Short-sightedness (myopia) is a problem of vision that causes distant objects to appear blurred, while close objects can still be seen clearly.
This is because light rays are being focused in front of the retina (the inside of the back of the eye), rather than directly on the retina.
The cause of myopia is the eye being too long from front to back, or the cornea (the front of the eye) being too steeply curved. The result is a mismatch between the length of the eye and its focusing power.
The degree of myopia is determined by the focusing power of the lens, which is measured in dioptres (D):
At least five million British people are short-sighted and about 200,000 of them have high-degree myopia.
There is a tendency for myopia to run in families, although it is believed that a number of factors combine to cause myopia.
A person's genetic make-up, along with the environment within which they are brought up, are considered to be linked to the development of myopia.
Blurred vision can also be a symptom of other health problems (see box).
Children have about a 30% chance of developing myopia if one of their parents has the condition, and a 55% chance if both have it.
People who spend a lot of time reading, working at a computer or doing other close-vision work may develop temporary myopia (where vision returns after resting the eyes). Also, they may be more likely to develop permanent myopia.
Some people develop 'night myopia' – blurred distance vision that only occurs at night. This is because blue light makes the eyes artificially short-sighted and a small degree of myopia becomes more significant at night.
An optician will usually diagnose myopia following a sight test. The optician will use a letter or symbol chart to record the level of vision and see if lenses are able to correct it to normal.
Children may have their vision checked at school, but if not, parents should book their children in for a sight test with an optician.
Children should have their sight tested at least every two years.
You qualify for a free sight test if you are:
The main symptom of myopia is distant objects appearing blurred, while near objects can still be seen clearly. Other symptoms can include headache and tired eyes.
Myopia usually appears around puberty, but may appear at any age from early childhood up to 25 years of age.
Usually, the amount of myopia increases with age; this is related to the eyeball increasing in size as the child grows. In most cases, myopia will stop getting worse when the person stops growing.
In older people, the onset of myopia can be the earliest indication that a cataract (cloudy lens) is starting to form.
Some children who develop myopia may not realise at first that their vision has been affected. They may be able to read books and do 'close work' well, but may find it difficult to see distant objects, such as the blackboard at school. They often think this is 'normal' and do not tell anyone.
Myopia can usually be corrected with glasses (spectacles) or contact lenses. These cancel out the increased curvature of your cornea or the increased length of your eye, so that distant objects no longer appear blurred.
Some people can get vouchers towards the cost of glasses or contact lenses from the NHS (see box).
Some people with severe myopia might be eligible to have contact lenses fitted for clinical reasons under the NHS contact lens scheme. To be eligible, you would have to be referred to an NHS eye clinic and be considered to need contact lenses as a clinical necessity.
Laser surgery works by altering the shape of the cornea so that the eye focuses correctly. It is generally suitable for treating low degrees of myopia.
Most people pay for laser treatment privately, as it is generally unavailable on the NHS.
Current evidence suggests that laser surgery for the correction of myopia is safe and effective for use in appropriately selected patients.
However, as with any surgery, there are possible complications you need to be aware of (see Complications page). The two available procedures are as follows.
A small flap is made in the cornea and a tiny piece of tissue is removed with a laser. It is usually performed as an outpatient procedure and takes about 15 minutes for each eye. Vision may be hazy or blurry for a few days after surgery.
A laser is used to remove tissue from the cornea and reshape it. The treatment may be painful for 48 hours afterwards and can only be used to treat less severe myopia.
Lens implant surgery involves implanting a contact lens into your eye to correct severe short-sightedness (high-degree myopia). It is sometimes referred to as intra-ocular lens insertion or corneal implant insertion.
There are two main ways of performing lens implant surgery:
This procedure is generally used for people aged 25-45 for whom laser surgery is not suitable, or for people who have difficulty wearing glasses, for example because of a disability or professional requirement.
It is the preferred technique for younger (aged under 45) patients because their natural, unaided reading vision is preserved.
The procedure is carried out using a local anaesthesic and you can go home the same day. Your pupil is dilated using eye drops and a phakic lens implant is inserted into your eye via a small cut in your cornea (front of the eye).
Depending on its design, the phakic lens is anchored to your iris (coloured part of the eye), placed in the angle between your cornea and iris, or positioned to float over the surface of your natural lens.
The second eye is usually treated about seven days after the first.
The National Institute for Health and Clinical Excellence (NICE) published guidelines on this procedure in February 2009 (see Useful links). It concluded that there is good evidence of short-term safety and efficacy, but there is an increased risk of cataract, corneal damage or retinal damage and there are no long-term data about this.
It is therefore important that you talk to your doctor about this procedure and understand the risks involved.
This procedure is essentially the same as cataract surgery – your natural lens is replaced with an artifical lens that corrects your short sightedness.
Refractive lens exchange is usually a day case procedure performed under general or local anaesthetic.
The operation is not painful and you will be able to see clearly within two days of the surgery. The second eye is usually treated about seven days after the first.