What is a refractive error?
A refractive error is an eyesight problem. Refractive errors are a common reason for reduced visual acuity (level of eyesight).
Refraction refers to the bending of light. In terms of the eye. A refractive error means that the eye cannot focus light on to the retina properly. This usually occurs either due to abnormalities in the shape of the eyeball, or because age has affected the workings of the focusing parts of the eye.
There are four types of refractive error:
- Myopia (short sight).
- Hypermetropia (long sight).
- Astigmatism (a refractive error due to an unevenly curved cornea).
- Presbyopia (an age-related refractive error).
In order to understand refractive errors fully, it is useful to know how we see.
When we look at an object, light rays from the object pass through the eye to reach the retina. This causes nerve messages to be sent from the cells of the retina down the optic nerve to the vision centres in the brain. The brain processes the information it receives, so that in turn, we can see.
The light rays have to be focused on a small area of the retina, otherwise what we look at is blurred. The cornea and lens have the job of focusing light. 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 (convex) 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. Tiny string-like structures called the suspensory ligaments are attached at one end to the lens, and at the other to the ciliary body. When the ciliary muscles in the ciliary body tighten, the suspensory ligaments slacken, causing the lens to fatten. This happens for near objects. For looking at far objects, the ciliary muscle relaxes, making the suspensory ligaments tighten, and the lens thins out.
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.
What is presbyopia?
Presbyopia is long-sightedness (hypermetropia), caused by age.
In order to see close-up objects, our eyes have to accommodate. This means that the lens changes its thickness. It can do this because of the ciliary muscles that attach to the suspensory ligaments at either end. As these muscles tighten, the lens becomes more thickened and curved, and the light rays from close objects are brought into sharp focus on the retina.
The lens of the eye becomes more stiff, and less elastic as we get older. This makes it more difficult for the lens to change shape. With the lens in its normal resting position you are still able to focus on objects in the distance – long sight.
There are five types of presbyopia:
- Incipient presbyopia is the very earliest stage. It may be a bit more difficult to read small print.
- Functional presbyopia is where you begin to notice more problems with near sight.
- Absolute presbyopia is when the eyes cannot focus on near objects at all.
- Premature presbyopia is that which occurs before the age of 40.
- Nocturnal presbyopia is where focusing on near objects is difficult in dull light conditions.
What causes presbyopia?
Presbyopia is caused by normal ageing.
Presbyopia usually begins at around 40 years of age. By the age of 45 years, most people will need glasses or contact lenses. If you already wear glasses or contact lenses, your prescription may alter as a result of presbyopia.
Presbyopia may develop at a younger age if you already have long sight (hypermetropia) and wear corrective glasses or contact lenses. People who have a job that requires a lot of close-up work, and people who live and work in a hot climate with lots of ultraviolet sunlight exposure, may also develop premature presbyopia.
What are the symptoms?
Initial symptoms may be difficulties with prolonged close-up work, with tiring (eyestrain) of the eyes. This may be worse in dim light. You may also notice problems if you look from a near object to a faraway one, particularly if you have been concentrating on something close by, for a while. This may progress to blurred vision when looking at objects close up, to double vision and to headaches.
What is the treatment for presbyopia?
Presbyopia can be corrected by wearing reading glasses or contact lenses. Laser eye surgery (for example, laser-assisted in situ keratomileusis (LASIK®)) is no use for presbyopia. This is because presbyopia is caused by age-related changes in the lens of the eye, whilst laser surgery alters the shape of the cornea to improve vision (refraction).
If you already have glasses, bifocal or varifocal lenses may be prescribed to treat your presbyopia. With these lenses, different parts of the lens are different prescriptions. Bifocal (‘bi’ means 2) lenses have a lens at the bottom for long sight, to allow close up vision, and the rest of the lens above allows distant vision.
For people who already have contact lenses, reading glasses may be prescribed in addition to these. Another option is bifocal contact lenses, or wearing a different contact lens in each eye – one to allow near vision, the other distance vision.
An operation called PRELEX can be performed in some cases for presbyopia. PRELEX stands for PREsbyopic Lens EXchange. It involves removing your old, inflexible natural lens and replacing it with an artificial one. The operation is performed under local anaesthetic, so you are kept awake but the eye is numbed. (Artificial lenses are quite often put in after cataract surgery.)
How often do I need an eyesight test?
This depends on your age, your family history and any pre-existing medical conditions.
People at high risk of sight problems need more frequent eyesight checks. If you have diabetes, glaucoma, macular degeneration, or a family history of these conditions, you should check to see what your optometrist recommends about regular check-ups. As a guide, if you fall into the high-risk group, you should have at least an annual (yearly) eye examination if you are over 60 years of age. If you are over 50 years of age it should be every two years, and over 40 years, with risk factors, then an eyesight check is recommended at least every three years.
Low-risk people with no symptoms of an eyesight problem, do not need to have their eyes tested so frequently. If you fall in this group and are aged between 19 and 40 years, an eye test is needed every 10 years. Between the ages of 41 and 55 years, it is recommended that you see an optometrist five-yearly. At any age between 56 and 65 years, two-yearly checks are needed, dropping to annual checks in low-risk people who are 65 years old or more.
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