Identification of different subtypes of auditory neuropathy using electrocochleography

Currently, the physiological mechanisms underlying auditory neuropathy are unclear, and there are likely to be multiple sites of lesion. A better understanding, of the disruption in individual cases may lead to more effective management and deviceselection. Frequency-specific round-windowelectrocochleography (ECochG) waveforms were used to assess local hair cell, dendritic, and axonal currents generated within the cochlea in 15 subjects with auditory neuropathy (16 ears). These results were compared with electrically evoked auditory brainstem response (EABR) measured after cochlear implantation. The results of this study demonstrate that predominantly two patterns of ECochG waveforms can be identified: (i) a prolonged latency of the hair cell summating potential (SP) waveform With Or Without residual CAP activity and (ii) a normal latency SP, typically followed by a dendritic potential (DP). We show that seven of eight Subjects with a prolonged SP showed a normal EABR waveform, consistent with a presynaptic lesion, whereas six of seven subjects with a normal latency SP showed poor morphology or absent EABR waveforms, consistent with a postsynaptic lesion. We suggest that a presynaptic and postsynaptic type of auditory neuropathy exist, which may have implications for the fitting of cochlear implants.