Laservibrometrie

Abstract

A complete battery of audiometric methods is required for the differential diagnosis of different hearing disabilities (including puretone audiometry, impedance, stapes reflex, speech audiometry, brainstam evoked response audiometry, otoacoustic emissions, etc.). In many cases, a comprehensive diagnosis is not possible. Here we describe a new technique based on a laser-Doppler vibrometer that has the potential for non-invasive diagnosis not only middle ear disease but also cochlear pathologies. Disturbance of cochlear function can be ascertained because the input impedance of the cochlea acts as a mechanical load on the middle ear and therefore influences motion of the umbo. In the present study vibration of the umbo and eardrum were measured with a commercially available laser-Doppler vibrometer coupled directly into a standard surgical microscope. The use of the microscope allowed non-invasive measurements of vibrations without having to introduce reflecting material onto the tympanic membrane. Sound pressure was measured with a calibrated probe microphone placed near the tympanic membrane. The displacement response and the specific acoustic impedance of the umbo were calculated from the velocity and sound pressure measured. For normal hearing subjects, the amplitude of the umbo's displacement for frequencies from 0.1 kHz to 1 kHz was 1 nm at 60 dB SPL and decreased with a slope of 6 dB/octave for frequencies between 1 and 5 kHz. A strong correlation was found between the specific acoustic impedance of the umbo and hearing thresholds for hearing-impaired subjects (having otosclerosis or sensorineural hearing losses). The frequency response of the umbo proved to be a means for evaluating the function of both the middle ear and the cochlea under pathological conditions. The measurement technique described is also suitable for intraoperative investigation of the frequency response of the opened middle ear, as well as for the in situ frequency response of partial and total ossicular replacement prostheses.

Distortion product otoacoustic emissions measured as vibration on the eardrum of human subjects

“…Although our early experiments provided initial evidence for a cochlear mechanical component in the frequency response of the umbo (20), more recent studies by others (25,30) have found no difference between umbo vibration in normal hearing subjects and in patients with sensorineural hearing loss. The problem with these latter studies is that they are conducted at sound pressure levels (SPLs) of 80-100 dB, where the cochlear amplifier is effectively short-circuited (3).…”

“…Instead of only using sound pressure measurements to assess the patency of the cochlear amplifier in humans, it has been proposed that vibration measurements of the eardrum, in particular at the umbo, might provide more reliable information (20), particularly at high frequencies where correctly calibrated sound fields are notoriously difficult to achieve (5,21,22). The most modern technology employs a laser Doppler vibrometer (LDV) to measure eardrum vibrations in human subjects (20,(23)(24)(25)(26)(27)(28)(29)(30).…”

“…The most modern technology employs a laser Doppler vibrometer (LDV) to measure eardrum vibrations in human subjects (20,(23)(24)(25)(26)(27)(28)(29)(30). The LDV is a heterodyne interferometer where the instantaneous frequency of the interferometric signal is shifted in direct proportion to the velocity of the vibrating object (31,32).…”

“…The problem with these latter studies is that they are conducted at sound pressure levels (SPLs) of 80-100 dB, where the cochlear amplifier is effectively short-circuited (3). Although our experiments were performed at lower sound intensities of Ϸ60 dB SPL (20), where there are some remnants of nonlinear cochlear amplification (3), even our LDV was far too insensitive to measure vibrations near auditory threshold. It is in the low-amplitude region below Ϸ40 dB SPL that the cochlear amplifier produces extraordinary sensitivity and frequency selectivity.…”

“…Stimulus levels of 60 dB SPL produce umbo displacements of Ϸ1 nm below 1 kHz (20,(23)(24)(25)(26)(27)(28)(29)(30). However, vibration measurements of Ͻ1 nm on the eardrum are difficult to perform in human subjects because the poor signal-to-noise ratio (SNR), due to poor optical reflection and extraneous noise such as heart beat, breathing or swallowing, necessitates inordinately long averaging times.…”