Sonotubometry is based on the principle that sound when applied to the nasopharyngeal ostium of eustachean tube will be conducted through the eustachean tube into the middle ear when it opens. This concept was first conceived durning the 19th century. It has undergone lot of changes like introduction of better microphones. Politzer should be credited for the introduction of sonotubometry for analysis of eustachean tube function.
In principle sonotubometry records changes in sound pressure level in the external auditory meatus when a constant sound source is applied to the nostril. In theory the eustachean tube opening will cause an increase in the sound pressure level of middle ear cavity causing a retrograde conduction of sound from the middle ear to the external canal which is recorded by a sensitive recording device. For this to occur the eustachean tube should open and close normally. By just recording the sound which is transmitted from the nose to the external canal via the eustachean tube it is possible to assess the functional integrity of the eustachean tube.
Diagram showing the placement of microphone in the external canal to record the transmitted sounds.
Advantages of sonotubometry:
The whole procedure is performed under physiological conditions
Static pressure is not applied over the external auditory canal
The whole procedure is independent of the status of ear drum
Pitfalls of sonotubometry device:
Leakage of sound from the nostrils and external auditory meatus
Identifying the optimal frequency of test sound that should be used
Results got corrupted due to the physiological noise of swallowing
These pitfalls were overcome by using inflatable nasal seals that would prevent leakage of sound from the nose and the use of pneumatic valve on the meatal microphone. This pneumatic valve would ensure that the external canal is sealed properly and there is no leakage of sound from it.
Regarding the frequency of test sound that should be used it has been demonstrated that frequencies between 6-8 kHz usually produced the best results. It has also been determined that frequencies of 5 kHz usually could be mistaken to that of physiological noises produced due to the act of swallowing.
The test is usually performed by placing a pneumatic microphone inside the external canal and a nasal sound probe which could generate sound at frequencies ranging from 6-8 kHz. Patient is asked to take about 20ml of liquid and hold it inside the oral cavity. When the sound is projected via nasal probe the patient is asked to swallow the liquid. This act of swallowing will open the normally functioning eustachean tube and is considered to be normal if the sound recorded at the external auditory canal reaches the level of 4dB sound pressure level. The duration of the eustachean tube opening can be calculated as the time difference between the onset of increase in the amplitude of sound and the return to its baseline.
Figure showing the sonotubometric curve. SPL reaches the peak level at the external canal 16 dB at 500 milliseconds. The base of the triangle formed by the curve is the time which eustachean tube is kept open. By looking at this curve eustachean tube functional abnormalities can be identified.