Refractive error

What is it?

Refractive error is the name for a common group of conditions in which the light hitting the retina – a layer of tissue at the back of the eye - is unfocused.  For clear vision, light should be clearly focused so that the retina sends the best possible electrical signals to the brain.  

For most people, mild refractive error is inconvenient, but the eyes are otherwise healthy – a bit like having large, small or very narrow feet. Some other eye conditions, certain medical conditions and conditions that affect child development are particularly linked to refractive error. Uncorrected refractive error can be more damaging to developing eyesight in children.

Detecting refractive error is part of any routine eye test by an optometrist. Refractive error in most adults and older children is managed very well by glasses prescribed by local high street optometrists.

Common types of refractive error are:

  • Short-sight (myopia). This happens when the front of the eye is too curved or the eyeball is too long. It means that light is focussed short of the retina so distance vision is blurred.
  • Long-sight (hypermetropia or hyperopia). If the front of the eye isn’t curved enough or the eyeball is too short, the focus point will be beyond the retina. Effort to do extra focusing (accommodation) may be able to make things clear, but this is only possible in mild cases and is harder for near vision.
  • Presbyopia. This is often described as being a type of long sight, but is actually a normal part of getting older. The lenses in everyone’s eyes get less flexible with age whether or not we need distance glasses. This makes focusing on near things harder and reading glasses become necessary in middle age. Long-sighted people may notice the effects of presbyopia earlier than usual.
  • Astigmatism. This is when the front of the eye is not evenly curved – a bit like a rugby ball. It means there will be more than one focus point on the retina and, for example, the upright edges of a window frame will be clear, but the horizontal edges will be blurred.

  • Causes

    There are many different causes of refractive error and for most people it runs in families. There are some types refractive error specifically linked to problems with development, for example being born prematurely, or specific conditions.

    Myopia is often blamed on too much close work, and often develops or gets worse in teenagers. But the major risk factor is a family history. Too much time indoors may make it worse for at-risk children, however. Myopia is very common in people from the Far East. Variations in several genes may affect the risk of developing myopia and it may develop at any age as a result of other eye conditions such as cataract.

    Hypermetropia is more common in people with a European heritage. Most babies are a little long-sighted at birth but most grow out of it in the first two years of life. If they don’t, it can increase the risk of squint (strabismus) or lazy eye (amblyopia).

    Astigmatism is also very common at birth and is usually only a problem if it doesn’t get better by age 2, but injury and surgery can lead to later astigmatism.

  • Symptoms

    The most common symptom of myopia is having blurred vision for distant objects. In severe cases (high myopia) there is an increased risk that the retina can tear or pull away from its blood supply (detached retina).

    The main symptom of both hypermetropia and presbyopia is having trouble with near vision. People may also experience eye strain and headaches when doing close work such as reading.

    Astigmatism can cause blurred vision, headaches and eye strain.

    Children rarely complain of poor vision because they don’t realise they have it. Parents may spot that their child has trouble seeing, but refractive error can be easily missed, so it’s important that all children are screened well before they are 6 years-old.

    Screening should be done by someone with special training in testing children e.g. screening services led by an orthoptist, or an optometrist experienced with young children. The correct treatment given in childhood can restore good vision, but this may not be possible if it is not picked-up until later, especially if it is only in one eye or if long sight or astigmatism are marked.

  • Treatments

    Most types of refractive error can be corrected with glasses or contact lenses. Glasses and contact lenses work on the same principle – the lenses are curved in a way that compensates for the eye’s curve.

    For adults, not wearing their glasses may cause blurred vision or lead to eye strain or headaches, but does not damage eyesight permanently. For children with anything but mild refractive errors, glasses for are much more important.

    Most of 'seeing' is done by the brain, not the eyes which only capture the image. Children can only learn to see well if the images collected by the eyes are clear and equal in both eyes.

    Refractive error can also be treated with surgery. Generally this is laser eye surgery to change the shape of the cornea but is usually only offered to adults as the eyes change shape during growth. Having artificial lens implants may be an option for some people with very severe short-sightedness and are also commonly used after cataract surgery.

  • Research

    Refractive error research is focused on identifying the causes, predicting when it will get worse and preventing it and on developing better treatment, for example with new surgical techniques and lens technology. Short sight is an increasing problem in the Far East and there is lots of research but few clear conclusions yet.

    The most promising research is showing that more time spent outdoors in childhood may slow down myopia in children. Very low dose atropine eye drops, and special contact lenses also show some effect, but are still being tested.

    There has been less research on long sight, but this is changing. It used to be thought that children would “make up the difference” if their long sight was under-corrected, but we’re not sure any more and are trying to find out.

    Read our research projects
  • Clinical trials

    You could play an important part in eye research by being a participant in clinical research study that may benefit many people. You could even help shape clinical research by becoming more actively involved and having a say. Patients, carer, or anyone with an interest can help.

    What are clinical trials

    Clinical trials are research studies that find out if a medical strategy, treatment, or device is safe and effective for humans. They are a key research tool for improving medical knowledge and patient care. The people who carry out research are mostly the same doctors and healthcare professionals who treat people. Their aim is to find better ways of treating patients and keeping people healthy.

    Taking part

    Here are some ways to find out about research projects and clinical trials that you can get involved in.

    UK Clinical Trials Gateway

    The UK Clinical Trials Gateway run by the National Institute for Health Research (NIHR) provides easy to understand information about clinical research trials running in the UK, and gives to a large range of information about these trials. It is designed to enable patients and clinicians to locate and contact trials of interest. Visit their website and select the eye condition that you are interested in.

    NIHR Clinical Research Network Portfolio

    The NIHR Clinical Research Network Portfolio is a database of high-quality clinical research studies in England, Northern Ireland, Scotland and Wales. Within this the Ophthalmology Specialty Group supports a national portfolio of research studies in ophthalmology and the vision sciences. See their website for details.

    If you wish to join a trial it is always best to discuss this with your doctor or clinical team first.

Last updated April 2016
Approved by Dr Anna Horwood, University of Reading

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