Worldwide, refractive error is a relatively common cause of vision being significantly reduced. Untreated refractive error can result in a person's vision being so poor that they are classified as being blind.
Refractive error is a deficiency or a failure of the optical surfaces of the eye to focus images clearly on the retina.
This results in a blurred image being perceived, even when all other parts of the eye and visual system are working perfectly.
Worldwide, it is a relatively common cause of vision being significantly reduced. Untreated refractive error can result in a person's vision being so poor that they are classified as being blind.
Addressing refractive error in the developing world has therefore become a priority for blindness-prevention organisations such as The Fred Hollows Foundation.
Types of refractive error
In the normal eye, light from a distant image is focused exactly on the surface of the retina, giving a clear image. This is called emmetropia.
There are four main types of refractive error:
Myopia (short-sightedness) - the optical surfaces of the eye are too powerful and the image focuses in front of the retina. In some severe forms of myopia, the eyeball grows too long as a developmental defect, so the retina is too far away from the focal point of the image. In this form of myopia (pathological myopia) there can also be degenerative damage to the retina from stretching.
Hypermetropia (long-sightedness) - the optical surfaces of the eye are not powerful enough to focus the image on the retinal surface and so the image is blurry.
Astigmatism - the optical surfaces of the eye are not spherical and are stronger in bending light in one axis than in the axis perpendicular to it. This effect is rather like light passing through a lens shaped like a rugby football. As the diagram shows, the image is focused more strongly in one direction than the other, causing it to be seen as a blur.
Presbyopia - in the normal young eye, images from near objects are actively focused onto the retina by an automatic change in the refractive power of the lens of the eye. With advancing age, this ability is progressively lost, so that most people from the age of about 45 need to have glasses to see near print and objects clearly. By the age of about 55 most people need glasses to see near print or objects well enough to be able to work with them.
With each of the types of refractive error, the greater its degree, the greater the blur and disability experienced.
Who gets refractive error?
Generally speaking, anyone can suffer from refractive error, however:
- Higher degrees of refractive error tend to run in families.
- Almost everyone gets presbyopia if they live long enough.
- Myopia is especially common in Asian peoples. It is a major cause of treatable visual disability in developing countries in Asia and other parts of the world.
- Myopia tends to begin in young people, at or before the age of puberty and tends to increase in degree until the age of about thirty, although this is variable.
- Hypermetropia tends to appear as people age because younger people have a natural mechanism by which they adjust to it. As in presbyopia however, there comes a point at which the eye is no longer able to adjust and blur is noted.
Can refractive error be treated?
For most forms of refractive error, glasses of the appropriate strength can relieve the problem and the person can see perfectly clearly when wearing them.
- For myopia, concave lenses are used.
- For hypermetropia and presbyopia, convex lenses are used.
- For astigmatism, cylindrical lenses are used.
Various combinations of lens shape are often used according to a person's individual measurements.
Contact lenses can also be used for the various types of refractive error and are especially useful for myopia. They have the advantage of giving a better quality of image when they are used to correct higher power refractive errors, as well as their cosmetic benefits. However, they can cause serious problems if they are not used properly.
Laser can be used to re-shape the front surface of the eye, the cornea, effectively making it into a corrective lens for the particular type of refractive error. A special computer processes measurements of the surface contour of the eye, and the shape to be ‘carved' into the eye is calculated. The laser is then used to vaporise the surface tissue of the cornea, resulting in a new shape.
This has been very effective and reliable particularly in people with myopia, and is a common, although expensive, procedure these days. In most advanced countries, an industry has grown up around this procedure.
Intraocular lens (IOL) surgery
In older people, removing the lens of the eye (in the same way as with cataract surgery) and replacing it with an intraocular lens of the appropriate power is also a viable way to address refractive error and this approach is becoming more common. It is usually cheaper than laser and avoids the complexity of the intraocular lens power calculations in someone who has already had a laser procedure, but it requires surgery to the inside of the eye.
Treatment in developing countries
The approach to dealing with refractive error in poorer countries is fairly straightforward.
Detection of refractive error as a cause of poor vision, including the use of school screening, needs to become part of comprehensive eye care delivery.
Training of non-medical staff in the ordering of appropriate glasses is part of the human resource development needed in all eye care programs.
Glasses can be provided to all who need them if they are sourced from appropriate manufacturers and a system is developed to make them reliably, consistently and cheaply available within communities.
Note: This information is general in nature and is not a substitute for specialist medical advice. Have your eyes checked regularly every two years, even if you have not noticed any symptoms or changes.
About the author
Dr David Moran, Ophthalmologist MB BS (Hons) FRANZCO
David first met Fred Hollows in the mid 1970s, and worked with him on the ground-breaking National Trachoma and Eye Health Program. David has been involved with The Foundation since its earliest days, and was a board member for five years until May 2010. A practising ophthalmologist, David has presented at numerous conferences and authored many papers and publications related to eye health.
Make a Donation
Call us: 1800 352 352