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Acoustic Reflex Testing

Table of contents

1. What is the acoustic reflex?
2. Why perform acoustic reflex measurements?
3. How to set up the reflex protocol
4. How to perform acoustic reflex testing
5. Reasons for repeating reflex measurements
6. Probe and contra headphone placement
7. Normal acoustic reflexes
8. Non-reflexes
9. Acoustic reflex patterns
10. Acoustic reflex decay
11. References

What is the acoustic reflex?

The acoustic reflex is the contraction of the stapedius muscle elicited by the presentation of an acoustically loud sound.

When either ear is presented with a loud sound, the stapedius muscles on both sides contract.

Contraction of the stapedius muscle tilts the anterior stapes away from the oval window and stiffens the ossicular chain.

This results in increased impedance which is measured as a small decrease in compliance by an ear canal probe.

The stapedius muscle is innervated by the seventh cranial (facial) nerve (CNVII).

So, in the presence of CNVII paralysis, the stapedius muscle is likely to be affected.

Why perform acoustic reflex measurements?

Acoustic reflex results make a major contribution to differential diagnosis and should be part of every basic audiological evaluation.

They can provide/confirm information about the type (conductive, sensory, neural) and degree of hearing loss.

An acoustic reflex will most likely be elicited if all the following conditions are met:

• Normal middle ear function
• Loud enough stimulus to elicit the response
• No abnormal adaptation to stimulus

Yet, about 5% of the adult population have absent acoustic reflexes.

The pure tone intensity range to elicit an acoustic reflex is 70 to 100 dB HL (median = 85 dBHL).

Ipsilateral acoustic reflex thresholds (ARTs) in patients with normal hearing are usually 70-80 dB above their pure tone thresholds, and about 5 dB greater for their contralateral threshold.

So, if pure tone thresholds were at 10 dB HL, you would expect ipsilateral ARTs between 80-90 dB HL and contralateral ARTs between 85-95 dB HL.

Contraindications for acoustic reflex testing are:

• Tinnitus
• Outer ear infection
• Severe recruitment
• Hyperacusis

How to set up the reflex protocol

There are several parameters you need to take into consideration when setting up the reflex protocol.

 

1. Ipsi- and contralateral pathways

You can either measure the ipsilateral pathway, contralateral pathway, or both.

Ipsilateral means ‘same side’ and contralateral means ‘opposite side’.

The reflexes are always indicated by the probe ear.

For both pathways, the loud sound travels through the outer, middle, and inner ear, then along the vestibulocochlear nerve (CNVIII) to the brainstem arriving at the cochlear nucleus.

From here the signal travels to the superior olivary complex and to the CNVII nuclei.

The signal is then sent down the CNVII causing contraction of the stapedius muscle.

 

2. Probe tone

Use a 226 Hz probe tone unless you are testing neonates.

In this case, use a high-frequency probe tone (1000 Hz).

 

3. Continuous or pulsed stimulus

The pulsed stimulus provides the opportunity to measure ipsilateral reflexes using a 1000 Hz tone.

Also, the pulsed stimulus lowers the risk of false reflexes that may occur at high intensities in cavities.

Yet, using the pulsed stimulus doubles the test time relative to the continuous stimulus.

Thus, the continuous stimulus is better in scenarios where you measure using a 226 Hz probe tone, at normal intensities, and when measuring contralateral reflexes.

The pulsed stimulus is better at high intensities, where there is a risk of false passes in cavities, and when measuring ipsilateral reflexes on infants using a 1000 Hz probe tone.

There is little difference in the morphology of the reflex when using a pulsed relative to continuous stimulus, so it is something to keep in mind in the interpretation of the test results (Figure 1).

Figure 1: Pulsed vs continuous reflex morphology.

 

4. Acoustic reflex threshold (ART)

The ART is the lowest intensity of an acoustic stimulus that elicits a measurable change in acoustic immittance.

A change or threshold criteria of 0.03 is usually taken as the smallest change required to confirm the presence of a reflex (Figure 2).

Figure 2: Threshold criteria.

 

5. Test frequencies

ART measurements are usually conducted at 500, 1000, 2000, and 4000 Hz.

Results are variable at 4000 Hz and many normal-hearing adults have elevated ARTs at this frequency.

Thus, you should view results with caution.

Some clinicians prefer to use a broadband noise as an alternative to 4000 Hz.

Generally, noise stimuli elicit reflexes at lower levels than pure tones do; approximately 20 dB lower.

 

6. Intensity

The intensity should start from 70-80 dB HL up to 105 dB HL in 5 dB steps until an acoustic reflex threshold is obtained.

Depending on the required outcome of testing (screening vs clinical), it is not recommended to go above 105 dB HL unless you suspect a conductive loss.

Going above this level can cause permanent hearing damage and tinnitus.

 

7. Positive or negative reflex display

Reflexes are either displayed positively or negatively, depending on your setup (Figures 3 and 4).

Figure 3: A negatively displayed reflex.

Figure 4: A positively displayed reflex.

How to perform acoustic reflex testing

The procedure is outlined below step-by-step.

 

Step 1

Alert the client that they will hear some loud sounds in either ear.

Ask them to sit still and quiet.

 

Step 2

Place the immittance probe (probe used for tympanometry) into the ear you want to test.

Place the contralateral probe into the other ear.

 

Step 3

Perform tympanometry first.

You should measure acoustic reflexes with the ear canal pressure set to the largest compliance in the presence of the 226 Hz probe tone.

 

Step 4

Press Start.

The measure will then run a reflex growth and stop when a reflex is present with the defined threshold criteria.

If the tone is loud enough and a contraction of the stapedius muscle occurs, the immittance probe will record that an acoustic reflex is present.

 

Step 5

The presence of a reflex is automatically confirmed by running the measure twice at the same intensity to confirm that the found threshold is reproducible.

Or you can repeat the measure manually 5 dB above the ART obtained to ensure it is a true ART.

Reasons for repeating reflex measurements

If any of the following occur during testing, it is wise to re-test to confirm your results are true:

(1) Patient swallows, talks, laughs, coughs or similar during the test.

(2) You get an odd result that does not look correct or does not match audiogram findings. When in doubt, repeat the test to check your results are repeatable.

(3) Collapsed canals can lead to false results, particularly if a headphone is used on the contralateral ear. Re-check results if they look suspicious or do not fit with other results in the test battery.

Probe and contra headphone placement

The reflexes are indicated by the probe ear.

So, when obtaining the reflexes for the right ear, place the probe in the patient’s right ear and the contra headphone on the left ear (Figure 5).

Figure 5: Probe and contra headphone placement for right ear acoustic reflexes.

When obtaining reflexes for the left ear, place the probe in the left ear and the contra headphone on the right ear (Figure 6).

Figure 6: Probe and contra headphone placement for left ear acoustic reflexes.

Normal acoustic reflexes

Your results should show:

• The presence or absence of the stapedial reflex
• An acoustic reflex threshold
• Acoustic reflex decay or adaptation (if tested)

The following is an example of what a reflex should look like (Figure 7).

Figure 7: Normal acoustic reflex.

The deflection value (0,07) is shown when a reflex is present.

But detection is based on the measurement matching an algorithm.

Thus, it is important to look at the morphology of the reflex in conjunction with your testing situation and decide if the reflex is in fact a ‘true’ reflex and not an artifact.

A reflex should show a negative deflection from 0,00 ml, which is time-locked to the stimulus presentation.

It will then hold the change in compliance before offsetting back to 0,00 ml (Figure 8).

Figure 8: Negative deflection at onset.

You will note in the example above that the reflex goes into the negative part of the graph before moving to the positive deflection point of 0,05 ml.

This type of response is biphasic and can occur at the onset or both onset and offset of the reflex.

The abnormal pattern of a biphasic response at both onset and offset is associated with otosclerosis, particularly in its early stages.

Non-reflexes

Non-reflexes occur when:

• The reflex does not meet the deflection criteria
• Patient moves, swallows, or speaks during the test, which creates artifacts

In the former, it may appear to be a reflex, but the deflection value is not large enough to meet the given criteria (Figure 9).

Figure 9: Non-reflex due to deflection value that does not meet the criteria.

In Figure 10, the system has detected this as a reflex (highlighted in green), but it is in fact an artifact.

Figure 10: Non-reflex due to artifact.

You can tell by looking at the shape that it does not match the pattern of a normal acoustic reflex.

In this instance, you should repeat the test at that frequency to confirm the true ART.

Acoustic reflex patterns

Below are various reflex patterns you may come across during testing.

These are not the results or patterns that you will see every time you test and real‑life clinical interpretations are much more complex.

Different authors publish patterns or record results in different ways.

Thus, the tables below are estimative.

Note that reflexes at 4000 Hz may or may not be present due to variability at this frequency.

You may wish to use broadband noise as an alternative to testing at 4000 Hz.

 

1. Normal hearing and middle ear function

In patients with normal hearing and middle ear function, both ipsilateral and contralateral reflexes will be present at all frequencies (Table 1).

	Frequency	.5 kHz	1 kHz	2 kHz	4 kHz
Probe R	Stim R (ipsi)	85	85	85	85
	Stim L (contra)	90	90	90	90
Probe L	Stim L (ipsi)	80	80	80	80
	Stim R (contra)	85	85	85	85

Table 1: Normal hearing and middle ear function.

 

2. Conductive hearing loss

Acoustic reflexes will be absent when a probe is placed in an ear with a middle ear disorder.

This is because middle ear disorders typically prevent the probe from measuring a change in compliance when the stapedius muscle contracts.

Reflexes will therefore be absent even in the case of a mild conductive hearing loss (Table 2).

In the presence of a Type C tympanogram, depending on the degree of negative pressure in the middle ear, reflexes can be either present or absent.

Conductive hearing loss is unlikely if acoustic reflexes are present in the probe ear, except in the rare case of superior semicircular canal dehiscence (SSCD).

	Frequency	.5 kHz	1 kHz	2 kHz	4 kHz
Probe R	Stim R (ipsi)	85	85	85	85
	Stim L (contra)	100	100	100	105
Probe L	Stim L (ipsi)	X	X	X	X
	Stim R (contra)	X	X	X	X

Table 2: Normal hearing in the right ear and a mild conductive loss in the left ear.

In Table 2, the raised left contralateral reflex thresholds (probe right, stimulus left) are due to the extra sound pressure level (SPL) needed to overcome the mild loss in the left ear.

The mild middle ear pathology may affect signals traveling through the left ear or being measured in the left ear.

They will either be absent or raised.

	Frequency	.5 kHz	1 kHz	2 kHz	4 kHz
Probe R	Stim R (ipsi)	85	85	85	85
	Stim L (contra)	X	X	X	X
Probe L	Stim L (ipsi)	X	X	X	X
	Stim R (contra)	X	X	X	X

Table 3: Normal hearing in the right ear and a moderate conductive loss in the left ear.

In Table 3, the stimulus was not loud enough to elicit the stapedius reflex in the left contralateral recording (probe right, stimulus left), as we are dealing with a moderate conductive loss.

 

3. Cochlear hearing loss

In ears with a cochlear hearing loss, it is possible for the acoustic reflex to be elicited at sensation levels of less than 60 dB.

The sensation level is the difference between the ART and the hearing threshold.

For example, if the hearing threshold at 1 kHz is 50 dB HL and the ART is 90 dB HL, the sensation level is 40 dB.

A sensation level of less than 60 dB is a positive Metz test.

This indicates a cochlear site of lesion (sensorineural loss) due to the loudness recruitment phenomenon.

	Frequency	.5 kHz	1 kHz	2 kHz	4 kHz
Probe R	Stim R (ipsi)	85	80	80	100
	Stim L (contra)	85	90	90	X
Probe L	Stim L (ipsi)	85	90	85	100
	Stim R (contra)	90	80	85	X

Table 4: A mild to moderate cochlear loss in both ears.

In Table 4, note that the ARTs occur at about normal levels.

This is because the ART in an ear with a cochlear loss may resemble the results of a normal ear when the air conduction thresholds are below 50 dB HL.

As the hearing threshold increases above this level, the chance of recording a raised or absent acoustic reflex increases.

	Frequency	.5 kHz	1 kHz	2 kHz	4 kHz
Probe R	Stim R (ipsi)	85	85	85	95
	Stim L (contra)	X	X	X	X
Probe L	Stim L (ipsi)	X	X	X	X
	Stim R (contra)	90	90	90	95

Table 5: Severe to profound cochlear loss in the left ear; normal hearing in the right ear.

In Table 5, the stimulus was not loud enough to elicit a stapedius reflex due to the severe to profound loss in the left ear.

Thus, whenever a stimulus is presented to the affected ear, reflexes will be absent/raised in both ipsilateral and contralateral recordings as shown above.

 

4. Retrocochlear hearing loss

ARTs in ears with a retrocochlear (CNVII) pathology are usually elevated above what they would have been for normal hearing or a cochlear hearing loss.

Often, they are absent at the greatest stimulus levels.

Keep in mind that you should analyze ART results in combination with the patient’s case history, audiogram, speech audiometry findings, and tympanometry findings for differential diagnosis.

Some things to note:

• Ears with a retrocochlear pathology and normal hearing do not have reflexes 30% of the time
• With a mild 30 dB hearing loss, the likelihood of absent reflexes increases
• The absence of reflexes at 0.5, 1, and 2 kHz in normal hearing patients must be considered suspicious unless proven otherwise
• The affected ear will show absent acoustic reflexes when a stimulus is presented to it in the case of a CNVIII lesion

	Frequency	.5 kHz	1 kHz	2 kHz	4 kHz
Probe R	Stim R (ipsi)	80	80	80	90
	Stim L (contra)	105	110	X	X
Probe L	Stim L (ipsi)	110	X	X	X
	Stim R (contra)	85	80	85	95

Table 6: Retrocochlear lesion in the left ear; normal hearing in both ears.

In Table 6, note the raised/absent acoustic reflexes with presentation to the left ear.

	Frequency	.5 kHz	1 kHz	2 kHz	4 kHz
Probe R	Stim R (ipsi)	80	80	85	85
	Stim L (contra)	X	X	X	X
Probe L	Stim L (ipsi)	X	X	X	X
	Stim R (contra)	85	85	90	90

Table 7: Retrocochlear lesion in the left ear; mild hearing loss in the left ear; normal hearing in the right ear.

In Table 7, note the absent acoustic reflexes when sound is presented to the left ear.

 

5. Facial nerve/CNVII involvement

Acoustic reflexes are absent when measured on the affected side in the case of a facial nerve disorder.

This is because the stapedius muscle is innervated by the CNVII.

Often, CNVII disorders are recognizable, such as facial paralysis in the case of Bell’s palsy.

Measuring the acoustic reflex is used as a tool to track the recovery process in such patients.

	Frequency	.5 kHz	1 kHz	2 kHz	4 kHz
Probe R	Stim R (ipsi)	80	80	85	85
	Stim L (contra)	85	85	85	90
Probe L	Stim L (ipsi)	X	X	X	X
	Stim R (contra)	X	X	X	X

Table 8: Facial nerve/CNVII lesion in the left ear due to Bell’s palsy; normal hearing in both ears.

In Table 8, note that the acoustic reflexes are absent when the probe is inserted into the left ear.

Also, you will recognize this is a similar pattern of results for a CNVIII lesion.

 

6. Intra-axial brainstem lesion

This is a very rare condition; about 1 in 10 million.

Acoustic reflexes are normal ipsilaterally and absent contralaterally (Table 9).

The left and right pathways are disrupted by a lesion involving the auditory fibers.

	Frequency	.5 kHz	1 kHz	2 kHz	4 kHz
Probe R	Stim R (ipsi)	80	80	85	85
	Stim L (contra)	X	X	X	X
Probe L	Stim L (ipsi)	85	80	80	85
	Stim R (contra)	X	X	X	X

Table 9: Intra-axial brainstem lesion; normal hearing in both ears.

Acoustic reflex decay

Acoustic reflex decay testing can be useful in detecting/confirming a retrocochlear pathology.

Generally, patients will present with typical retrocochlear indicators, such as unilateral tinnitus, asymmetrical hearing loss, and vertigo.

You will often have enough information to warrant a referral to an ENT specialist without needing to do this test.

But the test may be useful when the audiogram and case history are normal, but reflex results show a retrocochlear pattern.

So, let’s dive in.

 

What is the acoustic reflex decay test?

The acoustic reflex decay test measures whether a reflex contraction is maintained or weakens during continuous stimulation (usually 10 seconds).

The test is usually conducted at 500 Hz and 1000 Hz, but not above these frequencies as even normal ears can show decay at higher frequencies.

The test is performed by presenting a continuous stimulus 10 dB above the ART for the given frequency for a period of 10 seconds.

The size of the reflex response will either stay the same or decrease during the 10-second period.

What you are looking for is whether the response decays to half its original size.

So, if the reflex response decreases to 50% of its original size within the 10 seconds of testing, the test would be positive for reflex decay.

Figure 11: Examples of acoustic reflex decay tests. Source: Gefland, 2001.

In Figure 11 (Gefland, 2001), the acoustic reflex decay is considered negative if the reflex response does not decrease (a) or if it decreases by less than half of its original size (b).

Reflex decay is positive if the magnitude falls by 50% or more (c).

 

How to perform the acoustic reflex decay test

The procedure is outlined below step-by-step.

 

Step 1

Perform tympanometry and reflex measurements first.

 

Step 2

Take the acoustic reflex threshold at 500 Hz or 1000 Hz in the ear you want to test and add 10 dB.

This is the stimulus level you will use for testing.

 

Step 3

Make sure you have a good probe seal and press start to run the test.

 

Caveat

If the reflex decay test is positive, you should check that it was not due to an improper seal, which might produce an artifact like a decaying curve.

 

Interpretation of reflex decay

The decay value is the percentage difference between the two reflex deflection values taken half a second after the stimulus started and half a second before the stimulus stopped.

The test is either negative (Figure 12) or positive (Figure 13).

 

Figure 12: Negative reflex decay test, as the response did not decay by more than 50% (green dotted line).

Figure 13: Positive reflex decay test, as the response decayed by more than 50% (green dotted line).

References

Bess, F.H., & Humes, L.E. (2003). Audiology: The Fundamentals (3rd ed.). Baltimore: Lippincott Williams & Wilkins.

Campbell, K.C., & Mullin, G. (2018). Impedance Audiometry. Retrieved March 29, 2021, from https://emedicine.medscape.com/article/1831254-overview.

Emmanuel, C. D. (2009). Acoustic Reflex Threshold (ART) Patterns: An Interpretation Guide for Students and Supervisors. Retrieved March 29, 2021, from https://www.audiologyonline.com/articles/acoustic-reflex-threshold-art-patterns-875.

Fowler, C. G., & Shanks, J. E. (2002). Tympanometry. In J. Katz (Ed.), Handbook of Clinical Audiology (5th ed.). (pp. 175-204). Baltimore: Lippincott Williams & Wilkins.

Gefland, S.A. (2001). Essentials of Audiology (2nd ed.). New York: Thieme Medical Publishers, Inc.

Margolis, R. H., & Hunter, L. L. (2000). Acoustic Immittance Measurements. In R. J. Roeser, M. Valente & H. Hosford- Dunn (Ed.), Audiology Diagnosis. (pp. 381 - 423). New York: Thieme Medical Publishers, Inc.

Srireddy, S. V., Ryan, C. E., & Niparko, J. K. (2003). Evaluation of the patient with hearing loss. In J. Niparko & L. R. Lustig (Ed), Clinical neurotology: Diagnosing and managing disorders of hearing, balance and the facial nerve. (pp. 65-80). London: Martin Dunitz Publishing.

Zito F, Roberto M. The acoustic reflex pattern studied by the averaging technique. Audiology. 1980;19(5):395-403. doi: 10.3109/00206098009070073. PMID: 7436858.