When an optical system is out of focus, blur is present. Refractive Error is the measurement and quantification of blur present in the human optical system. Referencing the human eye, the specific types of blur are also qualified and each condition is named respectively.
To see clearly, light rays pass through the eye and are focused on the retina that lines the back wall of the eye. The cornea, which is the clear front window of the eye, and the lens inside the eye, are responsible for bending or "refracting" light rays so that they focus properly. The retina receives the picture formed by the light rays and sends the image to the brain through the optic nerve.
Myopia (nearsightedness), hyperopia (farsightedness), and astigmatism (distorted vision) are caused by differences in the length or the shape of the eye. Presbyopia (aging eye) occurs when the lens inside the eye loses its focusing ability for near vision. These conditions are called "refractive errors" because the shape of the eye affects the way the eye "refracts" or bends light and focuses it on the retina. It is usual for refractive errors to occur within a certain range. Outside of this range, the error is considered high, as in "high" myopia or "high" hyperopia. Other medical problems within the eye occur more often when the refractive error is in the "high" range. The major symptoms of refractive errors are decreased vision, eye discomfort, or eye strain. They are entirely correctable with the appropriate glasses or contact lenses.
Myopia is the medical word for nearsightedness, a condition in which the eyes can see objects that are close but are unable to see distant objects clearly. The word "myopia" comes from a Greek word for "closed eyes" because people with this condition often squint their eyes to see better in the distance. Most commonly, myopia is an inherited trait that often becomes evident in children at about eight to twelve years of age. Typically, the condition increases as the body grows during teenage years before leveling off in adulthood. The major factor influencing nearsightedness is heredity or a family history of nearsightedness. Few factors other than heredity influence nearsightedness. Reading too much, using eyes in dim light, or deficiencies in nutrition do not cause or affect myopia. Usually myopia is the result of a lengthened eyeball. The eye becomes more oval than round, and because of this increased length, it is impossible for the lens to change its shape sufficiently to focus light from distant objects clearly on the retina. Rarely, myopia can also be caused by a change in the curvature of the cornea or a change in the shape of the lens in the eye. Nearly everyone with myopia has what is called simple myopia which increases as the body grows. Although myopia increases, it is the result of normal growth changes. As a child's body grows during adolescence, there are often changes in the length of the eye requiring new eyeglasses as frequently as every six months, much like getting larger shoes to fit growing feet. Historically, myopia may progress rapidly for several years and then change very little thereafter. Between the ages of 20 and 40, there has been usually very little change, although an adult form of myopia may occur in the early twenties. More recently, this trend has changed to include changes into the thirties, and there has also been some correlation to computer use.
Myopia is best treated with eyeglasses or contact lenses. The lenses do not reverse the condition but they compensate for the longer shape of the eye by "refracting" or bending light rays to focus on the retina. There is recent overwhelming evidence that contact lenses and more specifically, Orthokeratology, slows or stops the progression of myopia. Taking a more active role in the prevention of myopia increasing has been shown to be very important because it decreases the risk of sight-threatening conditions. This is termed "myopia management." You can read more about Orthokeratology and the current studies examining Orthokeratology's effectiveness in controlling myopia and its progression in the related links section below. Medical treatment for myopia with the use of special drops (atropine) has also been shown to be effective like Orthokeratology in slowing or halting the progression of myopia. Bifocal glasses have been studied but has not been shown to be very effective at slowing or halting the progression of myopia. Surgical treatment for myopia is also available although this does not apply to children and is only available for adults. There are several different surgical procedures available to reduce myopia and they each have pros and cons in addition to the inherent surgical risk accompanying all of these procedures. There is also significant limitation to any and all of the surgical procedures addressing all of the visual needs of the individual. Even those people that achieve very satisfactory visual results through surgical intervention, will find themselves needing an optical corrective device (glasses, etc.) within some time frame. It has been shown that as many as 50% of people reporting good outcome initially from refractive surgery find themselves wearing glasses for at least some activities within one year of surgery. Additionally, although the reduction of myopia may be achieved through one of these surgical procedures, the slowing or stopping progressing myopia will not be addressed by any of the currently available surgical procedures. Most doctors feel that eyes with simple myopia are best treated with glasses or contact lenses. Myopia is usually detected in children during periodic school eye examinations or during the course of routine physical examinations. Those who have myopia should have yearly vision tests particularly during their growth years. Unfortunately, myopia in on a sharp rise worldwide. Not only are more people becoming myopia, but their amounts are also increasing. Between the 1960s and the 1990s, the United States of America has seen the incidence of myopia increase from 32% to 42% of the population. This alarming increase is not confined to the USA, but is happening worldwide. In Taiwan, the incidence of myopia is now nearly 90% of the population. Since myopia is not just an "inconvenience" requiring optical correction, but also increases the risks of other associated potentially blinding conditions such as cataract, glaucoma, and retinal detachments, there is a great deal of study occurring now to find ways to curtail this "myopic epidemic." Currently, the most exciting information to come from all of this research is that treatments such as Orthokeratology seem to be very effective at reducing, if not totally arresting, the progression of myopia.
Pathologic myopia is a rare form of myopia which may cause sight loss that cannot be corrected by glasses or contact lenses. The condition is usually inherited and may progress rapidly causing the retina to tear and detach from the back of the eye. Frequent medical eye examinations by an eye doctor are needed to watch for signs of these changes. Although pathologic myopia may cause impairment of vision, new methods of treatment usually prevent this condition from resulting in blindness.
Hyperopia is the medical term for farsightedness. In this condition, the eyeball is usually shorter than normal (opposite from myopia, where the eye is often too long). This shortness makes it difficult for the lens to focus light from close-up objects clearly on the retina. Rarely, a flattening of the cornea or thinning of the lens in front of the eye may also cause farsightedness. Normally, all young children are farsighted to a moderate degree, and the condition decreases during the first few years of life. As the small farsighted eye of a baby grows and becomes longer, farsightedness lessens. Young people who are farsighted usually see well for both distance and near, since the focusing ability of the lens is strong enough to adjust for the shortness of the eye. Sometimes farsightedness may be associated with crossed eyes in children, since the muscles of the eyes have to contract tightly to see up close. In children, such non-visual symptoms as headaches and lack of interest in reading may be warning signs of a severe degree of hyperopia. Like nearsightedness, farsightedness is usually inherited. A recent study showed that lack of hyperopia in the very early years i.e. six years of age, is actually a strong predictor that progressive myopia is highly likely.
Hyperopia is most often corrected with the use of glasses or contact lenses. Correction of small degrees of farsightedness in children without other visual or non-visual symptoms is often not necessary, and only those with severe hyperopia or crossed eyes need to be given glasses for this condition when they are young. As the individual ages however, the focusing ability decreases and the person can become symptomatic. At this point, correction of the hyperopia does become important. Other options to correct hyperopia also include Orthokeratology, although this type of Orthokeratology can be a bit more challenging.
Astigmatism is most often caused by distortion or an irregularity of the cornea, the front surface of the eye. For normal undistorted vision, the cornea should be smooth and equally curved in all directions. When an individual as astigmatism, the cornea is "warped" and it curves more in one direction than in the other. In other words, the cornea is shaped more like a football than a basketball. The effect of astigmatism is to change vision similar to that seen when looking at a mirror with a wavy surface, like the "funhouse" mirrors that make you seem too tall or too wide or too thin. Astigmatism is usually inherited, may be present at birth, and frequently remains unchanged throughout life. Small amounts of astigmatism are very common and often do not require correction with glasses or contact lenses.
Correction of astigmatism is not difficult if the distortion proceeds in a regular or straight line across the cornea. In such instances, a similar wave can be ordered as a prescription in the glasses to neutralize or offset the distortion of the cornea. In some instances, however, such as a scar following an injury to the eye, the distortion of the cornea may be irregular. So-called irregular astigmatism is more difficult to correct and often can be improved only by making a totally new surface on the cornea. This may be accomplished either by applying a special rigid contact lens, or by performing a corneal transplant, which involves removing the old scarred cornea and replacing it with donated tissue. Normally, the blur from astigmatism can be corrected with glasses, contact lenses, or even Orthokeratology. Rigid contact lenses improve astigmatism better than soft contact lenses, but special soft contact lenses which correct astigmatism are sometimes helpful. If the amount of astigmatism is very large, glasses that correct the condition may cause some distortion of side vision. Very large amounts of astigmatism are not easily corrected with a contact lens either since the lens may wobble on the wavy surface of the cornea. In such instances a special lens, called a toric lens, in both soft and rigid designs, may be made with a curve in the back surface to stabilize the lens on the cornea.
As people become older and experience more difficulty in focusing their eyes for close objects, glasses are necessary for reading and close-up work. This condition is called presbyopia. Most people are not bothered by presbyopia until after the age of 40, when the lens of the eye loses enough of its flexibility that it cannot focus on near objects held at their normal working distance. Presbyopia is an inevitable problem because it occurs as a result of the natural growth of the lens inside the eye. It is usually corrected with reading glasses. No treatment including diet or exercise is known to either increase or decrease its progress. Bifocal, trifocal, or progressive addition spectacles may be prescribed for persons with presbyopia who have other errors of refraction such as myopia, hyperopia, or astigmatism. Presbyopia can also be corrected using multifocal contact lenses and multifocal Orthokeratology.
More Information About Children, Myopia, Myopic Progression, and Myopia Management.
Information About Wave Technololgy (Contact Lenses For Refractive Errors).
Information About ZWave For Astigmatism (Regular and Irregular).
What Are Refractive Errors?
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