7The acoustic reflex
A loud sound makes a tiny muscle in the middle ear contract, stiffening the ossicular chain, and that involuntary twitch — measurable in seconds, with no cooperation at all — turns out to be one of the most informative signs in the battery. Because the reflex depends on a long loop running from cochlea to brainstem and back out along the facial nerve, where it fails tells you where the lesion is: a conductive block on the measured side, a stressed nerve on the stimulated side, a crossed-pathway brainstem problem. Its threshold rises in a predictable way as cochlear hearing worsens, so a reflex present at a normal level quietly argues against a severe loss. This module covers the reflex as a diagnostic instrument — and points forward to its electrical cousin, used through the implant to set the map.
TWhat the reflex is
The stapedial (acoustic) reflex is the impedance change caused by stapedius contraction to a loud sound, elicited both ipsilaterally and contralaterally. Threshold sits about 85 dB above hearing threshold in a normal ear, with large between-subject but small within-subject variability.
CThe most sensitive test for conductive loss
The reflex is the single most sensitive audiologic measure for conductive loss: about 80% of ears with even a 10 dB air–bone gap show no recordable reflex on the probe side (Jerger 1974), and fixation or discontinuity abolish it too.[1974]
CThresholds & cochlear loss
With cochlear loss, reflex thresholds rise predictably: ≥100 dB HL in 90% of adults with hearing poorer than 50 dB, and ≥120 dB HL when poorer than 70 dB (Gelfand 1990). So a reflex present at normal levels argues against a severe sensorineural loss and can call a candidacy referral into question.[1990]
CThe arc & site of lesion
Because the reflex depends on neural synchrony, an injured auditory nerve elevates threshold and flattens amplitude growth and shows reflex decay — a valuable retrocochlear / ANSD sign — whereas a pure cochlear loss keeps the threshold near normal. The arc (cochlea → cochlear nucleus → superior olive → facial nerve → stapedius) and its crossed/uncrossed pattern localise the lesion.
CInfants & non-organic loss
Being objective and cooperation-free, the reflex helps identify non-organic loss and is depressed by central depressants (alcohol, barbiturates). Infants under 6 months are tested with a 1000 Hz probe and with broadband-noise elicitors as well as tones, since some normal infants show absent tonal reflexes but respond to noise. The electrically evoked stapedial reflex (eSRT), used through the implant to set MAP comfort levels, is covered in the Objective Measures chapter (Chapter 27).
What does this finding imply?
Why is the acoustic reflex so sensitive to conductive loss?
What does a present reflex at a normal level imply about degree of loss?