The Tympanometry represents an objective measurement method in audiology, with the help of which mechanical-physical sound conduction problems of the ear can be measured and localized.
In the automated process, the eardrum is exposed to changing differential pressures via the external auditory canal while simultaneously being exposed to a continuous tone. During the procedure, the acoustic impedance of the ear is continuously measured and recorded (tympanogram).
What is tympanometry?
Hearing ability is determined by the physical-mechanical sound conduction in the middle ear and the subsequent neural conversion of sound into sound perception. Tympanometry is an objective measurement method for sound conductivity.
It does not require the help of the test person or patient, so that no subjective sensations are included in the measurement result. First and foremost, it is a matter of measuring the acoustic impedance and thus the functionality of the mechanical-physical part of the hearing ability. The acoustic impedance is a measure of how high the reflected part of the sound is, or how high the absorbed part is, which is conducted via the sound conduction of the middle ear into the cochlea and converted into nervous signals there.
Secondly, tympanometry can also be used to measure the stapedius reflex, which can protect the ear within certain limits from damage caused by very loud noises. During the tympanometric measurements, the eardrum is exposed to different pressures via the external auditory canal and simultaneously sonicated with a test tone of different frequencies. During the automatic measurements, the proportion of the reflected sound is continuously recorded and recorded in a tympanogram.
Function, effect & goals
If a hearing loss is suspected, it is first ensured that the external auditory canal is free of foreign bodies or ear wax (cerumen) in order to ensure unimpeded sound conduction from the auricle to the eardrum.
One of the most important diagnoses for determining a possible reduction in sound conduction is to examine the acoustic impedance of the eardrum. The acoustic impedance (resistance) of the eardrum is a measure of the sound absorption capacity. Good absorption capacity, i.e. low impedance, correlates with good sound conduction and good hearing - as long as the hearing perception is not impaired.
A generally accepted method for the objective measurement of acoustic impedance is tympanometry. The external auditory canal is sealed by a small balloon with a hole in the middle through which the measuring probe is passed. The probe itself has three holes and is connected to the tympanometer with three thin tubes. An alternating slight overpressure or underpressure compared to the pressure prevailing in the middle ear can be generated in the external auditory canal via bore 1. The bore 2 houses a small loudspeaker via which a continuous tone with a selectable frequency and a selectable sound pressure level can be generated.
In the bore 3 there is a small microphone with which the portion of the continuous tone reflected by the eardrum can be measured. Normally, the eardrum has the lowest acoustic impedance with complete pressure equalization between the external auditory canal and the middle ear. The acoustic impedance measured at these pressure conditions is taken as a reference point in tympanometry and is given the value zero.
Then the elasticity (compliance) of the eardrum is measured in different over- and underpressure states via the reflected portion of the continuous tone. In an automatically generated tympanogram, in which the compliance is plotted as a function of the differential pressure, there is a clear maximum at a differential pressure of zero. With increasing positive or negative differential pressures of up to ± 300 mm water column or 30 hectopascals (hPa), the compliance of the eardrum strongly decreases non-linearly.
The tympanogram allows conclusions to be drawn about the cause of any malfunction or reduced function within the sound conduction chain in the middle and inner ear. For example, otosclerosis (ossification in the inner ear), tympanosclerosis (ossification in the area of the ossicles), cholesteatoma (ingrowth of squamous epithelium of the external auditory canal into the middle ear) or a tympanic effusion can be diagnosed.
In a tympanic effusion, the middle ear is filled with a secretion that can be serous to bloody or even purulent and can lead to considerable sound conduction problems. A malfunction of the Eustachian tube that compensates for pressure, a perforation of the eardrum and an otitis media can also be determined by means of tympanometry. The tympanogram then shows a typical course.
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Tympanometry is a procedure that was introduced in the 1930s and was originally based on the work of K. Schuster. Until 1960 the procedure was revised and adapted several times. The risks and side effects of tympanometry are unknown.
The changing differential pressures between the external auditory canal and the middle ear up to a maximum of 30 hPa are similarly perceptible as e.g. B. Changes in cabin pressure in the passenger aircraft during a steep descent or climb. A special feature of tympanometry is that not only specific sound conduction problems can be diagnosed, but also the proper functioning of the stapedius reflex.
The reflex is triggered by noises with a sound pressure level of over 70 to 95 dB and takes effect around 50 ms after the loud noise begins. The reflex causes a contraction of the stapedius muscle, which tilts the stapes a little and significantly worsens the sound transmission. Due to the stapedius reflex, both ears are reduced in their sensitivity to sound at the same time and, to a certain extent, protected from damage caused by loud noises.
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