Eustachian tube catheterization has been used as a simple test of Eustachian tube function. This test, however, consists of merely listening to the sound of inflation. The first purpose of this study is to investigate the reliability of the test by means of soundspectrography which permits an objective analysis of the sound. Secondly, an attempt was made to locate the origin of inflation sound in the tube, using a plastic model of the tube and tympanum.
Method 1: Soundspectrographic analysis of inflation sound was carried out in 130 ears, which consisted of 100 with occluded, 10 with patent and 20 with normal tubes. Their tubal function had been examined in advance by aerodynamic and impedance tests.
Result 1: The averaged sonagraphic feature in the 20 normal tubes was as follows. 1. The sound consisted of noise components with frequencies below 2kHz. 2. Acoustic power decreased as the sound frequency increased. 3. A small noise component existed in the high frequency range. The occluded tubes were divided into three types (L, LH and LMH types). The L-type was characterized by an abnormal bar which appeared in the low (L) frequency range. The sonagram for the LH-type was characterized by a dominant noise component which appeared in the high (H) frequency range, in addition to the abnormality in the low (L) frequency range. The sonagram of the LMH type displayed noise components not only in the low (L) and high (H) frequency ranges, but also in the middle (M) frequency range. A sonagram of an abnormally patent tube was characterized by the appearance of slight but regular vertical filaments in the middle frequency range.
Method 2: In order to study the relationship between acoustic findings of inflation sound and specific conditions of the tube, a model experiment was carried out. Under various conditions of stenosis of a plastic model of the tube and tympanum, inflation sounds were analyzed by sonagraphy.
Result 2: Normal inflation sound was obtained when the diameter of the tube was within the range of 0.4mm to 0.7mm, while stenotic sound was produced when the diameter was below 0.4mm. When the tube was obstructed along its whole length, the sonagram closely resembled that of the LMH type of the occluded tube. When stenosis was formed around the tubel orifice, the sonagram resembled that of the LH type of the occluded tube. When the stenosis of the orifice was significant to result in an axial deviation of the catheter from the Eustachian tube, sonagrams similar to those of the L type of occluded tube were obtained consistently.
Comparison of the results from the model experiment with those from clinical cases suggested that about 70% of stenotic inflation sound originated from the pharyngeal orifice of the tube. Through additional model experiment using smoke, it was demonstrated that the air in the catheter hardly flowed into the tube, but formed marked air turbulence around the orifice. Thus it was revealed that the origin of the inflation sound of the tubal stenosis was turbulent air at the pharyngeal orifice of the tube.
