By Dr. Colin Michie FRCPCH
Our brains are filled with sounds: Speech, tunes, language, noises around us. We are really good at recognising voices from family, friends, activities and celebrities. We can recall music on our playlists and media, and recognise birds, bells, and aircraft. Listening provides many gifts that are intertwined with our past – some difficult to remember, some recur as pesky earworms.
Our impressive sense of hearing is based on some 3,500 specialised hair cells deep within each ear – a tiny number of receptors for such a powerful sense! By comparison, we have many millions of detectors in our eyes, up our noses, or within our skin to deliver other sensations.
Outer ears – whatever their shape or size – direct sound towards the eardrums. Small bones then transmit sound from each tympanum into the inner ear or cochlea, a spiral-shaped organ about 2cm across, buried within our skull bones. Cochleae support the crucial sensory hair cells, suspended in salty fluid, that amplify, process and filter – the sound engineers. They turn sound waves into electrical signals for the auditory nerves leading into the brain. Similar cells line the semi-circular canals next to the cochlea, where they direct our balance and which way is up! Similar cells are found on the sides of fish to keep them upright: those mechanisms to convert fluid movements into nerve impulses are ancient biological processes.
We take for granted being able to hear where a pin drops, what is being said to us in a restaurant, or notice an approaching vehicle in the street. These capacities depend on those hair cells picking out specific sounds against a background of others, while the brain coordinates signals from both ears. This processing has other uses, which we do not normally hear, blood flowing through the arteries in our skull, or our breathing, swallowing and gut noises.
Those small armies of inner hair cells cannot be replaced. Unlike scalp hair or red blood cells, they last for life. Losing them causes deafness. They are most often damaged by age; the hair cells detecting higher-pitched sounds tend to disappear first. Approximately 5% of the world’s population suffers with significant hearing loss. In the elderly, this is often put down to ageing, is neglected and not treated. Those with limited hearing often become progressively more socially isolated, mobilise less and have greater risks of developing dementia. These are growing challenges on St. Maarten, where the population is ageing.
The second most common enemy to cochlear hair cells is noise. Blast exposures in military conditions, mining or construction are extreme examples, but damaging volumes of sound over long periods are experienced by many. Noise causes hair cells to lose their connections with the brain, leading to difficulty hearing speech in noisy environments. It can also cause annoying tinnitus (the perception of a sound, often a ringing or buzzing, when there is no source). Protecting workers from disc jockeys to dentists is important: Disposable or customised earplugs are a practical solution.
Of all our senses, hearing is the most often compromised at birth. Mutations in the genes building hair cells compromise hearing in one in every 500 infants. If a baby cannot hear well, they face enormous challenges understanding sounds and speech – hearing is crucial for listening, understanding, communicating and developing. All infants should receive hearing tests shortly after birth and treated if required.
Hair cells can be damaged by trauma to the middle and inner ears, as well as infections such as meningitis, viruses (in particular cytomegalovirus) or certain medications (including some antibiotics and chemotherapy agents). Although hair cells cannot be replaced, hearing aids or assistive listening devices have become very useful in supporting their functions. These are mostly digital, allowing a healthcare professional to customise an individual sound profile. Since the 1980s, cochlear implants have been transformative for children and many adults by bypassing the hair cells altogether with an electrode connected to a microphone placed under the skin. These require teaching and support to work well and allow children to comprehend and produce normal speech. Trials of gene therapies and stem cell strategies to repair or regenerate damaged hair cells have not yet reached clinics.
Technical interventions work better together with assistance with lip reading and often sign language too. Community approaches bridge language divides: Signing is a language we can all employ. It is important too in reducing discrimination against those with hearing difficulties. The sounds, voices, music and language we perceive and collect are brain foods; essential nutrition for our emotional and social health. By supporting cochlear hair cells we can all enrich the sound tracks of our lives, and those of others.
Dr. Colin Michie specializes in paediatrics, nutrition, and immunology. Michie has worked in the UK, southern Africa and Gaza as a paediatrician and educator and was the associate Academic Dean for the American University of the Caribbean Medical School in Sint Maarten a few years ago. Useful resources: Royal National Institute for the Deaf: rnid.org.uk ~ World Federation of the Deaf: This email address is being protected from spambots. You need JavaScript enabled to view it. ~ Hearing Loss in Children: cdc.fob/hearing-loss-children
