Changes in distortion of two-tone cochlear microphonic and otoacoustic emission signals during an acute endolymphatic hydrops in the guinea pig

An acute endolymphatic hydrops was induced by the injection of 1.1 l of artificial endolymph into the scala media of guinea pig cochleas. This volume corresponds with an acute endolymphatic hydrops of 23%. During and after the injection, cochlear function was assessed by measuring the 2f₁-f₂ and f₂-f₁ distortion products in cochlear microphonics (CMDP) and the 2f₁-f₂ distortion product otoacoustic emission (DPOAE). A reversible pressure increase of 23 Pa and a relatively stable endocochlear potential (EP) were accompanied by a mean decrease in 2f₁-f₂ DPOAE of only 3.4 dB. Similarly, the 2f₁-f₂ CMDP amplitude change was minimal during and after the injection. The only substantial change was measured in the f₂-f₁ CMDP amplitude. The measured range of distortion amplitudes during an acute endolymphatic hydrops can be related to small changes in the cochlear transducer operating point.