What is a Cataract?

This section provides more information about the nature of cataract. It expands on the brief explanation given on the previous page. It includes information about:

• Types of cataract
• Causes of cataract
• Prevention of cataract

To Recap:

The eye is rather like a camera. Inside a camera is a lens. Similarly there is a lens within the eye. This lens sits behind the pupil and is about the size and shape of a smartie sweet, i.e. like a little discus. In young children the lens of the eye is crystal clear. As we age the lens gradually becomes less clear and yellows. In the elderly the lens has a brown hue. When this clouding of the lens starts to affect and compromise vision the patient is then said to have a cataract.

Types of Cataract

The clouding or haziness of the lens of the eye may take different forms. For example the opacity may affect the whole lens or only part of it. Eye doctors therefore describe different types of cataract. Sometimes there may be a mixture of more than one form of lens opacity.

Nuclear sclerotic cataract

In this form of cataract the lens becomes yellow-brown and this discolouration is most dense within the core (or nucleus) of the lens. It is one of the most common forms of cataract in the UK and particularly affects the elderly. As it develops the eye may become more short sighted or less longsighted.

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Cortical cataract

In this type of cataract the opacity affects the outer softer layers of the lens. This may take the form of patches or spokes within the lens that seem to point towards the centre of the pupil. This type of cataract is also very common and often occurs along with nuclear sclerotic cataract.

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Posterior subcapsular cataract

Here the opacity is limited to an area just beneath the back surface of the lens in a plate like manner. The rest of the lens may be quite clear. It usually starts centrally on the line of vision and extends outwards. Because the opacity is close to the point where light rays entering the eye cross even small amounts of posterior subcapsular cataract may affect vision severely. This is particularly so under certain lighting conditions, e.g. bright sunshine, when the patient may experience disabling glare or dazzle. However under subdued indoor lighting the vision may be relatively unaffected. Although this form of cataract can occur for no particular reason it may be caused by long term use of systemic steroid medication or trauma to the eye.

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Anterior subcapsular cataract

This is similar to posterior subcapsular cataract but here the opacity is just beneath the anterior surface of the lens.

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Posterior polar cataract

This is a dot like opacity at or just beneath the centre of the back of the lens. The rest of the lens may be clear although further cataract can develop into the lens from this point over time. This form of cataract is a witness to the development of the eye in the womb. As the lens of the eye grows within the early embryo it is fed by a blood vessel that connects to rear of the lens. This blood vessel disappears before birth but may leave behind a posterior polar lens opacity.

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Anterior polar cataract

This is similar to posterior polar cataract but here the dot opacity is at or just beneath the centre of the front of the lens. In very early development the lens is in contact with the front surface of the eye and this form of cataract is a remnant of that. Polar cataracts are relatively uncommon. They will be present throughout life and may be first observed in childhood. If small they may not affect vision.

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Zonular or Lamellar cataract

The lens of the eye is made up of many layers pressed onto each other; rather like an onion. A zonular cataract is where one layer within the lens is hazy whilst the rest of the lens is clear. In some cases it is caused by a disturbance to the development of the lens at a certain point in time. The layers of the lens that formed before and after this disturbance are healthy and clear. In some cases this form of lens opacity runs in the family and is then caused by genetic factors. It is a rare form of cataract present from early childhood.

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Foetal cataract

This is an opacity of the very central of the lens, the part formed in the foetal stage of development within the womb. The rest of the surrounding lens may be clear. It is rare.

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Causes of Cataract

A cataract may also be classified according to its cause.

Age related cataract

The vast majority of cataracts in the UK are related to age. The older we get the more likely we are to develop cataract. The lens of the eye is mostly made from special proteins. The lens actually grows in size very slowly with age as new layers of protein are added. However because the lens is an isolated structure within the eye these proteins cannot be broken down and renewed. Slowly over time they are compacted and become denatured and start to loose their transparency. They change from being crystal clear to yellow to brown. This is similar to what happens to egg white when it's heated. Egg white is also mostly protein and nearly transparent. When it's cooked the protein is denatured (altered) and it becomes white and opaque.

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Diabetes

People with diabetes tend to develop age related cataract at an earlier age than those without diabetes. This statement is generally true although in an individual patient the development of cataract cannot be accurately predicted. Very rarely a person with diabetes may develop snowflake like opacities within the lens of the eye which occasionally can quickly worsen to a general white out of the lens. This is a true Diabetic Cataract.

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Steroid Induced Cataract

Cataract may be the result of a drug side effect. The most common example of this is cataract caused by corticosteroids. The characteristic form of cataract caused by steroid is a posterior subcapsular lens opacity (see above). Systemic steroid (e.g. steroid taken as tablets) is more likely to cause cataract than steroid eye drops or steroid by any other route of administration. The risk of developing a steroid induced cataract increases with the length of time the drug is taken for. As a general guide the risk is significant with a daily dose of 10mgs or more of Prednisolone taken for a year or more. There is probably no dose below which cataract will definitely not occur and susceptibility to steroid cataract probably varies between individuals.

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Traumatic Cataract

An injury to the eye if it is sufficiently violent can cause a cataract. This is especially likely if the eye and the lens have been punctured. This is called a perforating injury. If the thin coat of the lens, called the capsule, is breached then the fluid that surrounds the lens can seep into it. This will cause the lens to swell and will alter the molecular structure of the proteins that make up the lens. These proteins then change form being transparent to become white.

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Inherited Cataract

Sometimes cataract runs in the family and is due to a genetic abnormality. Such cataracts tend to occur earlier in life than the usual age related cataract. They may appear in childhood. These cataracts are often very similar in each of the affected individuals of the same family. The are relatively uncommon.

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Congenital Cataract

This is cataract that is present at birth. It may be due to a genetic abnormality (i.e. a form of inherited cataract) or it may be due to some disturbance to the development of the lens in the womb, e.g. an infection. One of the most well known examples of this is Rubella (German Measles). If the mother contracts Rubella for the first time early in pregnancy the virus may get into the eye of the developing baby and causes cataract and this may be severe. There is also a risk that the baby will be born deaf. It is therefore vital that girls are vaccinated against rubella.

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Metabolic Cataract

This is a cataract that is caused by an abnormality of the general metabolism of the body. These disorders are due to a genetic fault and may be inherited. The body may be unable to breakdown or process certain chemicals (often sugars) which then accumulate in the tissues. This chemical imbalance affects the nutrition of the lens which may then become cloudy. This is a rare cause of cataract.

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Radiation Cataract

If the eye and lens are exposed to ionising radiation (e.g. X-rays) a cataract may form. This is because such radiation has enough energy to damage and alter the proteins within the lens. It is extremely unlikely that sufficient amounts of such radiation will be encountered during normal living to cause a cataract. However if radiotherapy is needed to treat a malignant cancer in or near to the eye such a cataract can occur. For this reason wherever possible the lens is shielded during such treatments.

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Infrared Cataract

In the UK this is now a very rare cause of cataract. Infrared light is emitted by very hot objects. If enough infrared enters the eye it can denature the proteins of the lens by slightly cooking them. Again this will cause the lens to become cloudy. In the days before adequate eye protection this form of cataract was relatively common amongst steel furnace workers. There is a tale that it occurred in old ladies who spent their days sitting by and staring into their coal fires but whether that is true or not is debatable.

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Other causes

There are many other causes of cataract. It may develop following some other eye disease. It may be one feature of a systemic disease or syndrome. A "Syndrome" is an illness with several features spread around the body.

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Prevention of Cataract

The ultimate solution to cataract is to prevent it occurring in the first place. To achieve this it is essential to understand the factors that cause a cataract to develop. In the UK the vast majority of cataracts are age related and there is probably very little that can be done to influence their development. It is also very difficult to prove that any remedy has any definite effect in preventing cataract because studies need to be performed over long periods to give a reliable scientifically valid result. Some believe that certain dietary supplements may help but this is controversial. Not smoking will probably lessen the chance of cataract.

In some parts of the developing world cataract is more common and occurs at an earlier age. It has been proposed that this may be due to repeated severe diarrhoea. These illnesses dehydrate the lens of the eye (as well as the rest of the body) and this may denature the protein that makes up the lens of the eye causing it to become less clear. A safe water supply, better sanitation and improved general health may therefore help prevent cataract in these parts of the world.

It has been suggested that increased exposure to sunlight in the tropics may cause cataract but this theory does not stand up to vigorous scientific analysis.

A small number of cataracts are caused by a general illness. If these illnesses can be prevented then so can the cataract. The best example is Rubella (German Measles). If a woman contracts this illness during the early part of pregnancy then the baby may be born with cataracts (& deafness). Vaccination of girls against Rubella should prevent this form of cataract in their children.

Some rare cataracts are caused by an inherited biochemical abnormality. If this is treated, or in some cases if certain foods are avoided, then the cataract can be prevented. Patients with diabetes also suffer cataracts more than those without the condition. This is another reason to control diabetes well, and in the case of Type 2 diabetes, to avoid the life style that causes it.

Other Cataract Terminology

An immature cataract is one where the lens opacity is not fully developed. The patient can still see to some degree. A mature cataract is one where the lens is now completely opaque. It may be completely white or dense brown. The patient can see virtually nothing out of the eye and the doctor is not able to see through the cataract to examine the back of the eye. A hyper mature cataract is one where the cataract has become so advanced that the proteins that make up the lens are starting to break down and leak out of the lens. This can cause the eye to become inflamed (red & angry) and may cause the pressure within the eye to rise (i.e. the eye may develop a form of glaucoma). This is one rare situation where a cataract becomes directly harmful to the eye, rather than just obscuring vision. A Morgagnian cataract is one form of hyper mature cataract where the softer outer proteins of the lens break down into a soup and the harder brown nucleus of the lens sinks within this.

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