“Hearing” and Auditory Neuropathy: Lessons from Patients, Physiology, and Genetics

Abstract

I review auditory neuropathy (AN), an auditory temporal processing disorder, drawing upon lessons from patients, from temporal bones and peripheral nerves, and from the genetics of the disorder. The auditory temporal processing disorder affects speech comprehension and localization of sounds that can be disabling. Audibility is typically not the majoy problem. The criteria for diagnosis are physiological and include (1) abnormal auditory nerve function reflected by absent or abnormal auditory brainstem responses (ABRs) and (2) normal cochlear outer hair cell functions reflected by cochlear microphonics (CMs) and/or otoacoustic emissions (OAEs). The tests are relatively simple, and the results are typically unambiguous, encouraging the recognition of AN from diverse etiologies. The cochlear sites that are affected include auditory nerve, inner hair cells, or their synapses. Type I AN is a postsynaptic disorder involving both the number and functions of auditory nerves; Type II AN is a presynaptic disorder affecting inner hair cells’ ability to form and/or release neurotransmitters. Inherited forms of AN are diverse. Temporal bone studies of postsynaptic forms of AN show a marked loss of auditory nerve fibers with accompanying demyelination whereas both the number and morphology of inner and outer hair cells are preserved. There are as yet no temporal bone studies of presynaptic forms of AN.

To honor Kimitaka Kaga, scientist-clinician