What is refractive error?
Refractive error refers to a range of common conditions, including short-sightedness and long-sightedness, that mean light isn’t focusing correctly when it hits the retina – the light-sensitive layer at the back of the eye. This can cause vision to become blurred (out of focus).
Refractive errors are very common and affect millions of children and adults in the UK. A lot of the time it only causes mild eyestrain or the inconvenience of having to wear glasses or contact lenses. But for some people the impact is much more severe.
Severe short-sightedness (high myopia) is associated with an increased risk of retinal degeneration (myopic maculopathy) or retinal detachment – a medical emergency that requires urgent treatment. And uncorrected refractive error can be damaging in children whose eyesight is still developing. This is why funding vital research into refractive error plays a big part in our mission to create a future everyone can see.
Fight for Sight’s goal is to help further our understanding of refractive error and develop new prevention techniques and better treatments.
What causes refractive error?
Refractive error is mainly associated with small variations in the shape of the eye. These variations aren’t really noticeable without close-up examination - but they are significant enough to affect how light is focused on the retina as it passes through the eye.
The retina is responsible for converting light into signals carried along the optic nerve to the brain, enabling us to process vision. But for vision to be processed clearly, light needs to be focused accurately.
Family history is the biggest risk factor for refractive error. Developmental factors may also play a role (for example, being born prematurely may increase the risk) and having other eye conditions can make refractive error more likely.
The four most common types of refractive error are:
Short-sight occurs when light focuses just short of the retina, because the front of the eye is too curved or the eyeball is too long. Distant objects become blurred, while more close-up things can be seen more clearly.
This often runs in families and is also more common in people with Far Eastern heritage. Research suggests too much close-up working can cause short-sightedness, and it often develops or get worse during teenage years. Not spending enough time outdoors is another important risk factor during childhood. Myopia can develop in later life too, including as a result of other eye conditions, such as cataract.
Long-sight means light is focusing too far beyond the retina, because the front of the eye isn’t curved enough or the eyeball is slightly short. As a result, close-up objects become blurry, while far-away things can be seen more clearly.
Long-sight often runs in families too and tends to be more common in people with European heritage. Babies are often born with some degree of long-sight but grow out of it by around age two as their eyes develop. If they don’t, there’s an increased risk of developing a squint or ‘lazy eye’ (amblyopia).
Many people with the condition find their eyes are able to adapt by focusing harder while they are young, but in adulthood, particularly beyond 40.
This is similar to long-sightedness but occurs as a result of ageing. Our lenses become less flexible as we age, which makes focusing harder. Reading glasses or contact lenses are needed by almost all of us after the age of 40-50.
This happens because the front of the eye is not evenly curved – a bit like a rugby ball compared to a football. As a result, as light passes through the eye, there’ll be more than one focus point on the retina, which means certain areas of a person’s field of vision will become blurry.
As with long-sightedness, astigmatism is very common at birth but often corrects itself as the eye develops. If by around age two this hasn’t happened, problems can occur. Astigmatism can develop in adulthood too, particularly after an eye injury or surgery, and it often occurs alongside short or long-sightedness, rather than on its own.
What are the signs and symptoms of refractive error?
Symptoms can vary depending on which type of refractive error somebody has and how severe it is. For some people symptoms are very mild and only noticeable during tasks that require a lot of focusing, such as reading, using a computer or driving. For others vision can be severely affected.
These are the signs and symptoms typically associated with refractive error:
- Blurred vision. This may affect distant or close-up objects, or even both.
- Headaches. Having to try harder to focus and adjust the eyes often causes headaches.
- Tired, strained eyes. This can happen with tasks that involve lots of focusing.
- Squinting. This can be a sign somebody’s having to work harder than normal to focus their vision (note that this is different from an ‘eye turn’, also known as a squint).
How is refractive error diagnosed?
Refractive error is generally diagnosed during routine eye tests. These will look at any difficulty or changes with focusing on both near and distant objects, to determine whether somebody is short or long-sighted. Eye tests will also look at how well your eyes are working together and how they react to light.
It’s recommended that everybody has a routine eye test every two years, although those known to be at higher risk may be eligible for more frequent free tests.
Children often aren’t aware if there’s a problem with their vision as they may never have known any different. So it’s important to look out for possible signs that they’re struggling to see clearly – such as sitting very close to the television, squinting, and rubbing their eyes a lot. The sooner problems are detected, the better, so it’s always best to get things checked out.
How is refractive error treated?
Most of the time refractive error is managed by wearing glasses or contact lenses. These will help correct blurred vision and may be required all the time or just while carrying out certain tasks, like reading and driving. Some people find their vision worsens as they get older and they may need stronger prescriptions. For refractive error associated with ageing, reading glasses can help.
If refractive error is very mild, treatment may not always be required, as the eyes may be able to adjust. However, having to work too hard to focus can result in eye strain and headaches, so glasses may still be required even if the impact on vision seems small.
It’s very important that young children receive appropriate treatment, as not treating refractive errors while their eyes and brains are still developing may lead to further problems down the line. This is because in order for their brains to ‘learn’ how to see correctly, both eyes need to be working clearly and equally. Treatment for children is usually glasses.
For adults with more severe refractive error, laser eye surgery to correct the shape of the cornea (front of the eye) may be an option. However, this isn’t suitable for everybody and will generally need to be paid for privately. Short-sight is sometimes also treated with surgery to replace the lens with an artificial one - a relatively new procedure that can be very effective but isn’t always suitable.
What research is underway?
Fight for Sight’s goal is to help further our understanding of refractive error and develop prevention techniques and better treatments.
Research into refractive error is focused on improving understanding of underlying causes, as well as developing prevention techniques and better treatments, such as using new surgical techniques and lens technology. Fight for Sight is supporting a range of projects in this area. Promising research has shown that more time spent outdoors in childhood may slow down short-sightedness in children. Very low dose atropine eye drops and special contact lenses also show some effect, but more research is needed.
Fight for Sight, along with the Thomas Pocklington Trust, is also funding research at Ulster University, to understand more about the role our ‘natural body clock’ plays in the development of short-sight. This includes looking at the impact of disrupted sleep quality, with the aim of increasing understanding of the condition and how to prevent it.
In the past, there has been less research on long-sightedness - but this is changing. It used to be thought that children would “make up the difference” if their long-sightedness wasn’t sufficiently corrected, but researchers are unclear about whether this is really the case and aim to continue in-depth research into this condition..
What can I do?
Have your eyes tested every two years, even if you think your vision is fine. If you or your child experiences any vision changes or loss of sight such as blurring, however minor, always get things checked right away – even if it’s less than two years since you/they last had tests. Anybody who may be more at risk of refractive error, such as people with a family history or children, may need more frequent tests.
Last updated June 2019
Approved by Professor Jeremy Guggenheim, Cardiff University
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