What is a Cataract?
Normal eye.
Eye with cloudy lens (cataract).
A cataract is a clouding of part of the eye called the lens, sufficient to reduce vision. Vision becomes blurred or dim because light cannot pass through the clouded lens to the back of the eye.
The lens
In a normal eye, the lens is clear, lying behind the iris (the coloured part of the eye) allowing the passage of light rays through it. The lens also contributes to the bending of light rays onto the retina (the light sensitive part at the back of the eye).
The lens can also change its shape to focus objects at different distances. If images are not focused onto the retina properly then vision is affected. Thus, if the lens is cloudy as in cataracts, light cannot pass through it.
Consequently, light images do not reach the retina, thus impairing vision. The first image below illustrates light rays passing through the lens and onto the retina of a normal eye, the second shows a cloudy lens (cataract) which is blocking light from reaching the retina.
Example of vision with a cataract.
How can cataracts affect my vision?
An opacity in the lens of the eye...
- causes a gradual painless loss of vision.
You may notice that some things seem blurred round the edges, or that your glasses seem dirty or scratched. - causes glare, particularly in bright light
You may find that bright light or very sunny days make it more difficult to see. - may cause haloes to be seen
- may alter the degree of long or short-sightedness
- may cause double vision
The cloudiness in the lens may occur in more than one place, so that the light rays reaching the retina are split, causing a double image. - may alter colour vision.
As the cataract develops its centre becomes more and more yellow, giving everything you see a yellowish tinge.
Patients may have difficulty in recognising faces, reading or achieving the standard of vision required for driving. Both eyes can be affected simultaneously, but usually one eye worsens quicker than the other.
How does the doctor know that I have a cataract?
Using a special instrument to look at the eye called an ophthalmoscope, opacities, if present, can be seen. Similarly, the exact location of a cataract within the lens itself can be determined using another special piece of equipment known as a slit-lamp.
What causes cataracts?
Most cataracts develop slowly as a result of ageing. 65% of people aged 50 to 59 have lens opacities and all of those aged over 80 years.
The formation of a cataract occurs more rapidly in patients with a history of eye injury, inflammation of the eye, or diabetes - clinical studies strongly suggest that lens opacities are positively related to poorly controlled and unrecognised diabetes. Certain drugs, particularly steroid tablets and eye drops can increase the chance of cataract formation; however, there are still many individuals who do not develop cataracts following prolonged steroid administration. Similarly, certain occupations impose a greater risk for the development of cataracts including those who have worked with x-rays, microwaves, explosives, dyes and pesticides. Recent studies have also shown that smoking increases the chance of opacities developing in the lens.