Myopia is commonly known as short-sightedness. Short-sightedness occurs because the eye is too long and / or the cornea is too curved. This causes light rays entering the eye to form an image of an object at a point before it reaches the retina and as a result, the brain interprets it as a blurred image. As the object is moved closer towards the eye, it becomes clearer and more defined because the image ‘moves’ towards the retina, bringing it into better focus.
LASIK can be performed for myopia of power ranging from 0.50 to 20.00 dioptres (50 to 2,000 degrees). The procedure corrects myopia by removing a thin layer of tissue from the middle of the cornea. This results in central corneal flattening, allowing the point of focus to move closer to the retina, thereby improving a person’s distance vision.
Among people who suffer from myopia, some may have what is known as pathological myopia, a condition which results in ever increasing short-sightedness. This is usually due to the eyeball continuously growing in length.
Hypermetropia is caused by a person’s eye being too short, the cornea being too flat, or a combination of both. In the case of hypermetropes, light rays focus behind the retina, and only by moving the objects further away can the image become clearer. This gives them poor near vision as well as poor distance vision.
However, many people can accommodate and “focus away” their hypermetropia. They use their eye muscles to adjust the shape of the lens to bring their point of focus forward, onto the retina. LASIK is able to treat hypermetropia of powers ranging from 0.50 to 6.00 dioptres (50 to 600 degrees).
Hypermetropes may not have blurred vision when young, but as they age, their condition will become more evident as they begin to lose their ability to compensate for their hypermetropia by accommodation (focusing).
LASIK corrects the excessively flat cornea of a hypermetrope by removing peripheral parts of the cornea to form a circular ditch. When the LASIK flap is replaced after the procedure, the cornea becomes more curved, moving the point of focus from behind the eye onto the retina, improving both near and distance vision.
Presbyopia – also commonly known to the Chinese as “lao hua yan” (old age eyes) – is a condition which every person will eventually suffer. Presbyopia sets in when we pass the age of 40. As a person ages, the lens of the eye becomes harder and less elastic. This leads to increasing difficulty in focusing on near objects, allowing presbyopes to focus only within a narrow range of vision. Though presbyopes have poor near vision, their distance vision is still good if they do not have myopia, hypermetropia or astigmatism.
To date there is no cure for presbyopia, though it is being vigorously studied worldwide. Nothing has been found to replace or fully compensate for the loss of flexibility of the natural human lens. The only methods available are temporary measures like multi-focal spectacles, bi-focal contact lenses and various forms of experimental corneal and scleral surgery.
There are many claims of cures for presbyopia made every year. All have had limited success, unable to completely eliminate the condition. Presbyopia has nothing to do with the shape of the eye or the cornea, but with the loss of elasticity of the lens of the eye. This makes treatment by LASIK impossible.
Normal eyes have a spherical surface with regular curvature similar to the surface of a bowling ball. Astigmatism occurs when the shape of the eye’s curvature is irregular. This causes light rays to focus haphazardly at different points in the eye, leading to distorted vision. Most people have regular astigmatism. This means the eye has only 2 different curvatures. Light focuses at one point from the first curvature and at another point from the second curvature. This leads to the formation of a doubled image, giving the appearance of ‘shadowing’ or multiple images.
LASIK is able to correct astigmatism of a range from 0.25 to 6.50 dioptres (25 to 650 degrees).