Refractive Errors*

The most common eye disorders are refractive errors, which mean that the image of the object a person is looking at is not focused properly onto the retina (the light-sensitive tissue in the back of the eye). There are three main types of refractive errors which can affect people at any age: ·  Myopia or shortsightedness ·  Hyperopia or longsightedness ·  Astigmatism There is also a fourth type of related eye focusing disorder which is related to age, and affects everyone over the age of forty: ·  Presbyopia

                  

What are refractive errors? For perfect clear vision, the image needs to be focused onto the retina, just as a camera has to be focused properly in order to take a clear picture. If the image is not focused exactly on the retina, then the image will be blurred, just like an out-of-focus photograph. In this case, the person is said to have a refractive error. Refractive errors occur when there is a mis-match between the length of the eye, and its optical power. These mismatches usually originate during childhood, when the eyes are growing. The exact causes of refractive errors are still being studied, but it is known that both hereditary and environmental influences can affect their development. Most people have some refractive error, but in most cases the error is small, and does not cause any problems. In fact, the average person is slightly long-sighted. Return to Top Myopia What is myopia? Myopia is what most people call shortsightedness. Shortsighted people do not see distant objects clearly. The eye's lens and cornea normally focus light into an image on the retina. In a myopic eye the light is focused in front of the retina and so the image is blurred. Does a shortsighted person see close objects more clearly than a person with normal vision? No. Shortsighted people see close objects equally as well or often slightly worse. When their myopia is corrected, they see objects equally well. How can I tell if I am shortsighted? Shortsighted people have difficulty in seeing distant objects clearly. They find it hard to read road signs and scoreboards and to play ball games. Recognising people in the distance may be a problem for many shortsighted people. Often a person will not realise that they cannot see clearly but an eye examination by an optometrist will reveal the problem. How can I tell if my child is shortsighted? A complete eye test is the only sure way of determining whether your child's vision is normal. Some clues to myopia in a child are: screwing up eyes to see distant objects difficulty in reading the blackboard at school poor posture while reading a lack of interest in playing outdoor games. What causes myopia? Myopia is caused by a mismatch between the power of the optical components of the eye and the length of the eye (the 'axial length'). Either the power is too high, or the length is too long, or both. Usually the cornea (the front surface of the eye) is curved more steeply than average, increasing its optical power. The exact causes of the mismatch between power and axial length are unknown. There is some genetic influence (if your parents are shortsighted, you have a better than average chance of being shortsighted yourself), and there is some evidence for environmental factors, such as excessive amounts of near work, having some influence. Return to Top Can myopia be cured? No, but properly prescribed spectacles or contact lenses will enable the person to see clearly. The lenses diverge incoming light rays, so that they will be properly focused on the retina. There are also a number of techniques available for re-shaping the cornea (the front surface of the eye), in order to reduce its power and thus correct the myopia. One technique (known as orthokeratology or 'Ortho-K') uses rigid contact lenses to change the shape of the cornea. Other techniques use surgery to remove tissue from the cornea, leaving it with a flatter surface. Can myopia be prevented? There is no certain prevention for myopia. However, in some cases, treatment can be prescribed to stop or slow its progression. How common is myopia? It is a very common condition. About 15 per cent of the population are shortsighted. Usually myopia begins to develop in teenage years and it may get worse over the following few years. Can myopia cause other problems? High levels of myopia can cause other, more serious problems. People with high levels of myopia often have very large, elongated eyes, and their retinas may be stretched and thinner than normal. This increases the risk of the retina developing holes and tears and the risk of retinal detachments. If you are myopic you should have regular eye examinations, and talk to your optometrist about the potential for problems.                                            Return to Top Hyperopia What is hyperopia? The eye's lens and cornea focus light into an image on the retina, just as a camera lens focuses light on to a film. In a resting hyperopic (longsighted) eye, the light is focused behind the retina and so the image is blurred. The perfect state of focusing exactly on the retina is unusual; the average person is a little hyperopic. How does hyperopia affect vision? A little hyperopia is not a problem because the eye can compensate easily. However, if there is a significant amount of hyperopia the effort of focusing (called accommodation) can lead to symptoms. A hyperopic person can have normal vision, but the greater the hyperopia the harder it is to focus. Vision may become blurry, especially for close objects, because the closer the object the more focusing is required. Hyperopic people may get tired eyes or headaches after a lot of visual work, even if their vision is clear. Reading is more difficult and school work can be affected. What causes hyperopia? Hyperopia is often thought to be hereditary, but no-one is certain. The eyeball may be a little smaller than average. Does hyperopia change with age? It tends to increase, but not always. We all find it harder to focus on close objects as we get older (due to presbyopia). Hyperopes have trouble sooner and may need reading spectacles earlier because they have to focus more to start with. How is hyperopia diagnosed? Because a hyperopic person often can see well in the distance, a letter chart test alone may miss hyperopia. Special tests have to be used, including retinoscopy and refraction. What do we do about hyperopia? The optometrist has many things to consider when making a decision and symptoms are very important. In general, young people who are slightly hyperopic do not have problems. If they do, they may need spectacles, mainly for close work such as reading and using computers. Older people, or young people with significant hyperopia, often have problems because focusing requires much effort. Their vision is more likely to be blurred, especially for close objects. They usually need spectacles for reading and sometimes for distance vision as well. The spectacle lenses converge the light rays, moving the focus back onto the retina. Why is hyperopia called longsightedness? Because hyperopic people can generally see better in the distance than close, but they cannot see better at any distance than someone who is not hyperopic. Return to Top Astigmatism What is astigmatism? Astigmatism is a focusing error which causes asymmetric blur. Some directions in an image are more out of focus than others. This can be contrasted with short-sightedness (myopia) where all directions are uniformly blurred. What does it look like? Astigmatism causes different amounts of blur in different directions. This causes images to appear distorted, or sometimes even double. Certain letters may be more difficult to read than others, depending on the orientation of the lines within them. One type of chart used to detect astigmatism uses a series of lines arranged in a fan shape - if you have astigmatism, some lines will appear clearer than others. If you don't have astigmatism (or it is corrected), you can see what it is like by looking at the chart while squinting your eyes. When you do this, the pressure from your eyelids distorts your corneas, causing some temporary astigmatism. When you open your eyes again, your cornea returns to its normal shape. What causes astigmatism? Most astigmatism is caused by the shape of the front surface of the eye (the cornea). It can also be caused by slight tilting of the lens inside the eye. It may be an inherited characteristic or a normal variation accompanying growth. Can you describe the shape? A magnifying glass focuses the sun to a point image because its two surfaces are spherical, each like the surface of a basketball. Now imagine a transparent surface shaped like the side of an Australian Rules or rugby football. It has two different curvatures. These result in light focusing at two different locations. The image does not focus to a point and so is blurred. How does astigmatism affect me? Objects at all distances are indistinct or blurred and the eye cannot focus. Even slight degrees may encourage headaches, fatigue and reduce concentration. This is because the eyes may try, without success, to correct the blur, and because there is a tendency to screw up the eyes to try to see better, producing discomfort in the muscles of the eyelid and face. How is astigmatism corrected? Spectacles and contact lenses (hard and soft) can correct astigmatism. Sometimes correction of astigmatism can cause change in the apparent size and shape of objects and may affect judgement of distance. A patient may feel taller or shorter, or walls may appear to slope and floors curve. In most cases, adjustment to these side effects takes only a week or so. Astigmatism correction may involve a compromise between optimal clarity and visual discomfort. How is astigmatism detected? Some people notice blur themselves. Only a proper eye examination will determine for certain if you have astigmatism. Astigmatism is not an eye disease and any changes are generally gradual and not necessarily for the worse. Most people have at least very slight astigmatism. Return to Top Presbyopia What is presbyopia? Presbyopia is a common condition which makes vision difficult at a normal reading distance. It is not a disease. How will presbyopia affect my vision? Close tasks such as reading and sewing become difficult, particularly under poor lighting conditions. For example, you may find that you are holding your newspaper further away from your eyes to make the print clearer. Presbyopia does not affect distance vision. You may also have difficulty concentrating when reading, or you may find periods of close work result in sore eyes, headaches or tiredness. What causes presbyopia? It is important to understand how your eyes change their focus for viewing close objects. Normally they are focused for distance vision. Inside the eye there is a lens about the size of a pea. To focus on close objects, a special muscle in the eye changes the shape of the lens. This process is called accommodation. With age the lens loses its flexibility and is less able to change its shape. This is a completely normal ageing change, just like stiffening joints or greying hair. The loss in lens flexibility is the reason that close focusing becomes more difficult. Who is likely to become presbyopic? Everyone experiences the ageing process which causes presbyopia. The condition cannot be prevented. Does presbyopia come on quickly? No. Presbyopia is usually first noticed around the age of 40 to 45 years. Although difficulties with close work may seem to come on suddenly, the ageing process which causes presbyopia is gradual and has been going on since childhood. Between the ages of 45 and 65 the amount of presbyopia increases, making near work more difficult. From 65 years onwards, there are unlikely to be any further significant changes to vision due to presbyopia. How is presbyopia treated? Presbyopia is corrected by a spectacle prescription designed especially for close distances, in the form of spectacles or contact lenses. It is important that the prescription is calculated for the distance at which you do your close tasks. It is not possible to treat presbyopia by surgery. Through discussion with your optometrist you will be able to decide which is the best way of preparing your prescription. The correction for presbyopia will make near objects clear but distant objects blurry. This means that if you have a pair of spectacles just for reading you will not be able to watch television while wearing them. Having different prescriptions for distance and reading can be a nuisance, especially if you have to change spectacles all the time. One way around the problem is the use of bifocals. These are special lenses which have a prescription for distance vision in the top half of the lens and the prescription for near vision (reading) in the lower half. Another form of spectacles used to correct presbyopia are `look-overs' or half-glasses. Other options include the use of trifocals, progressive lenses and some special contact lens prescriptions. Does presbyopia mean that my eyes are deteriorating? No. Although your close focusing system is not functioning as well as it used to, once presbyopia has been corrected with spectacles you will be able to see close things as well as you always did. Presbyopia does not represent a threat to your eyes' health. When should I have my prescription for presbyopia renewed? Between the ages of 45 and 65 your prescription is likely to change significantly. It is sensible to have your eyes examined every two to three years to review your correction and your general eye health. If you experience vision problems within two or three years of your previous examination you should make a review appointment with your optometrist. Your optometrist will advise you of the most appropriate period between consultations. Will wearing spectacles weaken my eyes? No. Presbyopia will continue regardless of whether spectacles are worn. Wearing spectacles will not accelerate or slow the development of presbyopia. *This page was reproduced with permission © Optometrists Association Australia

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