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Endolymphatic hydrops (ELH) is a disorder of the inner ear that causes tinnitus, vertigo, and hearing loss. An elevated ratio of the summating potential (SP) to the action potential (AP) measured by electrocochleography has long been considered to be the electrophysiological correlate of ELH-related clinical conditions, such as Meniere's disease, but in vivo confirmation and correlation between an elevated SP/AP ratio and ELH has not yet been possible. Confirming this relationship will be important to show that elevated SP/AP ratio is indeed diagnostic of ELH. Here, we sought to confirm that an elevated SP/AP ratio is associated with ELH and test the hypothesis that severity of ELH and hearing loss would also correlate with the SP/AP ratio in vivo using the Phex(Hyp-Duk)/Y mouse model of postnatal ELH. In addition, we describe a minimally invasive approach for electrocochleography in mice. Auditory brainstem responses and electrocochleography data were collected from controls and Phex(Hyp-Duk)/Y mutants at postnatal day 21 and the mice (all male) were euthanized immediately for cochlear histology. Our results show that (1) the SP/AP ratio was significantly elevated in mice with histological ELH compared to controls, (2) the SP/AP ratio was not correlated with the severity of histological ELH or hearing loss, and (3) the severity of hearing loss correlated with the severity of histological ELH. Our study demonstrates that an elevated SP/AP ratio is diagnostic of ELH and that the severity of hearing loss is a better predictor of the severity of ELH than is the SP/AP ratio.

Objectives/hypothesis: Excitotoxic and related inflammatory injury are implicated in the spiral ganglion degeneration seen with Meniere's disease and endolymphatic hydrops (ELH). Excitotoxicity is initiated with glutamate elevation and associated with downstream increases in reactive oxygen species resulting in inflammation-mediated neuronal degeneration. This study tests the hypothesis that interruption of the initial and/or downstream aspects of excitotoxicity should provide hearing protection in ELH-associated hearing loss., Study Design: This study tests whether riluzole, a glutamate release inhibitor, and dimethylsulfoxide (DMSO), an anti-inflammatory and antioxidant solvent with favorable properties at the level of glutamate receptors, can protect against early-stage hearing loss in a mouse model of ELH., Methods: The Phex(Hyp-Duk) mouse spontaneously develops ELH and postnatal hearing loss. Starting at postnatal day 6 (P6), daily injections of riluzole + DMSO or just DMSO were administered. Untreated mutants served as controls. At P21, P25, and P30, hearing function was assessed by recording auditory brainstem responses. A cochlear function index was developed to assess global cochlear function at each time point., Results: Compared to no treatment, DMSO provided significant hearing protection (P < .05). The riluzole + DMSO also showed protection, but it was statistically indistinguishable from DMSO alone; a synergistic increase in protection with riluzole was not observed., Conclusions: This study demonstrates pharmacological hearing protection in an animal model of ELH. The results support the assertion that inflammatory (reactive oxygen species) injury, which is part of the excitotoxic pathway, contributes to the development of ELH-associated hearing loss.

Objectives/hypothesis: Neuronal toxicity is thought to be important in Meniere's disease and experimental endolymphatic hydrops (ELH). This study quantifies the relationship between neuronal degeneration and hair cell degeneration in ELH to evaluate the hypothesis that a primary neural insult would yield greater loss in the spiral ganglion than at the inner hair cell level., Study Design: Following induction and histopathologic confirmation of endolymphatic hydrops in guinea pigs, the degree of hydrops, spiral ganglion loss, and hair cell degeneration were quantified and compared., Methods: Guinea pigs with surgically induced unilateral hydrops were sacrificed and their cochleas preserved. Hydrops severity and spiral ganglion density were quantified using automated methods. Hair cells were counted manually. Values were normalized against the contralateral ear to create loss indexes., Results: Inner hair cell (IHC) loss at the apex is significantly lower than corresponding neuronal loss. IHC loss at the base is also lower than neuron loss, although not significantly. Regression analysis shows a significant, positive correlation between neuron loss severity and IHC loss severity at the apex, but not at the base. There is no correlation between hydrops severity and inner hair cell loss., Conclusions: By confirming that spiral ganglion loss is more severe than hair cell loss, and that hair cell loss appears to worsen with neuronal degeneration, this study supports the theory that neuronal toxicity is the primary insult in ELH-related disorders, such as Meniere's disease, and may provide the basis for designing treatment strategies.