Acoustic immittance testing evaluates the eardrum and the middle ear space behind the eardrum, as well as a muscle reflex that involves the eardrum and middle ear, the inner ear, the auditory nerve, the auditory brainstem pathways and the facial nerve. Acoustic immittance testing helps to localize what part of the ear may be involved in hearing loss. During the test, a small plastic probe is placed in one or both ear canals. During eardrum testing, also calledtympanometry, a slight pressure is placed on the eardrum to determine how well the eardrum vibrates and how much air is behind the eardrum.
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The stapedius muscle in the middle ear will contract in response to a loud sound; a reflex action designed to dampen loud sounds before they arrive at the cochlea or inner ear.
Absence or presence of acoustic reflexes for various presentations may be important in the differential diagnosis of both peripheral and central disorders of the auditory system such as vestibular schwannoma link or facial nerve disorders link.
A third test is the Acoustic Reflex Adaptation or Decay. Reflex decay is defined as a decline in the contraction of the muscle during a sustained stimulating signal.
This is also helpful in interpreting and differentiating auditory test results. In patients with normal middle ear and reflex thresholds, there is no decay during the presentation of the tone. Patients with lesions of the cochlea, eighth nerve or auditory brainstem may have evidence of decay. The Fistula Test is a special application of the tympanogram to test for certain causes of dizziness.
Pressure is placed against the eardrum and decreased slowly. A positive response may be constituent with a perilymph fistula link which is an abnormal opening in the inner ear associated with dizziness and fluctuating hearing loss.
Acoustic Immittance Testing
Adult Acoustic Immittance Measures