Eyes, Ears, and Teeth

TL;DR. These three get a fraction of the attention that hearts and cancers receive, and between them they cause more years lived with disability than almost anything else in this book. Cataract is the leading cause of blindness worldwide and is cured by a 20-minute operation, which means most of the world's blindness is an access problem rather than a medical one. Hearing loss affects roughly 1.5 billion people, and untreated hearing loss is the single largest modifiable midlife risk factor for dementia. Untreated tooth decay is the most prevalent disease on the planet, affecting an estimated 2 billion people in their permanent teeth. All three are largely preventable or treatable with cheap, established interventions, and all three are routinely deprioritised because they rarely kill anyone.

Key takeaways

• Cataract surgery is one of the highest-value operations in medicine: about 20 minutes, usually under local anaesthetic, and it restores sight. It remains undelivered to millions.
• Glaucoma destroys peripheral vision silently, and roughly half of people with it do not know. The only way to catch it is an eye examination.
• Short-sightedness is rising steeply worldwide, and the best-supported preventive factor in children is time spent outdoors, roughly two hours a day.
• Inner ear hair cells never regenerate in humans. Noise damage is permanent, painless, and entirely preventable.
• Dental caries is a bacterial disease driven by how often you eat sugar, not how much. Fluoride works, and its addition to water and toothpaste is one of the largest public health wins of the twentieth century.
• Gum disease is associated with cardiovascular disease and diabetes, in both directions, and it is the leading cause of tooth loss in adults.

The eyes

In short: Five conditions cause almost all vision loss, and four of them are treatable if caught.

Refractive error

The commonest eye problem and the easiest to fix: the eye focuses light in front of the retina (short-sighted, myopia), behind it (long-sighted, hyperopia), or unevenly (astigmatism). Glasses, contact lenses, or laser surgery correct it.

Uncorrected refractive error is nonetheless a leading cause of vision impairment worldwide, purely because glasses are unavailable or unaffordable. A pair of glasses costing a few dollars is one of the cheapest disability-preventing interventions that exists.

Myopia is rising steeply. Rates in parts of East Asia have reached 80 to 90 percent of young adults, and projections suggest roughly half the world's population could be myopic by 2050. This matters beyond glasses: high myopia stretches the eye and raises the risk of retinal detachment, glaucoma, and myopic macular degeneration.

The best-supported preventive factor is time outdoors in childhood. Cluster-randomised trials adding roughly 40 to 80 minutes of outdoor time to the school day reduced the incidence of new myopia meaningfully. The mechanism is thought to involve bright light triggering dopamine release in the retina, which slows eye elongation. Low-dose atropine eye drops also slow progression in children.

Presbyopia is the universal stiffening of the lens from around 45 (Chapter 6). It is not a disease and it happens to everyone.

Cataract

What it is. Clouding of the lens. Because lens cells are never shed (Chapter 6), their proteins accumulate a lifetime of oxidative damage and eventually scatter light.

What it does. Gradual blurring, glare and haloes around lights, faded colours, and difficulty driving at night. Painless and progressive.

Risk factors: age above all, plus ultraviolet exposure, smoking, diabetes, corticosteroids, and eye injury.

Treatment. The clouded lens is removed by ultrasound and replaced with a plastic one. It takes about 20 minutes, is usually done under local anaesthetic as a day case, and restores vision in the large majority. It is among the most cost-effective surgical procedures ever measured.

And it is the leading cause of blindness in the world, responsible for a large share of global blindness, almost entirely in countries where surgical capacity is insufficient. That is a distribution problem, not a scientific one.

Glaucoma

What it is. Progressive damage to the optic nerve, usually associated with raised pressure inside the eye when fluid drainage is impaired.

Why it is dangerous. It destroys peripheral vision first, and the brain fills in the missing areas so convincingly that people do not notice until a great deal is gone. It is painless in its common form. Roughly half of people with glaucoma are undiagnosed, and the vision lost never comes back.

The acute form is different and is an emergency. Acute angle-closure glaucoma causes sudden severe eye pain, a red eye, blurred vision with haloes, headache, and vomiting, and it can destroy sight within hours without treatment.

Risk factors: age, family history, African or Caribbean ancestry (earlier onset and more severe disease), high myopia, diabetes, and long-term steroid use.

Treatment: eye drops that reduce fluid production or increase drainage, laser treatment, or surgery. The treatment does not restore lost vision; it prevents further loss, which is why detection before symptoms is the entire game. Regular eye examinations are the only route to that.

Age-related macular degeneration

What it is. Damage to the macula, the small central area responsible for detailed vision, and the leading cause of blindness in high-income countries.

Two forms:

• Dry (about 90 percent): gradual accumulation of deposits and thinning. Slow. No cure, though a specific high-dose antioxidant and zinc formulation (the AREDS2 combination) slows progression in people with intermediate disease.
• Wet (about 10 percent): abnormal leaky vessels grow under the retina. Fast, and it causes most of the severe vision loss. Treatable with regular injections into the eye of drugs blocking VEGF, the signal that drives vessel growth, which stabilise or improve vision in the majority. This transformed the outlook for a disease that was untreatable twenty years ago.

What it does: central vision is lost while peripheral vision remains, so people cannot read or recognise faces but can navigate. Straight lines appearing wavy is a characteristic early sign, and noticing it is worth acting on immediately, because wet AMD treated early does far better.

Risk factors: age, smoking (a major one), family history, and cardiovascular risk factors.

Diabetic retinopathy

Covered in Chapter 18: damage to retinal capillaries from chronic high glucose, and a leading cause of blindness in working-age adults. It is silent until late, which is why annual retinal photography is standard for everyone with diabetes, and why AI-based screening is being deployed where specialists are scarce.

What to do about your eyes

• Get examined regularly, roughly every two years for adults and annually over 60 or with risk factors. Glaucoma and early retinopathy are found this way and by no other.
• Do not smoke, which is a major risk factor for macular degeneration and cataract.
• Protect from UV with sunglasses that specify UV protection.
• Get children outdoors, roughly two hours a day, which is the best available myopia prevention.
• Control glucose and blood pressure.
• Emergency signs: sudden vision loss, a sudden shower of new floaters or flashes, a curtain across vision, sudden painful red eye with visual loss, or straight lines becoming wavy. All of these mean the same day, not next week.

The ears

In short: 1.5 billion people with hearing loss, permanent damage from entirely preventable noise, and a decade of delay before most people act.

Hearing loss

Scale. Roughly 1.5 billion people have some hearing loss and around 430 million have disabling loss, projected to rise substantially with population ageing.

The two main types:

Age-related loss (presbycusis) affects high frequencies first, which is why the earliest complaint is not "everything is quieter" but "I can hear you but I cannot understand you," particularly in a noisy room. Consonants carry most of the information in speech and are high-frequency.

Noise-induced loss is the entirely preventable one. Damage begins with sustained exposure above about 85 decibels, and the safe exposure time halves for every 3 dB increase: 8 hours at 85 dB, 4 at 88, 2 at 91, and so on down to minutes at concert or power tool levels. It is painless, invisible, and cumulative.

The dementia connection is the most important recent finding. Untreated hearing loss is the largest single modifiable midlife risk factor for dementia in the Lancet Commission analysis, and the ACHIEVE randomised trial found hearing aids slowed cognitive decline substantially in older adults at higher risk (Chapter 37). The proposed mechanisms are increased cognitive load, reduced stimulation, and social withdrawal.

And yet people wait. The average delay between noticing hearing difficulty and getting help is around a decade, driven by stigma, cost, and the gradualness of the change. Over-the-counter hearing aids, now available in some countries, are intended to attack the cost and access part of that.

Cochlear implants bypass the damaged hair cells entirely, converting sound into direct electrical stimulation of the auditory nerve. They restore useful hearing in people with severe to profound loss, and outcomes in children implanted early are dramatically better, which is why newborn hearing screening exists.

Tinnitus

Perceived sound with no external source, affecting perhaps 10 to 15 percent of adults. It is usually generated by the brain in response to reduced input, which is why it commonly accompanies hearing loss.

There is no reliable cure. What helps: treating any underlying hearing loss (hearing aids often reduce it by restoring input), sound therapy, and cognitive behavioural therapy, which has the best evidence and works by changing the distress and attention rather than the sound itself.

Ear infections and vertigo

Otitis media (middle ear infection) is extremely common in young children because their Eustachian tubes are short and horizontal. Most resolve without antibiotics, and delayed prescribing is standard in many countries. Glue ear, persistent fluid, causes conductive hearing loss and speech delay if prolonged, and is treated with grommets when it persists.

Benign paroxysmal positional vertigo (BPPV) deserves specific mention as one of the best value-for-effort treatments in medicine. Displaced calcium crystals in the balance organs cause brief, intense, position-triggered vertigo. A bedside repositioning manoeuvre (the Epley manoeuvre) resolves it in a majority of people in minutes, at no cost. It is frequently misdiagnosed as something more sinister or dismissed as inevitable dizziness.

Ménière's disease causes episodic vertigo with hearing loss and tinnitus, and vestibular neuritis causes a single prolonged episode after a viral illness.

What to do about your ears

• Protect from noise. Earplugs at concerts, in workshops, and on machinery. The rule of thumb: if you must raise your voice to be heard by someone an arm's length away, it is loud enough to cause damage.
• Keep personal audio at 60 percent volume or below, for 60 minutes at a time, which is the commonly quoted guidance.
• Get hearing tested if you struggle in restaurants, turn the television up, or ask people to repeat themselves. Do not wait the average decade.
• Do not put anything smaller than your elbow in your ear. Cotton buds push wax inward and perforate eardrums.
• Get sudden hearing loss treated as an emergency. Sudden sensorineural hearing loss in one ear is treatable with steroids if started within days, and the window closes.

The teeth and gums

In short: The most prevalent disease on earth, entirely preventable, and driven by how often rather than how much sugar you eat.

Dental caries

What it is. Bacteria in dental plaque, principally Streptococcus mutans, ferment dietary sugars into acid. The acid dissolves the mineral of the tooth. Saliva neutralises it and returns minerals over the following 20 to 60 minutes.

So the critical variable is frequency, not quantity. Each sugar exposure starts an acid attack lasting roughly half an hour. A whole chocolate bar eaten in one sitting is one attack; six sweets spread across a day is six. This is the single most useful thing to know about teeth, and it is rarely explained.

Scale. Untreated caries in permanent teeth is estimated to affect around 2 billion people, making it the most prevalent condition in the Global Burden of Disease study. Caries in primary teeth affects hundreds of millions of children.

Fluoride works by three mechanisms: it incorporates into the tooth surface as a more acid-resistant mineral, it promotes remineralisation, and it inhibits bacterial metabolism. Water fluoridation and fluoride toothpaste together are among the largest public health achievements of the twentieth century. Excess fluoride in childhood causes fluorosis, mostly cosmetic mottling, which is why concentrations are controlled and children use smaller amounts.

Gum disease

Gingivitis is inflammation of the gums from plaque: red, swollen gums that bleed when brushed. It is reversible with proper cleaning.

Periodontitis is the progression: inflammation extends below the gum line, destroying the ligament and bone that hold teeth in place. Pockets form, teeth loosen, and eventually they are lost. Severe periodontitis affects roughly 19 percent of adults globally, over a billion people, and it is the leading cause of tooth loss in adults.

Bleeding gums are not normal. Healthy gums do not bleed when brushed, and treating bleeding as an expected part of brushing is how gingivitis progresses unnoticed.

Risk factors: plaque, smoking (a major one), diabetes, genetics, and immunosuppression.

The systemic connection is real and its direction is partly unresolved. Periodontitis is associated with cardiovascular disease, diabetes (bidirectionally: each worsens the other), adverse pregnancy outcomes, and rheumatoid arthritis. Treating gum disease in people with diabetes produces a small but measurable improvement in HbA1c, which is one of the more convincing pieces of causal evidence.

The rest

Tooth loss and edentulism affect a substantial proportion of older adults and impair nutrition, speech, and social confidence. Oral cancer is strongly linked to tobacco, alcohol, and betel quid, and increasingly to HPV for throat cancers (Chapter 25). Dry mouth, a very common side effect of many drugs, dramatically accelerates caries because saliva is the main defence.

What to do about your teeth

• Brush twice a day with fluoride toothpaste, and spit, do not rinse, so the fluoride stays in contact with the teeth. This last detail is genuinely important and almost never explained.
• Clean between the teeth daily, with floss or interdental brushes. Brushing reaches about 60 percent of tooth surface.
• Reduce the frequency of sugar exposure, more than the total. Confine sweet things to meals.
• Do not brush immediately after acidic food or drink, or after vomiting; wait about an hour, because enamel is temporarily softened.
• Do not smoke, which is a major driver of gum disease and oral cancer.
• See a dentist regularly, at intervals matched to your risk.
• Treat bleeding gums as a finding rather than a normality.
• If you have diabetes, treat your gums as part of your diabetes care, and vice versa.

Why these three are neglected

In short: They rarely kill anyone, so they are chronically underfunded relative to the disability they cause.

Eyes, ears, and teeth share a structural problem. They cause enormous disability and very little mortality, so they rank low in death-based priority setting and high in DALY-based measures (Chapter 61). In most health systems, dental and optical care are separately funded, partly funded, or not funded at all, which is a policy decision rather than a medical one and produces exactly the access gaps described above.

The result is that some of the cheapest, most effective interventions in this entire book, a pair of glasses, a 20-minute cataract operation, a hearing aid, fluoride toothpaste, remain undelivered to hundreds of millions of people.

Sources and notes

Global blindness and vision impairment causes: WHO World report on vision and the Vision Loss Expert Group. Myopia projections: Holden et al., Ophthalmology, 2016. Outdoor time and myopia incidence: He et al., JAMA, 2015, and subsequent cluster-randomised trials. AREDS2: JAMA, 2013. Anti-VEGF for wet AMD: MARINA and ANCHOR trials and successors. Hearing loss prevalence: WHO World report on hearing, 2021 (approximately 1.5 billion with some loss, 430 million with disabling loss). Noise exposure limits: NIOSH and WHO. Hearing aids and cognitive decline: ACHIEVE, Lin et al., The Lancet, 2023. Dementia risk factors: Livingston et al., Lancet Commission, 2024. Untreated caries prevalence of approximately 2 billion and severe periodontitis at about 19 percent of adults: WHO Global oral health status report, 2022, and Global Burden of Disease analyses. Periodontal treatment and HbA1c: Simpson et al., Cochrane review. Epley manoeuvre efficacy: Cochrane review and AAO-HNS guidelines. Spit-do-not-rinse fluoride advice: national dental guidelines including Public Health England's Delivering Better Oral Health.

Open questions. Whether treating hearing loss prevents dementia in the general population, as opposed to in higher-risk groups, is not yet established. The causal direction between periodontal disease and cardiovascular disease remains partly unresolved. The optimal frequency of dental check-ups for low-risk adults has surprisingly weak evidence.

Next: the organ you can see, and the diseases people notice first. 👉