Evaluation of cochlear function in an acute endolymphatic hydrops model in the guinea pig by measuring low-level DPOAEs

During and after microinjection of artificial endolymph into scala media of the guinea pig, the 2f(1) - f(2)-DPOAE at 4.5 kHz generated by low-level primaries was recorded. Reproducible changes were measured when 1.1 mul of artificial endolymph was injected at a rate of 1.65 nl/s (1.53-1.83). This volume corresponds with an acute endolymphatic hydrops of 23%. After the onset of injection the inner ear pressure immediately increased to a mean higher level of 22 Pa, whereas the 2f(1) - f(2)-amplitude and -phase did not change for about 1 min. Thereafter, the amplitude decreased 2.6 dB (+/-0.7) on average and slowly regained almost its initial value, with recovery frequently starting within the period of injection.

In an attempt to explain the observed changes in 2f(1) - f(2)-amplitude the basilar membrane displacement towards scala tympani at the 2f(1) - f(2) generation site is estimated to be 19 nm for a 1.1 mul increase of endolymph volume. A small deflection of the outer hair cell stereocilia and as a consequence a change in cell conductance may explain the 2f(1) - f(2)-amplitude changes. However, the precise mechanism of cochlear function change caused by endolymph volume increase (hydrops) remains to be elucidated. (C) 2004 Elsevier B.V. All rights reserved.