Introduction
The acoustic reflex is one of the most informative measurements in the immittance battery — a single objective test that probes the middle ear, the cochlea, the eighth nerve, the lower brainstem and the facial nerve all at once.
A reflex you can record
When a sufficiently loud sound reaches the ear, the stapedius muscle in the middle ear contracts. This contraction stiffens the ossicular chain, and that change in stiffness alters how readily sound energy flows into the middle ear. An immittance meter can record exactly that change. The acoustic reflex test is, in essence, the art of presenting a sound and watching for the small, characteristic dip in admittance that the muscle’s contraction produces.
What makes the test so valuable is that the reflex depends on an entire chain of structures working together. Sound must reach the cochlea; the cochlea must transduce it; the eighth nerve must carry the signal to the brainstem; brainstem nuclei must relay it; the facial nerve must carry the motor command back; and the stapedius must contract. A breakdown anywhere along that arc changes the result in a way that is often specific enough to localise the problem.
Where it sits in the test battery
The acoustic reflex is rarely interpreted alone. It belongs to a cross-check principle in which no single test is trusted without corroboration from others[4]. Tympanometry describes middle-ear status; pure-tone audiometry quantifies the hearing loss; otoacoustic emissions report on outer hair cells; the auditory brainstem response interrogates neural synchrony. The reflex threads through all of these, and its greatest power emerges when its result agrees — or pointedly disagrees — with the others.
How to use this atlas
The atlas is arranged so that each chapter builds on the last. Anatomy introduces the structures; Physiology explains what they do; Technique covers how the test is performed; Normal Reflex establishes the baseline; and Interpretation brings the patterns together. The eight condition pages then apply that framework to specific diseases.
Three features run throughout. The reader-level selector in the top bar re-pitches explanations between Foundation, Trainee and Clinician depth. The interactive tools — simulator, comparison, pattern trainer and self-assessment — let you test your understanding actively. And every substantive claim carries a citation you can hover to read.