Acoustic Reflexes

What is an acoustic reflex (AR)?

The reflexive contraction of the ME muscles in reaponse to a loud sound presented to either ear

What are the 4 clinically feasible AR measurements?

1. Acoustic Reflex Threshold (ART)

2. Acoustic Reflex Decay

3. Acoustic Reflex Amplitude

4. Acoustic Reflex Latency

What are the ME muscles? (2)

1. Tensor tympani

2. Stapedius

What is the overall action of the ME muscles in response to intense sounds?

• Stiffening of the ME transmission system

• Increasing the Za = decreasing the Ya

The bilateral response contractions…

Protects & dampens the ossicular chain

The stapedius muscle is the _________ muscle in the body

Smallest

Tensor tympani: origin

Wall of the ET

Tensor tympani: insertion

Manubrium of malleus

Tensor tympani: innervation

Trigeminal (V CN)

Tensor tympani: contraction

Mainly contracts to tactile stimulation

• Around the orbit/startle response

• Talking, yelling, chewing

• Acoustic: very intense sounds

Tensory tympani: action

Pulls the malleus inward → TM tightens → Za increases = Ya decreases

Stapedius: origin

Around VII canal (post wall of ME)

Stapedius: insertion

Head of stapes/post surface of the neck of the stapes

Stapedius: innervation

Stapedial (VII CN)

Stapedius: contraction

Mainly contracts to acoustic stimulation

• Tactile: stimulation around the pinna

Stapedius: action

Stapes is pulled down & out of the oval window → Attenuation of footplate vibration → Za increases = Ya decreases

Importance of AR: Theories (3)

1. Protection theory

2. Interference prevention (perceptual theory)

3. Desensitization

Protection theory

Protect the cochlea from sound sounds (low freq)

Interference prevention (Perceptual theory)

• Separate signal & noise

• Non-acoustic reflex (chewing, talking, etc.)

Desensitization

Unimportant sounds (talking, yelling, internal sounds, etc.)

To have an AR, what must work?

• Middle ear

• Cochlea

• VIII nerve

• Brainstem neurons (CN & SOC)

• FMN & VII nerve

• Stapedius muscles (ME)

Ipsilateral AR

Uncrossed

• Probe & stimulus ear

• Stimulus & recording in the same ear

Ipsilateral AR: Advantages (3)

1. Sensitivity to ME effects because the probe & activator are presented to the same ear

2. Ease of use in young children & individuals who are difficult to test because it requires one ear at a time

3. No concern for collapsed ear canals (for systems using supra-aural headphones to present a contralateral activator)

Contralateral AR

Crossed

• Stimulus presented to insert ear

• Recording from probe ear

Contralateral AR: Advantages (3)

1. Sensitivity to crossed pathways, and therefore, mid-brainstem pathologies

2. Less susceptible to artifact because the activator & probe stimulus are presented through separate transducers to the two ears

3. Having more normative data available

What are ARTs obtained at?

TPP

• Airtight seal is critical

What do ARTs measure?

Immittance changes

• Visually monitored/recorded by the equipment

A negative or positive reflex display is _______

Monophasic

What is the threshold of deflection?

0.02 - 0.03 mL

What are normal AR response findings? (2)

1. Monophasic

2. Biphasic (onset only)

What is a Monophasic response?

• Normal response

• Increase in Za

What is a Biphasic (at onset) response?

• Normal response

• Decrease in Za at the onset

  • Momentary contraction of the stapedius muscle → changes in the coupling b/w footplate & cochlear fluid

What is an abnormal AR response finding?

Biphasic (at onset & offset)

What is a Biphasic (at onset & offset) response?

• Abnormal response

• Using high frequency tones (600-700 Hz)

• Stiffness pathology

What is an abnormal biphasic response indicative of?

• Early otosclerosis

• Cogan’s syndrome

• Congenital stapes fixation

• Osteogenesis imperfecta

What is the normal ART range?

70 - 100 dB HL (mean = 85 dB HL)

OR

70 - 80 dB SL

In patients with normal hearing, ipsilateral ARTs should be…

0 - 80 dB above their pure tone thresholds

In patients with normal hearing, contralateral ARTs should be…

5 dB greater than ipsilateral ARTs

• i.e. if PT thresholds were at 10 dB HL, you would expect ipsilateral ARTs to be between 80 - 90 dB HL & contralateral ARTs between 85 - 95 dB HL.

If a patient’s PT thresholds were at 10 dB HL, what would you expect ipsilateral & contralateral ARTs to be?

Ipsilateral ARTs should be between 80 - 90 dB HL;

Contralateral ARTs should be between 85 - 95 dB HL.

What is the test-retest reliability for ART?

High; ± 2.4 dB

ART should be within ___ dB of eachother

< 10 dB

If using BBN to record ART, thresholds should be ___ dB lower than thresholds recorded using tones

20 dB

The ipsilateral threshold averages should be ___ - ___ dB lower than contralateral thresholds

3 - 5 dB lower

• With the exception of 2k Hz!!

What factors are dependent on probe tone (newborns)? (2)

1. Percent of reflex present

   • More with high-tone than low-tone

2. Mean threshold

   • Little elevated with low- vs. high-tone

What factors influence mean ART? (4)

1. Stimulator & step size

2. Age

3. Drugs

4. Non-acoustic, tactile reflex

How does stimulator & step size affect mean ART?

• Tone (5 dB step size)

• BBN (2 dB step)

• 20 dB lower with BBN stimulus

How does age affect mean ART?

BBN thresholds increase with age

How do drugs affect mean ART?

Drugs elevate ART

• Barbiturates → elevate contra ART (ipsi unaffected)

• Ethyl alcohol → elevate both ARTs (larger elevation for BBN than tones)

• Antipsychotic & chlorpromazine → elevate ARTs

How does non-acoustic, tactile reflexes affect mean ART?

Tactile reflexes are larger than the acoustic reflex

• Can interfere with accurate ART measurements in a wiggly child

You should never perform AR testing on pts with… (4) (contraindications)

1. Tinnitus

2. Severe recruitment

3. Hyperacusis

4. Outer ear infection (otitis externa)

Acoustic Reflex Decay is presented at ___ dB above the ART

10 dB above

AR Decay is presented for ___ seconds

10 seconds

Normally, the stapedius muscles can maintain _______ contraction

Sustained

AR Decay: Normal Findings

• No decay: 500 & 1000 Hz

• Normal decay: 2000 & 4000 Hz (neural adaptation)

If a patient has normal auditory nerve function & normal brainstem function…

The acoustic immittance change will be maintained throughout the duration of the stimulus.

AR Decay: Abnormal Positive Decay

Abnormal neural adaptation

• A decline in AR contraction for a sustained acoustic activating signal

What is Reflex Half-Life?

The time required for the response magnitude to decline to 1/2

Decay half-life (acoustic immittance change begins decreasing to baseline or > 50 %) of 10 seconds (or less) is a positive sign of what?

A retro-cochlear lesion

AR Amplitude: the Stapedius muscle response is _______

Graded

Normal Findings: AR Amplitude (regarding Linear Growth Function)

With increased stimulus levels, you should see an increase in AR amplitude (decreased Ya)

• Normal hearing or cochlear HL

Normal Findings: AR Amplitude (regarding Linear Growth Function)

No Linear Growth Function observed

• Question the threshold?

• Age: lower growth function?

• A retro-cochlear lesion?

  • VIII lesion/tumor?

Normal Findings: AR Amplitude (regarding amplitude ratio between ears)

Normal ratio: 1

Abnormal Findings: AR Amplitude (regarding Linear Growth Function)

Abnormal ratio < 0.7

• Retro-cochlear lesion?

What is AR Latency?

• How long it takes for the AR to occur after the stimulus is presented

• The time interval between the onset of an intense sound and the onset of ME muscle contraction

What is Onset Latency?

When AR magnitude rises from baseline to 10% of its eventual max magnitude

AR Latency Test (ARLT): Normal values

In normal, cochlear hearing loss, & cortical lesions…

• At 0.5 kHz: 102.8 ms

• At 1 kHz: 101.9 ms

• At 2 kHz: 127.7 ms

• At 4 kHz: 147.3 ms

AR Latency Test (ARLT): Abnormal values

Abnormally long latency > 200 ms

• Indicative of retro-cochlear pathology (8th nerve, CN, SOC)

ARLT: Advantages

• Valid, cost-effective

• Simple clinical procedure

ARLT: Limitations

• Age effect

• Averaging techniques

• Test-retest reliability

ARLT: Most effective when…

When comparing both absolute latency & ILD (between ipsilateral and contralateral AR)

In conclusion: The presence of an ipsilateral AR is usually reassuring of…

Normal middle ear function

In conclusion: On average, ART = ___

85 dB HL

In conclusion: Normally, AR amplitude increases with…

Stimulus level

In conclusion: Normally, at 500 & 1000 Hz reflex decay is…

Negative

• The presence of decay may suggest a retro-cochlear (neural) lesion

In conclusion: AR Latency > 200 ms is…

Abnormally delayed