Tympanometry represents an objective measurement procedure in audiology that can be used to measure and localize mechanical-physical sound conduction problems of the ear. In the automated procedure, the tympanic membrane is subjected to changing differential pressures via the external auditory canal with simultaneous exposure to a continuous tone. During the procedure, the acoustic impedance of the ear is continuously measured and recorded (tympanogram).
What is tympanometry?
Tympanometry represents an objective measurement procedure in audiology that can be used to measure and localize mechanical-physical sound conduction problems of the ear. Hearing is determined by the physical-mechanical conduction of sound in the middle ear and the downstream neural conversion of sound into auditory sensation. Tympanometry is an objective method of measuring sound conduction. It does not require the assistance of the test person or patient, so that no subjective sensations are included in the measurement result. The primary objective is to measure acoustic impedance, and thus the functionality of the mechanical-physical part of hearing. The acoustic impedance is a measure of how high the reflected portion of the sound is, or how high the absorbed portion is, which is conducted via the sound conduction of the middle ear into the cochlea, where it is converted into nervous signals. Secondarily, tympanometry can also be used to measure the stapedius reflex, which can, within certain limits, protect the ear from damage in the presence of very loud sounds. During tympanometric measurements, the eardrum is exposed to different pressures via the external auditory canal and simultaneously exposed to a test tone of different frequencies. During the measurements, which run automatically, the proportion of reflected sound is recorded continuously and recorded in a tympanogram.
Function, effect and objectives
If hearing loss is suspected, the first step is to ensure that the external auditory canal is free of foreign bodies or earwax (cerumen) to ensure unobstructed sound conduction from the auricle to the eardrum. One of the most important diagnoses to determine if conductive hearing loss may be present is by examining the acoustic impedance of the eardrum. The acoustic impedance (resistance) of the eardrum is a measure of sound absorption capacity. Good absorptive capacity, i.e., low impedance, correlates with good sound conduction and good hearing – as long as hearing sensitivity is not impaired. A generally accepted method for objective measurement of acoustic impedance is tympanometry. The external auditory canal is sealed by a small balloon, which has a hole in the middle through which the measuring probe is passed. The probe itself has three holes and is connected to the tympanometer by three thin tubes. Through bore 1, an alternating slight positive or negative pressure can be created in the external auditory canal relative to the pressure prevailing in the middle ear. Bore 2 houses a small loudspeaker through which a continuous tone with selectable frequency and sound pressure level can be generated. Hole 3 houses a small microphone that can be used to measure the portion of the continuous tone reflected from the eardrum. Normally, the eardrum exhibits the lowest acoustic impedance when the pressure between the external auditory canal and the middle ear is fully equalized. The acoustic impedance measured at these pressure conditions is taken as the reference point in tympanometry and is assigned the value zero. The elasticity (compliance) of the tympanic membrane at various overpressure and underpressure conditions is then measured via the respective reflected portion of the continuous tone. In an automatically generated tympanogram, in which compliance is plotted as a function of 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), tympanic membrane compliance decreases sharply in a nonlinear fashion. The tympanogram allows conclusions to be drawn about the cause of a possible malfunction or reduced function within the sound conduction chain in the middle and inner ear.For example, otosclerosis (ossifications in the inner ear), tympanosclerosis (ossifications in the area of the auditory ossicles), a 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 cause significant sound conduction problems. A malfunction of the eustachian tube, which provides pressure equalization, a perforation of the eardrum and an inflammation of the middle ear can also be detected by means of tympanometry. The tympanogram then shows a typical course in each case.
Risks, side effects and dangers
Tympanometry is a procedure that was introduced as early as the 1930s and was originally based on the work of K. Schuster. By 1960, the procedure had been revised and adapted several times. Risks and side effects of tympanometry are not known. The changing differential pressures between the external auditory canal and the middle ear, up to a maximum of 30 hPa, are perceptible in a similar way to, for example, changes in cabin pressure in a passenger aircraft during a sharp descent or climb. A special feature of tympanometry is that not only specific sound conduction problems can be diagnosed, but also the proper function of the stapedius reflex. The reflex is triggered by sounds with sound pressure levels above 70 to 95 dB and becomes effective about 50 ms after the onset of the loud sound. The reflex causes contraction of the stapedius muscle, which causes the stapes to tilt slightly and significantly worsens sound transmission. The stapedius reflex virtually down-regulates both ears simultaneously in their sensitivity to sound and, to some extent, protects them from damage caused by too-loud sounds.
