Wave V Latency Shift During Retrosigmoid Tumor Dissection
A 52-year-old woman undergoes retrosigmoid resection of a 2.2 cm left vestibular schwannoma. Left ear BAEPs are obtained with a click at 90 dB nHL; baseline wave V latency is 5.7 ms, interpeak I-V is 4.1 ms, and amplitude is 0.35 microV. Stimulus rate is 11.1 per second with contralateral masking, and the ear insert is taped in place. During drilling of the internal auditory canal the technologist notes wave V at 6.6 ms with amplitude reduced by half, with wave I latency unchanged at 1.6 ms. Warm saline is running into the field, and the patient's end-tidal volatile concentration and temperature have not changed.
1. Wave I is unchanged while wave V has shifted. How do you use that relationship, and what technical checks come before you call the surgeon?
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The I-V interpeak interval has widened from 4.1 to 5.0 ms while wave I is unchanged. A preserved wave I is strong evidence that the cochlea, the transducer, and the stimulus delivery are intact, because wave I arises from the distal eighth nerve near the cochlea. That makes a purely conductive or stimulus-delivery problem unlikely and points to slowed conduction between the distal nerve and the lower brainstem - that is, to the segment being manipulated.
Even so, complete the technical checks quickly, because the occipitomeatal approach creates specific artifacts: fluid in the external canal or a partially dislodged foam insert (with the saline irrigation running, this is a live possibility), a kinked or fluid-filled tubing, a change in stimulus polarity or rate, loss of contralateral masking, and electrode impedance at the recording site. Re-seat and dry the insert, verify the tubing, confirm rate and polarity, and repeat the average. If wave I stays at 1.6 ms through these maneuvers, the peripheral chain is confirmed sound.
Only then notify: give the latency values, the interpeak change, the amplitude change, and the fact that wave I is stable, so the surgeon knows the finding localizes to the operative region.
2. What are reasonable warning criteria for BAEP in this case, and how do you weight latency versus amplitude?
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Latency is the more reliable and more reproducible measure for BAEP; amplitude varies considerably between averages and between patients and is a poor sole criterion. Commonly used warning thresholds are an absolute wave V latency increase of about 0.5 to 1.0 ms, or an interpeak I-V increase of similar magnitude, sustained on repeat averaging. A 0.9 ms shift with a 50% amplitude drop in this case is at or past that threshold and should be treated as a warning rather than a curiosity.
Context modifies the call. A change that appears during direct dissection or drilling near the internal auditory canal carries far more weight than the same number appearing while the patient is being positioned. The trend matters more than the single value - two successive averages each 0.4 ms worse is a more convincing signal than one noisy 0.8 ms jump that resolves on the repeat. And BAEP is relatively resistant to anesthetic effects, so an anesthetic explanation for a latency shift is weak; the usual confounders are temperature, fluid in the ear, and stimulus parameters.
Practical approach: use a warning threshold of 0.5 ms latency or a sustained 50% amplitude loss for notification, and reserve a stronger "stop and reassess" statement for changes approaching 1 ms or loss of a previously reproducible wave V.
3. The shift is confirmed on repeat. What do you recommend, and what would you do if wave V disappears entirely?
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Communicate the finding with its localization: "Wave V is 6.6 ms, up 0.9 ms from baseline, wave I stable at 1.6 ms, amplitude down 50%, confirmed on two averages." Then offer the mechanism-linked requests: pause drilling and dissection, release or reposition the retractor for a few minutes, irrigate with warm rather than cool saline, check that no fluid or muscle is compressing the nerve, and consider whether hemostatic material near the canal is impinging. Re-average every one to two minutes to see whether the change reverses with release. Reversal after release is itself useful information for the surgeon about how much retraction the nerve tolerates.
If wave V disappears, first confirm it is not a recording failure: check the insert and tubing, raise the click level, increase the number of sweeps, and verify wave I is still present. Total loss with wave I preserved is a genuine and serious warning - notify immediately, ask for release of retraction and cessation of manipulation, and state clearly that the monitor can no longer detect further deterioration. Document the time of loss, the stimulus level used, and whether any recovery occurred before closure, because that sequence is directly relevant to the postoperative hearing discussion.