Low-frequency masking for detection of endolymphatic hydrops in patients with glaucoma.

The coincidence of various eye and ear abnormalities has been described in the literature. Some authors discuss the possible existence of endolymphatic hydrops in patients with glaucoma. Whereas the current diagnostic tests for glaucoma are well-defined and evident, those for endolymphatic hydrops are not so reliable. This has made it difficult to accurately study the coincidence of endolymphatic hydrops and glaucoma. For better detection of endolymphatic hydrops, we performed low-frequency masking tests in 23 patients with primary open-angle glaucoma without signs of Ménìere's disease. The phase dependent sensitivity of the organ of Corti to a short test stimulus can be measured by applying a low-frequency masker tone to determine the modulation depth. Whereas the modulation depth in the normal hearing population is around 20-35 dB, the modulation depth in patients with Ménière's disease may be significantly decreased (5-10 dB), depending on the stage of disease. A decreased modulation depth was found in at least one ear in 19 of our 23 patients with glaucoma. Correlations between homeostatic mechanisms and their histological characteristics, e.g. melanocytes and their hormonal and enzymatic regulation, will be discussed. If the common pathogenesis of eye and ear lesions is better understood, it may be possible to develop new and more effective strategies for prevention and therapy.

Diagnosis of endolymphatic hydrops by low-frequency masking.

Low-frequency masking is a new method for the diagnosis of endolymphatic hydrops. A short acoustic stimulus and a low-frequency masker tone are applied to the same ear in an adjustable phase relationship. We recorded phase-dependent masked thresholds from normal-hearing subjects, and patients with Ménière's disease and sensory hearing loss without vertigo. In normal hearing, there is a mean maximal difference in masking (modulation depth) of 28 dB between the phase delays of 0 degree and 270 degrees. In patients with sensory hearing loss without vertigo, modulation depth is reduced due to recruitment. In Ménière cases, the phase dependence may be totally absent and varies as the disease progresses. Therefore, repeated measurements of masking are required: patients and subjects with normal hearing were tested for a period of 1 year. Also, modulation depth is significantly reduced in the contralateral nonsymptomatic ears of Ménière patients. The results indicate that low-frequency masking is a quick, noninvasive and relevant method for the diagnosis of endolymphatic hydrops.

[Diagnosis of endolymphatic hydrops with low tone masked otoacoustic emissions]. / Diagnostik des endolymphatischen Hydrops mit tieftonmaskierten otoakustischen Emissionen.

BACKGROUND: A new method of diagnosis of endolymphatic hydrops by recording low-tone masked evoked otoacoustic emissions (TEOAE) is presented. METHODS: A short acoustic stimulus and a masker tone of 30 Hz are applied in an adjustable phase relation simultaneously to the same ear. In the normal hearing ear the masker shows little influence on the TEOAE at 0 degrees, whereas the suppression at phase 270 degrees (maximal rarefaction at the eardrum) is nearly complete. However, in cases of endolymphatic hydrops this masking effect is reduced or absent, indicating impaired mobility of the basilar membrane. RESULTS: The masked TEOAE were recorded of patients with normal hearing, Menière's disease, and sudden hearing loss without vertigo. In Menière cases with supposed endolymphatic hydrops, the amplitude modulation of the emissions was found to be much less than in the other groups. CONCLUSIONS: Where TEOAE can be recorded, low-tone masking is a quick, objective, and noninvasive method for the diagnosis of endolymphatic hydrops.

Masking and pitch shift of tone bursts and clicks by low-frequency tones.

From experiments in animals and investigations in humans it is known that the normally phase-dependent masking of a short stimulus by a low-frequency continuous tone does not occur in the case of endolymphatic hydrops. The recording of the masked threshold of short tone stimuli in a loud tone of 30 Hz is to be evaluated for the clinical diagnostics of Ménière's disease. To this purpose, the main parameters of the measurements (type, frequency, duration of the stimulus, and intensity of the masker) and their effect of phase-dependent masking and pitch-shift are investigated. Stimuli above 2 kHz are masked less than those of lower frequencies. Wide-band stimuli are less useful, since only the low-frequency component of their spectrum is masked. The tone stimuli should be short (1 - 2 ms) in order to make the measurement of the phase dependence more accurate. With increasing masker level the masking at phase 0 degree corresponds to the increase in level, at phase 270 degrees the amount is twice as much. The pitch shift which is perceived in low-tone masking depends on the phase of the stimulus, and on the levels of the stimulus and the masking tone. The use of brain stem recordings in the investigation of phase-dependent low tone masking is problematic since well-synchronizing stimuli with high frequency spectral components are masked poorly.

Low-frequency masking of brainstem potentials.

The phase and intensity dependence of masking a click by a loud low-frequency tone was examined with brainstem potentials. Wave V latency is practically unaffected; its amplitude, however, is maximally suppressed at a phase of 270 degrees, i.e. rarefaction in the ear canal and basilar membrane displacement towards the scala tympani. Corresponding to subjective threshold, a minor suppression is also observed for the opposite deflection. In the phase of maximal suppression, wave V can be cancelled by a 30 Hz tone of 115 dB SPL up to click intensities of 40 dB HL. With cochlear damage, total suppression can be achieved at even higher click intensities as long as they are close to the subjective threshold. Low-frequency suppression is discussed as a tool applicable for early diagnosis of endolymphatic hydrops.