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 astigmatism?
Regular astigmatism usually occurs when the cornea at the front of the eye is not perfectly rounded. The cornea should be ball-shaped. In astigmatism this curve is not evenly rounded; one side may be steeper or flatter than the other. So, light rays coming through the cornea and lens are not focused on to one sharp spot on the retina. This lack of ‘point focus’ means that images received by the brain are blurred.
Astigmatism is a bit more complicated than either short sight (myopia) or long sight (hypermetropia). All the refractive errors can reduce eyesight (visual acuity). Although astigmatism is very common (about 9 in 10 people have some degree of astigmatism), for most it is only a very mild, minor problem.
Irregular astigmatism is usually caused by a scar on the cornea.
What causes astigmatism?
Astigmatism is usually present at birth but it can result from an injury or operation to the eye, if the corneal surface is damaged.
Astigmatism can run in families and seems to be more common in premature or low birthweight babies, perhaps because the cornea does not have enough time to develop properly. Keratoconus is a relatively common eye condition that can cause significant astigmatism (as well as short sight – myopia, and corneal scarring). In keratoconus there is a pointed cornea which is thin. About 1 in 5 people with Down’s syndrome have a significant astigmatism.
What are the symptoms?
The main symptoms are blurred vision, light sensitivity (photophobia), eye strain and fatigue (especially after long periods of concentration, such as when using a computer), and headaches. You may have difficulty seeing one colour against another (contrast) or you may experience distorted images, such as lines which lean to one side. Severe astigmatism can cause double vision.
Astigmatism usually occurs with either short sight (myopia), long sight (hypermetropia) or presbyopia. (See separate leaflets called ‘Short sight – Myopia’, ‘Long sight – Hypermetropia’ and ‘Presbyopia’ for more information.)
Are there any complications?
Astigmatism in only one eye may cause amblyopia (lazy eye). The affected eye does not ‘learn’ how to see because the brain ignores the signals it receives. Amblyopia can be treated with eye patching if diagnosed early enough, before the vision pathways in the brain are fully developed. (See separate leaflet called ‘Amblyopia’ for more information.)
What is the treatment for astigmatism?
In many cases the symptoms of astigmatism are so mild that no treatment is needed. If vision is more significantly affected, glasses, contact lenses or surgery can correct the vision.
The simplest, cheapest and safest way to correct a regular astigmatism is with glasses. The lenses of the glasses adjust the direction of the incoming light rays, correcting the uneven curve of the cornea. There is an enormous choice of spectacle frames available, to suit all budgets. An irregular astigmatism cannot be corrected by a lens. Glasses are seen as more acceptable these days, and younger people may even regard them as a fashion accessory.
These do the same job as glasses. Many different types of contact lenses are available. Lenses may be soft or rigid gas-permeable. Toric lenses are used to correct astigmatism. Contact lenses can be daily disposable, extended wear, monthly disposable, or non-disposable. Your optometrist can advise which type is most suitable for your eyes and your astigmatism prescription. Contact lenses often work out more expensive than glasses and they require more care, and meticulous hygiene. They are suitable for older teenagers and adults, rather than children.
Surgery is an option for some people to cure their astigmatism and any associated short or long sight. Generally, these operations are not available on the NHS, and so are a private option. Laser eye surgery is expensive but offers the chance to restore normal sight permanently. The cure is usually instant and the procedure is generally painless.
However, a small number of people develop complications after surgery to correct astigmatism. Some people develop hazy vision, a problem with night vision, or problems with bright light haloes in their peripheral vision. Sometimes improved but not perfect vision is achieved, and glasses may still be needed.
Many private companies aggressively market their laser eye surgery. Before embarking upon this type of treatment you need to do some research. Try to go with personal recommendations, preferably a recommendation by an NHS ophthalmologist (eye surgeon). It is important that you know your facts – the failure rate, the risk of complications, level of aftercare and what the procedure involves, before submitting yourself to an irreversible, costly treatment.
Several types of laser surgery have been developed. These include: LASIK®, PRK® and LASEK®. They are similar because the basic idea is to reshape the cornea using the laser to remove a very thin layer. The reshaped cornea allows the refraction of the eye to be corrected.
LASIK stands for Laser-Assisted In situ Keratomileusis. This is the most popular form of laser eye surgery. The laser is used to lift and remove a very thin layer of the cornea. This helps to alter the shape of the cornea (making it more regular), so that the light rays can be focused to a single point on the retina. Any associated myopia (short sight) or hypermetropia (long sight) can also be corrected at the same time.
PRK stands for Photo-Refractive Keratectomy. It is an older surgical operation, which has mostly been replaced by newer techniques.
LASEK stands for LAser Sub-Epithelial Keratomileusis. It is an improved form of PRK with some similarities to LASIK. Most of the outer layer of the cornea (the epithelium) is left intact. The LASEK procedure tends to be more painful, and discomfort can last longer than with LASIK.
- Other techniques are available, most are variants of laser surgery. Implantable contact lenses (refractive lens exchange) and corneal grafts are options in very severe or specialised cases of astigmatism.
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, an eye test is needed every 10 years. Between the ages of 41 and 55, it is recommended that you see an optometrist five-yearly. At any age between 56 and 65, two-yearly checks are needed, dropping to annual checks in low-risk people who are 65 or more.
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