Safe and reliable sound threshold measures with direct vibration of the ossicular chain

OBJECTIVES: The aim of the study was to evaluate the safety and feasibility of piezoelectric malleus vibration audiometer (MVA), which presents micromechanical vibrations to the umbo membranae tympani. STUDY DESIGN: Phase I study performed in a tertiary referral center (University Hospital). METHODS: The coupling rod of the MVA was moved slowly through the outer ear canal toward the eardrum with a micromanipulator. Coupling was completed when the rod tip touched the umbo membranae tympani. Basic audiologic measures of sound threshold obtained with direct stimulation of the malleus are presented. We used MANOVA (multivariate repeated measures ANOVA) to investigate the repeatability of MVA thresholds from one day to the other and when decoupling and retracting the coupling rod 2 mm off the umbo. We also selected the MANOVA to test for unwanted bone-conduction threshold shifts after MVA application. We assessed normality of the data by quantile-quantile plots of the residuals. RESULTS: Twenty-eight male and 10 female subjects with normal hearing, 22.2 to 34.6 years old (median age, 27.2 yr) underwent an examination. Thirty-six subjects underwent MVA, because 2 of the 38 subjects who volunteered for the study have not undergone the procedure due to the external auditory canal anatomy preventing application of the MVA. The results show that it is possible to safely and reliably measure thresholds of direct vibration of the ossicular chain. Using pure tone audiograms, no pure tone bone- and/or air-conduction threshold shifts occurred after the procedure. None of the subjects reported any other ear-related symptoms such as vertigo, tinnitus, or dizziness. Geometric mean vibratory displacements at threshold ranged from 0.55 nm at 250 Hz to 0.03 nm at 6 kHz. MANOVA demonstrated a repeatability of MVA thresholds. CONCLUSION: Malleus vibration audiometry will not allow exact linkage of actual implantable hearing aid. But the present study demonstrates that MVA can provide an audiometric tool for assessing ossicular function and integrity prior to implantation of an electronic hearing amplifier.