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Your Ears Are Whispering

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Your eyes do not produce light, nor does your tongue have a flavour, but your ears do make sounds! In 1978, acoustician David Kemp inserted a miniature microphone into a human ear canal and, for the first time, recorded nearly inaudible sounds originating from the inner ear (Kemp, 1978). These sounds have since been called otoacoustic emissions (OAEs). Although most OAEs are softer than a whisper, they are a remarkably useful indicator of healthy ears (Yongbing and Martin, 2006).

Understanding OAEs requires basic knowledge of acoustics and ear anatomy. As you read this, sound vibrations in the air around you are propagating through your ear canal, initiating a domino effect of vibrations through the intricate structures comprising your middle and inner ear (Figure 1) (Young and Ng, 2022). The vibrations reach your basilar membrane, stimulating sensory hair cells that transmit electrochemical signals to your brain. A certain class of hair cells—the outer hair cells—receive direct neural feedback from the auditory system. In response, the outer hair cells rapidly contract and elongate to amplify signals to the brain (Brownell, 1990). The action of the outer hair cells also generates a vibration of the basilar membrane that travels backwards out of your ear. Consequently, outer hair cells produce OAEs detectable with a sensitive microphone. 

Figure 1: A diagram of the ear showing structures involved in OAE production. Sound vibrations travel from the air in the ear canal to the eardrum, along the malleus, incus, and stapes, then through the spiralling fluid-filled cochlea. The cochlea contains the Organ of Corti, a cell structure that sits on the basilar membrane. When the basilar membrane vibrates due to incoming sounds, sensory hair cells send signals along the spiral ganglion to the auditory system of the brain. OAEs are produced when the outer hair cells cause the basilar membrane to vibrate, and that vibration travels in reverse from the cochlea to the ear canal. Created in BioRender.com

OAE amplitude is a clinical indicator of ear health, which can be compromised due to hereditary factors, excessive noise, hypoxia, aging, and drug side effects (Hopkins, 2015; Young and Ng, 2022). Many commonly-used drugs are ototoxic, meaning that they damage and impair the inner ear (Bisht and Bist, 2011). For instance, the common chemotherapeutic drug cisplatin is infamously ototoxic, presenting a near-certain chance of hearing loss. Cisplatin destroys tumour cells by forming cross-links with purine bases of DNA, damaging the DNA and triggering cell self-destruction (Sheth et al., 2017). Unfortunately, cisplatin typically also damages outer hair cell DNA, causing cell destruction and hearing loss. 

Since damaged outer hair cells no longer produce OAEs, drug ototoxicity can be monitored non-invasively using OAE tests (Young and Ng, 2022). An audiologist uses a small speaker and microphone to play an acoustic stimulus to a patient and measure the amplitude of the patient’s OAE (Figure 2). A 2.4 dB minimum decrease in OAE amplitude compared to a baseline measurement indicates clinically significant hearing loss (Bader et al., 2021). Patients should undergo OAE tests before taking ototoxic drugs so that audiologists can compare their baseline OAE intensity with OAE intensity after each drug dose. 

Figure 2: An example of spontaneous otoacoustic emissions (SOAEs) measured with a microphone. The amplitude, or level, of SOAEs is presented in dB SPL, where SPL stands for sound pressure level and simply indicates that all dB levels are relative to the threshold of human hearing (20 µPa). In this case, SOAEs of many frequencies were measured across two trials in a newborn, indicating healthy hearing. Figure attributed to Dreisbach et al., 2019.

OAEs are the unsung hero of the auditory world. They may be soft and subtle, but with an astonishingly simple test, OAEs can notify you and your audiologist of ear damage due to ototoxic drugs and many other factors. Just as you listen with your ears, be sure to listen to your ears!

References

Bader, K., Dierkes, L., Braun, L.H., Gummer, A.W., Dalhoff, E. and Zelle, D., 2021. Test-retest reliability of distortion-product thresholds compared to behavioral auditory thresholds. Hearing Research, 406, p.108232. https://doi.org/10.1016/j.heares.2021.108232.

Bisht, M. and Bist, S.S., 2011. Ototoxicity: The Hidden Menace. Indian Journal of Otolaryngology and Head & Neck Surgery, 63(3), pp.255–259. https://doi.org/10.1007/s12070-011-0151-8.

Brownell, W.E., 1990. Outer Hair Cell Electromotility and Otoacoustic Emissions. Ear and hearing, 11(2), pp.82–92.

Dreisbach, L., Portugal, N., Aguilar, E., Hansen, W., Kite, D., McIntyre, S. and Minasian, C., 2019. Acquisition and Repeatability of High-Frequency Distortion Product Otoacoustic Emissions Using Two Different Calibration Methods in Newborns. Applied Sciences, 9, p.2947. https://doi.org/10.3390/app9152947.

Hopkins, K., 2015. Deafness in cochlear and auditory nerve disorders. In: M.J. Aminoff, F. Boller and D.F. Swaab, eds. Handbook of Clinical Neurology, The Human Auditory System. Elsevier. Ch. 27 https://doi.org/10.1016/B978-0-444-62630-1.00027-5.

Kemp, D.T., 1978. Stimulated acoustic emissions from within the human auditory system. The Journal of the Acoustical Society of America, 64(5), pp.1386–1391. https://doi.org/10.1121/1.382104.

Sheth, S., Mukherjea, D., Rybak, L.P. and Ramkumar, V., 2017. Mechanisms of Cisplatin-Induced Ototoxicity and Otoprotection. Frontiers in Cellular Neuroscience, 11. Available at: <https://www.frontiersin.org/articles/10.3389/fncel.2017.00338> [Accessed 17 March 2023].

Yongbing, S. and Martin, W., 2006. Spontaneous Otoacoustic Emissions in Tinnitus Patients. Journal of Otology, 1(1), pp.35–39. https://doi.org/10.1016/S1672-2930(06)50006-6.Young, A. and Ng, M., 2022. Otoacoustic Emissions. In: StatPearls. Treasure Island (FL):

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