聴力改善手術前後の鼓膜インピーダンス

抄録

The impedance of the human tympanic membrane has conventionally been measured by the indirect methods developed by such researchers as Tröger, Schuster, Møller, Zwislocki, and Morton. As the result of the studies of these methods, Onchi proved that a modified Tröger's method is adequate in its clinical application, and deviced new equipment. Fig. 1 shows a block diagram of this equipment. A small earphone of high acoustic impedance is driven by an oscillator and sends sound waves to an acoustical tube, which is variable in length. At the opposite end of this tube, a perfect connection is made by an ear-piece to the external auditory canal of the test ear. A probe tube microphone is inserted into the acoustical tube immediately at the front of the earphone. Standing waves are brought about in the acoustical tube by changing its length, and then sound pressure at the tympanic membrane is measured by the microphone under two special conditions of resonance and antiresonance. The impedance measured by this device is calculated and divided into two components, such as resistance R and reactance j (Mω-S/ω). Where M is mass, S is stiffness, j √-1, and ω is angular frequency. The measured impedance of the tympanic membrane is represented by the equations described in this paper. These equations require that four values be measured by this device as follows: 1) The length of the acoustical tube d_T is measured by the scale (in mm) when the maximum sound pressure is produced in the tube connected to the auditory canal of the test ear. 2) The length of the acoustical tube d₀ is measured by the scale when the maximum sound pressure is produced under a special condition when the tube is closed with a rigid wall at its end; i.e., under a condition where the tympanic membrane is replaced by an infinite high impedance. It is necessary in this measurement to make a volume equivalent tube. This tube can be made variable in length by a sliding rod and made equal in volume to the air cavity of the test ear. Thus the length d required by the equations is obtained by the difference of d_T-d₀. 3) r_T is a ratio of the maximum sound pressure to the minimum in the acoustical tube when connected to the auditory canal. 4) r₀ is a ratio of the maximum sound pressure to the minimum when connected to the volume equivalent tube.
It was found by our experience that reactance is available for diagnosis of conductive deafness with nonperforated tympanic membrane. The measured reactance in cases of normal condition and otosclerosis, as well as in cases of ossicular chain separation is as shown in Fig. 5. Otosclerosis is higher in reactance, compared with the normal ear, about-j1500 acoustic ohms at 500Hz. On the contrary, the ear with ossicular chain separation is lower in reactance, such as-j300 acoustic ohms at 500Hz. The shadowed area in figures indicates the range of reactance of normal tympanic membrane. The resonance-frequency of the tympanic membrane is found by checking a frequency at which reactance becomes zero. The frequency of resonance is shifted to lower frequency in cases with ossicular chain separation, such as about 700Hz, while normal and otosclerotic ears in a range of 1200Hz to 1500Hz. The ear with ossicular separation shows a particular trend of reactance which is always in the direction of +j in a frequency range of 700Hz to 1500Hz.
The impedances were measured in otosclerosis before and after stapes mobilization operation. After the surgery, the reactance changed to normal value in successful cases (Fig. 6). The impedance after fascia-flap-used tympanoplasty were also measured (Fig. 8, 9, 10). Along the post operative course, the increase of stiffness and decrease of mass of the tympanoplastic membrane were generally observed. These findings run parallel with increase in better conductivity of the membrane.