Most hearing loss is caused by the death or damage to the sensory cells in the cochlea. This is called sensorineural hearing loss, and this makes up an estimated 90% of all hearing loss in humans. These sensory cells are of two types, both required for hearing; hair cells and auditory neurons. Both types are very easy to damage through exposure to noise, certain chemicals, or just as a natural consequence of ageing. These cells cannot be repaired or naturally regenerated in humans. They develop in utero: you are born with a compliment that you must retain for your entire life. If you lose them or they are damaged, you lose hearing function and therefore progressive loss results in progressive deafness.
There are no current treatments for hearing loss related to loss of auditory neurons. Hair cell damage can be partially circumvented by the use of hearing aids and cochlear implants, there are no treatment options that target auditory neuron loss. Hearing aids are sound amplifiers that detect sound and amplify frequencies which are useful for the perception of human speech. Cochlear implants are devices which detect the sound from the environment, convert it to electrical signals, and then send a signal to the internal electrode array of the implant, effectively bypassing the damaged hair cells to directly stimulate the auditory neurons. However, if the auditory neurons are lost or damaged, then these medical devices will not work well because you need the auditory neuron population to send the sound signal from the cochlea to the brain.
People with hearing loss related to auditory neuron function are particularly badly served by current healthcare solutions. This is particularly problematic as the most common form of hearing loss, presbycusis (also known as age-related hearing loss), auditory neuron loss has been shown to proceed hair cell loss. Published research¹ has shown that by 60 years of age, average loss of auditory neurons is around 46%. The first treatment in Rinri’s pipeline is focused on repairing damage to the auditory neurons, and has the potential to make a difference to the millions of people with age-related hearing loss, or more rare conditions such as auditory neuropathy.
¹ Wu, P.Z. et al. (2019) “Primary neural degeneration in the human cochlea: Evidence for hidden hearing loss in the aging ear,” Neuroscience, 407, pp. 8–20. Available at: https://doi.org/10.1016/j.neuroscience.2018.07.053.