Abnormal fast fluctuations of electrocochleography and otoacoustic emissions in Menière's disease.

The responses of cochlear hair cells to sound stimuli depend on the resting position of their stereocilia bundles, which is sensitive to the chemical and mechanical environment. Cochlear hydrops, a hallmark of Menière's disease (MD), which is likely to come with disruption of this environment, results in hearing symptoms and electrophysiological signs, such as excessive changes in the cochlear summating potential (SP) and in the postural shifts of distortion-product otoacoustic emissions (DPOAEs). Here, SP from the basal part of the cochlea and DPOAEs from the apical part of the cochlea were recorded concomitantly in 73 patients with a definite MD, near an attack (n = 40) or between attacks with no clinical symptoms (n = 33), to compare their sensitivities to posture and evaluate their stability. The phase of the 2f1-f2 DPOAEs was monitored during body tilt, with stimuli f1 = 1 kHz and f2 = 1.2 kHz at 72 dB SPL. Extratympanic electrocochleography was performed in response to 95-dBnHL clicks. The normal limits of the DPOAE phase shift with body tilt, [-18°, +38°], and of the SP to action-potential (AP) ratio, <0.40, were exceeded in 75% and 60% of patients, respectively, near an attack. In these patients, but not in the asymptomatic ones, both tests reveal fluctuating cochlear responses from one data sample to the next. They emphasize how hydrops hinders normal hair-cell operation and may generate fast fluctuations in inner-ear functioning. If these fluctuations also occur on shorter time scales, it might explain the imperfect diagnostic sensitivity of SP and DPOAE tests, as averaging procedures would tend to level out transient fluctuations characteristic of hydrops.

Unstable distortion-product otoacoustic emission phase in Menière's disease.

The presence of endolymphatic hydrops as a marker of Menière's disease (MD) suggests abnormal pressure in the intralabyrinthine compartments of patients and excessive stiffness of sound-sensitive structures. Otoacoustic emissions (OAEs) have been reported to respond to changes in the ear's stiffness, including those produced by intracranial pressure steps, by a characteristic phase shift around 1 kHz, thereby suggesting a noninvasive means of monitoring MD. Here, body tilt was used for modulating intracranial pressure in forty-one patients with definite MD who were tentatively measured at two stages, with and without active symptoms. Their distortion-product OAEs (DPOAEs) were dynamically monitored around 1 kHz every few seconds in response to body tilt. In a control sample of thirty normal ears, the maximum phase rotation of DPOAEs produced by body tilt was between -18° and +37°. In MD ears with the complete set of symptoms, the posture-induced phase shifts in 32 out of 35 tests fell outside the normative interval, and in 10 tests, although DPOAEs were well above noise floor, their phase was always so abnormally erratic that body tilt produced hardly any additional effect. When MD ears were asymptomatic, nine out of 32 posture tests were abnormal. The excessive DPOAE phase shift is consistent with either a too stiff cochlear partition or a displacement of the operating point of outer hair cells by endolymphatic hydrops.

Frequency specificity of distortion-product otoacoustic emissions produced by high-level tones despite inefficient cochlear electromechanical feedback.

Distortion product otoacoustic emissions (DPOAEs) are thought to stem from the outer hair cells (OHCs) around the normally narrow place tuned to the primary tone stimuli. They are thus said to be frequency-specific: their local absence should accurately pinpoint local OHC damage. Yet the influence of impaired tuning on DPOAE frequency specificity is poorly documented. Mice with local damage to OHCs were examined. Their DPOAEs were frequency-specific in that audiometric notches were accurately tracked. The same cochleae were further impaired by ischemia or furosemide injection inducing strial dysfunction with flat loss of sensitivity and tuning, while the preexisting pattern of damaged OHCs remained unaltered. Despite the loss of cochlear activity, DPOAEs produced by high-level (> or =70 dB SPL) primaries remained large in about the same interval where they had been initially normal, i.e., that with nondamaged OHCs, albeit with a slight frequency shift, of -1.1 kHz on average. Thus, the ability of DPOAEs to map structurally intact OHCs cannot be a mere consequence of cochlear tuning as it largely persists in its absence. The key element for this correct mapping is likely part of intact OHC structures (e.g., stereocilia bundles) and must have some tuning of its own.