Tympanometry

Tympanometry
Diagnostics
ICD-9-CM	95.41
MeSH	D000158
MedlinePlus	003390

Tympanometry is an examination used to test the condition of the middle ear and mobility of the eardrum (tympanic membrane) and the conduction bones by creating variations of air pressure in the ear canal.

Tympanometry is an objective test of middle-ear function. It is not a hearing test, but rather a measure of energy transmission through the middle ear. The test should not be used to assess the sensitivity of hearing and the results of this test should always be viewed in conjunction with pure tone audiometry.

Tympanometry is a valuable component of the audiometric evaluation. In evaluating hearing loss, tympanometry permits a distinction between sensorineural and conductive hearing loss, when evaluation is not apparent via Weber and Rinne testing. Furthermore, in a primary care setting, tympanometry can be helpful in making the diagnosis of otitis media by demonstrating the presence of a middle ear effusion.

Operation

A tone of 226 Hz is generated by the tympanometer into the ear canal, where the sound strikes the tympanic membrane, causing vibration of the middle ear, which in turn results in the conscious perception of hearing. Some of this sound is reflected back and picked up by the instrument. Most middle ear problems result in stiffening of the middle ear, which causes more of the sound to be reflected back.

Admittance is how energy is transmitted through the middle ear. The instrument measures the reflected sound and expresses it as an admittance or compliance, plotting the results on a chart known as a tympanogram.

Normally, the air pressure in the ear canal is the same as ambient pressure. Also, under normal conditions, the air pressure in the middle ear is approximately the same as ambient pressure since the eustachian tube opens periodically to ventilate the middle ear and to equalize pressure. In a healthy individual, the maximum sound is transmitted through the middle ear when the ambient air pressure in the ear canal is equal to the pressure in the middle ear.

Procedure

After an otoscopy (examination of the ear with an otoscope) to ensure that the path to the eardrum is clear and that there is no perforation, the test is performed by inserting the tympanometer probe in the ear canal. The instrument changes the pressure in the ear, generates a pure tone, and measures the eardrum responses to the sound at different pressures. This produces a series of data measuring how admittance varies with pressure, which is plotted as a tympanogram.

• 

  Type A tympanogram

• 

  Type B tympanogram

• 

  Type C tympanogram

Tympanograms are categorized according to the shape of the plot. A normal tympanogram (left) is labelled Type A. There is a normal pressure in the middle ear with normal mobility of the eardrum and the conduction bones. Type B and C tympanograms may reveal fluid in the middle ear, perforation of the tympanic membrane, scarring of the tympanic membrane, lack of contact between the conduction bones of the middle ear or a tumor in the middle ear.

The categorising of tympanometric data should not be used as a diagnostic indicator. It is merely a description of shape. There is no clear distinction between the three types, nor the two subtypes of type A, namely A${}_S$ and A${}_D$. Only measures of static acoustic admittance, ear canal volume, and tympanometric width/gradient compared to gender, age, and race specific normative data can be used to somewhat accurately diagnose middle ear pathology along with the use of other audiometric data (e.g. air and bone conduction thresholds, otoscopic examination, normal word recognition at elevated presentation levels, etc.).

External links

• Medscape article by Kathleen C M Campbel
• Basic Multifrequency Tympanometry: The Physical Background