耳小骨形成術後の気骨導差判定について

術前骨導値と術後骨導値による比較

抄録

It is controversial whether preoperative or postoperative bone conduction thresholds should be utilized in calculating postoperative air-bone gap for evaluation of ossiculoplasty results. In 1995, committee on Hearing and Equilibrium Guidelines (American Academy of Otolaryngology-Head and Neck Surgery) recommended reporting the air-bone gap by using air-and bone-conduction values determined at the same time, rather than comparing postoperative air-conduction thresholds with the preoperative bone-conduction thresholds. Whereas a Guideline in Reporting Hearing Results in Middle Ear and Mastoid Surgery (Otological Society of Japan) defined to utilize preoperative bone conduction thresholds in calculating postoperative airbone gap in 2000.
We compared the results of postoperative air-bone gap with pre- andpost-operative bone conduction thresholds in 121 ossiculoplasty cases to elucidate which value is suitable for evaluation of postoperative airbone gap.Audiometric evaluation was based on pure tone audiograms, taken before and 6 months after surgery, and the mean hearing levels at three speech frequencies were utilized for calculation. Cases with postoperativeair-bone closure less than 15dB were 64 ears (52.9%) when utilizing pre-operative bone conduction thresholds, whereas they were 62 ears (51.2%) when utilizig post-operative ones.In thesetwo groups, incidence of overclosure showing negative value was significantly higher in thecases calculated by pre-operative bone conduction threshold (34.4%) than in those by post-operative ones (6.5%)(Fisher's exact probability test; p=.0003).In addition, comparing the change of bone conduction values after surgery in these two groups, it was significantly better in the former with a mean of 8dB improvement (paired t-test;p<.0001) while it was not significant in the latter with a mean of 2.8dB improvement.On the other hand, cases showing deterioration of postoperative bone conduction thresholds more than 10dB were 1 ear (1.6%) in the former while 5 ears (8.1%) in the latter: no significant difference was noted between these two groups, indicating incidence of underestimation of air-bone gap due to worsening ofpostoperative bone conduction is not so different between two groups.
Cases which met any of the three criteria for success defined by a Guideline in Reporting Hearing Results in Middle Ear and Mastoid Surgery (2000) i.e.hearing gain of more than 15dB, postoperative hearing level within 30dB, and postoperative air-bone gap less than 15dB were 79 ears when utilizing pre-operative bone conduction thresholds, whereas they were 82 ears when utilizing post-operative ones, being no significant difference in success rate between two groups.
These results indicated that postoperative air-bone gap calculated by pre-operative bone conduction thresholds is apparently overestimated due to improvement of bone conduction after surgery in successful cases although overall hearing results were almost the same in comparison betweencalculation by pre-and post-operative bone conduction thresholds. Thus, post-operative air-bone gap should be calculated utilizing values examined at the same time i.e. postoperative air-and bone-conduction values as recommendation of Committee on Hearing and Equilibrium Guidelines in American Academy of Otolaryngology-Head and Neck Surgery.