Neural and receptor cochlear potentials in auditory neuropathy obtained by transtympanic electrocochleography (ECochG)

Auditory neuropathy (AN) is a recently identified disorder of auditory nerve characterized by prominent auditory temporal processing deficits. Both auditory nerve and brainstem activities recorded as far-field potentials are typically undetectable whereas cochlear outer hair activities (otoacoustic emissions and/or cochlear microphonics) are normal. We recorded cochlear receptor (summating potential [SP]; cochlear microphonics [CMs]) and auditory nerve (compound action potential [CAP]) activities by transtympanic electrocochleography (ECochG) in 15 adults and children with the clinical picture of AN. Data from eight children and adults with delayed onset AN will be presented. Test stimuli were 0.1 ms clicks which were presented in free-field from 60 to 120 dB SPL. Measures were compared with the ECochG results from 16 children who had normal thresholds of CAPs. Cochlear potentials from AN were significantly prolonged in duration and reduced in peak amplitude compared to controls. CAPs were identified in 4/16 AN cochlea only to high intensity stimuli. In the remaining ears it was difficult to separately identify the CAP and the SP within the broad cochlear potential. In contrast, CMs were identified in all AN subjects and did not differ significantly in amplitude from controls. Receptor summating potentials (SPs) when identified were of normal amplitude. Rapid stimulus rates were used in six of the AN subjects to help distinguish the generator sources of the prolonged cochlear potentials by taking advantage of different amount of adaptation involving CAP and SP. In controls, rapid stimulus rates were accompanied by amplitude reduction of CAP approximately twice that for SP while the duration of cochlear potentials was unaffected. In AN rapid stimulus rates were accompanied by a reduction of duration to control values in all and a reduction in amplitude in the majority consistent with a neural origins for the abnormal cochlear potentials. We suggest that the prolonged and attenuated cochlear potentials in AN reflect pre- and postsynaptic disorders of inner hair cell synapses with auditory nerve terminals.