Almost 100 million people worldwide are estimated to be affected by cataracts. In low-income and middle-income countries, cataracts remain the number one cause of blindness.1
Cataracts often develop gradually and can affect one or both eyes.2
Ageing is by far the most common risk factor for cataracts, accounting for 75% of cases. Other major cataract risk factors include short-sightedness (especially higher degrees of myopia), diabetes and prolonged use of steroids. 3
| TABLE 1: MOST COMMON CAUSES OF CATARACT | |
| AGE-RELATED | PROLONGED USE OF STEROIDS |
| DIABETES | SMOKING |
| FAMILY HISTORY OF CATARACTS | STATIN MEDICINES FOR CHOLESTEROL? |
| BEING VERY SHORT-SIGHTED | PREVIOUS EYE SURGERY |
| LIFELONG EXPOSURE TO SUNLIGHT AND UV | HORMONE REPLACEMENT THERAPY (HRT)? |
| HIGH BLOOD PRESSURE |
A recent analysis in the journal Nature (2020) showed the pooled prevalence estimate of any cataract found in the world’s population at 17.2%.
This was broken down into the types of cataract: nuclear cataract in 8.22%; cortical cataract in 8.05%; and posterior subcapsular (PSC) cataract in 2.24%.4
The essential cause of cataracts, while not completely understood, appears to be the clumping of proteins, with an unequal distribution of protein and water within the lens. 5
Such clumping will lead to areas of opacity within the body of the lens, which can expand over time with increasing age.
This clouding of the lens along with double vision and glare from bright light are common cataract symptoms, which are easily spotted by an eye doctor(ophthalmologist).
As lens clouding extends it will increasingly affect the central line of sight and vision will steadily decline.
Initially, this will be a partial reduction but advanced cataracts will cause blindness.
Most cataracts are produced by natural changes within your eyes as you get older. However, there are many conditions and factors that are associated with the development of cataracts. 6
Age‐related cataracts are essentially a result of ongoing chemical attacks on the lens through a process called oxidation (the same process that turns iron into rust).
Despite the lens’ natural defence mechanisms against oxidation, the accumulated damage eventually results in the nucleus or cortex developing cataracts in the later decades of life. 7
The good news is that whatever the cause of the cataract, the treatment is the same and highly effective – cataract extraction with an intraocular lens implant.
Modern cataract surgery is a safe procedure with a reliable restoration of vision.
More recent developments in intraocular lenses (IOLs) mean that patients are often also able to read once again without glasses after having their cataracts removed, as well as having clear distance vision.
Who gets cataracts?
There are many research studies that have investigated the various factors involved in cataract formation. As well as the most common causes listed above and in Table 1, we can see several groups who have a higher incidence of cataract.
Gender and cataracts
Women develop cataracts more often than men and this holds true across most types of cataract. This may be because of a drop in female sex hormones (oestrogen) after menopause.
Experimental research using rats that treatment with female sex hormones provided a 25% protective benefit.8
Race and ethnicity
Data from the United States found that Hispanics and African-Americans appeared to have double the chance of getting cataracts compared to the white population.
It was postulated to be because of a greater prevalence of other conditions, notably diabetes. Not having access to cataract-removal surgery could also play a significant role.9
A cortical cataract is the most common form of lens opacity in population studies of black persons,3 4 a group for whom cataract represents a particularly significant visual burden.5 6
Eye conditions linked with cataracts
Some eye conditions are known to be associated with an increased chance of developing cataracts. The most common conditions are:
Glaucoma – Filtering eye surgery to treat glaucoma (typically with a raised pressure inside the eye that leads to vision loss through damage to nerves at the back of the eye).
Inflammatory conditions of the eye – these can all lead to a risk of developing cataracts, both from the inflammation itself as well as the steroids used to treat the condition.
Among the most common are iritis (inflammation of the iris, also known as anterior uveitis), uveitis linked with autoimmune diseases, Fuch’s cyclitis and Still’s disease.
Myopia – short-sighted people have a significantly increased chance of getting cataracts.
1. Age-related cataracts
After the age of 40, proteins within the lens of your eye begin to break down. This is a natural process and most cataracts develop slowly over time because of these normal eye changes.
Ageing is a cause of cataract-related blindness on a global scale involving both genetic and environmental influences.10
It should be noted babies can be born with congenital cataracts as well although this is rare and the most common form of cataracts are due to the natural ageing process.
Prevalence of cataracts and cataract surgery
Cataract extraction is the most common surgical procedure carried out in the NHS. Approximately 400,000 cataract operations were performed in 2018. As the population ages, demand will only increase.
The Royal College of Ophthalmologists estimated growth of greater than 50% in cataract removal procedures over two decades between 2015 and 2035. There will be a 25% increase over the 10 years up to 2025.11
Cataracts are prevalent around the world, with a burden of sight loss within the developing world. Vision loss from cataract is, however, also found in the UK.
Population studies in the UK showed that just over one-third of people (35%) over the age of 65 had visually significant cataract changes (vision worse than the driving standard of 6/12) or had already undergone cataract surgery.
Furthermore, the severest cases of vision loss with documented legal blindness (less than 6/60 on the letter test chart) were not just restricted to poorer countries.
UK national data from as recently as 1999 highlighted that just over one in seven patients were legally blind from cataracts on the date they were put down to have cataract surgery.12
Mechanism of lens opacity in cataracts
Age-related cataracts are usually characterized by the scattering of light rays as they pass through the lens. Light scattering and opacity are the results of the buildup of high molecular weight proteins (HMW – typically measuring 1000 or more angstroms in size, for the scientists out there).
The build of large proteins can also disrupt the short-range ordered packing of lens crystallins (water-soluble proteins which make up 90% of the lens total proteins), which is critical for maintaining lens transparency.13
There is a common underlying mechanism in the development of cortical and nuclear cataract that is caused by the failure to control cell volume in the cortex part of the lens or to deliver antioxidants14 such as the potent protein glutathione (found in the cells of animals, plants and fungi 15) to the lens nucleus.
There is a great deal of research investigating the role of antioxidants in the formation of cataracts, and how they could be used as a treatment to prevent or delay the onset of cataracts, including dietary changes.16
How common are cataracts by age groups?
The North London Eye Study showed the prevalence of cataract steadily increases with age. A cataract that impaired vision was found in:
- 16% of people aged 65 to 69 years
- 24% of people aged 70 to 74 years
- 42% of people aged 75 to 79 years
- 59% of people aged 80 to 84 years
- 71% of people aged 85+
In the UK, age-related cataracts affect around one in five of those aged 70. This rises sharply to affect half of all those aged 80; increasing further, just over 70% of those aged 85 or older will have cataracts.17
Dysfunctional lens syndrome and ageing changes
A progressive loss of transparency with age occurs along with a steep fall in accommodation (the ability of the lens to change its shape and focus while reading).
This failing accommodation is the basis of presbyopia – the need to wear glasses for reading in middle-age – which is apparent in most people by the age of 50. 18
Dysfunctional lens syndrome (DLS) is a name that has been coined to describe these natural ageing processes in the crystalline lens that affect both transparency and the ability of the lens to accommodate. 19
They are all part of the same continuum of ageing changes.
It begins with losing the ability to see up close, e.g. reading clear print, then leads on to a reduced quality of the image you see and optical aberrations, also increased light scattering as rays pass through the lens, and ultimately ends with the formation of cataracts which develop slowly over time.20
Dysfunctional lens syndrome (DLS) describes the natural changes in the lens; it has been useful in educating patients about these changes and discussing potential treatments.
The ageing of the lens from presbyopia to cataracts is covered in a single term (DLS) which includes three overlapping stages:21
- Stage 1 has been proposed as between 42 to 50 years old presents as presbyopia when accommodation has been lost but image quality remains good, with a scattering of light by the lens relatively limited.
- Stage 2 (50 to 65 years old) where light scatters increase, presbyopia worsens and long-sightedness and/or astigmatism may alter because of the changes in the lens. Distance vision often is not as good as it was.
- Stage 3 (65 or older) when the ocular scattering of light has increased, classical changes of cataract may be seen and a lens replacement-based procedure may be the treatment of choice.
The treatment of lens-based problems including advanced presbyopia (Stage 2 DLS) and cataractous changes in the lens (Stage 3 DLS) is the same – surgery to remove the clouded lens and replacement with an intraocular lens implant.
If this occurs during stage 2, it is usually called Refractive Lens Exchange, or RLE for short. Other terms for the same procedure are clear lens exchange and lens replacement surgery.
If there is a noticeable cataract visible on the eye exam, the procedure is called cataract surgery. The steps performed in both operations are the same.
2. Use of steroids and cataract
Long-term use of steroids is a known cause of cataracts. Steroids are man-made and a prescribable variety of hormones normally made by the adrenal glands (small glands which live just above each of your kidneys).
Steroids come in many different forms. The main types are:
- Tablets (can also come as syrups and liquids): e.g prednisolone, hydrocortisone
- Nasal sprays, inhalers: e.g. fluticasone, beclometasone
- Injections (intramuscular, joints, veins): e.g. triamcinolone, methylprednisolone
- Skin creams: e.g. hydrocortisone, betamethasone
The majority of steroids (or sometimes corticosteroids) are only available from your doctor with a prescription, but some weaker steroids can be purchased over the counter, such as hydrocortisone skin cream.
Oral medications include prednisolone (patented in 1954 and approved for use in 1955 in the United States) and hydrocortisone (patented in 1936 and approved for use as a medication in 1941).22
Prednisolone (or prednisone in the US) is very widely used; as of 2018 it was the 20th most frequently prescribed drug in the United States being prescribed more than 25 million times.23
These medications are potent anti-inflammatory agents and are used in a wide variety of conditions where inflammation is a feature.
This includes chronic forms of arthritis, asthma, chronic airways diseases, lupus, psoriasis, immunosuppression after organ transplantation, asthma and inflammatory bowel diseases, such as colitis or Crohn’s disease.
Posterior subcapsular cataracts with steroids
The type of cataract associated with steroids is known as posterior subcapsular, where the opacity is a thin sheet towards the very back of the lens.24
It usually affects both eyes and is distinct from the more common nuclear or cortical cataracts.
Steroids may have a more exaggerated effect in children, with a more rapid rate of posterior cataract formation.25
The mechanisms behind the creation of opacification are not fully known, although several possible mechanisms have been put forward.26
Discovering the steroid connection to cataracts
The connection between the usage of steroids and the development of cataract was first noted by RL Black and colleagues back in 1960.
In a research study of 44 people suffering from rheumatoid arthritis, they noted that 39% of patients who had been taking systemic steroid treatment developed posterior subcapsular cataract (PSC). 27
Black and team also observed a dose-dependent effect where patients who took a higher dosage of steroids were more likely to have PSC cataracts.
Not only oral steroids and cataracts
The formation of cataracts was found to be not limited to only oral steroid medications. In 1963, Valerio and colleagues discovered that topically applied steroids could also induce posterior subcapsular cataracts.
Ophthalmologists frequently prescribe steroid eye drops for inflammatory conditions of the eyes.
Treatment of steroid-related cataracts
Despite not knowing the exact mechanism in cataract formation from the use of steroid medications, the treatment choice is always extraction of the cataract and replacement of the lens with an intraocular implant.
The posterior subcapsular steroid cataract is removed using phacoemulsification in a standard manner.
The procedure is no different here than when treating nuclear or cortical cataracts.
3. Diabetes and cataracts
Diabetes, also more fully known as diabetes mellitus, is a leading cause of visual reduction and blindness. Diabetes causes two major problems in the eyes, both of which can lead to loss of vision: cataracts and retinopathy (disease of the retinal blood vessels).
The longer that a patient has diabetes, the more likely they are to suffer from complications in the eyes.28
Prevalence of diabetes and cataracts
Around the world, more than 285 million people suffer from diabetes mellitus.29
According to the International Diabetes Federation, this will nearly double by the year 2030 to around 440 million diabetics.30
Cataracts occur two to five times more frequently in patients who suffer from diabetes.31 Poor control of diabetes is known to be a significant predictor of the development of cataracts.32
Numerous research studies have demonstrated that cataracts occur more often and at a younger age in diabetics.33 It is believed that up to 20% of all cataract removal procedures are carried out in diabetic patients.34
Diabetes in childhood
Insulin-dependent diabetes can come on during childhood. Diabetic disease in the retina is very rare before puberty but unfortunately is found in 70-90% of adolescents who have had diabetes for longer than 10 years.
The most severe form of diabetic retinopathy, known as proliferative retinopathy, also occurs in adolescents where it can cause blindness.35
Types of diabetic cataract
Diabetic cataracts usually have opacities in the cortical or posterior subcapsular types. If the cataract comes on in adulthood, a nuclear lens opacity can also be found.
Cataract surgery in diabetes
Cataract surgery is performed around two decades earlier in patients with type-1 diabetes, compared to patients without diabetes.36 Danish patient registry data showed that 1 in 5 patients (21%) with type-1 diabetes had cataracts removed during 25 years of clinical follow-up.
The average age at the time of surgery was 59 years old.37
Surgical removal of the cataract is the only way to provide a cure and restore vision lost from lens opacity. However, cataract extraction can bring about problems in diabetic patients.
Several research papers have stated that surgery for cataracts arising in patients with diabetes can lead to a relatively rapid worsening of diabetic retinopathy.38
Cataract surgery can also bring about a haemorrhage within the vitreous gel at the back of the eye or induce blood vessel growth on the iris. All this can eventually cause a decrease in sight or indeed complete loss of vision.
Even routine cataract surgery without apparent complications can lead to elevated levels of inflammatory molecules in the eye.
These so-called cytokines can bring on a subclinical or clinical deterioration in diabetic retinopathy, including disease at the macula (the critical central area of the retina responsible for detailed vision).39
4. Smoking and cataracts
According to the World Health Organisation:40
- Tobacco kills up to half of its users.
- Tobacco kills more than 8 million people each year. More than 7 million of those deaths are the result of direct tobacco use while around 1.2 million are the result of non-smokers being exposed to second-hand smoke.
Smoking is consistently amongst the most reliable risk factors for nuclear sclerotic cataracts in research studies on causation published across the globe. 41 42 43
There has not been much evidence that it causes the cortical type of opacities, although it can be linked with posterior subcapsular cataracts.44
Q: How much does smoking increase your risk of developing cataracts?
A: Smokers getting through one pack of cigarettes a day more than double their chance of cataract.45 Another study showed a three-fold increase in risk.46
Smoking most commonly induces the formation of nuclear (nuclear sclerotic) cataracts. The risk has been found to be dose-related. 47
It has been described as an early age-related cataract.
Aromatic compounds present in the inhaled smoke appear to damage parts of the lens through oxidation.48
Smoking and eye diseases
Whilst we know that smoking directly increases your risk of getting cataracts, this is certainly not its most damaging effect on the eyes: this title goes to age‐related macular degeneration (AMD).
Macular degeneration is a blinding disease of the vital central area of the retina that is untreatable in its most common dry form.
Compared to those who have never taken a cigarette, smokers have a two to three and a half times greater chance of suffering from AMD.49 50
Smoking increases your chance of developing or worsening several eye disorders with some known multiples of risk compared to non-smokers:
- Age-related macular degeneration: 2 – 3.5 times increased risk
- Cataract: two to three-fold elevated risk 51 52
- Diabetic retinopathy (potentially blinding disease of the retina)
- Thyroid eye disease (bulging eyes, dry eyes): four times increased risk
- Acute ischaemic optic neuropathy AION: 16 times increased risk.53
Quitting smoking and cataracts
What if you were to give up smoking – would that help to reduce your risk of lens opacities or needing early cataract surgery?
Fortunately, the risk can be decreased by giving up. Research demonstrated that stopping smoking considerably drops the risk of getting cataracts. 54
A Danish study in 2014 found that if you smoke more than 15 cigarettes a day then your likelihood of needing cataract surgery increased by 42% compared to those who have never smoked.
After quitting smoking for 20 years, the chance of needing cataract surgery did fall by half though it was still 21% above those who never used cigarettes.55
So quitting smoking does reduce your likelihood to get earlier cataracts, though it appears that the elevated risk persists for decades.
What can you do to prevent or delay cataracts?
Due to the prevalence of cataracts across the world and an increasing healthcare burden from an ageing population, prevention of cataracts is a very desirable goal.
Cataracts are a consistent public health issue due to a shortage of surgical facilities and surgeons, especially in developing countries.
However, the sheer number of cases also poses problems for developed nations.The Royal College of Ophthalmologists in the UK identified a major shortage of surgeons to perform future cataract surgeries.56
How can we help to delay the need for cataract surgery? Different types of cataracts can be caused by several lifestyle factors, including smoking as described above, have been identified that are known to accelerate the formation of cataracts.
Avoiding risk-increasing behaviours can help maintain better clarity of vision and delay the need for cataract extraction.57
The following guidance summarises what we know today are the best approaches that can help you prevent getting cataracts: 58 59
- Avoiding excessive exposure to ultraviolet light
- Not smoking
- Maintain a healthy weight
- Avoid excessive alcohol consumption
- Eat a diet with balanced nutrition full of fruits and vegetables
- Sensible use of antioxidants
5. Family history of cataracts
Most people will develop cataracts with age, with 70% affected by the age of 85 or older. 60
However, some people reach a very advanced age without suffering from significant lens opacity.
This has led several research investigators to look at the genetic basis of age-related cataract, as perhaps this could lead to innovative treatments as an alternative to surgery.
A number of research studies using population-based data indicate that cataracts do indeed cluster within family groups.61 62 63 64
Teasing out the genetic basis is complicated because if an association is found, it may be because of a shared environment as well as having genes in common.
Studies using twins may be able to shed light on the long-standing nurture versus nature argument. Twin studies have repeatedly indicated that there is a genetic basis for cataracts that develop with age.65 66
Evidence from the Salisbury Eye Evaluation (SEE) study carried out in Maryland, US showed that if you had a sibling who already had been diagnosed with a cataract, then your chance of getting a cataract was doubled.
Having twice the risk was still present even after other factors were taken into consideration, including risk factors from the environment and various personal factors.67
The factors which affected the outcome were:
- age
- gender
- race
- smoking
What does this mean for the heritability of a nuclear cataract as a percentage?
These data indicate that 36% of the variability in the severity of the cataract can be laid at the door of genetics.
A similar level of contribution of genes to the degree of getting cataract was found in the Beaver Dam Eye Study (35%), accounted for by a single gene. 68
Twin studies showed a somewhat higher contribution of 48% when looking at nuclear cataracts.69
The SEE results confirm previous findings using population-based studies70 71 and research using twins.72
Genetics appears to play a major role in the nuclear type of cataract, which is the most frequent form of lens opacity in European groups. 73
It is also the form of cataract most commonly needed to be surgically removed.74
6. Statin medicines for cholesterol and risk of cataracts
One of the most widely taken groups of medicines around the world is statins. 75
There has been quite a bit of discussion around whether or not statins contribute to the risk of developing cataracts.
In the UK, the statins currently available at the time of writing are:
- simvastatin
- atorvastatin
- pravastatin
- fluvastatin
- rosuvastatin
Statins are routinely prescribed by doctors to treat elevated fat (lipid) levels in the blood, for which the medical term is hyperlipidemia.
They decrease the chance of suffering from cardiovascular diseases such as heart attack, stroke and poor circulation by inhibiting a specific enzyme. (For the scientifically curious, it is called 3‐hydroxy‐3‐methyl‐glutaryl‐CoA reductase).76
In the United States, a remarkable one in four people over the age of 40 uses a statin to reduce their risk of suffering from cardiovascular disease. 77 78
Examples of cardiovascular diseases potentially treated by statins include:
- Heart attack (myocardial infarction, MI)
- Stroke (cerebrovascular accident, CVA)
- Irregular heart rhythms
- Narrowing of the arteries of the heart
- Heart failure
- Weakness of the muscle of the heart (cardiomyopathy)
- Blood vessel disease
Statins can reduce the levels of so-called bad cholesterol (LDL) by up to 60%. These drugs are now cheap and hence affordable for many people.
It is clear that statins can effectively decrease the risk of death from cardiovascular disease.
However, with such large numbers of patients using statins, there has been increasing awareness of adverse side-effects, including analysis of the risk of developing cataracts.79
Although statin medications are overall tolerated well by patients, serious (albeit rare) adverse events have been documented.
These side-effects include muscle, liver and nerve damage, diabetes and kidney failure.80
That statins could cause cataracts was put forward after researchers found that dogs developed this eye condition in accordance with the dosage they were given. 81
The greater the dose, the more likely the chance of getting a cataract.
When it comes to humans, the link between taking statins and developing cataracts has been looked at in some depth, although the studies have not to date provided a clear answer.
Some clinical studies actually found there was a lower probability of developing cataracts when taking statins.82 83 However, other researchers found no difference. 84 85
Still, others uncovered an elevated likelihood of cataracts from these lipid-lowering agents. 86 87
What should we take away from all this research on statins and the risk of developing cataracts?
Based upon the combination of studies, it would appear safe to conclude that there is as yet no definitive evidence indicating that taking statins might increase the risk of you getting cataracts.
The most probable outcome is that there is no link between the use of statins and the formation of cataracts.
However, the benefits of statins in terms of reducing LDL cholesterol are clear for patients with cardiovascular problems or risks of vascular disease.
There are potential downsides from the use of statins but it appears that developing cataracts is not one of them.88
If you are taking statins, or are considering taking them, you should do so without worrying about getting cataracts.
7. Myopia (short-sightedness) causing cataracts
Short-sightedness, also known as myopia, is a very common focusing (refractive) error in the eyes. Myopic people have blurred distance vision, although close vision typically remains clear.
However, with severe short-sight, the object has to be held so close to the eyes that even near vision is not functional.
In countries with developed healthcare, myopia is usually corrected in three ways:
- Glasses
- Contact lenses
- Laser eye surgery
High and extreme myopia is now correctable with Implantable Contact Lenses (ICL).
Orthokeratology (Ortho-K) is also used, where contact lenses are worn through the night to flatten the cornea, although with an increased risk of corneal infections.
In poorer countries, one or more of these methods may not be available.
In fact, uncorrected refractive error is one of the most common causes of blurred vision in the world.
Eye problems linked with short-sightedness
Myopia is much more than just needing glasses, contact lenses or laser eye surgery.
The changes found in the anatomy of a short-sighted eye mean the patient is at greater risk of a range of eye problems, some of which are potentially blinding. These include:
- Retinal tears & detachment
- Squinting (the eyes can look in different directions)
- Cataract
- Glaucoma
- Maculopathy
- Lazy eye (amblyopia)
Short-sight and risk of getting cataracts
Short-sightedness people have an elevated risk of getting cataracts.
An analysis of seven cross-sectional studies and a case-control study showed that myopic people were three times as likely to get nuclear cataracts, and twice as likely to get posterior subcapsular cataracts.
No link with the cortical cataract was found.89
Another research study from Salisbury, in the Eastern United States, showed the risk of cataract was linked to the severity of the short-sightedness.
For nuclear cataracts, those with mild myopia were just over twice as likely to get cataracts as those with normal vision without glasses.
Higher myopia had a greater risk of cataracts, with three to four and a half times the risk.
For posterior subcapsular cataracts, the study demonstrated the increased risk of cataracts was as follows:90
Table 2: The risk of cataracts with short-sightedness
| Degree of Myopia | Increased Risk of Cataracts |
| Mild myopia (low prescriptions up to -2.00D) | 1.6 times elevated risk |
| Lower Moderate myopia (-2.00 to -5.00 dioptres) | 3.2 times elevated risk |
| Higher moderate myopia (-4.00 to -6.00 dioptres) | 5.4 times elevated risk |
| High myopia (wearing glasses of -6.00 or greater) | 12.3 times elevated risk |
For high myopia of -6.00 dioptres or worse, patients were more than 12 times as likely to get posterior subcapsular cataracts.
Once again, no link was found between short-sight and cortical lens opacity.91
9. Cataracts and exposure to sunlight & ultraviolet (UV)
A cataract is an opacity of the lens inside the eye that reduces vision. It is the #1 cause of blindness in the world.92 93
The most common type of cataract comes from ageing and typically comes on after the age of 50.94
Reports that have linked prolonged exposure to the sun’s UV radiation to the development of cataracts have been written about since the 1800s.95
Several early studies of the distribution and pattern of cataract occurrence suggested an association between cataract and exposure to UV from sunlight.
A further study in the year 2000 showed that UVB (a higher energy form of ultraviolet than UVA – see below) was the most important personal and avoidable factor for the risk of getting a cortical cataract.96
Radiation and oxidative damage
Radiation coming from the sun that affects the eyes includes wavelengths from the:
- Ultraviolet spectrum (100-400 nanometres, nm)
- Light visible to the human eye (from 400 up to 760 nm)
- Infrared radiation (760 up to 10,000 nm)
Ultraviolet (UV) rays represent the part of the sun’s wavelengths hitting Earth that have the highest energy – they can be a major cause of oxidative damage to living tissues.97
Radiation with shorter wavelengths (ultraviolet and blue light) carry the most energy and can potentially do the most oxidative harm.
When body tissues absorb radiation from the sun, a photochemical reaction can occur leading to damage of cells, including DNA, proteins and fats (lipids).
One main mechanism is the energy absorbed creates so-called reactive oxygen species causing oxidative damage.98
Types of UV radiation
Ultraviolet radiation can be divided into three bands based upon the wavelength and energy of the UV rays:99
- UVA rays (315-400 nm, the longest wavelength and weakest energy – hard UV)
- UVB rays (280-315 nm, more damaging than UVA)
- UVC rays (100-280 nm, shortest wavelength and most damaging – soft UV)
The shorter wavelengths of UV are the most harmful because they have higher energy. The Earth’s atmosphere luckily does a very effective job at scattering or absorbing virtually 100% of UVC.
Although all UVC and most o the UVB is in fact absorbed by ozone, water vapour, oxygen and carbon dioxide gases, significant amounts of UVB and most UVA will still reach us at the planet’s surface.
UV radiation that does reach the eye can still be absorbed. The various components of the eye have different degrees of filtering and absorption of UV.
For shorter wavelengths, even though filtered by atmospheric gases, the cornea would absorb 100% of UVC. For medium and soft UV, around 90% of UVB, and 60% of UVA rays will also be absorbed by the cornea.
Most of the lower energy UVA that is not removed by the cornea is absorbed by the lens – only a small component of UVA radiation ultimately reaches the light-sensitive retina.
When it comes to visible light, the highest energy part is in the violet and blue range, known as high energy visible (HEV) wavelengths.
The cornea and lens do not provide much filtration of HEV rays so most of these wavelengths will arrive at and be absorbed by the retina and dark cells of the retinal pigment epithelium (RPE).
The damage to the eye from the energy of HEV and UV radiation comes from direct irradiation, oxidation, and thermal injury.
UV radiation and damage to the human eye
Human eyes are necessarily exposed to the sun’s rays and indoor lighting continuously during waking hours.
The passage of light into the eye is essential to its role in seeing our surroundings or objects of interest, as well as regulating the body’s circadian rhythm.
We need this allowed energy coming into the eye to do the least amount of damage possible.
Sunlight with its UV radiation can both improve and damage the health of human beings. This leads to a discussion of just how much UV and sun exposure is the right amount for optimal health.
It is now well known that UV can lead to negative effects, such as skin ageing, skin cancers including melanomas and squamous carcinomas, sunburn and cataracts.
On the other hand, being in the sun leads to vitamin D production important for healthy bones and likely other body processes.
Additionally, two proteins called serotonin and melatonin are influenced by the sun’s rays. Serotonin has multiple actions, including a feeling of well-being.
Melatonin is important in getting restful sleep and regulating the body’s circadian rhythms.100
It has become clear that ultraviolet can damage the eye and potentially causes cataracts to develop more rapidly. Factors that affect the degree of damage include:
- the wavelength of the radiation
- the intensity of the energy
- duration of the exposure
- the cumulative effect
- the angle that the radiation makes with the surface of the eye
- how high the sun is in the sky
- any ground reflection, e.g. from water or snow; altitude (higher is worse for exposure)
- the anatomy of the eyebrow and eyelids (deep-set eyes offer greater protection)
Due to these various factors, the need for protection from UV becomes essential for anyone who spends many hours in the sun.
This is exacerbated for activities where there is a high reflection, such as from snow or the surface of the water.
Acute and chronic UV eye damage
We know that acute high doses of exposure to UV rays lead to short-term damage to the surface of the eye, in effect a ‘sunburn’ of the eyes.
When it damages the cornea it is known as photokeratitis while UV acute damage to the conjunctiva is called photoconjunctivitis.101
Examples of damage include ‘arc eye’ where radiation from an electric arc weld burns the cornea if proper face shielding is not worn, and ‘snow blindness’ where the sun’s UV, both directly and reflected off snow on the ground, causes an overdose of UV energy hitting the eyes.
A good recovery can be expected in 1-3 days once exposure has ceased.102
However, even in doses that are low, over prolonged time exposure to UV radiation increases the risk of developing:
- Cataract
- Pterygium
- Squamous cell carcinoma (cornea and conjunctiva)
Many research analyses have shown that ultraviolet (UV) radiation is a risk factor for the formation of cataract. 103 104
Perhaps not surprisingly, studies looking at exposure to UV over time also reveal an increased risk for cataract extraction.105 106
Those with a history of cataract were 19% more likely to have a history of skin cancer, with UV radiation implicated in both conditions.107
However, the mechanisms of cataract formation from UV radiation are not fully understood. More research will be needed into the contributions of UVA and UVB over time.
Protecting ourselves against UV and cataract
Worldwide, there has been increasing government awareness over recent decades about the need for educating the public about protection from the sun.
Damage to the ozone layer and much greater incidence of various skin cancers has fed into the need to avoid excessive sunlight and UV radiation
There is also a major concern regarding deficiencies in vitamin D levels in many people.
Sunlight leads to the production of vitamin D within the skin following exposure to UV rays.
So the dilemma is now how much UV is good for us? Too little leads to vitamin D deficiency, and too much leads to disorders of the skin and eye.
UV-induced eye problems are widespread, potentially disabling, and represent a major burden of disease across countries of the world.
It becomes very difficult to establish a simple message with a large number of variables when it comes to UV exposure:
- Skin colour and type
- Age
- Weight and obesity
- Clothing and climate
- Medications
To complicate the matter further, the multiple suggested health benefits that may be possible from sufficient levels of vitamin D still need greater evidence.108
What we do believe to be clear is that eliminating UV radiation reaching the eye can have a major benefit in preventing early-onset cataracts and reduce the likelihood of UV-linked retinal disease.
Sunglasses and contact lenses
The simplest form of intervention, already very widely available, is to wear sunglasses that are designed to filter out UV light – anything with a wavelength of 400nm or shorter.
This will often be easily marked and labelled as “400” on the glasses themselves.
Having a wraparound design is necessary to block reflected rays coming off the ground (e.g. white concrete), water, sand or snow.
The amount reflected will vary according to the surface, often to a surprising degree:109
- Up to 90% of UV is reflected off snow
- Up to 30% of UV is reflected off water
- Up to 25% of UV is reflected off sand
Even at 50cm below the water’s surface, the strength of the UV radiation is still 40% when compared to being out of the water.
A common myth is that the greatest risk of UV exposure is at midday because this is the time when the sun is at its highest point in the sky.
While this is true for getting a sunburn on your skin, the eyes are recessed and shadowed by your eyebrows.
The greater time of danger for the eyes is at dusk and dawn when the lower sun can be seen directly by the eyes.110
Additional UV eye protection may be provided by contact lenses that avoid leakage around the edge of sunglasses and also absorb significant amounts of UV rays.111
It is not only UV radiation that can put our eyes at risk.
The first part of visible light within the spectrum that we can see is short-wavelength blue light (from 400 to 440 nm wavelength) – this is a known risk factor for damage to the retina at the rear part of the eye.
This so-called high-energy visible (HEV) light is not required for us to see; it is not required as part of the mechanism within the retina for control of our body’s circadian rhythms.
Removing HEV light from those at middle-age and beyond can reduce the chance of suffering from macular degeneration; this can be achieved with sunglasses, contact lenses or artificial lens implants for those who have had lens replacement or cataract surgery.
11. High blood pressure and cataracts
Cataracts are a leading cause of poor vision among the elderly worldwide. 112
In fact, according to statistics produced by the World Health Organization (WHO), cataract accounts for almost half of all blindness around the globe. 113
With populations in many countries ageing quickly, we know that cataracts will become increasingly prevalent. 114
A great many research studies have been published, looking at how cataracts arise and what risk factors contribute to the formation of lens opacities.115 116 117 118 119 120 121
Being able to identify and target risk factors for lens opacity development could have major health, societal and economic benefits in the coming years.
Numerous analyses have found that elevated pressure in the arteries (high blood pressure – hypertension) contributes significantly to cataract development, while some other studies found no association.
To help answer the question, a recent meta-analysis looked at 25 different research studies (9 cohort papers, 5 case-control papers and 11 cross-sectional studies) taken from 23 articles, which summarised that hypertension in the arteries increases the risk of getting a cataract, especially the posterior subcapsular subtype. 122
Patients with high blood pressure were up to 28% more likely to develop cataracts, most commonly posterior subcapsular opacities.
No association of hypertension with the risk of nuclear cataracts was found.123
This study found that the link between high blood pressure and cataracts was independent from diabetes, raised blood lipids and obesity.
Furthermore, patients with more severe high blood pressure have a greater chance of getting cataracts when compared to those with mild hypertension.124
This finding was backed up by additional research demonstrating a direct correlation between the severity of hypertension and the risk of developing cataracts.125 126
How long you have suffered from high blood pressure also plays a role – the longer you have been hypertensive, the greater your chance of getting lens opacities.127
Achieving better control of hypertension would help reduce the burden of cardiovascular disease, including heart attacks and strokes, and would also help to reduce the prevalence of cataracts and associated costs of cataract surgery.128
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