Clin Exp Otorhinolaryngol.  2023 Feb;16(1):20-27. 10.21053/ceo.2022.01039.

Improved Bone Conduction Hearing After Middle Ear Surgery: Investigation of the Improvement Mechanism

• Kim H
• Ha J
• Gu GY
• Choung YH

• KMID: 2539765
• DOI: http://doi.org/10.21053/ceo.2022.01039

Abstract

Objectives
. When performing middle ear operations, such as ossiculoplasty or stapes surgery, patients and surgeons expect an improvement in air conduction (AC) hearing, but generally not in bone conduction (BC). However, BC improvement has often been observed after surgery, and the present study investigated this phenomenon.
Methods
. We reviewed the preoperative and postoperative surgical outcomes of 583 patients who underwent middle ear surgery. BC improvement was defined as a BC threshold decrease of >15 dB at two or more frequencies. Subjects in group A underwent staged ossiculoplasty after canal wall up mastoidectomy (CWUM), group B underwent staged ossiculoplasty after canal wall down mastoidectomy (CWDM), group C underwent ossiculoplasty only (thus, they had no prior history of CWUM or CWDM), and group D received stapes surgery. We created a hypothetical circuit model to explain this phenomenon.
Results
. BC improvement was detected in 12.8% of group A, 9.1% of group B, and 8.5% of group C. The improvement was more pronounced in group D (27.0%). A larger gain in AC hearing was weakly correlated with greater BC improvement (Pearson’s r=0.395 in group A, P<0.001; r=0.375 in group B, P<0.001; r=0.296 in group C, P<0.001; r=0.422 in group D, P=0.009). Notably, patients with otosclerosis even experienced postoperative BC improvements as large as 10.0 dB, from a mean value of 30.3 dB (standard error [SE], 3.2) preoperatively to 20.3 dB (SE, 3.2) postoperatively, at 1,000 Hz, as well as an improvement of 9.2 dB at 2,000 Hz, from 37.8 dB (SE, 2.6) to 28.6 dB (SE, 3.1).
Conclusion
. BC improvement may be explained by a hypothetical circuit model applying the third window theory. Surgeons should keep in mind the possibility of BC improvement when making a management plan.

Keyword

Bone Conduction; Stapes Surgery; Otosclerosis; Ossicular Replacement; Otologic Surgical Procedures

Figure

• 

  Fig. 1. Audiograms of subjects exhibiting air conduction hearing gains >30 dB. (A) Staged ossiculoplasty after canal wall up mastoidectomy (CWUM) cases. (B) Staged ossiculoplasty after canal wall down mastoidectomy (CWDM) cases. (C) Cases of ossiculoplasty without any prior middle ear surgery. (D) Stapes surgery cases. The bars are the standard errors. AC, air conduction; BC, bone conduction.

• 

  Fig. 2. Correlations between the average air conduction (AC) hearing gains (500–4,000 Hz) and the average bone conduction (BC) hearing gains at each frequency. (A-C) In groups A to C, the highest correlations were apparent at 1,000 Hz. (D) In group D, the highest correlation was evident at 4,000 Hz. Group A underwent staged ossiculoplasty after canal wall up mastoidectomy (CWUM), group B underwent staged ossiculoplasty after canal wall down mastoidectomy (CWDM), group C underwent ossiculoplasty only (thus, they had no prior history of CWUM or CWDM), and group D received stapes surgery.

• 

  Fig. 3. Audiograms of subjects in group D (stapes surgery group). (A) Eighteen subjects underwent stapes surgery to treat otosclerosis. (B) Nineteen underwent stapes surgery to treat other conditions such as congenital stapedial fixation. The bars are the standard errors. AC, air conduction; BC, bone conduction.

• 

  Fig. 4. Hypothetical circuits showing the pressure differences between the scala vestibuli and scala tympani. The tops of the circuits represent the scala vestibuli and the bottoms the scala tympani. The values are arbitrary. (A) In the normal state, the gradient is 6V. (B) The model when superior semicircular canal dehiscence (SSCD) is in play. Another resistance (the third window) is added. The gradient is 8V; bone conduction (BC) hearing increases because of the larger pressure difference. (C) The model when otosclerosis is in play. The oval window (OW) has experienced a sclerotic change. Vibrational energy that normally passes through the OW now travels via other bony regions. In patients with otosclerosis, the OW does not serve as BC resistance. Schematics of cochlear air conduction (AC) and bone conduction (BC) (D, E). (D) The pressure gradient is created by the impedance difference between the scala vestibuli and scala tympani sides. BC* indicates conduction through the skull to the OW. (E) In patients with otosclerosis, BC* does not move through the OW but, rather, through other body regions, because the impedance of a sclerotic OW is very high. Conduction follows the path of least resistance. RSV, resistance of the scala vestibuli; ROW, resistance of the oval window; RST, resistance of the scala tympani; RRW, resistance of the round window; RW, round window.

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