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Sensorineural Hearing Loss
Published in R James A England, Eamon Shamil, Rajeev Mathew, Manohar Bance, Pavol Surda, Jemy Jose, Omar Hilmi, Adam J Donne, Scott-Brown's Essential Otorhinolaryngology, 2022
Linnea Cheung, David M. Baguley, Andrew McCombe
Initially, excessive noise exposure causes a temporary threshold shift (TTS), resulting in a temporary HL. The high-frequency regions of the cochlea are most sensitive (between 3 and 6 kHz). Recovery of the TTS occurs over hours, days, or weeks following exposure. Expected recovery time is dependent on the loudness and duration of the noise presented. However, a permanent threshold shift (PTS) can occur at the initial insult, or it may evolve where there is continuous or repeated excessive noise exposure at levels that would only have otherwise caused a TTS. It is thought that this may be caused by metabolic factors such as excessive neurotransmitter release, changes in cochlear blood flow, and oxidative stress within the hair cells. Structural factors like depolymerisation of actin filaments in the stereocilia, swelling of the stria vascularis, and damage to afferent nerve endings and supporting cells are also thought to play a part. Synaptic connections between inner hair cells and spiral ganglion cells may also be susceptible to noise damage. Importantly, in animal studies, these synapses can be the first site of damage, even without an HL (‘hidden hearing loss’, or synaptopathy). Additionally, genetic susceptibility, smoking and cardiovascular disease, and diabetes have been implicated as risk factors.
Tinnitus and Hyperacusis
Published in John C Watkinson, Raymond W Clarke, Christopher P Aldren, Doris-Eva Bamiou, Raymond W Clarke, Richard M Irving, Haytham Kubba, Shakeel R Saeed, Paediatrics, The Ear, Skull Base, 2018
Outer hair cells have been shown to be more susceptible than inner hair cells to damage by certain agents including noise and aminoglycoside antibiotics. It has been suggested that, in areas where outer hair cells have been damaged but inner hair cells remain, the tectorial membrane is no longer supported by the outer hair cells and can sag onto the inner hair cells, causing them to depolarize.38
Stem Cells
Published in John C Watkinson, Raymond W Clarke, Louise Jayne Clark, Adam J Donne, R James A England, Hisham M Mehanna, Gerald William McGarry, Sean Carrie, Basic Sciences Endocrine Surgery Rhinology, 2018
Sensorineural hearing loss resulting from inner-ear cochlear dysfunction involves the loss of sensory hair cells and/or spiral ganglion neurons. Loss of inner hair cells is currently irreversible but a range of stem cell procedures that aim to regenerate cochlea structures, such as hair cells and associated peripheral nerves, has been reported.9 A primary issue is identifying types of stem cells appropriate for differentiation into sensory and neural progenitor cells, but equally challenging secondary issues are the differentiation of stem cells into the desired structures in situ, or the delivery of in vivo or in vitro differentiated stem cells to the right location. Rodent MSCs and some other stem cell types have been shown to differentiate in vitro into cells with hair cell properties indicating the feasibility of stem cell use. Of particular interest, a stepwise in vitro process that mimics normal developmental interactions, can differentiate murine ES cells into both functional mechano-sensitive hair cells and into sensory neurones that interact to form specialized synapses.16
The applications of targeted delivery for gene therapies in hearing loss
Published in Journal of Drug Targeting, 2023
Melissa Jones, Bozica Kovacevic, Corina Mihaela Ionescu, Susbin Raj Wagle, Christina Quintas, Elaine Y. M. Wong, Momir Mikov, Armin Mooranian, Hani Al-Salami
The sensory hair cells transduce auditory signals, with mechanosensitive stereocilia bundles positioned on their apical surface. The bundles themselves are arranged in a highly uniform chevron shape, with each bundle containing 50 to 200 stereocilia [39]. Outer hair cells contribute to the process of cochlea amplification, assisting in the selectivity and diversity of hearing via the mechanical boosting of sound-induced vibrations [44]. This offers a wide dynamic range, sharp frequency tuning and overall high sensitivity. Inner hair cells possess mechanotransduction channels, indicating their ability to result in a biological response from a physical stimulus. Which, in the case of inner hair cells, occurs via the auditory nerve fibre synaptic connection, permitting the detection of sound, and transmitting information about the acoustic environment to the central auditory system. Overall, sound vibrations are detected by inner hair cells following amplification by outer hair cells [45–47]. One key element of significance to note is the inability for the regeneration of mammalian ear hair cells. This is in contrast to non-mammalian vertebrate species which have the capacity for hair cell restoration via the regeneration of supporting cells. Hence, damage to mammalian hair cells is currently considered permanent and leads to varying degrees of hearing loss [48–50].
Experimental drugs for the prevention or treatment of sensorineural hearing loss
Published in Expert Opinion on Investigational Drugs, 2023
Judith S Kempfle, David H. Jung
Inner hair cells have the ability to transduce mechanical movements into electrical signals via fiber-like structures at the apical end, the stereocilia [12]. Ribbon synapses at the basal end of hair cells release the neurotransmitter glutamate in response to sound-evoked stereocilia motion. Afferent spiral ganglion neurons are stimulated via their α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) glutamate receptors, and the electrical signal is then propagated along the auditory pathway. Outer hair cells act in response to auditory signals and function as modulators and amplifiers of sound [13]. The protein Prestin in the membrane of outer hair cells can undergo conformational changes in response to electrical potential, and this electromotility contributes to frequency selectivity and helps to amplify the auditory signal [13]. The electrical gradient between endolymph and perilymph is maintained by the stria vascularis, which flanks the scala media and organ of Corti, and which harbors various ion pumps to restore the resting potential [14].
Non-linguistic auditory speech processing
Published in International Journal of Audiology, 2023
Robert H. Margolis, Aparna Rao, Richard H. Wilson, George L. Saly
A candidate for a peripheral cause of disturbed spectro-temporal processing that is independent of threshold shift is the synaptopathy that has been observed in animal models in the synapses between auditory nerve fibres and inner hair cells. The nerve fibres that synapse with inner hair cells are high-threshold, low-spontaneous-rate fibres that do not contribute to auditory sensitivity but encode temporal characteristics of sounds that reach the cochlea (Parthasarathy and Kujawa 2018). Widespread synaptopathy of inner hair cell synapses in animal models results from noise exposure and ageing (Kujawa and Liberman 2009, 2015; Sergeyenko et al. 2013). Heeringa and Köppl (2019) suggested that the fifth category of presbycusis – synaptopathy presbycusis – should be added to the classical categorisation of age-related hearing loss proposed by Schuknecht (1964).