Acoustic Reflex Testing: A Complete Guide

Table of contents

• What is acoustic reflex testing?
• What is the acoustic reflex?
• The acoustic reflex arc
• Why perform acoustic reflex testing?
• Criteria and contraindications
• Equipment considerations
• How to perform acoustic reflex testing
• How to interpret acoustic reflex results
• What is the acoustic reflex decay test?
• How to perform the acoustic reflex decay test
• How to interpret acoustic reflex decay results

What is acoustic reflex testing?

Acoustic reflex testing provides information about the neural pathway involved in the acoustic reflex. It can help to differentiate between middle ear vs conductive issues, inner ear vs sensorineural problems, and eighth cranial nerve pathologies.

The acoustic reflex test is a form of tympanometry, whereby the compliance of the middle ear is recorded first, then a loud stimulus is presented which triggers the acoustic reflex, and then the resulting change in compliance is recorded.

Acoustic reflex testing can be thought of as recording two different tympanograms and measuring the difference between the two (Figure 1).

This difference between the two is the deflection value and is represented in an acoustic reflex response as the deflection of the response line (the blue line seen in the Figure 2 below) from the baseline.

What is the acoustic reflex?

The acoustic reflex is the contraction of the stapedius and tensor tympani muscles in the middle ear in response to the presentation of a loud sound. The contraction of these muscles leads to a stiffening of the ossicular chain and causes a change in compliance of the middle ear system.

This reflex is thought to offer some protection to the inner ear against loud stimulus levels and occurs bilaterally. As a result, you can measure the acoustic reflex both ipsilaterally and contralaterally.

The acoustic reflex arc

The following diagram provides a schematic of the acoustic reflex arc, also known as the acoustic reflex pathway (Figure 3). Although this is a simplified version, the key components of the pathway are displayed and can help in interpreting the results of acoustic reflex testing.

Ipsilateral reflexes

Ipsilateral reflexes refer to when the sound stimulus and probe measurement are placed in the same ear. The right and left ipsilateral pathways are highlighted in red and blue, respectively, in Figure 4.

Contralateral reflexes: Theory vs clinical practice

The theoretical perspective of contralateral acoustic reflexes has historically involved referring to the acoustic reflex according to the ear that received the stimulus. For example, the right contralateral pathway would refer to the stimulus being presented to the right ear, with the left ear reflex being measured.

In modern, PC-based tympanometers, this is handled differently. To improve the workflow and efficiency, many clinicians prefer to place the probe in one ear and perform all the testing that requires that arrangement before moving the probe to the other ear.

A common workflow would be to perform ipsilateral reflexes followed by contralateral reflexes. To avoid having to move the probe back and forth between ears, the probe remains in the same ear as the ipsilateral reflexes were recorded in.

This means that the reflex is recorded in the original ear, with the stimulus being presented to the opposite side. This is therefore described differently to the theoretical explanation above, with all testing referring to the ear containing the probe.

Figures 5 and 6 show the different methods of describing the contralateral pathways.

It is important to know which descriptors are implemented by the equipment you are using to make accurate and appropriate interpretation of the results.

Why perform acoustic reflex testing?

Acoustic reflex testing is quick, easy to perform, well tolerated, non-invasive, and can elicit a significant amount of audiological information. By utilizing the acoustic reflex arc, it is possible to use the results of acoustic reflex testing in differential diagnosis. You should interpret the results within the context of a wider test battery for most clinical utility.

Testing criteria and contraindications

You will most likely elicit an acoustic reflex if all the following conditions apply:

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

Acoustic reflex testing is contraindicated in the following circumstances:

• Tinnitus
• Hyperacusis
• Severe recruitment
• Outer ear infection or (excessive) wax

It is important to be aware that approximately 5% of the population have absent acoustic reflexes.

Equipment considerations

Acoustic reflex testing is performed using a tympanometer. To ensure optimal test results, you should consider the following:

• Daily checks
• Calibration

Daily checks

You should check your tympanometer daily using a hard walled cavity. These are normally provided with the system or integrated into the tympanometer in some way.

A hard walled cavity is a small chamber of fixed, known volume (usually 2cc, 0.5cc or 4cc), with a port or adapter that allows you to insert the tympanometer probe with a perfect seal. After performing a standard tympanometry measurement in the hard walled cavity, the results should read the correct volume of the cavity within a tolerance which is normally printed on the cavity itself.

The results should also show that no compliance value is measured due to the hard walled nature of the cavity. If the results fall outside of this tolerance, then you should check the tympanometer for any obvious damage or fault.

You should also check the tympanometer with a known normal ear to prove you’re measuring correct compliance.

Calibration

Your tympanometer should be calibrated annually using the necessary equipment to ensure calibration of the probe tones, the compliance (volume) and the pressure sensor. This should be performed by a qualified technician.

When you use your tympanometer for acoustic reflex testing, the reflex pure tones should also be calibrated as part of the annual calibration. These tones are calibrated as normal audiometer tones with the required coupler chosen based on the transducer.

Ipsilateral tones are calibrated as inserts in a 2cc coupler, and contralateral tones are calibrated in either a 2cc or 6cc coupler depending on the transducer.

How to perform acoustic reflex testing

In this section, we’ll cover the testing parameters followed by a step-by-step testing procedure.

Parameters

There are several testing parameters you should be aware of before performing the test, outlined below.

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’. In modern clinical practice, acoustic reflexes are referred to according to the ear that contained the probe.

The pathway(s) chosen for measurement may depend on the case presentation and/or clinical question.

2. Probe tone

Acoustic reflex testing is typically performed using a 226 Hz probe tone unless you are testing neonates under the age of 6 months. In this case, a high-frequency probe tone (1000 Hz) is used.

The probe tone used is different to the stimulus tone. The probe tone delivers sound energy to the ear, which allows for the measurement of the change in compliance. The stimulus tone is the pure tone frequency at which you perform the measurement. The probe tone remains the same while you select and/or vary the stimulus tone.

Other probe tones can be used but this is not standard practice.

3. Continuous vs pulsed stimulus

You can perform acoustic reflex testing using either a continuous or a pulsed stimulus. Table 1 summarizes the differences between the two stimuli.

Table 1: Comparison of continuous vs pulsed stimulus for acoustic reflex testing.

The difference in the morphology of the reflex when using a pulsed stimulus compared with a continuous stimulus is something to keep in mind when interpreting the test results (Figure 7).

4. Test type

Acoustic reflex testing, or acoustic reflex threshold testing, establishes the lowest intensity at which an acoustic reflex was recorded.

There is also an acoustic reflex test called the acoustic reflex decay test. This test measures whether the reflex contraction is maintained or weakens during a continuous stimulation of typically 10 seconds. It is usually performed to confirm or rule out retrocochlear pathologies.

This article will focus on acoustic reflex threshold testing. For more information on acoustic reflex decay testing, please go to this section of the article.

5. Test frequencies

You can perform acoustic reflex threshold testing using 500, 1000, 2000 and 4000 Hz pure tone stimuli. Many normal hearing adults present with elevated acoustic reflex thresholds at 4000 Hz, meaning you should interpret results for this frequency with caution.

You can also use a broadband noise stimulus for testing. Generally, broadband noise stimuli elicit reflexes at lower levels than pure tones do; approximately 20 dB lower. One advantage of using broadband noise stimuli is that the lower intensity levels used can be more comfortable for the patient.

6. Intensity

Acoustic reflex threshold testing is typically started at an intensity level of 70-80 dB HL and increased in 5 dB steps until you’ve recorded an acoustic reflex threshold. The Titan tympanometer will repeat the measurement to ensure a reproducible reflex is recorded before accepting the response.

Many clinicians also choose to perform a test at 5 dB above the threshold to check for growth of the acoustic reflex. Be careful when testing at loud intensities, as this increases the risk of noise induced hearing loss (NIHL) and tinnitus. Testing above 105 dB HL is not recommended.

7. Positive vs negative reflex display

The acoustic reflex is displayed as a deflection from the baseline. You can display this as either a positive or a negative deflection, depending on your setup (Figures 8 and 9).

The display preference is generally country-specific and doesn’t impact the measurement in any way. But you must be familiar with which display you’re using when interpreting the results.

Test procedure

The acoustic reflex testing procedure is outlined below.

Step 1: Prepare and instruct your patient

Perform otoscopy before middle ear testing to check for any physical contraindications and to decide which size probe tip is best for the patient. Identify whether your patient presents with any of the following contraindications in the case history:

• Tinnitus
• Hyperacusis
• Reduced dynamic range

Instruct your patient to sit still and remain quiet throughout the test, without coughing, moving their jaw, or swallowing. Let your patient know that you will present a series of loud sounds via the probe tip in their ear(s).

Step 2: Insert the probe tip

Insert the probe tip into the test ear. For contralateral reflexes, insert the probe into the opposite ear.

Step 3: Perform tympanometry

Perform 226 Hz tympanometry before progressing to acoustic reflex testing. You should consider any abnormal tympanometry results when interpreting acoustic reflex testing results. The tympanogram’s peak pressure value is used for acoustic reflex testing, and most tympanometers apply this value automatically.

Step 4: Perform acoustic reflex testing

Start the measurement at the chosen frequency and intensity. Depending on your equipment setup, the protocol may automatically progress through increasing intensity levels. Otherwise, increase the intensity level manually.

Step 5: Record the acoustic reflex

If the change in compliance is large enough to meet the sensitivity criteria, then you have an acoustic reflex. To confirm the presence of the reflex, the threshold should be reproducible at the same intensity.

Depending on your equipment setup, the protocol may automatically progress to 5 dB above threshold to check for growth. Otherwise, increase the intensity level manually.

How to interpret acoustic reflex results

In this section, we’ll cover the following to help you interpret your acoustic reflex results:

• Data quality
• Normal acoustic reflexes
• Absent acoustic reflexes
• Artifacts
• Differential diagnosis

Review data quality

As with all audiological results, it is important to review the data quality before making any interpretations. If any of the following occur during testing, re-test to confirm your results are true:

• Ear-canal collapse
• Patient swallows, talks or similar
• Results don’t look correct or don’t match audiogram findings

Normal acoustic reflexes

A normal acoustic reflex will show a deflection from the baseline (Figure 10). Using a pulsed stimulus will result in a more pointed morphology than a continuous stimulus.

You can adopt the sensitivity value according to your preference. However, the literature shows a value of 0,02 and above to be significant.

Typically, patients with normal hearing present with pure tone ipsilateral acoustic reflex thresholds at 70 to 80 dB above their audiogram thresholds. Contralateral acoustic reflex thresholds are approximately 5 dB greater. Broad band ipsilateral thresholds are approximately 20 dB lower than their pure tone counterpart.

See Table 2 for an example with audiogram thresholds at 10 dB HL.

Table 2: Expected acoustic reflex thresholds with audiogram thresholds at 10 dB HL.

Absent acoustic reflexes

An absent acoustic reflex has a deflection value that is smaller than the sensitivity criterion (Figure 11).

Artifacts in acoustic reflexes

It is also possible to record an artifact during acoustic reflex testing. If the patient moves, speaks, swallows or the probe dislodges, this movement may be recorded as a response (Figure 12).

The unusual morphology shown in Figure 12 is an indicator of the artifact recorded. Repeat the acoustic reflex test for that condition to confirm whether it is an artifact or not.

Differential diagnosis

Much of the literature on acoustic reflex testing results focuses on different types of pathologies and the likely pattern of results for these pathologies. However, this section will focus on differential diagnosis based on the actual results.

Note: In the below examples, ‘normal hearing’ diagnoses do not rule out mild hearing losses. Mild hearing losses can elicit normal acoustic reflex thresholds. Thus, you should always interpret acoustic reflex thresholds in conjunction with results from other tests in your test battery.

Example 1: Normal acoustic reflex thresholds

If both ipsilateral and contralateral reflexes are present at all frequencies (Table 3), then this indicates normal hearing and normal middle ear function. Refer to the results of other tests within the test battery to confirm this.

Table 3: Normal acoustic reflex thresholds. ‘R’ = right ear, ‘L’ = left ear, and ‘stim’ = stimulated (ear).

Example 2: Absent ipsilateral and contralateral reflexes in one ear and raised contralateral reflexes in the other ear

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

Example 3: Absent contralateral reflexes in one ear and absent ipsilateral reflexes in the other ear

Diagnosis: Severe to profound sensorineural hearing loss / possible retrocochlear pathology.

Example 4: Unilaterally absent ipsilateral and contralateral reflexes

Diagnosis: Unilateral facial nerve pathology.

Example 5: Bilaterally absent contralateral reflexes

Diagnosis: Intra-axial brainstem pathology.

What is the acoustic reflex decay test?

The acoustic reflex decay test measures whether the reflex contraction is maintained or weakens during a continuous stimulation of typically 10 seconds. It is usually performed to confirm or rule out retrocochlear pathologies.

Retrocochlear pathologies are usually indicated by other tests in the test battery. Yet, in some cases where audiometry is normal or unfeasible, acoustic reflex decay testing can add valuable information.

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.

How to perform the acoustic reflex decay test

The procedure for how to perform acoustic reflex decay testing is outlined below.

Step 1: Same preparation steps as acoustic reflex testing

Follow the same preparation steps as required for acoustic reflex threshold testing, including history taking, otoscopy and tympanometry.

Step 2: Perform acoustic reflex threshold testing

Perform acoustic reflex threshold testing to establish the acoustic reflex threshold levels.

Step 3: Note the value of acoustic reflex thresholds

Note the value of the acoustic reflex thresholds at 500 Hz and 1000 Hz and add 10 dB – this will be your stimulus intensity for acoustic reflex decay testing.

Step 4: Perform the acoustic reflex decay test

Start the acoustic reflex decay test at the appropriate stimulus intensity, ensuring you have a good probe seal within the ear canal. A poor seal can lead to artifact.

How to interpret acoustic reflex decay results

In this section, we’ll cover the following to help you interpret your acoustic reflex decay results:

• Data quality
• Time base
• 50% mark
• Extent of acoustic reflex decay

Review data quality

As with acoustic reflex threshold testing, it is important to review the results of acoustic reflex decay testing to ensure you have a good quality measurement. Repeat the test if you’re in doubt about the quality of the results.

A false positive acoustic reflex decay result can occur when there is a poor seal, leading to the introduction of artifact. Any movement, talking, swallowing, coughing or similar activity can also lead to false positives.

Select the time base

You should base your interpretation of the acoustic reflex decay results on two reflex deflection values:

• One recorded at half a second after the stimulus onset.
• One recorded at half a second before the stimulus offset.

See Figure 13 for an example.

Determine the 50% mark

The first deflection value, recorded at half a second after the stimulus onset, is the starting point for determining the 50% mark. 50% of this deflection value is the significance point for acoustic reflex decay interpretation.

The green dotted lines in Figures 14 and 15 show the 50% mark.

Establish the extent of acoustic reflex decay

If the response decayed by greater than 50% of the first deflection value, then the acoustic reflex decay test is positive. If the response decayed by less than 50% of the first deflection value, then the acoustic reflex decay is negative.

Referring to Figures 14 and 15, Figure 14 shows a negative result and Figure 15 shows a positive result. The Interacoustics Titan device calculates the decay value automatically and displays it within the software screen.

References

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

Fowler, C. G., & Shanks, J. E. (2002). Tympanometry. In J. Katz (Ed.), Handbook of clinical audiology (5th ed., pp. 175–204). Lippincott Williams & Wilkins.

Gelfand, S. A. (2001). Essentials of audiology (2nd ed.). Thieme Medical Publishers.

Margolis, R. H., & Hunter, L. L. (2000). Acoustic immittance measurements. In R. J. Roeser, M. Valente, & H. Hosford-Dunn (Eds.), Audiology diagnosis (pp. 381–423). Thieme Medical Publishers.

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