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Refractive Disorders of the Eye
Dioptres
Refractive eye disorders such as short, long sight and astigmatism are measured in units known as dioptres. The bigger the number, the larger your refractive error and the thicker your glasses or contact lenses.
Your glasses prescription is written as a series of numbers e.g. -4/-3 x 180.
The first number shows the amount of short or long sight you have. A minus sign in front of the number shows you are short sighted while a plus sign shows you are long sighted.
The second number shows how much astigmatism (ovalness) your eye has.
The third number shows the axis or direction of your astigmatism.
Myopia - short sight
Myopia is the medical term for short sightedness. Myopia occurs when the eye is too long for the eye's focusing system, causing light to be focused in front of the retina, making distant objects appear blurry. However near objects may be focused clearly.
About 90% of myopes have a refractive error of less than -6.00 dioptres. Myopia is classified according to the corrective power needed.
- For example: Mild myopia less than -3.00 dioptres
- Moderate myopia -3.00 to -6.00 dioptres
- Severe myopia -6.00 to -9.00 dioptres
- Extreme myopia more than -9.00 dioptres
Hypermetropia - long sight
Hypermetropia (also known as hyperopia), or long sight, occurs when the eye is too short for the eye's focusing system, causing light rays to be focused behind the retina.
Younger people who are not very long sighted may be able to see clearly by using their focusing muscles. As people get older and approach middle age, their focusing ability deteriorates, and presbyopia begins to develop. Long sighted people who could focus to see clearly when younger, may find their vision deteriorates and they need glasses as they get older. Some people who are very long sighted may need glasses from a young age.
Presbyopia
With increasing age, the lens of the eye becomes harder and is less able to change shape. This results in increasing difficulty in seeing at close distances, and is known as presbyopia.
This is a normal ageing process. Long sighted people will experience this at an earlier age than myopic people. This is corrected with reading glasses or bifocal glasses.
Monovision
People who are short sighted will retain the ability to see at close distances after the age of 45 (the short sight counteracts the presbyopia). If the short sight is removed by refractive surgery, glasses will be required for near vision after middle age. This need for reading glasses can be reduced by having one eye deliberately undercorrected, leaving it mildly myopic and able to still see reasonably well at near distances. This is known as monovision.
Astigmatism
Astigmatism is very common and often occurs in association with myopia. Astigmatism occurs when the cornea has an uneven curvature, being shaped more like a rugby ball than a soccer ball. As a result, patients may experience tilting or distortion of images. With high amounts of astigmatism, vision may be blurred at close as well as distance.