How the eye works
We need light to see what is around us and to see colour. Light bounces off the objects we look at. These reflect different amounts of light which we see as different colours.
Front of the eye
Light rays enter the front of our eye through the clear cornea and lens. It is very important that both the cornea and lens are clear as this allows the light to pass directly through the front of the eye to the retina.
The cornea and lens bends light so that it can focus on the retina at the back of our eye. This gives us a clear, precise image. The cornea focuses the light towards our retina. The lens fine tunes the focussing of this light.
Our tears form a protective layer at the front of the eye and also help to direct the light coming into our eye.
The iris, the coloured circle at the front of our eye, changes the size of the pupil which allows different amounts of light into our eye.
The pupil is the dark hole in the middle of the coloured part of our eye. The pupil gets smaller in bright conditions to let less light in. The pupil gets bigger in dark conditions to let more light in.
Middle eye
The middle of our eye is filled with a jelly-like substance called the vitreous. The vitreous is clear and allows light to pass directly from the front to the back of our eye.
Back of the eye
The retina at the back of the eye is a light-sensitive layer which consists of rod and cone cells. These cells collect the light signals directed onto them and send them as electrical signals to the optic nerve at the back of our eye.
Rod cells are concentrated around the edge of the retina. They help us to see things that aren't directly in front of us, giving us a rough idea of what is around us. They help us with our mobility and getting around by stopping us from bumping into a things. They also enable us to see things in dim light and to see movement.
Cone cells are concentrated in the centre of our retina where the light is focused by the cornea and lens. This area is called the macula. Cone cells give us our detailed vision which we use when reading, watching TV, sewing and looking at people's faces. They are also responsible for most of our colour vision.
The optic nerve is made up of thousands of nerve fibres. These fibres pass the electrical signals along to our brain where they are processed into the image we are looking at.
How we see
Seeing can be likened to the process of taking pictures on a film with a camera which you then get developed. The retina is like a camera film which stores an image of what we are looking at. The image directed onto the retina is then sent along to the brain where it is processed, like developing a camera film. Therefore we actually "see" in our brain with the light information sent to it from our eyes. This whole process happens very quickly so that everything we see is in focus.
What happens during cataract surgery
The lens of your eye helps to focus light onto the retina. The lens is a small clear structure which sits just behind your iris (the coloured part of your eye). It is held in place by a sack of clear tissue called the lens capsule. Usually the lens and the capsule are both clear allowing light to pass directly through them.
A cataract is a clouding of the eye's lens and normally occurs as part of the ageing process. When the lens has a cataract then the light does not pass smoothly through it resulting in poor, blurred vision.
Cataracts are removed by surgery. During the operation the natural lens of the eye is replaced by an artificial lens. This artificial lens is held in place by the same structure, the lens capsule, which held your natural lens. Most people have a good level of sight following the operation.
Clouding of sight after surgery
Within a few months, sometimes years, following the cataract operation people can start to have difficulties with their vision again.
Sight can become blurred or people can have problems with bright lights and glare. It is almost as though the cataract is coming back. However this is not a re-growth of the cataract but is due to a thickening of the back of the lens capsule. Medically this is known as posterior lens capsule opacification.
About the posterior lens capsule
The artificial lens is placed in the lens capsule which held your natural lens in place. This capsule is made of very thin clear tissue. In some people this capsule thickens after the cataract operation. Sometimes this happens within a few months of the operation but it is more likely to occur a number of years afterwards.
Thickening of the capsule
The capsule thickens because sometimes lens cells grow across the back of the artificial lens. These cells are the natural lens cells which can sometimes be left behind following the surgery. The younger you are the more likely these cells are to grow.
This causes problems with the light entering the eye and therefore problems with your vision.
Treatment
Usually the posterior lens capsule opacification can be dealt with quite simply. Using a laser the doctor can make a hole in part of the capsule so that the light can once again pass directly to the back of the eye. This can improve vision in the vast majority of cases. The procedure is called YAG laser capsulotomy. YAG is the type of laser used for the surgery.
How laser surgery works
Lasers are beams of energy which can be targeted very accurately. The YAG laser is a focused laser with very low energy levels which can be used to cut structures inside the eye without any risk to the other parts of the eye. The doctor aims the laser exactly onto the posterior lens capsule in order to cut away a small circle shaped area.
This leaves some of the capsule to keep the lens in place (like a cuff around the lens) but removes enough to allow the light to pass directly through the eye to the retina. The very small part of the lens capsule which is cut away falls harmlessly to the bottom of the inside of the eye.
The procedure is quick and painless. It is usually done in an outpatient clinic and normally takes about fifteen minutes the actual laser part of the procedure may only take five minutes.
In most cases the doctor will use an eye drop to dilate your pupil before the treatment. This may make vision more blurry. Sometimes but not always the doctor may use a contact lens to stabilise the eye, if the doctor decides this is necessary then they will also use a drop to anaesthetise (numb) the front of your eye. If no contact lens is used then the eye isn't always anaesthetised.
Once your pupil is dilated you will have to place your head on the headrest of the laser machine to help keep your eye still. Then the doctor uses the laser to remove part of the capsule. The laser uses a wavelength that cannot be seen, but there is a light which you may notice that helps the doctor see what they are doing. Each laser shot is over in a fraction of a second and you shouldn't feel anything but you may notice a few flashing lights as the laser is fired and you may hear some faint clicks as the laser is working.
After the laser surgery
For most people there is an immediate improvement in sight within a few minutes of treatment with vision improving again once the dilating drop has worn off, but for some people it can take a few days for the sight to become clear again.
You may notice that you have a few more floaters in your eye but this is normal and they should improve with time. Following the laser surgery you should have sight similar to that which you had following the original cataract operation providing you do not have anything else wrong with your eye.
Because the laser surgery doesn't require any incisions or stitches you are normally able to return to your daily activities straight away. There is also no risk of introducing an infection inside the eye. If at all possible it may be best to arrange for someone to drive you to and from the hospital, since it can take some time for the drop that dilates your pupil to wear off.
Risks for this type of surgery
If a contact lens is used the eye may be a little sore following the treatment but this soon wears off. The laser procedure is considered very safe. Though there are some risks, serious side effects are very rare.
Sometimes the surgery can cause the eye pressure to rise and this can be a particular worry if you already have raised eye pressure such as in glaucoma. If the doctor is worried about this they will check the eye pressure soon after the laser surgery and if it is found to be high may give you some eye drops or a tablet to bring it back down.
It is also possible to develop some fluid in the central part of the retina following the cataract surgery. The medical term for this condition is cystoid macular oedema. This is particularly likely to happen if it also occurred following the original cataract operation. Cystoid macular oedema causes the vision to become blotchy, usually after a few weeks of good vision. If this happens then you should contact the doctor who performed the laser treatment for advice.
Very occasionally the laser treatment can cause some damage to the retina. This may happen some months after the operation as a little tear forms in your retina. You can tell this may be happening because you tend to get a new batch of floaters and flashing lights.
You should see your doctor at the hospital urgently if this happens. If you experience something like a curtain falling or rising across your vision then you should attend your local casualty department. It may mean a retinal detachment has taken place and these need to be seen quickly.
Note: These three problems are extremely rare complications of the laser surgery and most people have no complications at all.
How laser surgery affects sight
After the operation your sight should be restored to the level you had following your original cataract surgery. You will still have to use any glasses you may have needed but your vision should be clear again. The only thing to bear in mind is that the cataract surgery and the laser treatment will not help any other eye problems you may have, such as macular degeneration or damage from glaucoma.
What next?
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