5Electrical stapedius reflex (ESRT)
The ECAP tells you the nerve fires; it does not tell you how loud the stimulus feels. The electrical stapedius reflex does — indirectly. Because the stapedius muscle contracts in response to loud sound (and loud electrical stimulation), the lowest level that triggers it sits near the upper, comfortable end of the dynamic range. That makes the ESRT the single most useful objective anchor for setting C/M levels, and one of the few obtainable under the microscope before the patient even wakes up.
FWhat the reflex is
The acoustic stapedius reflex is a bilateral brainstem reflex: a sufficiently loud sound triggers contraction of the stapedius muscle, stiffening the ossicular chain. The electrical stapedius reflex is the same reflex arc driven by electrical stimulation from the cochlear implant instead of sound. Its defining property for our purposes is that it is loudness-dependent — it appears only at relatively high stimulus levels, near what the recipient would report as loud-but-comfortable.[1997]
The ESRT (electrical stapedius reflex threshold, also eSRT) is the lowest stimulus level that elicits a visible or measurable stapedius contraction. Because it lies near the top of the dynamic range, it offers an objective ceiling for programming — the complement to the ECAP threshold, which sits lower in the range.
TIntra-operative observation
A distinctive advantage of the ESRT is that it can be observed in theatre, directly. With the middle ear exposed during (or at the end of) surgery, the surgeon watches the stapedius tendon under the microscope while the implant delivers increasing stimulation; the threshold is the level at which a visible twitch of the tendon first appears. This intra-operative ESRT gives an early, objective estimate of comfortable levels before the patient can give any behavioural report.[1997]
Intra-operative reflex thresholds are sensitive to the anaesthetic regimen — muscle relaxants abolish the reflex entirely, and depth of anaesthesia shifts the threshold. Intra-operative ESRTs are therefore systematically higher (and more variable) than awake post-operative values and should be treated as a rough early anchor, to be refined once the patient is awake.
TPost-operative measurement
After implantation the reflex is measured with standard immittance (tympanometry) equipment recording from the contralateral (non-implanted, or opposite) ear: the CI delivers the electrical stimulus, the reflex contracts the stapedius bilaterally, and the probe detects the resulting change in admittance in the ear it is sitting in. The stimulus level is raised until a reliable admittance deflection appears — that level is the ESRT.[2000]
It is not always obtainable: a non-functional or absent contralateral middle ear, an effusion, or a patent ventilation issue can block the recording, and a minority of recipients simply do not show a clear electrical reflex.
TCRelationship to comfortable (C/M) levels
The clinical workhorse role of the ESRT is anchoring the upper end of the electrical dynamic range — the C-level (Cochlear) or M-level (MED-EL/AB). Across studies the ESRT sits close to, and correlates with, behavioural comfortable levels, typically near or just above C/M, making it a sensible objective ceiling — especially in young children who cannot scale loudness.[1994, 2000]
A common practical approach is to set C/M levels a defined offset below the measured ESRT on each tested electrode, then refine behaviourally. Because the reflex is measured only on a subset of electrodes, the ESRT informs the shape and ceiling of the MAP rather than every individual value — see Module 9.
| Objective anchor | Where it sits in the dynamic range | Primary use |
|---|---|---|
| ECAP threshold | Within the range, generally between T and C | Estimating overall level / profile shape |
| ESRT | Near the top, around C/M | Anchoring the comfortable-level ceiling |
CStrengths & limits
Strengths: the ESRT uses standard immittance equipment, requires no behavioural response, can be obtained in theatre, and — unlike the ECAP — sits at the clinically critical upper end of the range. It is one of the best objective predictors of comfortable level available.
Limits: it cannot be measured in every patient (middle-ear status, absent reflex), intra-operative values are anaesthesia-dependent, the offset between ESRT and behavioural C/M varies between individuals, and it samples only some electrodes. As always in this atlas, it is an anchor to be refined behaviourally, not a final answer.
Several of those limits are addressable with technique. The reflex is easier to elicit and read when the recording is optimised — choice of probe-tone frequency, stimulus parameters, and which electrodes are tested all affect how often a usable ESRT is obtained, and attention to these raises the measurable-reflex rate considerably.[2017]
How should the ESRTs inform your initial MAP?
The electrical stapedius reflex is most useful for anchoring which part of the MAP?
Post-operatively, the electrical stapedius reflex is typically recorded with immittance equipment in the:
Why are intra-operative ESRTs generally higher and more variable than awake post-operative values?