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Daniel Brown

Daniel Brown, PhD.

Curtin University

Topic:The hydrostatic effects of endolymphatic hydrops on cochlear and vestibular function.

Whilst the relationship is not straightforward, endolymphatic hydrops (EH) is a characteristic feature of Meniere’s Disease. EH appears to be present in almost all cases of Meniere’s, even when sufferers are asymptomatic. Importantly, this means that EH alone does not underlie the hearing fluctuation, tinnitus, ear fullness or vertigo attacks. The most common theory of the cause of vertigo attacks and fluctuating auditory symptoms in Meniere’s disease is a rupture of the membranous labyrinth, and K+ toxicity of the vestibular hair cells. However, the rupture theory is not wholly supported on clinical, experimental, and theoretical grounds, and several other theories exist.

Research has demonstrated that cochlear function is sensitive to an increased endolymphatic pressure, causing displacement of the organ of Corti, changes in the mechanoelectrical transduction (MET) channel gating, and changes in the gain of the cochlear amplifier. On the basis that hearing sensitivity fluctuates in Meniere’s, presumably either the hydrostatic pressure of EH also varies over the time course of hearing fluctuation, or hair cells adapt to pressure induced organ of Corti displacement. Experimental evidence supports both phenomena, where cochlear MET channels are known to adapt to static displacements of their bundles, and endolymph volume and pressure can vary during development of EH.

Whilst there is evidence to explain how EH can result in discontinuous cochlear function, it is less clear how it affects vestibular function. That is, does EH cause a displacement of the vestibular hair cells, and if so, does this alter their function? Recent evidence highlights how EH can directly affect semicircular canal and otolith hair cell function, through mechanical rather than ionic disturbances in the vestibule. Understanding these mechanisms is important in the context of accurately interpreting clinical measures of inner ear dysfunction, and this is important in developing tools for objectively monitoring the progress of EH in Meniere’s patients, particularly those undergoing new therapeutic strategies.