Water transport in the inner ear considered from the endolymphatic hydrops model animals

―Relationship with Ménière's disease―

Abstract

　A typical abnormality in water transport in the inner ear is endolymphatic hydrops (EH), which is a critical component of Ménière's disease (MD). Regarding the flow of the endolymph, there are three aspects, i.e., the longitudinal flow theory, the radial flow theory, and the dynamic flow theory. The dynamic flow theory has both longitudinal flow and radical flow components. Maintenance of ion composition and resting potential is performed at each site (radial flow), and the flow from the cochlear, vestibule and semicircular canal to the endolymphatic sac (ES) (longitudinal flow) is necessary for absorption of endolymph and transport of polymeric substances. ES plays an important role for the water transport in the inner ear. The function of endolymph absorption is conventionally reported as a role for the ES. In recent years, it has been demonstrated that the ES actively regulates the homeostasis of the endolymphatic environment in response to changes in endolymphatic volume, pressure, and blood flow in the inner ear. EH is caused by impaired absorption of endolymph, overproduction, or both. Concerning the EH models, surgical obliteration of the ES sac and duct model is most promising, and various studies have been conducted on the pathology and treatment of MD using this model. On the other hand, EH can be induced by systemic administration of vasopressin (VP) as an overproduction mode. In this study, we compare the process of EH formation and features of each model using the obliteration model and the endolymph overproduction (VP administration) model. The results show that, in the obliteration model, severe EH occurs in all sites. In the overproduction model, the EH of the cochlea, utricle, and semicircular canals is severe, but is mild in the saccule. Overproduction of endolymph alone could not explain the severe saccular EH of MD, and it seemed that some inhibitory factors for absorption of endolymph are necessary in addition. Based on these results, the causes of MD and the relation with stress are discussed.