Abstract
As a reason why hearing loss in the damaged middle ear is often more serious than 30db although normal stapes function is preserved, the interference of sound waves through both oval and round windows should be considered.
Cochlear microphonics of cats stimulated through both windows were measured to clarify the effect caused by interference.
The amplitudes of cochlear microphonics produced by 40db pure tones through the external auditory canal were equal to the amplitudes produced by 60db pure tones through the round window at 500cps, 1000cps and 2000cps. The amplitudes produced by 90db pure tones through the oval window were equal to the amplitudes produced by 75db pure tones through the round window at 2000 cps.
When contrary phase tones were applied through both pathways simultaneously, the amplitudes showed a remarkable decrease. At 4000cps, however, this decrease was not observed.
When same phase tones were applied simultaneously, the amplitudes increased remarkably. From these results, it became clear that this interference caused a more serious hearing loss when the perforated ear drum was exposed to tones through the external auditory canal.
That there is no decrease at 4000cps is due mainly to different reactions of both pathways in transmitting high frequency sound waves.
In most cases of tympanoplasty in Japan the first aim is to heal chronic inflammation. So it is difficult to reserve the ossicles chain completely. Therefore, exclusion of the round window from the tympanic cavity, preserving the mobility of its membrane, should be attempted.
Judging from the experimental results, microphonics stimulated through the round window are greater than those through the oval window with stapes. Sonoinversion by exclusion of the oval window (Garcia-Ibanez, 1961) is also profitable.
These two methods can lessen deafness to the degree of 30db in spite of the simplicity of the technique.
It should be noticed here that the thicker the graft it is better for alleviating the hearing loss, since such a graft is for keeping sounds from entering the round window.
