Learning Objectives: The better bone conduction threshold at low-tone frequencies immediately after tympanoplasty with mastoidectomy and no preoperative fistula symptoms might imply the third mobile window theory. The worse bone conduction threshold in high-tone frequencies with spontaneous nystagmus after surgery might indicate inner ear damage.

Objective: To understand the third mobile window effect of chronic otitis media with cholesteatoma with inner ear fistula on the bone conduction threshold, we examined changes in the bone conduction audiogram after tympanoplasty with mastoidectomy for chronic otitis media with cholesteatoma with canal fistula.

Study Design: Retrospective case review.

Patients: According to the intraoperative classification of Dornhoffer and Milewski, we focused especially on type IIa (anatomical bony fistula with no perilymph leak). We checked the bone conduction threshold at least three times: just before, just after, and 6 months after surgery in 20 ears with type IIa lateral semicircular canal fistula.

Results: Compared with the preoperative bone conduction threshold, six cases were better, 12 cases were unchanged, and two cases were worse within the first postoperative week. Finally, one case was better, 15 cases were unchanged, and four cases were worse at the sixth postoperative month. Patients with a better bone conduction threshold in the low-tone frequencies immediately after surgery had a tendency to show no preoperative fistula symptoms. Postoperative spontaneous nystagmus had a tendency to be observed in patients with a worse bone conduction threshold in the high-tone frequencies.

Conclusion: The better bone conduction threshold at low-tone frequencies immediately after tympanoplasty with mastoidectomy and no preoperative fistula symptoms might imply the third mobile window theory. The worse bone conduction threshold in high-tone frequencies with spontaneous nystagmus after surgery might indicate inner ear damage.