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Refractive Errors |
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This article was provided by AllAboutVision.com. Follow the links below for more information on eye health and vision correction.
Refractive errors often are the main reason a person seeks the services of an optometrist or ophthalmologist. But what does it really mean when we're told that our vision is blurry because we have a refractive error?
We see the world around us because of the way our eyes bend (refract) light. Refractive errors are optical imperfections that prevent the eye from properly focusing light, causing blurred vision. The primary refractive errors are nearsightedness, farsightedness and astigmatism.
Causes of Refractive Errors
The eye's ability to refract or focus light sharply on the retina primarily is based on three eye anatomy features: 1) the overall length of the eye, 2) the curvature of the cornea and 3) the curvature of the lens inside the eye.
- Eye Length: If the eye is too long, light is focused before it reaches the retina, causing nearsightedness. If the eye is too short, light is not focused by the time it reaches the retina. This causes farsightedness or hyperopia.
- Curvature of the Cornea: If the cornea is not perfectly spherical, then the image is refracted or focused irregularly to create a condition called astigmatism. A person can be nearsighted or farsighted with or without astigmatism.
- Curvature of the Lens: If the lens is too steeply curved in relation to the length of the eye and the curvature of the cornea, this causes nearsightedness. If the lens is too flat, the result is farsightedness.
More obscure vision errors, known as higher-order aberrations, also are related to flaws in the way light rays are refracted as they travel through the eye's optical system.
These types of vision errors, which can create problems such as poor contrast sensitivity, are detected through new technology known as wavefront analysis.
Myopia
Nearsightedness, or myopia, is a vision problem experienced by up to about one-third of the population. In fact, a recent study* found that myopia is more common among Americans now than it was 30 years ago.
Nearsighted people have difficulty reading highway signs and seeing other objects at a distance, but can see for up-close tasks such as reading or sewing. Nearsighted people often have headaches or eye strain and might squint or feel fatigued when driving or playing sports. If you experience these symptoms while wearing your glasses or contact lenses, you may need a comprehensive eye examination as well as a new prescription.
Myopia occurs when the eyeball is slightly longer than usual from front to back. This causes light rays to focus at a point in front of the retina, rather than directly on its surface. Nearsightedness runs in families and usually appears in childhood.
Nearsightedness may be corrected with glasses, contact lenses or eye surgery. Depending on your vision problem, you may need to wear your glasses or contact lenses all the time or only when you need distance vision, like driving, seeing a chalkboard or watching a movie.
If you're nearsighted, your prescription is a negative number. The higher the numeral, the stronger your lenses will be.
Refractive surgery can reduce or even eliminate your need for glasses or contacts. The most common procedures are performed with an excimer laser.
- In PRK the laser removes a layer of corneal tissue, which flattens the cornea and allows light rays to focus closer to or even on the retina.
- In LASIK — the most common refractive procedure — a flap is cut through the top of the cornea, a laser removes some corneal tissue, and then the flap is dropped back into place.
- Then there's orthokeratology, a non-surgical procedure where you wear special contact lenses that slowly reshape the cornea over time to correct your myopia. When the lenses are removed, the cornea temporarily retains the new shape, so you can see clearly without the lenses.
- With orthokeratology or corneal refractive therapy (CRT), an orthokeratology-like procedure approved by the FDA in 2002, you wear cornea-shaping lenses at night, so you have daytime vision without contacts or glasses.
- Implantable lenses known as phakic IOLs are a newer surgical option for correcting nearsightedness, particularly in more extreme cases that may be unsuitable for LASIK or other vision correction surgery.
- Phakic IOLs work like contact lenses, except they are surgically placed within the eye and typically are permanent, which means no maintenance is needed. Unlike IOLs used in cataract surgery, phakic IOLs do not replace the eye's natural lens, which is left intact.
Hyperopia
Hyperopia, or farsightedness, is a common vision problem, affecting about a fourth of the population. People with hyperopia can see distant objects very well, but have difficulty focusing on objects that are up close.
Farsighted people sometimes have headaches or eye strain and may squint or feel fatigued when performing work at close range. If you get these symptoms while wearing your eyeglasses or contact lenses, you may need an eye exam and a new prescription.
This vision problem occurs when light rays entering the eye focus behind the retina, rather than directly on it. The eyeball of a farsighted person is shorter than normal. Many children are born with hyperopia, and some of them "outgrow" it as the eyeball lengthens with normal growth. Sometimes people confuse hyperopia with presbyopia, which also causes near vision problems but for different reasons.
You may need to wear your glasses or contacts all the time or only when reading, working on a computer or doing other close-up work.
Refractive surgery, such as LASIK , is another option for correcting hyperopia. Surgery may reduce or eliminate your need to wear glasses or contact lenses. Investigational procedures involving corneal inlays and onlays may be a future option for correcting hyperopia.
Astigmatism
Astigmatism might be the most misunderstood vision problem. Even the name is challenging to many people, who incorrectly call it "stigmatism."
Like nearsightedness and farsightedness, astigmatism is a refractive error, meaning it is not an eye health problem; it simply is a problem with how the eye focuses light.
In an eye with astigmatism, light fails to come to a single focus on the retina to produce clear vision. Instead, multiple focus points occur, either in front of or behind the retina (or both).
Astigmatism usually causes vision to be blurred or distorted to some degree at all distances.
Symptoms of uncorrected astigmatism are eye strain and headaches, especially after reading or other prolonged visual tasks. Squinting also is a very common symptom of uncorrected astigmatism.
Astigmatism usually is caused by an irregularly shaped cornea. Instead of the cornea having a symmetrically round shape (like a baseball), it is shaped more like a football, with one meridian being significantly more curved than the meridian perpendicular to it.
(To understand what meridians are, think of the front of the eye like the face of a clock. A line connecting the 12 and 6 is one meridian; a line connecting the 3 and 9 is another.)
The steepest and flattest meridians of an eye with astigmatism are called the principal meridians.
In some cases, astigmatism is caused by the shape of the lens inside the eye. This type of astigmatism is called lenticular astigmatism, to differentiate it from the more common corneal astigmatism.
Astigmatism, like nearsightedness and farsightedness, usually can be corrected with eyeglasses, contact lenses or refractive surgery.
Presbyopia
Presbyopia usually occurs beginning at around age 40, when people experience blurred near vision when reading, sewing or working at the computer.
You can't escape presbyopia, even if you've never had a vision problem before. Even people who are nearsighted will notice that their near vision blurs when they wear their usual eyeglasses or contact lenses to correct distance vision.
Presbyopia Symptoms and Signs
When people develop presbyopia, they find they need to hold books, magazines, newspapers, menus and other reading materials at arm's length in order to focus properly. When they perform near work, such as embroidery or handwriting, they may develop headaches, eye strain or feel fatigued.
What Causes Presbyopia?
Presbyopia is caused by an age-related process. This differs from astigmatism, nearsightedness and farsightedness, which are related to the shape of the eyeball and are caused by genetic and environmental factors. Presbyopia generally is believed to stem from a gradual thickening and loss of flexibility of the natural lens inside your eye.
These age-related changes occur within the proteins in the lens, making the lens harder and less elastic over time. Age-related changes also take place in the muscle fibers surrounding the lens. With less elasticity, the eye has a harder time focusing up close. Other, less popular theories exist as well.
Articles on our website are courtesy of AllAboutVision.com. Visit their site for more information about eyeglasses, types of contact lenses and various eye conditions.
Article ©2011 Access Media Group LLC. All rights reserved. Reproduction other than for one-time personal use is strictly prohibited.
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