Intraoperative Monitoring
Hearing-preservation surgery in the cerebellopontine angle relies on a real-time ABR feed. Knowing when to alarm — and the AICA blind spot — matters.
Where it's used
Posterior fossa surgery in the CPA: vestibular schwannoma resection with hearing preservation, microvascular decompression for hemifacial spasm or trigeminal neuralgia, and other lesions in or adjacent to the eighth nerve and brainstem[3]. ABR is the workhorse modality; CNAP (cochlear nerve action potential) from a directly placed electrode gives faster feedback when feasible.
Alarm criteria
Consensus thresholds for warning the surgeon: wave V latency prolongation of 1.0 msfrom the patient's own opening baseline, wave V amplitude reduction of 50%, or loss of wave V. Any of the three triggers an announcement and a pause; combinations escalate urgency[22].
| Change | Alarm threshold |
|---|---|
| Wave V latency | +1.0 ms |
| Wave V amplitude | −50% |
| Wave V presence | loss |
Mitigation, in order: pause and announce the change; release any retractor near the nerve; irrigate with warm saline; reposition and consider papaverine for suspected vasospasm; document the opening, change, and recovery points on the trace.
Anaesthetic considerations
ABR is robust to most agents. Inhalational anaesthetics (sevoflurane, isoflurane) produce small dose-dependent latency increases; TIVA with propofol gives the cleanest, most stable baselines[11]. Core temperature is the variable to watch: every degree of hypothermia adds roughly 0.2 ms to wave V latency, and the team should distinguish surgical from thermal drift.
The AICA blind spot
The cochlea has a private blood supply via the labyrinthine artery (a branch of AICA). An AICA-territory infarct can abolish cochlear function entirely, leaving the auditory nerve and brainstem structurally intact but no signal to recover. The ABR can then disappear suddenly and irreversibly without any traction or cautery near the nerve — the team should know that vascular events in the field can mimic mechanical injury.
Baseline-to-closing comparison
A stable, well-defined opening trace at the start of the case is the reference for every later trace. If the opening trace is poor, no later change can be confidently flagged. Time invested establishing a clean baseline pays back through the rest of the operation.
Postoperative correlation
Postoperative audiometry remains the definitive outcome measure. An intact closing ABR does not guarantee preserved hearing — occasional patients lose hearing without an obvious intraoperative event. Conversely, transient intraoperative changes that recover often correlate with preserved hearing.