In the automated procedure, the eardrum is exposed via the external auditory canal to changing differential pressures while sounding a continuous tone. During the procedure, the acoustic impedance of the ear is continuously measured and recorded (tympanogram).
What is tympanometry?Tympanometry is an objective measurement method in audiology, with the help of which mechanical-physical sound conduction problems of the ear can be measured and localized.
The hearing is determined by the physical-mechanical sound conduction in the middle ear and the downstream neuronal conversion of the sound into the sensation of sound. Tympanometry represents an objective measurement method for the sound conductance.
It does not require the help of the subject or patient, so that no subjective sensations are included in the measurement result. The primary goal is to measure the acoustic impedance, and thus the functionality of the mechanical-physical part of the hearing. The acoustic impedance is a measure of how high the reflected portion of the sound is, or how high the absorbed portion, which is passed via the sound conduction of the middle ear in the cochlea and converted there into nervous signals.
Secondarily, by means of tympanometry it is also possible to measure the stapedius reflex, which, within certain limits, can protect the ear from damage in the case of very loud noises. During the tympanometric measurements, the eardrum is exposed to different pressures through the external auditory canal and simultaneously sonicated with a test tone of different frequencies. During the automatic measurements, the proportion of 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 earwax (cerumen) in order to ensure unimpeded conduction of sound from the auricle to the eardrum.
One of the most important diagnoses for the detection of a possible reduction in conductive conduction is the examination of the acoustic impedance of the eardrum. The acoustic impedance (resistance) of the eardrum is a measure of the sound absorption capacity. Good absorption capacity, ie low impedance, correlates with good sound conduction and good hearing - as long as the hearing sensitivity is not disturbed.
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 probe passes. The probe itself has three holes and is connected to the tympanometer with three thin tubes. About hole 1 can be generated in the external auditory canal an alternating slight positive or negative pressure relative to the pressure prevailing in the middle ear. The hole 2 houses a small speaker, via which a continuous tone with selectable frequency and selectable sound pressure level can be generated.
In the hole 3 is a small microphone, with which the part of the continuous sound reflected by the eardrum can be measured. Usually, the eardrum exhibits the lowest acoustic impedance with complete pressure equalization between the external ear canal and the middle ear. The acoustic impedance measured under these pressure conditions is taken as the reference point in tympanometry and is given the value zero.
Thereafter, the elasticity (compliance) of the eardrum is measured at different over- and under-pressure conditions via the respectively reflected portion of the continuous tone. In a self-made tympanogram plotting compliance versus differential pressure, there is a clear maximum at zero differential pressure. With increasing positive or negative differential pressures of up to ± 300 mm of water or 30 hectopascals (hPa), the compliance of the eardrum decreases non-linearly.
The tympanogram allows conclusions to be drawn about the cause of a possibly existing malfunction or inferior function within the sound conduction chain in the middle and inner ear. For example, it is possible to diagnose otosclerosis (ossification in the inner ear), tympanosclerosis (ossification of the ossicles), cholesteatoma (ingrowth of squamous epithelium of the external auditory canal into the middle ear), or a tympanic effusion.
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 problems with the conduction of sound. Malfunctioning of the eustachian tube for pressure equalization, perforation of the eardrum and middle ear inflammation can also be detected by means of tympanometry. The tympanogram then shows a typical course.
Risks, side effects & dangers
Tympanometry is a technique that was already introduced in the 1930s and was originally based on the work of K. Schuster. Until 1960, the process was revised and adapted several times. Risks and side effects of tympanometry are unknown.
The varying differential pressures between the external auditory canal and the middle ear up to a maximum of 30 hPa are similar to perceptible as z. B. Changes in cabin pressure in the passenger aircraft in heavy descent or climb. A peculiarity 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 about 50 ms after the beginning of the loud noise. The reflex causes a contraction of the stapedius muscle, which causes the stapes to tilt a little and the sound transmission is significantly worsened. Due to the stapedius reflex, both ears are at the same time downsized in their sonic sensitivity and, to a certain extent, protected against damage due to loud noises.