Auditory Nerve Monitoring Techniques

Overview

1. Auditory Brainstem Response (ABR)

Basis and Waveform Generators

• Scott-Brown's Otorhinolaryngology, Vol. 1, p. 575-576
• Shambaugh Surgery of the Ear, p. 374

Intraoperative Technique

• Electrodes: Subdermal needle electrodes are preferred over surface electrodes for impedance stability over long surgical periods.
• Active electrode: Placed at or near the vertex (Cz).
• Reference electrode: Placed on the mastoid, earlobe, or neck on the operative side.
• Stimulus: Several hundred to several thousand click stimuli (100 μV square wave pulse) delivered at 60-80 dB nHL via insert earphones (insert phones better than cushion phones for operating room use).
• Response: Averaged responses; small amplitude (usually less than 1.0 μV). Wave I latency normally occurs at or before 2 milliseconds after stimulus onset; subsequent waves at approximately 1-millisecond intervals.

Parameters Monitored Intraoperatively

• Prolongation of wave V latency (the most commonly used warning criterion): a 1 ms increase is a widely cited alert threshold
• Increase in I-V interpeak interval
• Decrease in wave V amplitude (>50% reduction is a common alert criterion)
• Loss of wave V - considered a serious warning sign
• Preoperative baseline ABR should always be obtained, as knowledge of the patient's individual waveform helps choose optimal monitoring strategy (e.g., whether to track wave V latency or the interaural wave V latency difference)

Advantages

• Non-invasive, no direct manipulation required
• Provides anatomically specific information - each wave corresponds to a known structure
• Widely available and well-understood technology

Limitations

• Slow: Multiple averages are required, so meaningful changes take 5-10 minutes to detect. This delayed feedback is its main clinical limitation.
• ABR waveforms may be absent preoperatively in some patients (typically larger tumors), yet these patients may still have cochlear nerve action potentials detectable by DENM
• Susceptible to electrical artifact from cautery and drilling

2. Direct Eighth Nerve Monitoring (DENM) / Direct Cochlear Nerve Recording

Historical Development

• Shambaugh Surgery of the Ear, p. 373

Technique

• A recording electrode (monopolar ball electrode or bipolar electrode) is placed directly on the cochlear nerve surface, typically at the root entry zone near the brainstem.
• Click stimuli similar to those used in ABR elicit a compound action potential (CAP) from the nerve.
• Because it is a near-field recording, amplitude is large (no averaging needed in the same way as ABR), giving a much more rapid response.

Parameters Monitored

• Changes in CAP latency, amplitude, and morphology are all reported to the surgeon.
• Sensitivity 100%, specificity 70% in predicting hearing outcome (Ehrmann-Muller et al., cited in Scott-Brown's).

Advantages Over ABR

• Speed of feedback: DENM provides near real-time feedback - the most rapid available. This is the primary clinical reason it is now preferred.
• Can detect cochlear nerve action potentials when ABR is absent preoperatively.
• Less affected by background noise.
• Hearing preservation rates of 80-85% have been reported in acoustic neuromas ≤10 mm using DENM.

Limitations

• Requires direct contact with the nerve - electrode placement and stability can be challenging, though modern electrode designs have mitigated this.
• Potential for mechanical nerve injury from the electrode.
• Records from the root entry zone, not from the intracranial portion during all stages of dissection.
• Shambaugh Surgery of the Ear, p. 373-374
• Scott-Brown's Otorhinolaryngology, Vol. 1, p. 576

3. Electrocochleography (ECochG)

Technique

• Extratympanic intra-meatal placement (less invasive)
• Transtympanic placement (closer to cochlea, larger amplitudes)

Role in Monitoring

• Records the cochlear microphonic (CM), summating potential (SP), and N1 component of the compound action potential (representing the same generator as ABR Wave I).
• The large amplitude of ECochG responses allows interpretation after fewer averages, enabling more rapid decision-making than ABR alone.
• ECochG is less compromised by acoustic noise or electrical artifact from cautery, making it potentially useful during these periods.

Limitations

• Records activity from the cochlea and distal cochlear nerve - not the intracranial portion at risk during CPA surgery. It can therefore give misleading reassurance if used alone.
• Research has shown ECochG responses can persist after complete transection of the eighth nerve in animal models - meaning isolated ECochG monitoring could falsely suggest preserved function.
• Most authorities regard ECochG as an adjunct to ABR, not a primary monitoring tool.
• Scott-Brown's Otorhinolaryngology, Vol. 1, p. 575

4. Otoacoustic Emissions (OAE)

• Reflect outer hair cell / cochlear function, not direct cochlear nerve function - so they may not indicate acute, reversible nerve injury at the site of surgery (the internal auditory canal).
• Could theoretically detect ischemic or noise injury to the cochlea.
• Have not been demonstrated to identify reversible injury at a stage when intervention is possible.

• Scott-Brown's Otorhinolaryngology, Vol. 1, p. 575

5. Combined (Multimodality) Monitoring

• ABR provides anatomically specific waveform data and a more global view of the auditory pathway.
• DENM provides the speed of feedback needed for real-time surgical guidance.
• Several studies have reported that direct cochlear nerve recordings are statistically more effective than ABR alone in maintaining hearing function, attributed to the immediacy of the response.
• The person undertaking monitoring must be experienced in the operating theatre environment, able to interpret waveform changes rapidly, and communicate findings clearly and concisely to the surgeon.

"A consensus has not yet been reached on techniques for monitoring auditory function, but it looks likely that a combination of techniques will be optimal." - Scott-Brown's Otorhinolaryngology, Vol. 1, p. 577

Clinical Applications of Auditory Nerve Monitoring

1. Newborn infant auditory screening (ABR-based)
2. Estimation of auditory sensitivity (threshold ABR)
3. Neurodiagnosis:
   • Eighth nerve lesions (e.g., vestibular schwannoma): I-III interpeak interval is the best descriptor
   • Auditory brainstem dysfunction
4. Intraoperative monitoring:
   • Eighth nerve and auditory brainstem status during posterior fossa surgery
   • Cochlear implant / brainstem implant placement
   • Vestibular nerve section
   • Acoustic tumor removal
   • Eighth nerve microvascular decompression

• KJ Lee's Essential Otolaryngology, p. 336

ABR Parameters Used in Clinical Assessment and Monitoring

• KJ Lee's Essential Otolaryngology, p. 336

Evidence Base and Best Practice

• Facial nerve monitoring: There is strong evidence that monitoring improves facial nerve outcomes in vestibular schwannoma surgery (multiple RCTs and prospective series).
• Auditory nerve monitoring: The evidence base is less robust due to heterogeneity of patient selection, tumor size, surgical technique, and outcome reporting. However, consensus supports its use as best clinical practice during CPA surgery. Well-designed clinical trials remain a priority.
• DENM has been demonstrated as superior to ABR alone in facilitating hearing preservation during CPA surgery.

"The evidence base for monitoring auditory nerve function is less robust, but it should be regarded as best clinical practice." - Scott-Brown's Otorhinolaryngology, Vol. 1, p. 577

Summary Table