Cochlear blood flow measured by averaged laser Doppler flowmetry (ALDF)

Revealing the morphology and function of the cochlea and middle ear with optical coherence tomography

Optical coherence tomography (OCT) has revolutionized physiological studies of the hearing organ, the vibration and morphology of which can now be measured without opening the surrounding bone. In this review, we provide an overview of OCT as used in the otological research, describing advances and different techniques in vibrometry, angiography, and structural imaging.

Usefulness of Intravital Multiphoton Microscopy in Visualizing Study of Mouse Cochlea and Volume Changes in the Scala Media

Conventional microscopy has limitations in viewing the cochlear microstructures due to three-dimensional spiral structure and the overlying bone. But these issues can be overcome by imaging the cochlea in vitro with intravital multiphoton microscopy (MPM). By using near-infrared lasers for multiphoton excitation, intravital MPM can detect endogenous fluorescence and second harmonic generation of tissues. In this study, we used intravital MPM to visualize various cochlear microstructures without any staining and non-invasively analyze the volume changes of the scala media (SM) without removing the overlying cochlear bone. The intravital MPM images revealed various tissue types, ranging from thin membranes to dense bone, as well as the spiral ganglion beneath the cochlear bone. The two-dimensional, cross-sectional, and serial z-stack intravital MPM images also revealed the spatial dilation of the SM in the temporal bone of pendrin-deficient mice. These findings suggest that intravital MPM might serve as a new method for obtaining microanatomical information regarding the cochlea, similar to standard histopathological analyses in the animal study for the cochlea. Given the capability of intravital MPM for detecting an increase in the volume of the SM in pendrin-deficient mice, it might be a promising new tool for assessing the pathophysiology of hearing loss in the future.

Two-photon microscopy allows imaging and characterization of cochlear microvasculature in vivo

Impairment of cochlear blood flow has been discussed as factor in the pathophysiology of various inner ear disorders. However, the microscopic study of cochlear microcirculation is limited due to small scale and anatomical constraints. Here, two-photon fluorescence microscopy is applied to visualize cochlear microvessels. Guinea pigs were injected with Fluorescein isothiocyanate- or Texas red-dextrane as plasma marker. Intravital microscopy was performed in four animals and explanted cochleae from four animals were studied. The vascular architecture of the cochlea was visualized up to a depth of 90.0±22.7 μm. Imaging yielded a mean contrast-to-noise ratio (CNR) of 3.3±1.7. Mean diameter in vivo was 16.5±6.0 μm for arterioles and 8.0±2.4 μm for capillaries. In explanted cochleae, the diameter of radiating arterioles and capillaries was measured with 12.2±1.6 μm and 6.6±1.0 μm, respectively. The difference between capillaries and arterioles was statistically significant in both experimental setups (P<0.001 and P=0.022, two-way ANOVA). Measured vessel diameters in vivo and ex vivo were in agreement with published data. We conclude that two-photon fluorescence microscopy allows the investigation of cochlear microvessels and is potentially a valuable tool for inner ear research.

An animal model for the analysis of cochlear blood flow [corrected] disturbance and hearing threshold in vivo

Impairment of cochlear blood flow (CBF) is considered to be important in inner ear pathology. However, direct measurement of CBF is difficult and has not been investigated in combination with hearing function. Six guinea pigs were used to show feasibility of an animal model for the analysis of cochlear microcirculation by intravital microscopy in combination with investigation of the hearing threshold by brainstem response audiometry (ABR). By the application of sodium nitroprusside (SNP), CBF was increased over 30 min. Reproducibility of measurements was shown by retest measurements. Mean baseline velocity of CBF was 109 +/- 19 mum/s. Vessel diameters had a mean value of 9.4 +/- 2.7 mum. Mean hearing threshold was 19 +/- 6 dB. In response to SNP, CBF velocity increased significantly to 161 +/- 26 mum/s. Mean arterial pressure decreased significantly to 36 +/- 11 mmHg. After the end of the application, CBF velocity recovered to a minimum of 123 +/- 17 microm/s. Within the retest, CBF velocity significantly increased to a maximum of 160 +/- 31 microm/s. Second recovery of CBF velocity was 125 +/- 14 mum/s. Within the second retest, CBF increased significantly to 157 +/- 25 microm/s. ABR thresholds did not change significantly. The increase in blood flow velocity occurred in spite of substantial hypotension as induced by a vasodilator. This may explain the fact that ABR threshold remained unchanged reflecting a maintained blood supply in this part of the brain. This technique can be used to evaluate effects of treatments aimed at cochlear microcirculation in inner ear pathologies.

Focal damage to cochlear microcirculation measured using a non-contact laser blood flowmeter in guinea pigs

The focal microcirculation damage induced by a photochemical reaction in the stria vascularis (SV) of the guinea pig cochlea was evaluated using a non-contact laser blood flowmeter (NCLBF) and the endocochlear potential (EP). Focal degeneration, including vascular thrombosis in the SV produced by the systemic infusion of rose bengal, and the illumination of green light in the second cochlear turn were observed with scanning and transmission electron microscopy. The NCLBF probe was placed at a position 10 mm from the cochlear surface, and the diameter of the laser light was focused to 1 mm in the green light illumination area. The change in NCLBF values induced by the loading of anoxia and administration of epinephrine agreed very well with those obtained with a conventional contact-type laser Doppler flowmeter. Significant decreases in the cochlear blood flow (CBF) (p < 0.01) and EP (p < 0.01) were observed at the site of the photochemical injury compared with the values at the non-illuminated area. CBF gradually decreased (82.0+/-7.3% at 10 min, 71.2+/-5.5% at 20 min, 64.3+/-11.2% at 30 min from the baseline, n=7), but blood pressure was stable. The EP values also decreased gradually during the first 13 min (79.9+/-3.7 mV at pre-illumination, 11.4+/-10.7 mV at 13 min, n=7). The gradual decline in the EP was comparable to the changes in the CBF. The NCLBF was useful for evaluating the haemodynamic properties of the cochlear microcirculation disorders, and this animal model is expected to be suitable for studying the pathology of focal cochlear vascular disease.

Basilar membrane velocity noise

Basilar membrane (BM) noise, measured as a velocity signal under the quiet acoustic condition, was investigated in the guinea pig. The cochleas of anesthetized young healthy guinea pigs were surgically exposed and a hole was made on the lateral wall of the scala tympani of the first cochlear turn for visualization of the BM and measurement of the BM velocity with a laser interferometer. The amplitude and frequency of the BM velocity noise were analyzed by a spectrum analyzer under different conditions. The spectrum of the BM velocity noise was a band limited function with a peak velocity at the topographic best frequency of the measured location on the BM. The peak velocity ranged to about 8 microm/s and depended on the physiological condition of the cochlea. Saline blockage of the external auditory canal or the middle ear did not change the BM noise. BM noise was much smaller, or was not evident, when the cochlear sensitivity decreased. The suppression tuning curve of the BM velocity noise indicates that the maximum suppression caused by an acoustic pure tone occurred at the best frequency location. A low sound level wide band acoustic noise given to the external ear canal produced a spectrum function having the same frequency and amplitude response as the BM noise. Electrical stimulation of the crossed olivocochlear bundle significantly depresses the BM velocity noise. These data demonstrate that the BM noise is a representation of internal rather than external noise. The amplitude and frequency of the BM noise reflect the usual cochlear sensitivity and frequency selectivity. Since the organ of Corti in the sensitive cochlea is a highly sensitive and tuned mechanical system, the internal (to the animal) noise responsible for the BM noise may originate from mechanical vibrations remote from the cochlea and propagated to the ear, or may be caused by Brownian motion of cellular structures in the cochlea.

Heart beat modulation of spontaneous otoacoustic emissions in guinea pig

It has been reported that background variation in the frequency of spontaneous otoacoustic emission (SOAEs) may arise from the cardiovascular system and that the side-bands in the spectra of SOAEs may be modulated by heartbeat. For better understanding of the mechanical influence of the cardiac cycle on cochlear functions under physiological and pathophysiological conditions, this study investigated heartbeat-induced modulation of SOAEs in guinea pig. Possible mechanisms of these phenomena are also discussed. The external ear canal acoustic signal, round window electric signal, and the ECG were recorded for off-line analysis from five pigmented guinea pigs with SOAEs. Time and frequency domain averages with synchronization of the heartbeat indicate that both the external ear canal acoustic and round window electric signal contain some component contributed by ECG and phonocardiography. High resolution FFT spectra suggest that SOAEs were modulated by heartbeat and respiration. Theoretically the change in the SOAEs could be from frequency modulation or a combination of frequency and amplitude modulations. Results of a heartbeat-synchronized average of a wave analyzer output at SOAE frequency indicates that amplitude modulation of SOAE does occur. Data obtained in the current experiment support the hypothesis that the heartbeat-related modulation of SOAE in guinea pig is a combination of frequency and amplitude modulation. The proposed mechanism is that pulsatile cochlear blood flow may cause oscillations in cochlear pressure and affect cochlear performance, resulting in heartbeat modulation of SOAEs.