Experimental and clinical studies on electrocochleographic evaluation of endolymphatic hydrops

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Electrocochleographic indicators potentially available for the evaluation of endolymphatic hydrops (ELH) were studied using experimental animals and clinical materials.Although the enlarged summating potential (SP)/action potential (AP) ratio has been known to be frequently associated with Meniere's disease, this abnormality was found not only in ELH but also in some other inner ear pathological models such as the strial damage due to furosemide. Since the guinea pig ELH model showed a significant decrease in the degree of low-frequency modulation of AP amplitude and the tendency of increase in the AP-latency difference between rarefaction and condensation clicks, these two parameters seemed to be more sensitive indicators for ELH than any other clinically available parameters.In order to evaluate new electrocochleographic parameters not only in patients with sensorineural hearing loss but also in normal-hearing subjects, we have developed a special electrode designed for tympanic electrocochleography. This method is found to be more convenient and less traumatic than recording with a transtympanic or an ear-canal electrodes, even allowing repeated measurements during the glycerol dehydration test. The abnormal increase in the AP-latency difference to condensation and rarefaction clicks was found in patients with high-frequency hearing・loss as well as in those with acute low-tone sensorineural hearing loss and with threshold fluctuation.Further studies are needed in a larger number of clinical cases before concluding whether such AP-latency difference is caused by ELH.Improvements must also be made on better recording of the low-frequency cochlear microphonics using tympanic electrocochleography.