The medical name for short sight is myopia. Eyesight problems, such as myopia, are also known as refractive errors. Short sight leads to blurred distance vision, whilst close vision is usually normal. Short-sightedness is a very common problem that can be corrected by glasses or contact lenses, or cured with laser eye surgery.
What is a refractive error?
A refractive error is an eyesight problem. Refractive errors are a common reason for reduced level of eyesight (visual acuity).
Refraction refers to the bending of light, in this case by the eye, in order to focus it. 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:
- Short sight (myopia).
- Long sight (hypermetropia).
- Age-related long sight (presbyopia).
- Astigmatism (a refractive error due to an unevenly curved cornea).
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.
Light rays come off an object in all directions, as they result from the light around us from sun, moon and artificial light bouncing back off the object. The part of this bounced light that come into the eye from an object needs to be focused on a small area of the retina. If this doesn't happen, what we look at will be blurred.
The cornea and lens have the job of focusing light. The cornea does most of the work, as it (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. This is a bit like a trampoline with the central bouncy bit being the lens, the suspensory ligaments being the springs and the ciliary muscles being the rim around the edge.
When the ciliary muscles in the ciliary body tighten, the suspensory ligaments slacken, causing the lens to become fatter. 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)?
Short sight occurs when light coming from distant objects is 'overfocused', so that the point of focus is 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.
Light coming from near objects requires a stronger focusing activity anyway, so in myopia light from near objects is more likely to be focused in the right place.
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 above shows the differences in focusing between a normal and a short-sighted (myopic) eye.
What causes short sight (myopia)?
Short sight 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 of short sight (myopia)?
The main symptom is a difficulty with distance vision. The earlier short sight starts, the more severe it is likely to become. By the time early adulthood is reached, the level of short sight has usually reached its peak. This means that the vision does not generally become any worse.
Some children do not realise 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 children 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 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 with short sight (myopia)?
The vast majority of people with short sight have no associated problems.
However, people with severe short sight have a slightly increased chance of developing some other eye conditions. These include raised pressure in the eye (glaucoma), a detached retina and macular degeneration. These are serious eye conditions, so regular eye checks are advisable and new or changing vision symptoms should be reported promptly to an optician. See separate leaflets called Chronic Open-angle Glaucoma, Retinal Detachment and Age-related Macular Degeneration for more details.
What is the treatment for short sight (myopia)?
The simplest, cheapest and safest way to correct short sight is with glasses. Concave prescription lenses (called minus lenses) are used to bend light rays slightly outwards to counteract the over-focusing tendency. As a result, the light rays focus further back in the eye on the retina. There is an enormous choice of glasses frames available, to suit all budgets; younger people may even regard them as a fashion accessory.
In the developing world there is relatively poor access to vision testing. Some people still believe that glasses can make the eyes worse. Globally most short sight is unrecognised and untreated, even in schoolchildren.
These do the same job as glasses but they sit right on the surface of the eye. Many different types of contact lenses are available. Lenses may be soft or rigid gas-permeable. They can be daily disposable, extended wear, monthly disposable, or non-disposable. Your optician can advise which type is most suitable for your eyes and your prescription.
Contact lenses tend to be more expensive than glasses. They require more care and meticulous hygiene. They are more suitable for older teenagers and adults, rather than very young children.
Laser eye surgery is an option for some people with short sight. Generally, this type of surgery is not available on the NHS. Laser eye surgery is expensive but offers the chance to restore normal sight permanently. The procedure is generally painless.
Complete and permanent resolution of the refractive error is possible in a number of people. Others have a significant improvement even though perfect vision is not achieved and glasses or contact lenses may still be needed.
A small number of people develop complications. Some develop hazy vision, a problem with night vision, or problems with bright light haloes in their peripheral (edge) vision.
Many private companies advertise laser eye surgery. Before embarking upon this type of treatment you should do some research. You only have one pair of eyes and you need to find the best treatment for you. This may not be the cheapest. Try to go with personal recommendations, preferably a recommendation by an NHS eye surgeon (ophthalmologist). 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 all similar, typically taking about ten minutes per eye and aiming to reshape the cornea by using the laser to remove a very thin layer of corneal tissue. The reshaping of the 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 thin flap of the cornea.
- This helps to flatten the cornea so that the light rays can be focused further back, and on to the retina.
- The flap is then replaced, and sticks spontaneously to the underlying cornea. The flap serves as a natural bandage, keeping the eye comfortable as it heals. Healing occurs relatively quickly.
- This is the most popular and common type of laser eye surgery.
- Vision recovery time is said to be around 24 hours.
For people who are not suitable for LASIK® the following two options are sometimes offered.
- PRK stands for Photo-Refractive Keratectomy.
- During PRK®, instead of creating a corneal flap as in LASIK®, the surgeon completely removes the extremely thin outer layer of the cornea, using an alcohol solution, a 'buffing' device or a blunt surgical instrument. The underlying cornea is then reshaped with a laser. A new epithelial layer grows back within five days.
- The healing time in PRK® is faster than in LASEK®.
- LASEK stands for LAser Sub-Epithelial Keratomileusis.
- The LASEK® procedure involves taking a thinner flap of corneal epithelium than in LASIK®. The cornea underneath is then treated as in LASIK® and the thinner flap is repositioned and held in place with a bandage contact lens.
- The hinged flap made in LASEK® surgery is much thinner than the corneal flap created in LASIK® (which contains both epithelial and deeper stromal tissues).
- The LASEK® technique lessens the likelihood of removing too much cornea. There is also slightly less risk of developing dry eyes afterwards.
- Patients with a naturally thin cornea may be more suited to this treatment.
- LASEK® can be a better option for patients with a high degree of myopia, which requires more tissue removal from the central cornea.
- LASEK® tends to be more painful and discomfort can last longer than with LASIK®. Visual recovery time can be up to a week.
- In some cases, the thin flap created during LASEK® is not strong enough to be replaced and will be removed completely as it would have been in PRK.
- The alcohol solution used during LASEK® can irritate and slow the healing process immediately after surgery.
Side-effects of all laser surgery may include blurred vision, over-correction or under-correction of short sight, eye infection and dry eyes.
There are other surgical methods available to correct short sight - 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, raised pressure in the eye (glaucoma), macular degeneration, or a family history of these conditions, you should check to see what your optician recommends about regular check-ups.
If you fall into a high-risk group, you should have at least a two-yearly (biennial) eye examination if you are over 50 years of age and a yearly (annual) one if you are over 60 years of age.
If you have more than one risk factor then an eyesight check is recommended at least every three years once you reach 40 years of age.
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 optician 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 aged 65 years or more.
Further reading and references
Cochrane GM, du Toit R, Le Mesurier RT; Management of refractive errors. BMJ. 2010 Apr 12340:c1711. doi: 10.1136/bmj.c1711.
Shortt AJ, Allan BD, Evans JR; Laser-assisted in-situ keratomileusis (LASIK) versus photorefractive keratectomy (PRK) for myopia. Cochrane Database Syst Rev. 2013 Jan 311:CD005135. doi: 10.1002/14651858.CD005135.pub3.
Standards for laser refractive surgery; Royal College of Ophthalmologists (May 2009)
Corneal implants for the correction of refractive error; NICE Interventional Procedure Guidance, July 2007
Laser Refractive Surgery; Royal College of Ophthalmologists
Resnikoff S, Pascolini D, Mariotti SP et al: Global magnitude of visual impairment caused by uncorrected refractive errors in 2004: Bulletin of the World Health Organisation Volume 86, Number 1, January 2008, 1-80
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