5Reflex and Brainstem: Intraoperative ESRT and eABR

CWatching the stapedius tendon: the electrical reflex on the table

The electrically-evoked stapedius reflex (ESR) is a contraction of the stapedius muscle triggered through the brainstem reflex arc when an electrode is stimulated loudly enough; in theatre it can be seen DIRECTLY as the tendon twitches in the open middle ear. The level at which that twitch first appears is the electrically-evoked stapedius reflex THRESHOLD (ESRT), an objective marker of a comfortably-loud sensation. Because the surgical field already exposes the round window and stapes region, intraoperative ESRT needs only visual observation — no acoustic immittance probe is required. Unlike the ECAP, the ESR depends on the whole reflex arc (nerve to cochlear nucleus to superior olive to facial nucleus to stapedius), so a present reflex verifies more of the pathway.[2018][2020]

CESRT anchors the upper stimulation level

ESRT correlates strongly with behavioural maximum-comfort (C/M) levels and is used to ANCHOR the upper end of the map, guarding against setting uncomfortably high levels — invaluable when the patient cannot report loudness. Typically ESRT sits at or just above the behavioural C-level, so it is treated as a ceiling reference rather than the exact comfort setting. Combining ESRT with the ECAP profile gives both ends of the map objectively: ECAP orients the shape and approximate threshold, ESRT caps the loud end. The detailed psychophysics of loudness scaling and C-level setting belong to the Programming/Objective Measures chapters; the intraoperative ESRT simply provides an early, behaviour-free upper bound.[2018][2020]

TThe eABR: did the signal reach the brainstem?

The electrically-evoked auditory brainstem response (eABR) is a far-field scalp recording of brainstem activity in answer to implant stimulation; its hallmark is wave eV (the electrical analogue of acoustic wave V). Where the ECAP stops at the nerve trunk, the eABR confirms central conduction — that stimulation actually propagates UP the auditory brainstem, not merely that the local nerve fires. A stimulus artifact obscures the earliest peaks, so eABR interpretation centres on the later eIII-eV complex and the eV threshold. eABR requires scalp electrodes and averaging, making it slower and more setup-heavy than ECAP — it is reserved for cases where central conduction is genuinely in doubt.[2015][2020]

CeABR where the diagnosis is uncertain: CND, ANSD and the CI-vs-ABI decision

In cochlear nerve deficiency (hypoplasia/aplasia) and inner-ear malformations, intraoperative eABR objectively confirms whether a usable auditory signal reaches the brainstem — present, repeatable eV supports proceeding with the cochlear implant. An absent or grossly abnormal eABR in a deficient nerve flags that the cochlear implant may underperform and informs the cochlear-implant-versus-auditory-brainstem-implant (ABI) decision, sometimes intraoperatively. In auditory neuropathy spectrum disorder, where the acoustic ABR is dys-synchronous, a clean electrically-evoked eABR is reassuring evidence that direct electrical stimulation bypasses the synaptic/dys-synchrony lesion and synchronises the pathway. Series in malformed cochleae report eABR as a reliable, effective way to confirm device function and pathway integrity on the table — verifying the WHOLE route from electrode to brainstem before closure.[2015][2020]