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Science Behind Hearing Loss

Background Information on Hearing Loss:

  • Hearing loss, otherwise known as hearing impairment, is the partial or total inability to hear. 

  • Hearing loss is diagnosed when one fails to hear 25 decibels in at least one ear. 

  • The categories of hearing loss are mild (25 - 40 dB), moderate (41 - 55 dB), moderate-severe (56 - 70 dB), severe (71 - 90 dB), or profound (greater than 90 dB). 

  • In children, hearing disabilities may affect their ability to learn spoken language, leading to deaf-mute children. Sign language is a way to communicate with them. 

  • Around 466 million people, 6% of the world's total population as of January 2020 [1], have disabling hearing loss, and 34 million of these are children. By 2050, the number is predicted to rise to 900 million.  

    • Disabling hearing loss - hearing loss greater than 40dB in adults' stronger perceiving ear and a hearing loss greater than 30dB in children's greater perceiving ear. 

  • 60% of hearing loss during childhood could have been prevented through practices such as immunization, proper pregnancy care, avoidance of loud noises, and the appropriate use of medication. 

  • Early identification, hearing aids, cochlear implants, sign language, and other assistive devices and equipment are of benefit to those with hearing loss. [2] 

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How we hear: 

Anatomy of an ear: 

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[3] 

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OUTER EAR: 

The pinna/auricle catches sound waves and passes them along deeper into the ear. The sound that is captured by the pinna/auricle is funneled down into the external auditory canal towards the inner ear. The tympanic membrane, which is the boundary between the outer and middle ear, vibrates and passes the vibration down to the middle ear. 

 

MIDDLE EAR: 

Vibrations are passed to tiny bones in the middle ear. This relay station amplifies the sound waves so that they are stronger when they enter the inner ear. Amplification is essential because the inner ear moves sound though fluid, not through air. And, moving through a liquid is a lot harder. The tympanic cavity focuses the pressure of the sound waves so that they are strong enough to move through the fluid, using the auditory ossicles - a trio of the smallest bones in the human body: malleus, incus, and stapes. 

 

One end of the malleus links to the inner eardrum and moves back and forth when the drum vibrates. The other end is connected to the incus, which is also connected to the stapes. They form a chain that conducts eardrum vibrations over to another membrane - the superior oval window, where they cause the fluid in the inner ear to move. 

 

INNER EAR: 

The inner ear's job is to turn physical vibrations into electrical impulses the brain can identify as sounds. 

 

The cochlea consists of three chambers - scala vestibuli, scala media, scala tympani - separated by sensitive membranes. The most important membrane for hearing is the basilar membrane: a stiff band of tissue that runs between the scala media and the scala tympani. It can decipher every sound in the range of human hearing (20 - 20,000 Hz) and communicate it to the nervous system. And it does it like this: vibration causes certain parts of the basilar membrane to move back and forth. The membrane is covered with more than 20 thousand fibers. Those near the base are short and stiff and resonate at high frequencies; those near the end are longer and looser and resonate at lower frequencies. 

 

When the membrane moves, it tickles the neighboring organ of corti. When the hair cells on the organ of corti are triggered, it opens mechanically gated potassium channels. The influx of potassium leads to graded potentials, which leads to action potentials. Those electrical impulses go from the organ of corti across the cochlear nerve and up the auditory pathway to the cerebral cortex. (You won't hear anything until the brain knows!!!) 

 

Overall, all the sounds we hear and how you recognize them comes down to precisely the section of the membrane that vibrates at any given time. 

 

[4] 

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Types of Hearing Loss: 

  1. Conductive hearing loss: when sound has trouble moving from the outer ear to the eardrum and middle-ear bones. Soft sounds will be hard to hear, and louder sounds may appear muffled. [5]

  2. Sensorineural hearing loss (SNHL): due to nerve damage in the inner ear. Hearing aids or a cochlear implant may alleviate this condition. [6]

  3. Mixed hearing loss: both sensorineural hearing loss (SNHL) and conductive hearing loss.

 

Causes of hearing loss: (organized into two broad types of causes) 

  • Congenital causes: those present from birth 

    • Infections during pregnancy 

    • Low birth weight 

    • Birth asphyxia - a lack of oxygen at times of birth 

    • Inappropriate use of particular drugs during pregnancy 

    • Genetics

 

  • Acquired causes: those that may lead to hearing loss at any age

    • Aging, mainly due to degeneration of sensory cells 

    • Infectious diseases 

    • Chronic ear infections 

    • Otitis media - Collection of fluid in the ear 

    • Use of certain drugs, such as drugs used to treat neonatal infections, malaria, drug-resistant tuberculosis, and cancers. 

    • Injury to the head or ear 

    • Excessive noise (occupations involving machinery and explosions) 

    • Ear wax or foreign body blocking the ear canal 

    • Genetics 

[2] 

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Causes of hearing loss: organized into individual causes

  • Age: The ability to hear high-frequency sounds deteriorates with age and is known as presbycusis. This progressive loss of hearing function can begin at the age of 25 for men and 30 for women. 

  • Noise: Noise exposure accounts for approximately 50% of all hearing loss. If one is exposed to loud sounds - music, gunshots, explosions - for an extended period, hearing loss will occur. 

  • Genetics: hearing loss can be inherited. Approximately 75-80% of all hearing loss cases are inherited through recessive genes; 20-25% are inherited through dominant genes. 1-2% are inherited by X-linked chromosomes, and less than 1% are inherited by mitochondrial inheritance. 

[7] 

  • Perinatal issues (relating to the time, usually number of weeks, immediately before and after birth): premature birth is associate with sensorineural hearing loss due to potential complications and infections. The risk of hearing loss is highest for newborns weighing less than 1500 grams at birth. 

  • Disorders: (there are A LOT, I'm not going to list them all. Here are some:) Auditory neuropathy; cholesteatoma; otosclerosis; strokes. 

  • Medication: ototoxic medication may adversely affect hearing abilities. Medicines for HIV/AIDS has been said to induce hearing problems. 

  • Physical trauma: Physical damage to the ear or the brain centers that process the aural information conveyed by the ears may cause hearing problems. 

  • Chemical substances: ototoxic chemicals in the environment 

[8] 

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Cochlear Implants: 

  • Cochlear implant surgery is an operation to implant an electronic hearing device intended to create hearing sensations to someone with severe to profound nerve deafness by electrically stimulating nerves inside the inner ear. 

  • The surgery takes approximately an hour to do and involves making an incision behind the ear. The device is set next to the skull, and the electrode is delicately inserted into the inner ear using a soft-surgery approach. Then, the incision is closed. It is not significantly painful, and patients can resume their normal activities within 2-3 days typically. The implant is not activated on until the incision has completely healed, usually 2-4 weeks after surgery.

[9] 

  • A cochlear implant receives sound from the external environment, processes it, and sends small electric currents near the auditory nerve. These electrical currents activate the nerve, which then sends a signal to the brain. The brain recognizes this signal and experiences this as "hearing". 

[10] 

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References

[1] Current World Population. (2020). Retrieved July 07, 2020, from https://www.worldometers.info/world-population/

 

[2] (WHO), W. (2020, March 01). Deafness and hearing loss. Retrieved July 07, 2020, from https://www.who.int/en/news-room/fact-sheets/detail/deafness-and-hearing-loss

 

[3] Deruty, E. (2020, July 01). How The Ear Works. Retrieved July 07, 2020, from https://www.soundonsound.com/sound-advice/how-ear-works

 

[4] YouTube Crash Course Channel & Thought Café. (2015, May 4) Hearing & Balance: Crash Course A&P #17. Retrieved July 07, 2020, from https://www.youtube.com/watch?v=Ie2j7GpC4JU. 

 

[5] Conductive Hearing Loss. (n.d.). Retrieved July 07, 2020, from https://www.asha.org/public/hearing/Conductive-Hearing-Loss/

 

[6] Types of Hearing Loss. (2019, August 01). Retrieved July 07, 2020, from https://www.webmd.com/healthy-aging/types-hearing-loss

 

[7] Heidi L. et.al. Understanding the Genetics of Deafness. A Guide for Patients and Families. Retrieved July 07, 2020, from https://web.archive.org/web/20131019153702/http:/hearing.harvard.edu/info/GeneticDeafnessBookletV2.pdf

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[8] Hearing loss. (2020, July 06). Retrieved July 07, 2020, from https://en.wikipedia.org/wiki/Hearing_loss

 

[9] Cochlear Implantation for Sensorineural Hearing Loss. (2018, January 17). Retrieved July 07, 2020, from https://ent.keckmedicine.org/treatment/cochlear-implantation-for-sensorineural-hearing-loss/

 

[10] Center for Devices and Radiological Health. (2018, April 02). What is a Cochlear Implant? Retrieved July 07, 2020, from https://www.fda.gov/medical-devices/cochlear-implants/what-cochlear-implant

 

[11] Zych, A. (2019, April 11). What Do Cochlear Implants And Hearing Aids Sound Like? Retrieved July 07, 2020, from https://www.sciencefriday.com/educational-resources/cochlear-implants-hearing-aids-sound-like/

 

[12] Corwin, J. (2018, July 20). Opinion | The lonely world between the hearing and the Deaf. Retrieved July 07, 2020, from https://www.washingtonpost.com/opinions/the-lonely-world-between-the-hearing-and-the-deaf/2018/07/20/6628571c-86d8-11e8-8f6c-46cb43e3f306_story.html

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