| *Intended for a Canadian audience only |
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| *Intended for a Canadian audience only |
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Most of us inherit the basic shape of our eyes which determines how we focus. As we reach adulthood, changes in the shape of the eye stabilize and there is typically minor variation, if any, in visual acuity. Well over half the population will require no external device to assist with clear vision. The remainder, however, will seek some form of correction to see clearly.
Defocus errors produce distorted vision that can be corrected with glasses, contacts, and possibly nonsurgical corneal reshaping such as Orthokeratology. Through a specially shaped external lens in your glasses or contacts, defocus errors are corrected and functional vision is attained. Orthokeratology is nonsurgical corneal reshaping that involves intermittent wear of tightly fitted contact lenses which reshape the cornea for temporary periods. With Laser Eye Surgery, however, the cornea is permanently reshaped to compensate for the type of defocus error you have. These errors may include the following:
A nearsighted person can see close images well but has difficulty with distant images. This is because images are best focused in front of, or short of the retina (hence, the term "short-sightedness"), usually the result of an elongated eye. Treatment flattens the corneal curvature at the centre of vision, resulting in an extended angle of refraction so that images focus best on the retina.
A farsighted person can see distant images well but has difficulty seeing near images. This is because images are best focused beyond the retina, usually the result of a shortened eye. Treatment creates a steeper corneal curvature at the edges of the centre of vision, resulting in a shortened angle of refraction so that images focus best on the retina.
Unlike nearsightedness and farsightedness, astigmatism is not considered a distance error. An astigmatic person has blurred vision at any distance because the cornea or structures within the eye have an irregular shape that results in more than one focal point, producing multiple images within the eye. Treatment creates a symmetrically shaped corneal curvature, resulting in a single focal point and therefore, a single image. When astigmatism exists in conjunction with a distance error, both are treated at the same time.
In youth and early adulthood, our eyes have a remarkable ability to adjust the strength for varying distances of vision. This is because the natural crystalline lens inside the eye is flexible. It's composed of a gel-like material, enclosed in a delicate membrane surrounded by ligaments attached to eye muscles. As the muscles contract or relax, the lens can be stretched thinner or relax thicker for clear vision at varying distances. This process is called accommodation.
As we mature, however, the lens and its supporting structures become less malleable and we have increasing difficulty accommodating near images such as print. This normal phenomenon, called presbyopia, typically becomes noticeable sometime toward our mid-to late 40's. We then turn to progressive lenses, bifocal lenses or reading glasses to assist with focusing near images.
Surgical vision correction procedures that involve only the cornea of the eye, such as Laser Eye Surgery, do not alter presbyopia because the surgery does not reach the source of the problem: the natural lens of the eye. A more comprehensive solution to resolve both distant and near vision may be desired. That exists in the form of Refractive Lens Exchange, using the ReSTOR lens implant.