Acoustic Reflexes

The stapedial reflex, threshold testing, reflex decay, and site-of-lesion logic.

A sufficiently loud sound presented to either ear triggers a bilateral, involuntary contraction of the stapedius muscle. This stiffens the ossicular chain and changes middle-ear immittance — a change the probe microphone can detect. The acoustic reflex therefore extends immittance testing from the middle ear outward to the cochlea, auditory nerve and brainstem. [Hunter LL 2013][Borg E 1973]

The arc has an afferent and an efferent limb. The afferent limb runs from the cochlea along the auditory nerve to the ventral cochlear nucleus; from there, projections reach the superior olivary complex on both sides. The efferent limb runs from the facial nerve nucleus along the facial nerve to the stapedius muscle. The crossover at the superior olivary complex is what makes the reflex bilateral — a loud sound in one ear contracts both stapedius muscles. [Mukerji S 2010][Borg E 1973]

Acoustic reflex thresholds

The acoustic reflex threshold is the lowest stimulus level at which a reliable immittance change is seen, typically measured at 500, 1000, 2000 and 4000 Hz. In a normal ear the reflex appears at a high sensation level. Reflexes can be tested ipsilaterally and contralaterally, and the pattern across these conditions helps localise a lesion. [Hunter LL 2013][Katz J (Ed.) 2015]

What abnormal reflexes mean

• Absent reflexes with a conductive loss point to a middle-ear cause — effusion, fixation or discontinuity.
• Elevated reflexes can accompany cochlear hearing loss, though they often appear at a reduced sensation level.
• Elevated or absent reflexes with normal middle-ear function raise the question of a retrocochlear lesion. [Katz J (Ed.) 2015]

Reflex decay

Reflex decay testing presents a sustained tone (commonly at 500 and 1000 Hz) and watches whether the reflex contraction is maintained. A normal (negative) result holds the contraction; a positive result is a fall in reflex magnitude of 50% or more within the first five seconds. Positive reflex decay is highly suggestive of retrocochlear pathology and warrants onward referral. [Hunter LL 2013][Katz J (Ed.) 2015]

In advanced otosclerosis the reflex may show a characteristic biphasic on–off pattern before disappearing as fixation progresses — a reminder that reflex morphology, not just presence or absence, carries information. [Hunter LL 2013]

The reflex arc — animated

Trace the pathway below. Use the selector to place a lesion at different points and watch which arcs fall silent — the predicted reflex grid updates to match.