerve function was maintained in 88% of patients with anatomically preserved facial nerves in both Grade I and II tumors. The remaining 12% of patients retained partial function of the facial nerve. Two patients in the series lost anatomical integrity of the nerve due to surgery.

Management of vestibular schwannomas (acoustic neuromas): the value of neurophysiology for intraoperative monitoring of auditory function in 200 cases

OBJECTIVE

The present study investigated the significance of the presence or absence of auditory brain stem response (ABR) Waves I, III, and V as functional representatives of the cochlea, the nucleus cochlearis, and the colliculus inferior, respectively, and attempted to identify the microsurgical maneuvers that were especially likely to cause isolated or combined component losses and subsequent hearing losses.

METHODS

Based on the previously described ABR classification system, 201 patients with preserved Waves I, III, and V or Waves I and V were investigated for the peak latencies and amplitudes of the waves at 15 defined microsurgical stages. Analysis was performed with respect to the presence or absence of ABR components during specific microsurgical actions and the related danger of deafness.

RESULTS

Temporary or permanent losses of Waves V, I, and III occurred with 21, 27, and 29% of surgical actions, respectively, leading to deafness in 65 to 78% of the patients. Wave III disappearance was identified as the earliest and most sensitive sign. Wave V loss was usually preceded by disappearances of Waves I and III. During the most dangerous actions (drilling, pulling downward, medially, or laterally, and direct nerve manipulation), special attention ws paid to deterioration of Wave-III and then Wave I; if impairment was seen, intermittent breaks or changes in the type or site of microsurgical action were used to enable wave recovery. Acute, simultaneous, and permanent loss of all waves occurred in 27.5% of postoperatively deaf patients, whereas stepwise wave deterioration and losses occurred in 72.5%.

CONCLUSION

Useful (in-time) recognition of significant waveform changes is possible and enables a change of microsurgical maneuvers to favor ABR recovery.

Advantages of a new, atraumatic, self-retaining electrode for direct cochlear nerve monitoring

Direct cochlear nerve monitoring during posterior fossa surgery offers the surgeon real-time information concerning auditory stams. However, routine utilization of this monitoring technique has been hampered by electrode designs that have not allowed the maintenance of a consistent contact between the nerve and electrode. We report on our experience with a new electrode designed to maintain consistent, atraumatic contact with the cochlear nerve and discuss the advantages of this electrode over existing wick and ball type electrodes.The utilization of this electrode during 18 posterior fossa surgeries, performed at Kaiser Permanenie Hospital, San Diego, including 8 vestibular schwannoma resections, allowed for consistent recording of high amplitude cochlear compound action potentials. Long-term exposure to pulsating cerebrospinal fluid (CSF) did not displace the electrode. Minimal cochlear nerve action potential amplitude change was noted with the electrode imrnersed in CSF. The electrode caused no trauma to the nerve, even in cases where it was accidentally dislodged from the nerve. It is hoped that by overcoming the problems previously associated with direct cochlear nerve monitoring, this electrode will allow for increased use of this advantageous monitoring technique. As a by product of the real-time data provided to the surgeon, we anticipate increased rates of hearing preservation during cerebellopontine angle surgery.

Hearing preservation in vestibular schwannoma surgery: what factors influence outcome?

The goals in the management of patients with vestibular schwannomas have changed drastically over the past few decades, with preservation of useful hearing representing the newest challenge. The true incidence of preserved useful hearing, however, has become clouded by a lack of uniformity in reporting results. The authors have analyzed 56 consecutive cases, in which directed attempts were made to preserve hearing on the involved side, to understand what factors play a major role in postoperative hearing preservation. Of the 56 cases reviewed, there were 46 patients who had "good" preoperative hearing (pure tone average < 50 dB; speech discrimination score > 50%). We found that, in this group of patients, if the tumor was less than 2 cm in diameter from pons to petrous, there was a 52% (16 of 31 patients) chance of preserving good hearing and if the tumor was 1 cm or less, the chances increased to 83%. Factors such as preoperative hearing status, tumor size and location, tumor consistency, and preoperative and intraoperative brainstem auditory evoked potentials are discussed in detail as they relate to postoperative outcome.

Microsurgical anatomy around the foramen of Luschka in relation to intraoperative recording of auditory evoked potentials from the cochlear nuclei

Three cadaveric heads were dissected to investigate the microsurgical anatomy around the foramen of Luschka. It was found possible to place a recording electrode in proximity to the cochlear nuclei by inserting it in the lateral recess of the fourth ventricle through the foramen of Luschka. In operations of the cerebellopontine angle using the retromastoid approach, access to the foramen of Luschka and the lateral recess is obtained by retracting the biventral lobule of the cerebellum in a caudal-rostral direction under a caudal-rostral/medial field of vision. The craniectomy might need to be enlarged a few millimeters in the caudal direction. A wick electrode can be inserted in the lateral recess beneath the choroid plexus in a rostromedial direction and to a depth of approximately 3 to 5 mm from the foramen of Luschka without excessive retraction of the cerebellum. The optimum position for the recording electrode is in the triangle formed by the axis of the cochlear nerve and the glossopharyngeal nerve and by the lip of the foramen of Luschka. The caudal retromastoid approach is more suitable than the translabyrinthine technique for recording from the cochlear nuclei as well as for implantation of stimulating electrodes into the cochlear nuclei for use as hearing prostheses.

Preservation of hearing in operations on acoustic tumors: an alternative to recording brain stem auditory evoked potentials

The monitoring of auditory function by recording brain stem auditory evoked potentials in patients undergoing removal of acoustic tumors is hampered by the small amplitude of the brain stem auditory evoked potentials. Because several thousands of responses must be added, it takes several minutes to obtain an interpretable record. Recordings done directly from the exposed eighth nerve have much higher amplitudes, and, therefore, interpretable responses can be obtained after only a few responses have been added. However, it is difficult to place the recording electrode in an optimal position and the electrode may interfere with the removal of the tumor. In this report, we show that evoked potentials from the cochlear nucleus, which can be recorded by placing an electrode in the lateral recess of the fourth ventricle, have a large amplitude, and that the electrode placed in this way does not interfere with the removal of the tumor. This way of monitoring, therefore, yields interpretable responses within 15 to 20 seconds, or less, and makes it possible to detect injuries to the entire intracranial portion of the eighth nerve, just as brain stem auditory evoked potentials do, but 20 to 50 times faster.

Meningiomas of the cranial base

Treatment objectives for meningiomas of the cranial base include relief of neurologic disability and prevention of clinical progression or recurrence with the least morbidity. Recent advances in skull base surgical techniques, through an appreciation of skull base anatomy and institutional specialization, have contributed major improvements to the outlook for patients with these tumors, and previously inoperable cases may now often be removed completely with acceptable risk. Since significant morbidity may be incurred during surgical resection of these difficult lesions, especially in terms of cranial nerve dysfunction, the value of aggressive surgical resection must be weighted against the often indolent natural history of these lesions, and must be individualized in each patient. Completeness of resection is the major prognostic factor determining the outcome of patients with typical benign meningiomas in terms of length of survival, risk of recurrence, and neurological disability. Various means of prognosticating the growth potential of a given tumor are being investigated, though none have yet been confirmed for their predictive value in typical, histologically benign meningiomas. The role of external beam radiotherapy has not been subjected to adequately controlled, prospective studies, and there is currently insufficient followup to assess the risks and benefits of stereotactic radiosurgery. Advances in the clinical management of tumors of the skull base has had perhaps the greatest impact for patients with meningiomas who constitute a large portion of tumors seen in these locations. Although the majority have benign histological features, skull base meningiomas can present a formidable challenge due to their proximity to vital structures, surgical inaccessibility, and occasional aggressive features. The combination in recent years of advances in skull base surgical techniques, adjuvant therapy, and rehabilitation methods have dramatically improved the outcome for these tumors.

The impact of continuous electrophysiological monitoring on preservation of the facial nerve during acoustic tumour surgery

Continuous electromyographic (EMG) activity and responses to electrical stimulation of the facial nerve were monitored intraoperatively in 35 patients undergoing primary removal of acoustic neuroma. The rate of anatomical and immediate functional preservation of the facial nerve of these patients was compared with a matched population where a facial nerve stimulator and monitoring of facial muscle contraction by the anaesthetist only was employed. Overall, 94% (n = 33) of facial nerves were preserved anatomically in patients with continuous EMG monitoring, compared with 64% (n = 89) of patients (n = 139) without continuous monitoring. Amongst the 33 cases with anatomical facial nerve preservation, 64% (n = 21) had immediate functional preservation. Immediate functional preservation was only achieved in 39% (n = 35) of anatomically preserved facial nerves (n = 89) where conventional monitoring was used.