Understanding and Interpreting Acoustic

Reflexes

Jan PollardChief Audiologist

Sonic

Tensor Tympani Muscle

Extends from wall of Eustachian tube and connects to manubrium of malleus

Tensor Tympani Reflex

• Innervated by Cranial V (trigeminal)

• Part of startle response

• Pulling eyebrow upward, puff of air into orbital area- but not to sound

• Contraction draws malleus inward, tensing drum, stiffening ossicular chain, ossicular stiffening and canal volume change

Stadpedius Muscle

One end is connected near the facial nerve canal and the other connects to head of Stapes

Stapedial Reflex

• Innervated by stapedial (motor) part of Cranial VII (facial) nerve

• Tactile response to facial area around pinna (reflexogenic skin zone) or to sound (acoustic reflex)

• Pulls stapes down and out of oval window, BILATERALLY

• Stiffens ossicular chain but no canal volume change

The Model of the Acoustic Reflex Pathway

ME Inner Ear Cranial VIII (vestibulocochlear nerve) cochlear nucleus (CN) superior olivary complex (SOC) facial nerve (VII nerve) middle ear (Stapedius muscle)

Information crosses over to the other side

Ipsi- and Contra-lateral Reflexes

• Ipsi-lateral reflex

• Sound is put into the ear that the reflexes are being measured in. Stronger than contra-lateral.

• Contra-lateral reflex

• Sound is put into the test ear and the reflex is measured form the other (non-test) ear.

Ipsi and Contra-lateral

L

L

R

R

Ipsi Contra

IpsiContra

Normal Reflex range

General Rules• Normal reflex level is 85 dB

(70 to 100 dB)

• Ipsi-lateral thresholds occur at lower levels than contra-lateral reflexes (3- 5 dB)

• Hearing loss <50 dBHL , there should be no effect on the reflex and the results should resemble a normal ear.

• As the cochlear hearing threshold increases, the chances of an elevated or absent reflex increases

• 50-80 dB loss –elevated• >80 dB likely absent

Contra-lateral reflexes

3 General Rules for Eliciting a Reflex

NUMBER 1

Ear with the probe assembly in it must not have any outer or middle ear pathology

Middle Ear Pathology

• Probe Ear Principle• Usually ART absent when conductive component in

probe ear even though stapedial muscle working• 50% of ears have absent reflex with 5 dB Air/Bone gap

• 80% absent with 10 dB gap

• Stimulus Ear Principle• The air/bone gap decreases the intensity of sound

reaching the cochlea• Eg. 35 dB A/B gap = 115 dB threshold 45 dB = 130 dB

• Any degree of air/bone gap or an abnormal tympanogram can result in no reflex being measured.

• The gap can be as little as 5dB

• Eg perforation no A/B gap but abnormal tymp so no reflex.

Number 2

The stimulus ear must receive a tone that is loud enough

Conductive losses

• need to add that much to the tone to reach the cochlea at a level to elicit a reflex

Eg. 30 dB conductive loss and normal sn level

Expected level then 85 + 30 = 115 dB (may be outside limit of machine so NR)

• Negative ME pressure up to -240 daPa may elevate the reflex by about 10 dB, more negative than that can result in NR

Sensorineural Losses

• SN loss - now related to recruitment, not just added on as in a conductive loss

• For cochlear losses up to 70 dB HL it is expected that a reflex can be recorded.

NUMBER 3

The neural pathway must be adequate to activate the contraction of the stapedius reflex

VIIIth Nerve (8th ) involvement eg Acoustic Neuroma

• Compromise of neural impulses that can illicit ipsi- and contra- reflexes, so usually absent reflexes on that side.

• Can get reflexes sometimes between 100- 115 dB so need to look at 90th percentile figures (as long as no A/B gap)

VIIIth Nerve (8th ) involvement eg Acoustic Neuroma

L

L

R

R

IpsiContra

IpsiContra

VII th Nerve (7th ) involvement eg Bell’s Palsy

L

L

R

R

IpsiContra

IpsiContra

Low Brainstem Disorders

L

L

R

R

IpsiContra

IpsiContra

L ipsi Y, contra N R ipsi Y, contra N

Contralateral vs Ipsilateral Reflexes

Contralateral Reflexes

Advantages-

• sensitive to disorders involving crossed reflexed pathway eg miss intra-axial brainstems lesions if just used ipsi

• Contra less sensitive to artifacts than ipsi

• More data for contra reflexes

Ipsilateral Reflexes

Advantages-

• Sensitive to middle ear pathology

• Each ear tested independently of each other, so not confounded by disorders in the other ear

• Easier to do than contra for hard-to-assess children

Thank you