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The acoustic reflex: pathway, thresholds and patterns by lesion site
A loud sound activates the stapedius muscle in both ears through a pathway that runs in the auditory nerve, the brainstem and the facial nerve; a reflex needs an intact sensory and motor pathway and a normal middle ear on the probe side — so the pattern of absent reflexes points to the site of the lesion.
Why it matters in the exam
Exam questions ask you to interpret a pattern: in normal hearing the reflex threshold is about 95 dB; in a moderate cochlear loss the reflex appears at a normal threshold (recruitment); in a retrocochlear lesion the threshold is very high or the reflex is absent, and reflex decay of 50% or more within 10 seconds at 1,000 Hz requires work-up; in a conductive loss the reflex is absent when the probe is in the affected ear; and reflexes present only ipsilaterally in both ears and absent contralaterally point to the brainstem. Broadband noise elicits a reflex at a lower threshold than a pure tone.
More cases: with a right facial palsy the reflex is absent in every measurement with the probe in the right ear, so the left ipsilateral reflex and the contralateral reflex of right-ear stimulation remain; if the reflex is normal in a facial palsy, the nerve lesion is distal to the branch to the stapedius; when a reflex is present despite an air-bone gap, think of a third window or a test error, so a patient with a bilateral conductive loss and present reflexes is retested with insert earphones. The difference between reflex threshold and hearing threshold reflects the dynamic range, and a change of 0.02 mmho is the minimum criterion for a reflex. Reflex testing does not distinguish ossicular discontinuity from stapes fixation.
Key points
- A normal reflex at about 95 dB; broadband noise — a lower threshold.
- Cochlear — normal threshold; retrocochlear — raised, absent or decay.
- Conductive — absent on the probe side; nerve VII — absent on the affected probe side.
- Brainstem — ipsi present, contra absent.
- The difference between reflex threshold and hearing threshold reflects the dynamic range.
Summary table
| Condition | Reflex pattern | Exam point |
|---|---|---|
| Normal hearing | Present at about 85–100 dB, around 95 | Broadband noise — a lower threshold |
| Mild to moderate cochlear loss | Present at a normal threshold, that is close to the hearing threshold | Recruitment; narrow dynamic range |
| Retrocochlear lesion (nerve VIII) | High or absent when the affected ear is stimulated; positive decay | Decay of 50% or more within 10 seconds at 1,000 Hz — work-up |
| Conductive loss | Absent when the probe is in the affected ear; raised when that ear is stimulated | Even a small gap abolishes the reflex on the probe side |
| Nerve VII lesion | Absent in every measurement with the probe on the affected side | A normal reflex — the lesion is distal to the stapedius branch |
| Brainstem lesion (in the crossing pathways) | Ipsilateral present on both sides, contralateral absent | Points to the brainstem |
| Third window | Present despite an air-bone gap | Unlike otosclerosis, in which the reflex is absent |
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Sources
- Katz (ed.), Handbook of Clinical Audiology, 7th edition (2015) — Acoustic stapedius reflex measurements
- Gelfand, Essentials of Audiology, 4th edition (2016) — Acoustic reflex patterns and site of lesion
- Jerger & Jerger, Archives of Otolaryngology, 1977 — Diagnostic value of crossed versus uncrossed acoustic reflexes
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-02. The text may contain errors; when in doubt, check the source.