Vision problems

How the eye works

To see clearly, light rays need to be focused on to the retina at the back of the eye. This is achieved as the light rays pass through the transparent cornea, then through the fine-focusing lens.

The image formed on the light-sensitive retina is converted into electrical impulses, which pass through nerves to the part of the brain responsible for vision. Here, they're interpreted to tell us what we're seeing.

Short-sightedness (myopia)

Problem: the eyeball is too long compared with the focusing power of the cornea and lens. This means light rays from objects in the distance are focused in front of the retina because they've been bent too much by the cornea and lens.

Effect: getting a clear image of distant objects is difficult, but seeing things close up is fine. Children may perform poorly in class because they're unable to read what's on the whiteboard.

Who and when: often, the condition is first noticed in older children and young teenagers. Short-sightedness can run in families, so short-sighted parents are more alert to the possibility of their children having sight difficulties.

Treatment: short-sightedness can be corrected with a concave lens that makes the light rays diverge and focus accurately on the retina. Sometimes, laser surgery can reshape the cornea.

Long-sightedness (hypermetropia)

Problem: the eyeball is too short when compared with the focusing power of the cornea and lens. This means light rays from near objects are focused behind the retina because they haven't been bent enough by the cornea and lens.

Effects: getting a clear image of objects close up is difficult, so long-sighted people have difficulty reading or doing fine close-up work. Younger long-sighted people are able to see objects in the distance, but older people often find they develop problems seeing distant objects because the lens becomes less flexible as they age. This condition is called presbyopia.

Who and when: long-sightedness may be apparent in young babies and children who aren't interested in small objects. Older children may be reluctant to read, while adults may find they have difficulty reading or seeing a computer screen clearly. All children should be formally tested before the age of three if there's long-sightedness in the family. If the eyes aren't affected equally, a squint or loss of vision in one eye can occur if treatment isn't given at an early age.

Treatment: long-sightedness can be corrected with a convex lens that makes the light rays converge and focus accurately on the retina. As the lens becomes less flexible with age, a stronger prescription may be needed.

Double vision (diplopia)

Problem: people see two visual images of an object rather than one.

Effects: this tends to be like seeing a 'shadow' around every image rather than seeing two totally separate objects, but a few people with severe double vision do see two full and separated objects.

Who and when: It's essential you see your GP to establish why you have double vision. While it usually develops as a result of a squint, more worrying conditions, such as tumours, can also be the cause. When double vision develops in an adult who has never had it before, there's particular cause for concern.

A squint is the most common cause of double vision, especially in children. In a squint the two eyes don't look in exactly the same direction because some of the muscles are weak or paralysed. As each eye sees an object from a slightly different angle, this may result in a double image of the object (although what actually happens is a bit more complex than this). Not all squints cause double vision, but paralytic squints (where there's muscle paralysis) generally do.

Treatment: Squints can be corrected by surgery if it's performed early in life, and this should 'cure' the double vision. It's much more difficult to correct double vision from a squint in adult life.

Cosmetic surgery can cure the look of the squint, but by then the brain may have learned to see separately with the two eyes and can no longer fuse their two pictures into one. Fortunately, the brain usually learns to suppress the second image and the view from one eye dominates.

Other causes for double vision include:

• A tumour or cancer, in or behind the eye, which distorts the visual image. This will need very specialised treatment.
• A blood clot or tumour behind the eye which prevents normal movement of one of the eyes. Again urgent specialist treatment is essential.
• A condition called exopthalmos, where the eyeballs protrude and the eye muscles swell, disrupting the alignment and movement of the eyes. This is usually caused by problems with the thyroid gland, which needs medical treatment (but treating the thyroid doesn't always cure the eye problem).
• Abnormalities within the eye. Very occasionally double vision occurs because of problems such as dislocation of the lens in the eye, which needs specialist medical treatment.

Floaters

Floaters are common and most people will notice them at some point in their lives. They appear as tiny dark spots that float in front of the eye.

They tend not to be noticed all the time, and are often more obvious when someone is relaxing and gazing into space. In such instances, they float gently across the field of vision. When someone looks in different directions, however, they move quickly.

What people with floaters are seeing as solid blobs are actually shadows cast on the retina by pieces of tissue in the jelly-like material that fills the back of the eye, called the vitreous humour. No one knows why floaters occur and apart from just 'being there' they don't tend to interfere with someone's vision.

If a large number of floaters appear suddenly, it's important to consult a doctor immediately. This may be a sign of bleeding into the vitreous humour - a vitreous haemorrhage - or a sign that the retina is coming away from the back of the eye, which is known as a retinal detachment. Both these conditions need urgent treatment.

Eye checks

It's important everyone has their eyes checked regularly by an optician - every two years is recommended.

Many illnesses, such as diabetes and high blood pressure can cause changes in the eye which need to be caught early before the vision is damaged.