AUD6400 Acoustic Middle Ear Reflex

What are the two functions of the middle ear?

Conduction and transduction

How is the middle ear a conductor?

It allows sound energy to travel through to reach inner ear

How is the middle ear a transducer?

It converts acoustic sound energy to mechanical energy through ossicular chain

What is impedance matching of middle ear?

ME preserves acoustic energy and matches impedance between two media (air and fluid)

What are the middle ear muscles?

stapedius muscle and tensor tympani

Which of the two ME muscles are the dominant muscle?

stapedius muscle

Where is the stapedius muscle innervated?

Innervated by motor branch of CN VII

What is the stapedius muscle attached to?

stapes

What does the stapedius muscle do?

When innervated, it pulls on the stapes to reduce the sound energy being transmitted to the inner ear

What does the tensor tympani muscle do?

stiffens the tympanic membrane

What is the tensor tympani innervated by?

innervated by trigeminal nerve, mandibular branch

What is the tensor tympani attached to?

attached to the malleus

What is the stylomastoid foramen?

opening in temporal bone where facial nerve and stylomastoid artery pass through

What is a foramen?

structural opening in bone

What is the stylomastoid artery?

branch of posterior auricular artery which supplies facial nerve, stapedius muscle, bone and mucosa of adjacent mastoid region and otic capsule

What is the acoustic reflex?

an involuntary contraction of middle ear muscles in response to a loud sound, attenuates loud sounds (mostly low)

What is the acoustic reflex threshold?

lowest intensity level where middle ear immittance change can be detected in response to loud sound

What is the parotid gland?

large salivary gland that is located near each ear, facial nerve passes through

How high above normal hearing threshold does the acoustic reflex threshold usually stand (in young normal hearing)?

85 dB above normal hearing threshold

What is the clinical application of acoustic reflexes?

Can indirectly assess ME function, detection of hearing sensitivity loss, malingering

What are the three hypotheses for the function of the acoustic reflex?

1. protective function: reduced input of loud sounds by temporarily shifting the threshold (TTS)

2. Speech discrimination: Bell’s Palsy → reduced speech discrimination over 90dB when stapedius muscle is paralyzed

3. perceptual theory: attenuates low frequencies, separates specific sounds from noisy background

What reflex pattern would indicate cochlear or 8th nerve pathology?

absent/elevated reflexes when the stimulus is presented to the abnormal ear

What reflex pattern would indicate a 7th nerve pathology?

Absent reflexes when the probe is measuring the abnormal side

(left ipsi & right contra absent)

What reflex pattern would indicate a middle ear pathology?

absent reflexes for signals traveling through affected ear and signals measured in affected ear

(left ipsi, right contra, and left contra absent)

What reflex pattern would indicate a small interaxial brainstem lesion?

All contralateral reflexes are absent, issue with crossing the brainstem

What reflex pattern would indicate a large interaxial brainstem lesion?

all reflexes are absent