As originally described by Terkildsen1, tympanometry measurements evaluate how the eardrum impedance changes when different ear canal pressures are applied. Different pure tone signals, most typically 226Hz or 1kHz, are used to measure the acoustic admittance at the eardrum. Based on the condition of the ear drum and the middle ear (including the eustachian tube), various middle ear pathologies can be identified.
Without requiring any assistance from the patient during the test sequence, tympanometry is a useful objective way to confirm middle ear pathologies and understand findings in audiological measurements, such as:
- Inflammatory diseases of the middle ear, e.g. (acute) otitis media
- Perforation of the eardrum
- Ossicular chain dislocation
- Dysfunction of the eustachian tube
- Tympanoscleroris
- Cholesteatoma
How to perform tympanometry
Tympanometry tests are performed using a dedicated probe which is inserted into the ear canal. Different designs of probes exist, such as handheld tympanometers or pen probes, which are recommended for screening, whereas insert probes enable hands-free operation, which reduces the chance of artefactual measurements.
Choosing the right ear tip size is critical to be able to perform a successful tympanometry test. Umbrella ear tips are only recommended to be used with handheld devices or probes, whereas mushroom sized tips can be used with any kind of probe and will ensure a sufficient hold of the insert probe within the ear canal.
It is crucial to perform an otoscopy before placing a probe in the ear as this will ensure no debris is there to impact the measurement result or enter the probe tip. The probe should be inserted into the ear canal in order to close the canal airtight. All Amplivox tympanometers will guide the user to ensure the probe is placed correctly and will automatically start the measurement as soon as the probe is in position.
How to read a tympanogram
The most commonly seen tympanometry results are according to the Jerger classification type A, B and C. It is important to note, that depending on the middle ear condition, variations of these types have been introduced in recent years to better describe and differentiate selected middle ear disorders. The test frequency also plays an important role in the test results. The tympanometry curve plots the compliance behaviour of the eardrum, based on the applied pressure in the ear canal.
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In a healthy middle ear system (type A curve), the pressure in the middle ear is similar to the ambient pressure, allowing for normal mobility of the eardrum and ossicles. The ventilation through the eustachian tube functions as it should. When applying high positive (e.g. +200daPa) or high negative (-200daPa) pressure, the eardrum stiffens, which is reflected by a very low compliance. When moving towards ambient pressure, the eardrum becomes more flexible again, leading to a high and maximum compliance of around 0daPa.
It’s essential that a healthy ear drum shows maximum flexibility at ambient pressure. When allowing for maximum movability, sounds are ideally transferred towards the middle ear system. The stiffer the ear drum becomes, the more sound is reflected and not passed on, leading to a deficit in hearing.
Depending on how flexible the eardrum is, the tympanometry curve (compliance curve) can be high and shallow (type AD) or flat and wide (type AS). A type AD suggests a very compliant middle ear system, cause by either a very thin and flexible ear drum (often several peaks instead of just one) or an ossicular chain dislocation.
A type AS is often seen in stiff middle ear systems, for example when the membrane is thicker than normal, when there are scars in the ear drum or there is solidification of the ossicle chain.
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Curves shifted towards negative pressure (type C curve) are often caused by a dysfunction of the eustachian tube. Usually this means the tube is blocked and doesn’t allow for air to reach the middle ear cavity, leading to a negative pressure. The ear drum is pulled towards the middle ear, prohibiting normal movement.
Reviewing the height of type C curves can build on the diagnosis: A flat CS curve indicates that fluid is present in the middle ear in addition to the tube dysfunction, while a high CD curve suggests that an ossicular subluxation or a healed tympanic membrane perforation is present in addition to a tube dysfunction.
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A flat type B curve is often caused by secretory or serous otitis media, which can lead to a eustachian tube dysfunction if not treated. A flat curve can also appear when the ear drum is perforated, or it could also be the result of a blocked probe or too much debris in the ear canal.
Therefore, it’s important to not only review the shape of the tymp curve but also the parameters calculated from the measurement.
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In some circumstances , especially when using high test frequencies, notched audiograms can occur. Different pathologies can cause notched test results when using a 226Hz probe tone, such as an atrophic scar, abnormal increased tonus in the tensor tympani muscle, or ossicular chain dislocation after stapedectomy – to name a few.
Notched curves are expected to be seen in measurements conducted using high test frequencies. Further information about the interpretation of these curves can be found in part 2 of this series.
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Besides reviewing the shape of the compliance curve, several test parameters are given alongside the test result: This includes the maximum compliance curve values (air pressure in daPa and compliance value in ml), the width of the curve, also called gradient (in daPa) and the ear canal volume measured (in ml). The ear canal volume is of particular importance, as this value can be used to differentiate between a perforated ear drum (high volume) or an otitis media (regular volume), when a flat type B curve is seen on-screen.
Industry-leading tympanometers
Amplivox has worked hard to develop industry-leading handheld and desktop tympanometers that provide fast and accurate middle ear measurements for all age groups, including neonates.
With a compact and elegant design allowing for complete portability, our tympanometers are ideal for mobile use as well as clinical environments where space is at a premium. Including a wide range of test functionality, our devices ensure testing requirements can not only be met, but also exceeded.
For more information on our tympanometry products please visit our tympanometers webpage, contact our customer support team on +44 (0)1865 880 846 or email.
References
1Terkildsen K, Thomsen K A. The influence of pressure variations on the impedance of the human ear drum. A method for objective determination of the middle-ear pressure. J Laryngol Otol. 1959;73:409–418. National Library of Medicine. Accessible at:
https://pubmed.ncbi.nlm.nih.gov/13856410/