Intraoperative monitoring during surgery for acoustic neuroma: benefits of an extratympanic intrameatal electrode

Intraoperative brainstem auditory evoked potential monitoring during cerebellopontine angle surgery via retrosigmoid approach

OBJECTIVES

Brainstem auditory evoked potential (BAEP) monitoring was used to identify the influence of auditory function during cerebellopontine angle (CPA) surgery for cranial neuropathy via the retrosigmoid approach.

METHODS

This prospective study included 20 patients who underwent CPA surgery for cranial neuropathy via the retrosigmoid approach with intraoperative BAEP monitoring and pure tone audiometry (PTA). The latency and interpeak latency were analyzed at each surgical step in combination with the pre- and postoperative PTA.

RESULTS

Follow-up data were available for 17 patients. The mean pre- and postoperative PTA values were 25.65 dB and 20.70 dB, respectively. Two patients (2/17, 11.76%) developed hearing loss postoperatively. The latency of wave І significantly changed during direct auditory nerve manipulation and at the end of the surgery, while that of wave III only changed during direct auditory nerve manipulation. The appearance of wave V peak was delayed during CPA surgery.

CONCLUSIONS

CPA surgery for cranial neuropathy via the retrosigmoid approach can cause hearing loss to varying degrees, and intraoperative BAEP monitoring can reduce the occurrence of hearing loss. Intraoperative hearing function can be estimated by the latency of wave I. Hearing loss due to stretching of the brainstem can be estimated by the latency of wave III, and wave V is an early indicator of intraoperative hearing loss. Waves I and III remained stable both pre- and postoperatively, whereas wave V was unstable despite no surgery. Therefore, a precise operation and well-defined operative steps for surgeons during CPA surgery could facilitate maximal preservation of the anatomical structure and function.

Electrophysiology of Cranial Nerve Testing: Auditory Nerve

The electrocochleogram and brainstem auditory evoked potentials (BAEPs) are electrophysiologic signals used to assess the auditory nerve. The electrocohleogram includes the cochlear microphonic, the cochlear summating potential, and the eighth nerve compound action potential. It is used predominantly for hearing assessment and for diagnosis of Ménière disease and auditory neuropathy. Brainstem auditory evoked potentials are used for hearing assessment, diagnosis of dysfunction within the cochlea, the auditory nerve, and the brainstem auditory pathways up to the level of the mesencephalon, and intraoperative monitoring of these structures. The earliest BAEP component, wave I, and the eighth nerve compound action potential reflect the same process-the initial depolarization in the distal auditory nerve. Brainstem auditory evoked potential wave II receives contributions from the region of the cochlear nucleus and from the second depolarization in the distal auditory nerve. Wave III and later components are entirely generated rostral to the auditory nerve. Interpretation of BAEP studies is based on waves I, III, and V; auditory nerve dysfunction is manifested as prolongation of the I-III interpeak interval or absence of waves III and V. Eighth nerve tumors can cause a variety of BAEP abnormalities depending on which structures they affect. Adverse intraoperative BAEP changes can have many etiologies, including direct mechanical or thermal injury of tissue, ischemia (including cochlear ischemia or infarction due to compromise of the internal auditory artery), eighth nerve stretch, systemic or localized hypothermia, and artifactual BAEP changes due to technical factors.

Non-invasive intraoperative monitoring of cochlear function by cochlear microphonics during cerebellopontine-angle surgery

In vestibular-schwannoma (VS) surgery, hearing-preservation rate remains low. Besides damage to the cochlear nerve, intraoperative cochlear ischemia is a potential cause of hearing loss. Here, we used non-invasive cochlear microphonic (CM) recordings to detect the cochlear vascular events of VS surgery. Continuous intraoperative CM monitoring, in response to 80-95 dB SPL, 1-kHz tone-bursts, was performed in two samples of patients undergoing retrosigmoid cerebellopontine-angle surgery: one for VS (n = 31) and one for vestibular neurectomy or vasculo-neural conflict causing intractable trigeminal neuralgia, harmless to hearing (n = 19, control group). Preoperative and postoperative hearings were compared as a function of intraoperative CM changes and their chronology. Monitoring was possible throughout except for a few tens of seconds when drilling or suction noises occurred. Four patterns of CM time course were identified, eventless, fluctuating, abrupt or progressive decrease. Only the VS group displayed the last two patterns, mainly during internal-auditory-canal drilling and the ensuing tumor dissection, always with postoperative loss of hearing as an end result. Conversely, eventless and fluctuating CM patterns could be associated with postoperative hearing loss when the cochlear nerve had been reportedly damaged, an event that CM is not meant to detect. Cochlear ischemia is a frequent event in VS surgery that leads to deafness. The findings that CM decrease raised no false alarm, and that CM fluctuations, insignificant in control cases, were easily spotted, suggest that CM intraoperative monitoring is a sensitive tool that could profitably guide VS surgery.

Electrophysiologic auditory tests

Auditory stimulation elicits a complex series of electric signals in the ear and nervous system which can be used for hearing assessment, audiologic and neurologic diagnosis, intraoperative monitoring, and neurophysiologic research. The earliest components comprise the electrocochleogram. The cochlear microphonic arises from receptor potentials from cochlear hair cells, and is used to assess hair cell function in patients with auditory neuropathy. The summating potential becomes larger in Ménière's disease. The eighth-nerve compound action potential is useful for objective audiometry. Brainstem auditory evoked potentials, a series of components generated in the eighth nerve and the brainstem auditory pathways, can be used for diagnostic assessment and intraoperative monitoring of the ears and of the auditory pathways up through the mesencephalon. They are relatively easy to record, highly consistent in normal subjects, and little unaffected by surgical anesthesia. Middle-latency and long-latency auditory evoked potentials are generated in multiple areas of cerebral cortex that are activated by auditory stimulation. Anesthetic effects limit their utility for intraoperative monitoring, and substantial intersubject variability limits their utility as a diagnostic test in individual patients, but they are important research tools for the study of memory processes and the way in which the brain analyzes auditory stimuli.

Superiority of tympanic ball electrodes over mastoid needle electrodes for intraoperative monitoring of hearing function

Object

Recording the auditory brainstem response (ABR) is a common method for monitoring the integrity of auditory pathways during surgery in the cerebellopontine angle. Electrocochleography (ECochG) is an alternative means of intraoperative neuromonitoring. In the present study the authors compared the practicability and prognostic significance of these two methods by performing simultaneous recordings in the operating room.

Methods

Between 2006 and 2011, 125 patients (mean age 55 years) underwent surgery in the cerebellopontine angle. Seventy-one percent of the patients presented with a hearing deficit, and 37% had useful hearing but with slight functional impairment. Auditory brainstem response was recorded with a subdermal needle electrode at the mastoid. For ECochG recording, a noninvasive ball electrode was attached to the tympanic membrane. Amplitudes obtained from both ECochG and ABR audiometry were compared and correlated to pre- and postoperative hearing deficits.

Results

Simultaneous intraoperative monitoring via ABR and ECochG was possible in 114 cases (91%). Postoperatively, 42% of patients showed some degree of new hearing deficit, whereas 4% had improvement. The mean amplitudes in ECochG monitoring were significantly higher (0.18 ± 0.04 μV) than the ABR potentials (0.08 ± 0.006 μV; p < 0.05). All waves recorded at the mastoid needle electrode could be recognized in the potentials of the tympanic ball electrode. Hearing outcome correlated more reliably with the relative amplitude changes in Waves III and V in ECochG (III: p = 0.0008, V: p = 0.0015) than in ABR monitoring (III: p = 0.2075, V: p = 0.0398).

Conclusions

Intraoperative monitoring of the auditory system by recording with noninvasive tympanic ball electrodes is more practicable than with subcutaneous needle electrodes at the tragus. Since there is also a reliable correlation between ECochG and clinical outcome, the method can replace common ABR recording during surgery in the cerebellopontine angle.

Intraoperative monitoring of hearing during cerebellopontine angle tumor surgery using transtympanic electrocochleography

OBJECTIVE

To investigate the use of transtympanic electrocochleography (TT-ECochG) analyzed on-line by a detector strategy software that made possible automatic extraction of TT-ECochG components intraoperatively in real-time domain.

PATIENTS

Fifteen patients with cerebellopontine angle tumor among 50 patients were included in this study.

INTERVENTION

All subjects were operated on via middle fossa or retrosigmoid approach. Pure-tone average (PTA) was measured at 0.5, 1.0, and 2.0 kHz, and calculations were performed before and after surgery. Auditory function was monitored intraoperatively via TT-ECochG, and analyzed data were displayed on-line.

MAIN OUTCOME MEASURES

TT-ECochG changes in morphology were described. Postoperative PTA elevation level correlated with TT-ECochG morphology changes occurring intraoperatively.

RESULTS

Analyzed on-line, TT-ECochG data were displayed as first negative peak of compound action potential amplitude and latency in time domain every 3 to 5 seconds. A good correlation between postoperative PTA elevation and TT-ECochG morphology changes was showed (Spearman rank test, R = +0.93; t(N-2) = 9.00; p < 0.0001).

CONCLUSION

TT-EcochG seemed to effectively mirror even minimal changes in auditory function during intraoperative monitoring in real-time domain. Developed strategy of on-line analysis makes the intraoperative hearing status assessment faster and easier.

Neuromonitoring of cochlea and auditory nerve with multiple extracted parameters during induced hypoxia and nerve manipulation

A system capable of comprehensive and detailed monitoring of the cochlea and the auditory nerve during intraoperative surgery was developed. The cochlear blood flow (CBF) and the electrocochleogram (ECochGm) were recorded at the round window (RW) niche using a specially designed otic probe. The ECochGm was further processed to obtain cochlear microphonics (CM) and compound action potentials (CAP). The amplitude and phase of the CM were used to quantify the activity of outer hair cells (OHC); CAP amplitude and latency were used to describe the auditory nerve and the synaptic activity of the inner hair cells (IHC). In addition, concurrent monitoring with a second electrophysiological channel was achieved by recording compound nerve action potential (CNAP) obtained directly from the auditory nerve. Stimulation paradigms, instrumentation and signal processing methods were developed to extract and differentiate the activity of the OHC and the IHC in response to three different frequencies. Narrow band acoustical stimuli elicited CM signals indicating mainly nonlinear operation of the mechano-electrical transduction of the OHCs. Special envelope detectors were developed and applied to the ECochGm to extract the CM fundamental component and its harmonics in real time. The system was extensively validated in experimental animal surgeries by performing nerve compressions and manipulations.

Monitoring auditif et chirurgie du neurinome de l’acoustique

Intraoperative auditory monitoring in acoustic neuroma surgery is used to improve residual hearing. Three techniques are available: monitoring of brainstem auditory evoked potentials, of electrocochleography or of direct eighth nerve compound action potential. The three techniques with their advantages and disadvantages are discussed. The current trend is to monitor brainstem auditory evoked potentials using digital filtering or to monitor the eighth nerve compound action potential.

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.

Hearing preservation and facial nerve function in vestibular schwannoma surgery

The objective of this study was the assessment of hearing preservation in vestibular schwannoma surgery. This study reports a prospective cohort of 40 patients with clinically useful hearing from a consecutive series of 191 patients with unilateral vestibular schwannoma. The patients were managed in a tertiary centre by a combined team of Neurosurgeon, Otologist and Neuro-monitoring Scientist via a neuro-oto-surgical-retrosigmoid approach. Pure tone speech audiometry was conducted preoperatively and 6 months to 9 years following surgery. Using the AA0-HNS classification, useful hearing, i.e. grades A, B and C, was preserved in 47.5% of patients. Thirty-eight per cent achieved grade A or B. Using appropriate surgical and monitoring techniques, it is possible to preserve useful hearing in approximately 50% of patients following removal of a vestibular schwannoma via the retro-sigmoid approach.

Intraoperative neuromonitoring

BACKGROUND

Intraoperative neuromonitoring (IONM) has been a valuable part of surgical procedures for over 25 years. Insight into the nervous system during surgery provides critical information to the surgeon allowing reversal or avoidance of neural insults.

REVIEW SUMMARY

Electrophysiological tests including electroencephalography, electromyography, and multiple types of evoked potentials (somatosensory, auditory, and motor) are monitored during surgeries that involve risk to the nervous system. Deterioration of signals suggests a surgical insult and is associated with an increased risk of postoperative deficit. Intraoperative identification of this risk allows corrective action. In addition, IONM teams make use of their armamentarium of tests to evaluate anatomy or function of the nervous system in response to specific questions posed by the surgical team.

CONCLUSIONS

Intraoperative recordings are now a routine part of many surgical procedures. Their correct application leads to improved surgical outcome.