Where in the eye is a cataract located?

02 August 2018

Mr Jonathan Ross is a national expert in cataract surgery. Every year he performs over 1000 cataract operations on patients in Edinburgh and Glasgow. He is a specialist in performing cataract operations in difficult situations.

People often ask me what a cataract is and where in the eye it is located. I also hear many people describing cataracts as a cloudy film on the front of the eye. This is not quite accurate, so let’s talk about what a cataract is in more detail.

If you look at your eyes in a mirror you will not be able to see a cataract, even if it is quite severe. The only time when a cataract becomes visible is when it matures to the extent that it turns white. In these circumstances your pupil will turn from black to white, and you will be unable to see anything out of the affected eye. Other people will notice this and may advise you to seek advice from an eye specialist. Sometimes when a cataract develops in just one eye, the other eye can compensate so we do not notice the worsening cataract at first.

The front surface of your eye is extremely sensitive, but if it wasn’t and you were able to touch it, the first thing your finger would make contact with is the clear surface window of the eye, known as the cornea. This is a 1mm thick toughened structure, and does most of the focusing work of the eye to make sure you can see clearly. Behind the cornea is a 3mm deep pool of special fluid called the aqueous humour. This fluid circulates through the front of the eye and helps to pressurise the eyeball, and deliver nutrients to the cornea.

Going further back we come across the iris. This comes in a variety of colours, from green to blue to brown, and is the natural camera aperture of the eye. The iris changes shape to make the pupil bigger or smaller, depending on how much light we need to enter the eye in order to see well. Immediately behind the pupil we come to the lens, which is 4mm thick from front to back, and can change shape when we are young, in order to shift focus from distance to near. As the lens becomes cloudy with age we call this a cataract.

The front surface of the cataract should be thought of as lying in the same place as the coloured iris, occupying the space where the pupil is. The cataract also extends beyond the pupil and runs along the back of the iris until the equator of the cataract is reached. At the equator of the cataract, it is attached to the inner wall of the eye by several hundred guide wires called zonules. These look to the naked eye like strands of spider web, and are similarly tough and stretchy.

On reaching the back of the cataract, we are still only 8mm  from the front surface of the eye. The next 15mm inside the eye is a large space filled with jelly called vitreous humour. This clear jelly is the main structure that gives our eyes the right shape and pressure. The vitreous jelly is responsible for some people experiencing floaters, which are condensations of jelly inside the eye. Behind the vitreous we encounter the retina, its blood supply and then the back wall of the eye! And that’s it. The eyeball is a beautiful and complex structure and a small miracle of evolutionary biology!

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