Anatomy of the Eye

How We See
Sight is a complex process that begins when light enters the eye through the clear cornea, which covers the pupil and the colored iris. The cornea focuses the light through the pupil and onto the lens. Although the cornea provides about two-thirds of the eye's focusing power, the lens plays a vital role in fine-focusing the light onto the retina, an extremely delicate tissue that lines the interior walls of the back and sides of the eye. The sensitive retinal tissue is composed of photoreceptors that convert light rays into electrical impulses. These electrical impulses are transmitted through the optic nerve to the visual cortex of the brain where they are interpreted into sight. This amazing process, which involves processing billions of pieces of information, is routinely performed instantaneously with impressive accuracy.

BACK TO TOP

Focusing Problems
Because sight is so complex, it is not surprising that many people do not enjoy naturally perfect vision. In fact, approximately half of all Americans require glasses or contact lenses to correct a focusing problem. Variations in the shape of the eye interfere with the eye's ability to focus light onto the retina properly. These variations are called refractive errors and include nearsightedness (myopia), farsightedness (hyperopia), astigmatism and presbyopia.

BACK TO TOP

Myopia (Nearsightedness)
Myopia is the most common refractive error, affecting approximately 30% of North Americans, although it occurs in up to 75% of some populations. Myopia generally occurs when the eye is too long or when the curvature of the cornea is too steep. In these cases, the cornea and lens bring light to a focal point before it reaches the retina: by the time the light rays reach the retina, they have begun to spread out, giving the brain a blurred image. In mild or moderate cases of myopia, this is only a problem when the eye is looking at distant objects, but people with more severe myopia may have difficulty seeing clearly up close as well.

Refractive surgery is designed to correct myopia by reducing the curvature of the cornea. This brings light to a focal point directly on the retina.

BACK TO TOP

Hyperopia (Farsightedness)
Hyperopia occurs when the eye is too short or the cornea is too flat. The cornea and lens in a hyperopic eye are unable to bring light rays to a focal point on the retina, resulting in blurred vision. Depending on the degree of farsightedness, both distance and near objects may appear blurry. In general, near vision is less clear than distance vision.

Refractive surgery corrects hyperopia by steepening the corneal curvature, thereby bringing light to a focal point on the retina.

BACK TO TOP

Astigmatism
Astigmatism can occur alone or in combination with myopia or hyperopia. Astigmatism is not related to the length of the eye, but occurs when the cornea is not spherical. Spherical corneas are round like a baseball and allow light to focus evenly on the retina. Astigmatic corneas distort light rays, causing them to focus on different planes and resulting in blurred vision.

Astigmatism is corrected surgically by removing tissue from selected sections of the cornea, in order to make the cornea more spherical.

BACK TO TOP

Presbyopia (Aging Eye)
Presbyopia differs from other refractive problems in that it is part of the natural aging process and will eventually affect everyone -- even people who have never needed glasses or contacts. Over time, the weakening of the muscles surrounding the lens and/or hardening of the lens itself reduces the lens' ability to focus on near objects. This typically occurs between the ages of 40 and 50 and is usually treated with reading glasses or bifocals.

Unfortunately, there is no FDA approved surgery to correct presbyopia, although new surgical techniques are being researched. The only surgical option available is known as monovision, and although it does not eliminate presbyopia, it can be an effective way for some people to deal with this refractive error. With the monovision option, one eye is corrected for near vision and the other for distance vision. This is very successful for some people, but it does not work well for everyone. If you are interested in monovision, please discuss this with the staff at Casey Vision Correction Center during your pre-operative appointment.

People with presbyopia who are not interested in monovision, but still wish to have their distance vision corrected may be suited to laser vision correction. The surgery can improve their distance vision, but reading glasses will still be needed for tasks involving near vision.

BACK TO TOP