Refractive Errors

For the eye to present a clear image, the light rays entering the eye need to come to focus on the back of the eye. In 35% of people the rays of light do not focus on the back of the eye, giving a blurred image at the brain. This is correctable by glasses or contact lenses and is known as a refractive error. Refractive errors can be the result of a number of factors, which include:

• The curvature of your cornea
• The optical strength of your lens
• The length of your eye

The three categories of refractive errors are: myopia (short sightedness), hypermetropia (long sightedness), and astigmatism. All of these are usually able to be corrected by laser surgery.

How do I tell which refractive error I have:

There are two ways to tell which form of refractive error that you have. The first is to ask your eye specialist or look at your prescription.

There are three main numbers on a prescription. There is a sphere, a cylinder and an axis. The cylinder and the axis is indicating the presence of astigmatism. NOT everybody has a cylinder correction, as not everybody has astigmatism. The cylinder is the amount of astigmatism that you have, and the axis is the orientation of the astigmatism.

The sphere, the first number on the prescription tells you if you are long-sighted or short-sighted. If the sphere is a positive (+) number then you are Hypermetropic or long-sighted. If the sphere is a negative (-) number then you are Myopic or short- sighted.

The cylinder is the second number on the prescription, this can sometimes look like the bottom number of a fraction.

If your prescription has an Add, written below the main numbers, then this is the correction for your presbyopic or reading correction.

Myopia (Short-sightedness)

Myopia is caused by one or a combination of the following:

• Steep cornea
• Long eyeball
• Powerful lens

Consequently light rays focus before, instead of directly on the retina.

Short sighted people are able to see close up objects clearly, but objects in the distance are blurry and indistinct. People with moderate to high levels of myopia are unable to see anything of real value without their glasses.

Laser surgery corrects myopia by making the central cornea flatter, thereby refocusing light rays onto the retina.

There are different degrees of myopia. They are classified as:

• Low 0 to -4
• Moderate -4 to -8
• High -8 and above

All of the above can be improved, but those in the higher range of myopia, may still need to wear a small prescription for certain activities. The laser can successfully treat up to 15 dioptres of myopia.

Hypermetropia (Long-sightedness)

Hypermetropia is caused by one or a combination of the following:

• Flat cornea
• Short eyeball

Consequently light rays are focused behind instead of on the retina.

Long sighted people are able to see images in the distance more clearly than they can see images at close. Though most hypermetropic people are unable to see at either distance clearly without their glasses.

Laser surgery corrects hypermetropia by making the cornea steeper so the rays of light are refocussed on the back of the eye.

There is a limit on the degree of hypermetropia that can be fully corrected, approximately 6 dioptres. If you are above the level of treatment it is still worth while coming for a consultation. The Doctors may be able to give you less dependence on glasses and contact lenses, or outline other treatment options available to you.

Astigmatism

Astigmatism means that the front of the eye is not entirely spherical like a soccerball, but shaped more like a football, (ie. it is not equally curved in all places). This means that the rays of light are bent unequally, therefore giving a distorted or blurred image at all distances.

Astigmatism can be found either alone, or in conjunction with myopia or hypermetropia. Laser surgery can treat astigmatism alone, or when found in conjunction with myopia or hypermetropia.

Presbyopia

Presbyopia occurs in all adults approximately 45 years or older. Presbyopia is a natural aging process of the lens inside the eye. This means that distance vision will be unchanged, but near vision will become blurry.

People who are mildly short sighted will be unaffected by the symptoms of presbyopia, but people who have no prescription, are long sighted or strongly short sighted will suffer from the symptoms of presbyopia.

Presbyopia is able to be treated by Monovision.