Cochlear potentials recorded by transtympanic electrocochleography in patients with auditory neuropathy

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 8 children and adults with AN. Stimuli were 0.1 ms clicks 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. Receptor CMs were of normal or enhanced amplitude in AN. Neural CAPs and receptor SPs were separately identified in 5/16 AN ears. When identified SPs were of normal amplitude. In the remaining ears the ECochG potentials were of negative polarity and of normal or prolonged duration. Rapid stimulus rates were used in six of the AN subjects to help distinguish whether the generators of the negative potentials were of neural or receptor origin by taking advantage of different amount of adaptation involving CAP and SP. Adaptation in controls resulted in amplitude reduction of CAP twice that of SP without affecting the duration of ECochG potentials. In five AN ears with CAP, there was a reduction in amplitude of CAP and SP as controls but with a significant decrease in response duration consistent with post-synaptic disorder of proximal auditory nerve. In four ears without CAP and with normal duration potentials, adaptation was without effect consistent with receptor generation. In seven AN ears without CAP and with prolonged negative potential, adaptation was accompanied by reduction of both amplitude and duration of the negative potential to control values consistent with neural generation. We suggest that cochlear potential measures may identify pre- and post-synaptic disorders of inner hair cells and auditory nerve in AN.