If you can see objects nearby with no problem, but reading road signs or making out the writing on the board at school is more difficult, you may be near- or short-sighted.
Your eye care professional may refer to the condition as myopia, a term that comes from a Greek word meaning "closed eyes." The use of the word "myopia" for this condition may have grown out of one of the main indications of nearsightedness: Squinting to see distant objects clearly.
Myopia is not a disease, nor does it mean that you have "bad eyes." It simply refers to a variation in the shape of your eyeball. The degree of variation determines whether you will need corrective eyewear.
Myopia most often occurs because the eyeball is too long, rather than the normal, more rounded shape. Another less frequent cause of myopia is that the cornea, the eye's clear outer window, is too curved. There is some evidence that nearsightedness may also be caused by too much close vision work.
Our ability to "see" starts when light enters the eye through the cornea. The shape of the cornea, lens and eyeball help bend (refract) light rays in such a manner that light is focused into a point precisely on the retina.
In contrast, if you are nearsighted, the light rays from a distant point are focused at a place in front of the retina. As the light will only be focused in that one place, by the time it reaches the retina it will have "defocused" again, forming a blurred image.
Myopia usually occurs between the ages of 8 to 12 years. Since the eyes continue to grow during childhood, nearsightedness usually occurs before the age of 20. Often the degree of myopia increases as the body grows rapidly, then levels off in adulthood.
During the years of rapid growth, frequent changes in prescription eyewear may be needed to maintain a clear vision.
Myopia is often suspected when a teacher notices a child squinting to see a blackboard or a child performs poorly during a routine eye screening. Further examination will reveal the degree of the problem.
A comprehensive eye health examination will detect myopia. Periodic examinations should follow after myopia has been discovered to determine whether the condition is changing, and whether a change in prescriptive eyewear is needed. Eye exams also help to ensure that vision impairments do not interfere with daily activities.
Corrective concave lenses are prescribed to help focus light more precisely on the retina, where a clear image will be formed. Depending on the degree of myopia, glasses or contact lenses may be needed all of the time for clear vision. If the degree of impairment is slight, corrective lenses may be needed only for activities that require distance vision, such as driving, watching TV or in sports requiring fine vision.
If glasses or contact lenses are prescribed, it may take you a few days to adjust to them. After that, nearsightedness will probably not significantly affect your lifestyle. However, more severely nearsighted individuals may find the condition limits their choice of an occupation in some cases.
School-age children may have vision problems ranging from mild to severe. When problems are suspected, it is important that the child have a comprehensive eye health examination to determine the nature of the problem and to rule out serious eye diseases. When vision conditions are treated properly, the child will enjoy the best possible sight.
Hypermetropia is a form of refractive error, in which the power of the eye is too small for its length. As a consequence, light from a distant object does not come to a focus by the time it reaches the retina.
The refractive power of the eye's optical system is dependent upon the curvature of the optical surfaces. The curvature of the surfaces of the lens within the eye can be increased - thereby increasing the power of the eye - by a process known as accommodation. Although accommodation is used by emmetropes (those with no refractive error) to change their focus from a distance to a near object, it can often also be used by hypermetropic to overcome their refractive error. If it cannot, then the error is corrected by the positive spectacle, or contact, lenses, or by refractive surgery.
An uncorrected hypermetropia will be able to see clearly between infinity and their 'near point' (the closest they can see clearly). This 'near point' will generally be further away than that of an emmetrope. If some of the person's accommodation is being used to overcome the refractive error, there is less available for near-focusing. Consequently, a hypermetropia may suffer from presbyopia at an earlier age than normal.
Generally, uncorrected hypermetropic will not complain of blurred distance vision, but they may suffer from eyestrain and headaches (because of the continuous use of their accommodation) as well as blurred near vision.
For normal undistorted vision, the cornea should be smooth and equally curved in all directions. When astigmatism is present, the cornea is warped and curves more in one direction than the other. In other words, the cornea is shaped more like a rugby ball than a football. This causes objects at any distance to appear unclear, as the light will not focus to a point onto the retina. People with myopia or hyperopia can also have astigmatism.
Normally, light enters the eye and is focused precisely on the retina. With astigmatism, the warped cornea causes the light rays to bend improperly. They are not refracted equally in all directions and one focus point on the retina is not attained. Some light rays are not focused on the retina but are focused in front of or behind the retina. The result of multiple focal points is a distorted vision. Objects appear somewhat indistinct and slanted. Vision with astigmatism can be compared to what one sees when looking through a glass bottle, with images appearing too tall, too thin, too wide, or blurred.
Astigmatism is most often caused by a defect in the curvature of the cornea but may also occur from an unequal bending of light by the lens inside the eye. Astigmatism is usually inherited and may be present at birth. Astigmatism can occur in conjunction with nearsightedness or farsightedness and usually remains unchanged through life.
Astigmatism can also result from corneal scars or surgery. Eye surgery may cause the cornea to scar, changing the path of light as it enters the eye. Some patients experience surgically induced astigmatism after eye surgery. Tightly tied sutures can cause a slight wrinkling of the cornea. The patient will experience distorted vision or astigmatism until healing or loosening of the sutures relieves the problem.
Astigmatism is diagnosed with a routine eye examination. A visual acuity test is performed to determine the focusing power of the eye at different distances. A process called refraction is used to measure the refractive error of the eye and determine the prescription for corrective lenses.
A small amount of astigmatism is common and does not need correction. However, in cases where the problem is more severe, glasses and contact lenses are used to correct vision. To correct astigmatism, glasses or contact lenses are ground to neutralize the defective curvature of the cornea. Hard or gas permeable contact lenses generally improve astigmatism better than soft contact lenses. In some cases, however, soft contact lenses may be helpful.
Astigmatism can also be corrected with refractive surgery. Radial keratotomy, corneal transplantation and laser surgery are procedures used to correct the path of light as it enters the eye. The cornea is reshaped to allow light to focus properly on the retina. With refractive surgery, the eye regains its proper focusing ability and astigmatism is corrected.BL
Presbyopia is a vision condition in which the crystalline lens of your eye loses its flexibility, which makes it difficult for you to focus on close objects.
Presbyopia may seem to occur suddenly, but the actual loss of flexibility takes place over a number of years. Presbyopia usually becomes noticeable in the early to mid-forties.
Presbyopia is a natural part of the aging process of the eye. It is not a disease and it cannot be prevented.
Some signs of presbyopia include the tendency to hold reading materials at arm's length, blurred vision at normal reading distance and eye fatigue along with headaches when doing close work. A comprehensive optometric examination will include testing for presbyopia.
To help you compensate for presbyopia, your optometrist can prescribe reading glasses, bifocals, trifocals, varifocals or contact lenses. Since presbyopia can complicate other common vision conditions like nearsightedness, farsightedness and astigmatism, your optometrist will determine the specific lenses to allow you to see clearly and comfortably. You may only need to wear your glasses for close work like reading, but you may find that wearing them all the time is more convenient and beneficial for your vision needs.
Since the effects of presbyopia continue to change the ability of the crystalline lens to focus properly, periodic changes in your prescription may be necessary to maintain a clear and comfortable vision.