HHL can be detected physiologically and is characterized by normal auditory thresholds but reduced suprathreshold amplitude of the sound-evoked SGN compound action potential (AP) (the first peak of the auditory brainstem response (ABR) waveform) 3, 5, 6, 7, 8, and by the alteration in the ratio between the peak of the ABR waveform generated by hair cells (summating potential (SP)) and the peak I of the ABR waveform (AP) 9. However, recent animal and human studies indicate that moderate noise exposures or ageing can result in a new form of peripheral hearing loss called ‘hidden hearing loss (HHL)’ 3, 4, 5, 6. Until recently, it was believed that auditory processing is normal in subjects with normal thresholds, even after recovery from a temporary threshold shift (TTS) due to noise exposure, and that hearing deficits in those cases were due to central problems 2. This type of hearing loss, which affects 320 million people worldwide 1, is characterized by permanent elevation of auditory thresholds, that is, the minimal sound pressure levels (SPLs) that evoke an auditory or neural response at specific sound frequencies. Thus, proper function of both sensory cells and neurons is essential for normal hearing and auditory communication, and degeneration of either component causes sensorineural hearing loss. These neurons then relay the acoustic information to the central auditory circuits. In the mammalian cochlea, acoustic stimuli are detected by inner hair cells (IHCs), which transduce and transform them into synaptic signals to the primary auditory sensory neurons, the spiral ganglion neurons (SGNs). Thus, this study identifies a new mechanism for HHL, highlights the long-term consequences of transient Schwann cell loss on hearing and might provide insights into the causes of the auditory deficits reported in patients that recover from acute demyelinating diseases such as Guillain–Barré syndrome. This auditory neuropathy is not associated with synaptic loss, but rather with disruption of the first heminodes at the auditory nerve peripheral terminal. ![]() ![]() Here we report that transient loss of cochlear Schwann cells results in permanent auditory deficits characteristic of HHL. Currently, the only cellular mechanism known for HHL is loss of inner hair cell synapses (synaptopathy). Animals and humans with HHL have normal auditory thresholds but defective cochlear neurotransmission, that is, reduced suprathreshold amplitude of the sound-evoked auditory nerve compound action potential. Hidden hearing loss (HHL) is a recently described auditory neuropathy believed to contribute to speech discrimination and intelligibility deficits in people with normal audiological tests.
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