Acoustic reflex

ACOUSTIC REFLEX(STAPEDIAL REFLEX)
  • This reflex is based on the fact that a loud sound, 70-100 dB above the threshold of hearing of a particular ear, causes bilateral contraction of the stapedial muscles which can be detected by tympanometry.
  • Tone can be delivered to one ear and the reflex picked from the same or the contralateral ear.
  • 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 and other components.
  • Stapedial reflex is mediated by 7th and 8th Cranial Nerves.
  • Stapedial reflex is a protective reflex against loud sounds.
  1. Stapedius muscle is supplied by VII nerve (facial nerve).
  2. Lesions of facial nerve lead to loss of stapedial reflex and hyperacusis or phonophobia i.e. intolerance to loud sounds
  • As the stapedius muscle is innervated by the facial nerve,it can be used to locate the injury on the nerve.
  • If the injury is distal to the stapedius muscle, the reflex is still functional.
ACOUSTIC REFLEX THRESHOLD
  • The acoustic reflex threshold (ART) is the sound pressure level (SPL) from which a sound stimuli with a given frequency will trigger the acoustic reflex. Individuals with normal hearing have an acoustic reflex threshold (ART) around 70-100 dB .
  • People with conductive hearing loss(-i.e. bad transmission in the middle ear) have a higher acoustic reflex threshold.
RESULTS OF ACOUSTIC REFLEX THRESHOLD IN DIFFERENT PATHOLOGIES
Bilateral normal ART results.
Ipsilateral Cantratareal
Right Normal Normal
Lift Normal Normal
Cochlear pathology/VIII nerve pathology right ear
Ipsilateral Cantratareal
Right Elevaled/Absent Elevaled.Absent
Lift Narmal Narmal
Facial nerve pathology, right side
Ipsilateral Cantratareal
Right Absent/Elevaled Narmal/Elevaled
Lift Narmal Elevaled
Mild middle ear pathology, right ear
Ipsilateral Cantratareal
Right Absent Absent
Left Narmal Absent
Small intra-axial brainstem pathology (small)
Ipsilateral Cantareal
Right Narmal Absent
Left Narmal Absent
Larger intra-axial brainstem pathology
Ipsilateral Cantareal
Right Absent Absent
Left Absent Absent
ACOUSTIC REFLEX DECAY TESTING
  • The acoustic reflex decay test is used to assess the integrity of CN VIII.
  • Using a stimulus of either 500 or 1000 Hz, a contralateral continuous tone is presented for 10 seconds at a stimulus level 10 dB above the acoustic reflex threshold for that stimulus frequency in that ear.
  • This suprathreshold acoustic reflex then is recorded over the 10-second stimulation period.
  • If the amplitude of the recorded deflection on the screen decreases by 50% or more within 10 seconds, the test is considered positive.
  • Positive findings on this test may suggest a retrocochlear lesion (eg, vestibular schwannoma, acoustic neuroma).
Exam Question
  • Stapedial reflex is a protective reflex against loud sounds.
  • Stapedial reflex is mediated by 7th and 8th Cranial Nerves.
  • Acoustic/Stapedial reflex decay test is positive in lesion of Eighth nerve
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