How do we see?

When we look at an object, light rays from the object pass through the eye and hit the retina. This causes nerve messages to be sent from the cells of the retina down the optic nerve which we 'see' in the brain.

The light rays have to be focused on a small area of the retina otherwise what we look at is fuzzy and blurred. (This is similar to a camera lens that focuses light rays onto a film.) The focusing in the eye is done by the cornea and lens. The cornea partly bends (refracts) the light rays which then go through the lens which finely adjusts the focusing. The lens does this by changing its thickness. This is called 'accommodation'. The lens is elastic and can become flatter or more rounded. The more rounded (concave) the lens, the more the light rays can be bent inwards. The shape of the lens is varied by the small muscles in the ciliary body which is attached to the lens.

More bending (refraction) of the light rays is needed to focus on nearby objects, such as when reading. Less bending of light is needed to focus on objects far away.医学全在线www.med126.com

What happens in short sighted eyes?

People with short sight are not able to see distant objects clearly. This is because the light rays focus in front of the retina. The eye is not able to focus the light rays further back despite maximum flattening of the lens. It mainly affects looking at distant objects where the light rays come straight (parallel) into the eyes. Near objects (reading etc) can often be seen well. This is because when looking at near objects the light rays come into the eye going slightly outwards. These will focus further back in the eye than light rays that come in straight from distant objects.

What causes short sightedness (myopia)?

The eyeball tends to be long. The distance from the lens to the retina is further than average. Short-sightedness is very common. It tends to run in families.

What are the symptoms?

Difficulty with vision tends to start in the early teenage years - sometimes earlier. The earlier it starts the more severe it is likely to become. In early adulthood when growth stops, vision has usually become as bad as it is going to get and stabilises.

Some children do not realise at first that their vision is affected. They may be able to read books and do close work well. However, seeing distant objects such as the blackboard at school may become difficult. They may think this is 'normal' and not tell anyone. Schoolwork may suffer for a while before the condition is identified and treatment provided.

Are there any complications?

The vast majority of people with short sight have no other associated problems. However, people with severe short sight have a slightly increased chance of developing glaucoma, detached retina and macular degeneration. These are serious eye conditions. So regular eye checks are advisable and report any new changing eye symptoms promptly to an optometrist (optician).

What is the treatment for short sightedness?

Glasses - are the common treatment. They have a concave lens which bends the light rays slightly outwards. Therefore the light rays have a greater angle to bend back to focus when going through the cornea and lens. This means they focus further back - on the retina.

Contact lenses - are an option and do the same job as glasses.

Surgery - is an option. Various types of operations have been developed to 'cure' short sight. The most popular at present is a technique called laser-assisted in-situ keratomileusis (LASIK). In this procedure a computer guided laser removes part of the cornea. The cornea becomes flatter and the light rays focus further back in the eye. This is very successful at curing short sight in most cases. The cure is instant and the procedure is usually painless. Other techniques are also available and have their pros and cons.

However, a small number of people develop complications after surgery to correct short sight. For example, some people develop hazy vision. Also, the vision is sometimes improved but not made perfect and glasses may still be needed. Before embarking on an operation make sure you have all the facts, failure rate, possible complications, other options, etc, explained to you. Also, these operations are not available on the NHS and are expensive.