Micro-lesions in Reissner's membrane evoked by acute hydrops

Utricular Sensitivity during Hydrodynamic Displacements of the Macula

To explore the effects of cochlear hair cell displacement, researchers have previously monitored functional and mechanical responses during low-frequency (LF) acoustic stimulation of the cochlea. The induced changes are believed to result from modulation of the conductance of mechano-electrical transduction (MET) channels on cochlear hair cells, along with receptor potential modulation. It is less clear how, or if, vestibular hair cell displacement affects vestibular function. Here, we have used LF (<20 Hz) hydrodynamic modulation of the utricular macula position, whilst recording functional and mechanical responses, to investigate the effects of utricular macula displacement. Measured responses included the Utricular Microphonic (UM), the vestibular short-latency evoked potential (VsEP), and laser Doppler vibrometry recordings of macular position. Over 1 cycle of the LF bias, the UM amplitude and waveform were cyclically modulated, with Boltzmann analysis suggesting a cyclic modulation of the vestibular MET gating. The VsEP amplitude was cyclically modulated throughout the LF bias, demonstrating a relative increase (~20-50 %; re baseline) and decrease (~10-20 %; re baseline), which is believed to be related to the MET conductance and vestibular hair cell sensitivity. The relationship between macular displacement and changes in UM and VsEP responses was consistent within and across animals. These results suggest that the sensory structures underlying the VsEP, often thought to be a cranial jerk-sensitive response, are at least partially sensitive to LF (and possibly static) pressures or motion. Furthermore, these results highlight the possibility that some of the vestibular dysfunction related to endolymphatic hydrops may be due to altered vestibular transduction following mechanical (or morphological) changes in the labyrinth.

In vivo imaging of saccular hydrops in humans reflects sensorineural hearing loss rather than Meniere's disease symptoms

OBJECTIVES

A case-controlled imaging study demonstrated that saccular hydrops was specific to Meniere's disease (MD), but only present in a subset of patients. Here, we compared patients with definite MD, vertigo and sensorineural hearing loss (SNHL) to elucidate the relationship between saccular hydrops and extent of SNHL.

METHODS

In this prospective study, we performed 3D-FLAIR sequences between 4.5 and 5.5 h after contrast media injection in patients with MD (n=20), SNHL (n=20), vertigo (n=20) and 30 healthy subjects. Two radiologists independently graded saccular hydrops. ROC analysis was performed to determine the hearing loss threshold to differentiate patients with saccular hydrops.

RESULTS

Saccular hydrops was found in 11 of 20 MD patients, 10 of 20 SNHL patients and in none of the vertigo patients and healthy subjects. In SNHL patients, 45 dB was the threshold above which there was a significant association with saccular hydrops, with sensitivity of 100 % and specificity of 90 %. In MD patients, 40 dB was the threshold above which there was a significant association with saccular hydrops, with sensitivity of 100 % and specificity of 44 %.

CONCLUSIONS

Our results indicate saccular hydrops as a feature of worse than moderate SNHL rather than MD itself.

KEY POINTS

• MRI helps clinicians to assess patients with isolated low-tone sensorineural hearing loss. • Saccular hydrops correlates with sensorineural hearing loss at levels above 40 dB. • Vertigo patients without sensorineural hearing loss do not have saccular hydrops. • Saccular hydrops is described in patients without clinical diagnosis of Meniere's disease.

Changes in cochlear function during acute endolymphatic hydrops development in guinea pigs

Previous studies have injected artificial endolymph into scala media in anaesthetized guinea pigs as an acute model of endolymphatic hydrops. Here, we have injected artificial endolymph into scala media in guinea pigs at rates of 40-80 nl/min, whilst monitoring Compound Action Potential (CAP) thresholds, the Summating Potential (SP)/CAP ratio, Cochlear Microphonic (CM) distortion, low-frequency modulated Distortion Product Otoacoustic Emissions (DPOAEs), and the Endocochlear Potential (EP). We found that abrupt recovery of CAP thresholds, SP/CAP ratio, and CM and DPOAE asymmetric distortion could occur several times during a single injection of less than 3 μl, suggesting that endolymph pressure could periodically decrease while the injection was ongoing. Larger volumes are thought to produce a rupture of the membranous labyrinth, however, our results suggest that multiple injections, each larger than 3 μl and within 40 min of each other, cause multiple pressure-related changes, which are difficult to be explained on the basis of a simple labyrinth rupture. We have also examined the morphological changes of the temporal bones ex vivo using X-ray micro-tomography. Both the functional changes and the micro-CT images suggest ruptures of the membranous labyrinth may not always be responsible for abrupt changes in inner ear function. Our results provide a new insight into the changes in cochlear function occurring during acute hydrops development, which compares well to the clinical findings observed in Ménière's Disease. We suggest that hydrops development may be a continual process, yet cause discontinuous functional changes due to mechanisms other than a simple rupture of the membranous labyrinth.

MRI manifestation of novel superparamagnetic iron oxide nanoparticles in the rat inner ear

AIM

Superparamagnetic iron oxide nanoparticles hierarchically coated with oleic acid and Pluronic F127 copolymers (POA@SPION) have shown exceptional T2 contrast enhancement. The aim of the present work was to investigate the MRI manifestation of POA@SPION in the inner ear.

MATERIALS & METHODS

A total of 26 male Wister rats were selected for testing POA@SPION administered through intracochlear, intratympanic and intravenous routes. MRI was performed with a 4.7 T MR scanner.

RESULTS & CONCLUSION

POA@SPION can be introduced into the perilymph space, after which it becomes widely distributed and can demonstrate the integrity of the perilymph-endolymph barrier. Positive highlighting of the endolymph compartment against the darkened perilymph was visualized for the first time. POA@SPION passed through the middle-inner ear barriers in only small amounts, but stayed in the perilymph for 3 days. They did not traverse the blood-perilymph barrier or blood-endolymph barrier. The inner ear distribution of POA@SPION was confirmed by histology. POA@SPION is a promising T2 negative contrast agent.

The effect of changes in perilymphatic K+ on the vestibular evoked potential in the guinea pig

To investigate the effect on the functioning of the vestibular system of a rupture of Reissner's membrane, artificial endolymph was injected in scala media of ten guinea pigs and vestibular evoked potentials (VsEPs), evoked by vertical acceleration pulses, were measured. Directly after injection of a sufficient volume to cause rupture, all ears showed a complete disappearance of VsEP, followed by partial recovery. To investigate the effect of perilymphatic potassium concentration on the vestibular sensory and neural structures, different concentrations of KCl were injected directly into the vestibule. The KCl injections resulted in a dose-dependent decrease of VsEP, followed by a dose-dependent slow recovery. This animal model clearly shows a disturbing effect of a higher than normal K(+) concentration in perilymph on the vestibular and neural structures in the inner ear. Potassium intoxication is the most probable explanation for the observed effects. It is one of the explanations for Menière attacks.

Rupture of Reissner's membrane during acute endolymphatic hydrops in the guinea pig: a model for Ménière's disease?

CONCLUSION

The changes in cochlear function during a destructive acute endolymphatic hydrops were relatively small. This might be consistent with the hypothesis that an endolymphatic hydrops is a marker of disordered inner ear homeostasis rather than the cause of the clinical symptoms of Ménière's disease.

OBJECTIVE

Assessment of cochlear function during induction of a destructive acute endolymphatic hydrops.

MATERIALS AND METHODS

During repetitive microinjections of 0.5 microl of artificial endolymph at a rate of 50 nl/s the 2f1-f2 and f2-f1 cochlear microphonics distortion products (CMDP) and 2f1-f2 distortion products otoacoustic emissions (DPOAE) were recorded in the guinea pig.

RESULTS

A 'catastrophe' occurred in the inner ear when 2.5-3.5 microl of artificial endolymph was injected. A rupture of Reissner's membrane was then found, most often in the apical turn of the cochlea. This rupture had only minor effects on the endocochlear potential, whereas it caused a marked decrease in 2f1-f2 DPOAE amplitude. The 2f1-f2 and f2-f1 CMDP amplitude increased during each injection prior to the rupture. After the rupture the f2-f1 CMDP amplitude decreased during each injection, possibly due to a shift of the cochlear transducer operating point position.

High potassium concentrations protect inner and outer hair cells in the newborn rat culture from ischemia-induced damage

Several studies indicate that an increase in the extracellular potassium (K+) concentration is a factor exerting a damaging effect on cochlear hair cells (HCs). The present study was designed to examine the effects of high extracellular K+ concentrations on the HCs under normoxic and ischemic conditions. Organotypic cultures of the organ of Corti of newborn rats were exposed to normoxia and ischemia at K+ concentrations of 5-70 mM in artificial perilymph for 3-4h. The number of IHCs and OHCs in the apical, medial and basal parts of the cochlea were counted 24h later. The work resulted in two main findings: (1) extracellular K+ concentrations of 30-70 mM had no effect on the HCs under normoxic conditions; (2) under ischemic conditions, a clear HC loss, mainly in the medial and basal cochlear parts, was observed at 5 mM K+ as previously reported. In contrast, a high extracellular K+ concentration strongly attenuated the HC loss. This effect nearly completely disappeared by the addition of both eosin, an inhibitor of the plasma membrane calcium ATPase (PMCA), and linopirdine, an inhibitor of the KCNQ4 channel, indicating that a normal activity of the PMCA and the KCNQ4 channels are key factors for HC survival under ischemia and depolarizing conditions.

Ménière's disease: effects of glycerol on tasks involving temporal processing

The present study examined the effect of glycerol ingestion on aspects of auditory performance in subjects having Ménière's disease. It was hypothesized that Ménière's disease may be associated with abnormal firing in the auditory nerve and that this should result in a decreased ability to code the auditory temporal fine structure. Psychoacoustical measures of interaural time discrimination and quasi frequency modulation rate discrimination were used as measures of temporal coding, and performance on these tasks was examined both before and after glycerol ingestion. Pre- and postglycerol measures of speech recognition and audiometric thresholds were also obtained. In agreement with previous results, glycerol-related changes in audiometric thresholds were modest or absent, but improvements in speech recognition were relatively reliable. Improvements in interaural time discrimination and quasi frequency modulation rate discrimination were also observed. The results provide limited support for the hypothesis that Ménière's disease may be associated with a reduced ability to code the temporal fine structure of sound.