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How does impedance audiometry use the stapedius muscle response?

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In impedance audiometry, the stapedius muscle response is used to assess the integrity of the auditory pathway and the facial nerve that innervates it.1

The procedure functions as follows:
* Stimulation: A moderately intense sound is applied to one ear, which triggers a reflex contraction of the stapedius muscle in both ears.21
* Measurement: During this contraction, the stapedius pulls the stapes footplate outward, which changes the tension of the ossicular chain and increases the acoustic impedance of the middle ear.21
* Detection: Because the impedance has increased, more sound is reflected back from the tympanic membrane. This reflected sound is captured by a microphone in the ear canal, and the resulting change is recorded as a slight excursion on the audiometer.1

This reflex provides objective data regarding the condition of the cochlea, the auditory pathways of the medulla, and the function of the facial nerve.1