Light rays enter the eye, via the cornea and are refracted or ‘bent’ as they pass through the lens. This focuses the image sharply on the retina at the back of the eye, and the information is then transmitted to the brain via the optic nerve.
Refractive errors are a group of conditions referring to the inability of the eye to focus the light rays properly onto the retina, resulting in an image that is blurred or unclear.
There are four types of refractive errors :
This is also known as near-sightedness and is the most common refractive error in Singapore. It occurs when the eyeball grows too long, and as such, the light rays are focused in front
of the retina. There is difficulty seeing distant objects such as road signs or a movie screen. Near work such as reading or computer use is not affected.
Myopia usually develops in childhood and adolescence and progresses through to adulthood.
- Heredity, being more common in those who have one or both parents with myopia
- Frequent and excessive close work, especially in poor lighting
This is also known as far-sightedness and is uncommon in Singapore. It occurs when the eyeball is too short, resulting in the light rays converging behind the retina. There is poor near vision but no difficulty seeing distant objects.
It is relevant to note that we are born with eyeballs that are too short but which will elongate as we grow. When this elongation does not happen, hyperopia results and frequently may not be noticed until the onset of presbyopia in middle age.
This condition results from an uneven or irregular curvature of the cornea or lens. This causes the light rays entering the eye to be ‘scattered’, resulting in different focal points in front of, on or behind the retina. There is blurred and distorted vision for both near and far sight.
Astigmatism usually coexists with other refractive errors and is frequently not diagnosed until a child begins to read. It is thought to be hereditary but may also result from other eye conditions such as keratoconus or chalazion.
This condition is part of the normal aging process and is apparent usually from the early 40s onwards.
It results from the decreased elasticity of the lens and the weakening of the ciliary muscles which help the lens to ‘accommodate’ for near vision. The focal point of the light rays entering the eye is behind the retina (like in hyperopia), causing difficulty in near work like reading.
Distance vision is unaffected.
Other symptoms include difficulty seeing in dim light and eye fatigue/strain.
TREATMENT FOR REFRACTIVE ERRORS
The following are the commonly used means of correcting refractive errors:
- Corrective eyewear
- Contact lenses
- Laser surgery
- LASIK ie laser in-situ keratomileusis
- Photorefractive keratotomy (PRK)
Both of these procedures are similar in that the Excimer laser is used to reshape the cornea.
The difference lies in the access created on the corneal surface.
- Phakic intraocular lens implants
- radial keratotomy
Depending on the type of refractive error and your lifestyle needs, your optometrist and/or eye doctor will discuss with you the type of treatment most suitable for you.
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