Different mechanisms in temporal processing disorders accompanying 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 have recorded both receptor (summating potential [SP]; cochlear microphonics [CMs]) and auditory nerve (compound action potential [CAP]) activities by transtympanic electrocochleography (ECochG). We studied cochlear potentials bilaterally from eight children and adults with AN by an electrode placed on the promontory. Test stimuli were 0.1 ms clicks which were presented in free-field from 60 to 120 dB SPL. The results were compared with the ECochG results from 16 children who were tested because of a suspected hearing loss but were found to have normal thresholds of CAPs. Cochlear potentials were identified in all AN subjects and were prolonged in duration and reduced in 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. Also SPs when identified were of normal amplitude. We used rapid stimulus rates 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. We identified the presence of several different physiological abnormalities including preserved receptor and CAPs, preserved receptor and attenuated CAPs, and absence of CAPs. Adaptation studies were accompanied by a reduction of duration of cochlear potentials to control values consistent with their being atypical neural events. There are several mechanisms active in AN disrupting the function of synapses of inner hair cells, generation of nerve action potentials by auditory nerve, and transmission of nerve impulses centrally.