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 short sight (myopia)?
Myopia occurs when light is focused in front of the retina. It occurs because either the eyeball is too long, or because the cornea is too curved. Despite maximum flattening of the lens, the eye is not able to focus the light rays further back, and on to the retina.
People with short sight are not able to see distant objects clearly. Short sight or near sight mean exactly what the terms suggest. You are sighted (you can see), near (short) distance objects. Near objects (for example, when reading a book) can often be seen well. This is because when looking at near objects, the light rays come into the eye going slightly outwards. These will focus further back in the eye than light rays that come in straight from distant objects.
The diagram shows the differences in focusing between a normal and a short-sighted (myopic) eye.
What causes short-sightedness (myopia)?
Myopia tends to happen in children and young teenagers. It often runs in families. Temporary short-sightedness can also occur with certain illnesses – for example, in diabetes.
What are the symptoms?
The main symptom is a difficulty with distance vision. The earlier myopia starts, the more severe it is likely to become. By the time early adulthood is reached, the level of myopia has usually reached its peak. This means that the vision does not generally get any worse, and indeed stabilises.
Some children do not realise at first that their vision is not as good as it should be. They may be able to read books and do close work well. However, seeing distant objects such as the board at school may become difficult. They may think this is normal and not tell anyone. Schoolwork may suffer for a while before the condition is identified and treatment provided.
Children usually have a routine preschool or school-entry vision check. Your child’s teacher may notice that they are having difficulties in class reading the board. If you suspect your child has problems with his or her sight, you should arrange for an eyesight test with an optometrist (optician) who is happy to assess children. For young children and toddlers, your GP may be able to make arrangements for a sight test. Sight tests are free for children.
Are there any complications?
The vast majority of people with short sight have no other associated problems.
However, people with severe short sight have a slightly increased chance of developing glaucoma, detached retina and macular degeneration. These are serious eye conditions. So regular eye checks are advisable, and report any new changing eye symptoms promptly to an optometrist. (See separate leaflets called ‘Glaucoma (Chronic Open Angle)’, ‘Retinal Detachment’ and ‘Age-related Macular Degeneration’ for more information.)
What is the treatment for short-sightedness?
The simplest, cheapest and safest way to correct short-sightedness is with glasses. Concave prescription lenses (called minus lenses) are used to bend light rays slightly outwards. The light rays then have a greater angle to bend back to focus when travelling through the cornea and lens. As a result, the light rays focus further back – on the retina. There is an enormous choice of glasses frames available, to suit all budgets. 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 short sight 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 short 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 myopia. 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 flatten the cornea so that the light rays can be focused further back, and on to the retina.
PRK stands for Photo-Refractive Keratectomy. It is an older surgical operation, that 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 myopia.
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.
References and Disclaimer | Provide feedback
- Cochrane GM, du Toit R, Le Mesurier RT; Management of refractive errors. BMJ. 2010 Apr 12;340:c1711. doi: 10.1136/bmj.c1711.
- Photorefractive (laser) surgery for the correction of refractive error, NICE (2006)
- Laser in situ keratomileusis (LASIK), NICE (2004)
- Shortt AJ, Allan BD; Photorefractive keratectomy (PRK) versus laser-assisted in-situ keratomileusis (LASIK) for myopia. Cochrane Database Syst Rev. 2006 Apr 19;(2):CD005135. [abstract]
- A Patient’s Guide to Refractive Surgery, Royal College of Ophthalmologists, March 2006
- Taravella M; Myopia, LASIK, eMedicine, May 2009
- Murillo-Lopez FH; Myopia, PRK, eMedicine, Jan 2010
- Grewal S; LASEK, eMedicine, Feb 2010