10Vestibular schwannoma

The poster condition for why ECochG cannot replace imaging. The AP is generated in the most distal segment of the auditory nerve — peripheral to where most schwannomas sit. ECochG is therefore deceptively normal in many patients with substantial tumours.

FClinical presentation

Vestibular schwannoma is a benign tumour arising from Schwann cells of the vestibular division of the eighth cranial nerve, typically in the internal auditory canal or at the cerebellopontine angle. Audiometric features are notoriously variable: 95% of patients have some asymmetric SNHL, but pure-tone configuration ranges from high-frequency-only loss to substantial drops across all frequencies. About 5% have normal audiometric thresholds with subjectively symptomatic ears.[1993] Speech discrimination is often disproportionately poor relative to the audiogram — the most useful audiometric red flag.

Tinnitus is present in roughly two-thirds; vertigo and balance disturbance, despite the tumour being on the vestibular nerve, are surprisingly mild or absent in many because growth is slow enough for central vestibular compensation to keep up.

TWhy ECochG fails as a screening test

The auditory nerve compound action potential — ECochG's AP, equivalent to ABR wave I — is generated by the synchronous firing of distal auditory nerve fibres at the cochlear base, before the nerve enters the internal auditory canal.[2014] Vestibular schwannomas typically sit further proximally — within the IAC, at the CPA, or rarely at the brainstem. A tumour that does not actually compress the distal cochlear nerve does not necessarily affect the AP.

The consequence: a patient can have a 1.5 cm schwannoma compressing the eighth nerve at the CPA with a clinically significant hearing loss — yet a relatively preserved AP on ECochG, because the wave-I generator sits peripheral to the lesion. ECochG cannot rule out retrocochlear pathology.[2003]

FThe MRI gold standard

The Fortnum et al. systematic review established gadolinium-enhanced MRI as the diagnostic gold standard for vestibular schwannoma, with near-100% sensitivity for tumours as small as 2–3 mm.[2009]Modern high-resolution heavily T2-weighted sequences (CISS, FIESTA) detect even small intracanalicular tumours without contrast. The clinical question is therefore not "should I order MRI?" but "in which patients?" — typical indications include asymmetric SNHL ≥ 15 dB at any two contiguous frequencies, unilateral tinnitus, asymmetric speech discrimination, or SSNHL.

ECochG findings when schwannoma does affect cochlear potentials

When ECochG is performed in a known schwannoma patient — usually for intraoperative monitoring during tumour resection — the typical pattern is:

• AP often preserved or only mildly reduced at the diagnostic stage, because the generator is distal to the tumour.
• AP latency may be prolonged when there is substantial threshold elevation, but the prolongation is at the level expected from the audiogram alone (i.e. follows the latency-intensity function from Module 4) rather than reflecting retrocochlear conduction delay.
• SP may be normal or slightly elevated, reflecting the cochlear effects of the threshold elevation rather than the tumour itself.
• The ABR shows the retrocochlear signature — prolonged I–V interval, abnormal wave V latency — when the ECochG looks unremarkable.

Intraoperative hearing-preservation monitoring during schwannoma resection is the one current setting where ECochG retains a defined role; combined TT-ECochG and ABR give complementary cochlear and brainstem-level information, with society guidance recommending both channels.[2024][2023]

TCase 10.1 — asymmetric SNHL with normal ECochG