In-vivo assessment of osseous versus non-osseous transmission pathways of vibratory stimuli applied to the bone and the dura in humans.

BACKGROUND: Bone conduction (BC) is an alternative to air conduction (AC) for stimulation of the inner ear. Stimulation for BC can occur directly on the skull bone, on the skin covering the skull bone, or on soft tissue (i.e., eye, dura). All of these stimuli can elicit otoacoustic emissions (OAE). This study aims to compare OAEs generated by different combinations of stimuli in live humans, including direct stimulation of the intracranial contents via the dura, measured intraoperatively.

METHODS: Measurements were performed in five normal-hearing ears of subjects undergoing a neurosurgical intervention with craniotomy in general anesthesia. Distortion product OAEs (DPOAEs) were measured for f2 at 0.7, 1, 2, 3, 4, and 6 kHz with a constant ratio of the primary frequencies (f2 /f1 ) of 1.22. Sound pressure L1 was held constant at 65 dB SPL, while L2 was decreased in 10 dB steps from 70 to 30 dB SPL. A DPOAE was considered significant when its level was ≥6 dB above the noise floor. Emissions were generated sequentially with different modes of stimulation: 1) pre-operatively in the awake subject by two air-conducted tones (AC-AC); 2) within the same session preoperatively by one air- and one bone-conducted tone on the skin-covered temporal bone as in audiometry (AC-BC); 3) intra-operatively by one air-conducted tone and one bone-vibrator tone applied directly on the dura (AC-DC). A modified bone vibrator (Bonebridge; MED-EL, Innsbruck, Austria) was used for BC stimulation on the dura or skin-covered mastoid. Its equivalent perceived SPL was calibrated preoperatively for each individual by psychoacoustically comparing the level of a BC tone presented to the temporal region to an AC tone at the same frequency. Simultaneously with the DPOAEs, vibrations at the teeth were measured with an accelerometer attached using a custom-made holder.

RESULTS: It was possible to record DPOAEs for all three stimulation modes. For AC-DC, DPOAEs were not detected above the noise floor below 2 kHz but were detectable at the higher frequencies. The best response was measured at or above 2 kHz with L2  = 60 dB SPL. The acceleration measured at the teeth for stimulation on the dura was lower than that for stimulation on the bone, especially below 3 kHz.

CONCLUSION: We demonstrate a proof-of-concept comparison of DPOAEs and teeth acceleration levels elicited by a bone vibrator placed either against the skin-covered temporal bone, as in audiometry, or directly against the dura mater in patients undergoing a craniotomy. It was demonstrated that DPOAEs could be elicited via non-osseous pathways within the skull contents and that the required measurements could be performed intra-operatively.