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 long sight (hypermetropia)?
Hypermetropia occurs when light is focused behind the retina. In reality, light cannot actually be focused behind the retina. So, the lens changes its thickness (becomes fatter or more rounded) which aims to bring the light into focus on the retina – a process called accommodation.
But, people with long sight cannot accommodate fully and so the light does not focus sharply and vision is blurred. This occurs because either the eyeball is too short, the cornea is too flat, or the lens cannot become round enough.
People with a minor degree of long sight, can usually see at distance, and their near sight is clear. However, a person with long sight may get tiring of the eyes, often with a headache and vision discomfort. People with more severe hypermetropia are not able to see near objects clearly in focus. Long sight means exactly what the term suggests. You are sighted (you can see) distant (long) objects.
The diagram shows the differences in focusing between a normal and a long-sighted (hypermetropic) eye:
What causes long-sightedness?
The causes of hypermetropia are usually genetic (hereditary). Long-sightedness can occur at any age but it tends to become more noticeable above the age of 40 years.
In rare cases, hypermetropia is caused by other conditions such as diabetes, small eye syndrome (microphthalmia), cancers around the eye and problems with the blood vessels in the retina.
A particular type of age-related long-sightedness called presbyopia occurs because the lens of the eye becomes more stiff with age. (See separate leaflet called ‘Presbyopia’ for more information.)
What are the symptoms?
The main symptom is a difficulty with near vision. ‘Tiring’ of the eyes (asthenopia) is common and long-sighted people may have headaches and uncomfortable vision.
There may be difficulties with seeing with both eyes (binocular vision) as the brain will tend to ignore signals coming from the most long-sighted eye. Amblyopia (lazy eye) or strabismus (squint) can also occur in hypermetropia. Long-sighted people may have difficulty with depth perception (3-dimensional vision). (See separate leaflets called ‘Amblyopia’ and ‘Squint (Childhood)’ for more information.)
Are there any complications?
In severe cases of hypermetropia present from a very young age, amblyopia (lazy eye) can develop. The affected eye(s) 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.
What is the treatment for long-sightedness?
The simplest, cheapest and safest way to correct long-sightedness is with glasses. Convex prescription lenses (called plus lenses) are used to bend light rays slightly inwards. The light rays then have a lesser angle to bend back to focus when travelling through the cornea and lens. As a result, the light rays focus further forward – on the retina. There is an enormous choice of spectacle 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. 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 long-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 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 hypermetropia. 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. The shape of the cornea is altered to be more curved, so that the light rays can be focused further forward, 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.
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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