A guide to conducting acoustic immittance testing

Overview of this guideWelcome to this guide on acoustic immittance testing. It will help you to:

understand the purpose of immittance testing

clarify what client information you need

outline the equipment you need

conduct the tests

record the results, and

interpret the results.

Immittance testingImmittance testing (also known as impedance testing) consists of two types of tests:

tympanometry, and

acoustic reflex testing.

These tests are often conducted one after the other during a client consultation.

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Tympanometry

Tympanometry is conducted to measure:

the volume of the ear canal

middle ear pressure, and

the static compliance of the tympanic membrane or ear drum movement.

The test involves the use of a probe to increase and decrease air pressure in the ear canal which changes the stiffness of the tympanic membrane. The probe also emits a pure tone and measures how much sound is reflected back from the tympanic membrane at different pressures. Effectively, this is looking at how stiff or compliant the ear drum is. The stiffer the tympanic membrane the more sound is reflected back to the probe.

Before conducting the tympanometry test Before conducting the tympanometry test you will need the client’s otoscopy results. If necessary conduct an otoscope examination.

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Be sure to accurately write down your otoscopy observations. For example, is there evidence of external ear canal pathologies, perforated ear drum or a ventilation tube? What is the general size of the ear canal?

You will cross-check these observations later with the tympanometry results.

You also need to calibrate the tympanometer. The pressure at zero on the tympanometer needs to be set to the atmospheric pressure at the testing location.

What equipment will you need? To conduct tympanometry testing you need the following equipment:

a tympanometer (also known as an impedance meter)

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a tympanometer probe, and

clean probe tips of various sizes.

The tympanometer and probe primarily consists of a pump to increase and decrease air pressure in the ear, a speaker to generate a pure tone and a microphone to measure the amount of reflected sound.

Conducting the tympanometry testHere is an overview of the steps you need to take when performing tympanometry testing:

Step 1 - Make sure the client is seated comfortably before starting.

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Step 2 - Give the client an overview of how the test will be carried out and give them an opportunity to ask any questions.

Step 3 - Set the tympanometer.

Step 4 - Select the correct probe tip and place on the probe.

Step 5 - Place the probe in the client’s ear.

Step 6 - Conduct the tympanometry test.

Step 7 - Repeat the tympanometry test for the other ear.

Remember to tell the client

You will need to provide the client with test instructions. Proper instruction helps ensure that the test results are accurate and valid. There are a few important things to remember to tell the client:

tell them what you are going to do before you do it

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tell them what you expect from them during the test

during the test they must sit still

they should not swallow

they will hear a sound from the probe

they will feel pressure in their ear

both ears will be tested, one at a time

they need to tell you if they are not comfortable, and

encourage them to ask questions.

Here is an example of an audiologist instructing a client before a tympanometry test:

“Ok Audrey we're now going to test your middle ear function. The first test is called tympanometry. I'm going to put a probe in your ear and you'll hear a buzzing sound. You'll then feel some pressure on your ear, a little bit like when you're coming down into the airport when you're flying in a plane. During this test you don't need to do anything, just sit very still for me.”

Set the tympanometer

Before conducting the tympanometry test make sure that the tympanometer is:

set to tympanometry testing mode

zero pressure is equalised to the atmospheric pressure at the test location

set to auto rather than manual if available, and

set to the correct ear.

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Probe tips and placementTo obtain accurate results the probe tip used must be chosen carefully. Follow these steps:

Make sure all probe tips used are clean.

Choose a tip that you believe will best suit the size of the client’s ear canal. With experience you will become more accurate at estimating the correct size tip.

Place the tip on the probe.

Place the probe in the client’s ear. To do this, pull back and slightly up on the pinna to straighten the ear canal and insert the probe tip with a twisting motion to ensure an adequate seal. At a glance, you should be able to determine whether or not the probe tip is entirely within the ear canal. If the probe slides in too easily a larger probe tip should be used. If the probe tip won’t enter the ear a smaller probe tip should be used.

The probe tip should face the ear drum (towards the client’s nose).

The probe tip should not face the ear canal wall.

The path through the ear canal to the ear drum should not be blocked by excessive wax.

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The tympanometry test processHere is an overview of the steps you need to take when performing tympanometry testing:

Step 1 – With the probe inserted in the client’s ear hold the probe as steady as possible until the test is complete.

Step 2 – When an adequate seal is obtained the tympanometer can begin the test. Most tympanometers will automatically conduct the following test steps. Some tympanometers may require manual adjustments.

Step 3 – The pressure is increased to +200daPa and the compliance is measured. The compliance is based on the amount of sound reflected by the tympanic membrane.

Step 4 – The pressure is decreased until the pressure on both sides of the tympanic membrane is equal. This is the maximum compliance.

Step 5 – The pressure is incrementally decreased to -200daPa and the compliance is measured.

Step 6 – The pressure is returned to zero.

Step 7 – Plot a tympanogram. Most tympanometers will have automatically plotted a tympanogram during the test but you may have to plot one manually for some equipment.

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Step 8 – Repeat the test for the other ear.

Understanding the tympanogramA tympanogram plots the ear canal pressure on the horizontal x-axis against the immittance, or amount of sound that enters the middle ear on the vertical y-axis. The immittance is based on the amount of sound that is not reflected back to the probe.

Tympanograms contains several key pieces of information:

the ear canal volume - in the example the volume is 0.8 millilitres

the maximum compliance of the tympanic membrane - in the example this is 0.82 millilitres at -16daPa.

Normal results When interpreting your client’s tympanogram, you are looking for results that fall outside of the normal parameters for ear canal volume, static compliance of the middle ear and middle ear pressure.

Normative data Adults Children

Ear canal volume (cm3) 0.65 to 1.75 0.5 to 1.0

Static compliance (ml) 0.3 to 1.8 0.2 to 0.9

Pressure (daPa) -100 to 100 -150 to 100

There are three types of tympanogram results: Type A, Type B and Type C.

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Type A results Type A represents a normal set of tympanometry test results. There are three sub-types:

Type A - completely normal with a relatively sharp maximum compliance at or near zero pressure.

Type Ad - shows a maximum compliance near zero pressure but abnormally high compliance. These results indicate either a hyper mobile ear drum or ossicular discontinuity.

Type As - results show a maximum compliance that is close to zero pressure but abnormally low compliance. These results suggest some kind of stiffening within the middle ear system, such as otosclerosis.

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-400 2000

-400 2000

Type B resultsType B results can indicate:

Fluid in the middle ear indicated by a Type B graph with a normal ear canal volume.

A wax plug or foreign object blocking the outer ear canal indicated by a Type B graph with a small ear canal volume.

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-400 2000

-400 2000

Flat line near the bottom of the graph

A perforation or grommet in the tympanic membrane indicated by a Type B graph with a large ear canal volume.

Type B results can also indicate a wax blockage in the probe tip.

With cases of this sort you would expect to see some conductive hearing loss.

Type C resultsType C tympanometry results show a maximum compliance that is significantly shifted to the left beyond the normal -100daPa for adults or -150daPa for children.

This indicates negative pressure in the middle ear which can cause retraction of the tympanic membrane.

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-400 2000

This result is consistent with eustachian tube dysfunction.

This concludes the material on tympanometry. The following information outlines acoustic reflex testing.

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Acoustic reflex testing

Acoustic reflex testing involves subjecting the ear to a loud sound using a probe.

This triggers an involuntary contraction of the stapedius muscle which in turn tightens the stapes, ossicular chain and tympanic membrane.

As the tympanic membrane tightens it increases the reflection of sound generated by the probe.

What is it used for?Acoustic reflex testing is useful for diagnosing:

the function of the middle ear

the neurological functioning of the auditory system or reflex arc, and

auditory sensitivity.

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The acoustic reflex will be affected or absent if any of these three functions are impaired.

Reflex arcWhen a loud sound is presented into one ear the sound passes though the middle ear, the cochlea then passes this signal into the inner ear and then it travels along the 8th auditory nerve to the brainstem. The signal is received by the cochlear nucleus and passed onto the superior olivery complex. From here the signal is passed to higher centres in the brain and is also passed to the 7th facial nerve from where it descends to stimulate or innervate the stapedius muscle. This is an ipsilateral or same ear reflex.

However, when the signal arrives at the brainstem the signal also crosses over to the opposite or contralateral superior olivery complex and then on to the 7th facial nerve and innervates the opposite stapedius muscle. This is a contralateral or opposite ear reflex.

Acoustic reflex testing can help diagnose problems within these neural pathways.

This diagram represents the neural pathways involved in the reflex arc.

Conducting the acoustic reflex testHere is an overview of acoustic reflex testing:

Step 1 - Make sure the client is seated comfortably.

Step 2 - Give the client an overview of how the test will be carried out and give them an opportunity to ask questions.

Step 3 - Set the immittance meter to acoustic reflex mode, set to ipsilateral, select the correct ear and set to manual operation.

Step 4 - Select the correct probe tip and place on the probe.

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Step 5 - Place the probe in the client’s ear and the headphone over the other ear. Unlike tympanometry, which only requires a probe, acoustic reflex testing also requires headphones.

Step 6 – Start the test at 80dB at 500Hz. You are looking for the level at which the acoustic reflex occurs indicated by a change in immittance of 0.02.

Step 7 – If no acoustic reflex is detected keep incrementally increasing the presentation level by 5dB until an acoustic reflex is detected. Do not exceed 110dB. Also note that the stapedius muscle can fatigue. You should wait a few seconds between tests to allow the muscle to relax.

Step 8 – If no acoustic reflex is detected return the presentation level to 80dB and raise the frequency to 1000Hz. Repeat this process at 2000Hz and 4000Hz until a reflex is detected.

Step 9 – When the reflex is detected decrease the hearing level by 10dB. If an acoustic reflex is not detected raise by 5dB increments until the acoustic reflex threshold, or ART can be accurately determined.

Step 10 – Conduct the test contralaterally, that is present the stimulus sound to the ear opposite the probe. Be sure to change the immittance meter to contralateral.

Step 11 – Swap the probe and headphone around and repeat the test.

Step 12 – Interpret the results.

Remember to tell the client

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You will need to provide the client with test instructions similar to the tympanometry test.

tell them what you are going to do before you do it

tell the them what you expect from them during the test

during the test they must sit still

they should not make any sounds

they will hear loud sounds in each ear

they need to tell you if they are not comfortable, and

encourage them to ask questions.

Here is an example of an audiologist instructing a client before an acoustic reflex test:

“I'm now going to put some relatively loud, sharp sounds in your ear. They won't hurt your ear they'll just be quite loud but they'll be fairly short ones. They'll only last for about a second.”

Acoustic reflex graphAn acoustic reflex graph plots the immittance of sound into the middle ear on the y-axis against time on the x-axis.

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The immittance is based on the amount of sound not reflected back to the probe by the tympanic membrane.

In the example graph you can see that the 95dB, 1000Hz stimulus sound triggers a deflection in the immittance. This indicates the acoustic reflex.

Have a look at an example set of results. Notice that the example graphs are plotted downwards.

The first test is at 80dB. A deflection of 0.02 represents an acoustic reflex threshold. No acoustic reflex occurs.

The second test at 90dB shows an acoustic reflex.

The third test at 85dB shows an acoustic reflex at the 0.02 deflection criteria. This becomes the acoustic reflex threshold.

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0.000.02

80 dB HL

Deflection criterion

0.000.02

90 dB HL

The fourth test is at 95dB. It can be useful to check that the intensity of the acoustic reflex increases at higher presentation levels.

Interpreting resultsThe best means of determining a diagnosis is to compare acoustic reflex results with a table of possible conditions.

The table shows a list of conditions and the matching set of acoustic reflex results. As always a diagnosis should be based on a battery of tests.

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0.000.02

85 dB HL

0.000.02

95 dB HL

Acoustic reflex decay testIt is also useful to conduct an acoustic reflex decay test.

If the acoustic reflex stimulus is presented continuously over an extended period of time (10 seconds or longer), then in a normal ear the acoustic reflex will be sustained at the maximum amplitude.

If, however, a decay in the reflex occurs this can indicate cochlear or retrocochlear pathologies.

Decay is said to be present if the amplitude of the reflex decreases by over 50% within 5 seconds.

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The acoustic reflex decay test uses a 10 second stimulus. This is presented at 10dB above the acoustic reflex threshold at a low frequency of 500Hz and/or 1000Hz. Low frequencies are used because at higher frequencies decay may be evident in a client with normal hearing.

CreditsPublished by the Centre for Learning Innovation (CLI).

CLI would like to acknowledge the following people and organisations who have contributed to the development of this resource:

Subject consultants:

Signe Peitersen

Bettina Turnbull

Reviewer: Peter Robinson

Location photographs taken at Connect Hearing, Lane Cove.

Copyright© State of New South Wales, Department of Education and Training 2008

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This resource has been developed under the agreements and sponsorship of TAFE NSW. Copyright of this material is reserved to the Crown in the right of the State of New South Wales. Reproduction or communication in whole, or in part, other than in accordance with the provisions of the Copyright Act is prohibited without written authority of the copyright owners.

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