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. This 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 the evaluation of hearing loss, tympanometry is a valuable component of the distinction between sensorinueural and conductive hearing loss, when evaluation is not apparent via Weber and Rinne testing.
The basis of operation is that the probe tone is sent into the ear canal--the sound strikes the tympanic membrane causing vibration of the middle ear, which in turn results in conscious perception of hearing. Some of this sound is reflected back and picked up by the tympanogram which records the amplitude of feedback. The more sound that enters the middle ear, the less that is reflected back. Conversely, the less sound that enters the middle ear, the more that is reflected back. Most middle ear problems result in a stiffening of the middle ear, resulting in increased amplitude of reflection to the tympanogram.
The instrument does not plot the amount of sound reflected back--it plots some measure of admittance. Admittance is a general term that describes how energy is transmitted through the middle ear. The specific terms that we use typically in clinical settings are admittance and compliance.
The unit for admittance is the millimho (mmho) and cubic centimeters (cm3 or cc). So basically--the instrument measures the dB SPL reflected back and converts it into some measure of admittance. The relation between sound pressure and compliance, is fairly simple; when more sound is reflected back, it means the middle ear is stiffer than normal and therefore the compliance is lower.
Normally, the air pressure in the ear canal is the same as ambient pressure (air pressure in the area where you are). Also, under normal conditions, the air pressure in the middle ear is about the same as ambient pressure since the eustachian tube opens periodically to ventilate the middle ear and to equalize pressure.
In a healthy individual Maximum sound is transmitted through the middle ear when the air pressure in the ear canal is equal to air pressure in the middle ear. The air pressure in the middle ear is whatever is it is--when the air pressure in the ear canal is equal to it, maximum transfer of sound will occur and the most sound will go through the middle ear (and the least amount of sound will be reflected back).
After an otoscopy (examination of the ear with an otoscope) to ensure that the path to the eardrum is clear and that it isn't perforated, the test is performed by inserting a device in the ear canal that will change the pressure in the ear, produce a pure tone, and measure the eardrum responses to the sound and different pressures. Results are shown in a curve called: Tympanogram.
Type A tympanogram is considered normal, that means that there is a normal pressure in the middle ear with normal mobility of the eardrum and the conduction bones.
Type B or C tympanograms may reveal fluid in the middle ear, scarring of the tympanic membrane, lack of contact between the conduction bones of the middle ear or a tumor in the middle ear.
- Impedance audiometry at Otolaryngology Online by drtbalu