page_title: Acoustic reflex
section_title: Measurement
hierarchical_section_title: Acoustic reflex / Measurement
caption_attribution_description: English: The eardrum is excited with a 500 Hz probe tone that sets it into motion. A 1000 Hz elicitor tone is presented for 0.5 seconds (Blue bar indicates the presentation of the elicitor) and causes the acoustic reflex to activate. When the elicitor is turned off, the muscles of the middle ear relax and the vibration of the middle ear returns to normal. As the elicitor's presentation level increases, the effect of the acoustic reflex increases
context_page_description: The acoustic reflex is an involuntary muscle contraction that occurs in the middle ear in response to loud sound stimuli or when the person starts to vocalize.
  When presented with an intense sound stimulus, the stapedius and tensor tympani muscles of the ossicles contract. The stapedius stiffens the ossicular chain by pulling the stapes of the middle ear away from the oval window of the cochlea and the tensor tympani muscle stiffens the ossicular chain by loading the tympanic membrane when it pulls the malleus in toward the middle ear. The reflex decreases the transmission of vibrational energy to the cochlea, where it is converted into electrical impulses to be processed by the brain.
context_section_description: Most of the time, the stapedius reflex is tested with tympanometry. The contraction of the stapedius muscle stiffens the middle-ear, thus decreasing middle-ear admittance; this can be measured thanks to tympanometry.
  The acoustic stapedius reflex can also be recorded by means of extratympanic manometry (ETM).
  The stapedial reflex can be measured with laser Doppler velocimetry.  Jones et al. focused a laser on the light reflex of the manubrium in awake human subjects.  The amplitude of a 500 Hz probe tone was used to monitor the vibrations of the tympanic membrane.  Various elicitors were presented to the subjects: 1000 Hz tone-burst for 0.5 s at 100 dB SPL, recorded .22 caliber gunshot noise with a peak level of 110 dB SPL.  The amplitude of the 500 Hz probe tone was reduced in response to the eliciting stimuli.  Time constants for the rate of onset and recovery were measured to be about 113 ms for the tone and 60-69 ms for the gunshot recordings. 
  As the stapedius muscle is innervated by the facial nerve, a measurement of the reflex can be used to locate the injury on the nerve. If the injury is distal to the stapedius muscle, the reflex is still functional.
  A measurement of the reflex can also be used to suggest a retrocochlear lesion (e.g., vestibular schwannoma, acoustic neuroma).
  The acoustic reflex normally occurs only at relatively high intensities; contraction of middle ear muscles for quieter sounds can indicate ear dysfunction (e.g. tonic tensor tympani syndrome -TTTS).
  The pathway involved in the acoustic reflex is complex and can involve the ossicular chain (malleus, incus and stapes), the cochlea (organ of hearing), the auditory nerve, brain stem, facial nerve, superior olivary complex, and cochlear nucleus. Consequently, the absence of an acoustic reflex, by itself, may not be conclusive in identifying the source of the problem.
caption: Examples of the onset and recovery of the acoustic reflex measured with a laser Doppler velocimetry system.
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