The Eyes Have It

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The human eye is an amazing and complicated organ. To fully understand it requires some understanding of the physics of light, aka optics. For example, when light travels through a lens, the beams of light are bent by the curvature of the lens. The beams of light meet at a point known as the focal point. The focal point is where an image is at its sharpest. An image before or after the focal point will be blurry. When you look at an image through a magnifying glass, you move the glass up and down until the image becomes sharp. When the image is sharp you have found the focal point. The further an object is from the lens, the longer the distance to the focal point will be.

The outer clear part of the eye is the cornea. Light travelling through the cornea, which is a lens, is bent, and then bent again by the eye’s internal lens. A person with perfect vision will have the light beams passing through the eye meet directly at the retina, where cells convert the light to electrical signals that are transmitted to the brain via the optic nerve. Those signals are then interpreted by the brain as images. A person who is myopic, or nearsighted, has their focal point in front of the retina making distant images blurry. Someone with hyperopia, or farsightedness, has their focal point behind the eye making close objects blurry. Astigmatism, where the eye is irregularly shaped, makes all objects blurry. Age-related loss of near vision is called presbyopia, a condition that requires reading glasses to make near objects clear.

The most common procedure to correct these vision issues is LASIK, which stands for laser-assisted in situ keratomileusis. In LASIK, an ophthalmologist uses a laser to reshape the cornea. A small flap is made in the upper cornea, then the flap is lifted. The surgeon then uses to laser to reshape the deeper cornea by removing precise amounts of tissue. The goal of the procedure varies depending on the condition being corrected.

If the person is nearsighted, the goal is to flatten the cornea by removing tissue from the center. If farsightedness is being corrected, the goal is to steepen the cornea by removing tissue from the outer edges. LASIK can also correct astigmatism by reshaping the cornea into a more symmetric, spherical shape. While presbyopia cannot be reversed with LASIK, a procedure called monovision can be performed. This corrects one eye for distance and the other for near. LASIK is a quick procedure (often less than 15 minutes) that requires only numbing drops with no need for general anesthesia, stitches, or bandaging.

Complications from LASIK are rare. Some patients experience dryness, glare, light sensitivity, and halos around lights, although these tend to disappear over time in most people. Not everyone will have 20/20 vision after the procedure, but 95% of people are satisfied with its outcome. Some patients will require a touch-up treatment, called an enhancement, to make additional improvements, and others may still need glasses for activities such as reading. Rarely, some may find no vision improvement from the procedure.

For more information go to:                 www.medlineplus.gov/lasereyesurgery.html 

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By Peter Galvin, MD

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