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Lenses Medical Eye Info |
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 shortsighted. This condition is known as myopia, a
term that comes from a Greek word meaning "closed eyes."
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 or not you will need
corrective eyewear. What causes
nearsightedness? 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. How does myopia affect
sight? 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 almost always 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 clear vision. It is
important to bear in mind that the frequent changes in prescription are
not making the eyes "weaker". During the growth period that
occurs during the teen years the eye is also growing rapidly and hence
the degree of blur is also increasing. As the growth cycle slows the
prescription changes slow correspondingly. 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 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 (minus) 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. Nearsightedness
in children 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. To help a child cope
with nearsightedness:
If you can see objects at a distance clearly but have trouble focusing well on objects close up, you may be farsighted. Farsightedness or long-sightedness is often referred to by its medical names, hypermetropia or hyperopia. Hyperopia causes the eyes to exert extra effort to see close up. After viewing near objects for an extended period, you may experience blurred vision, headaches and eyestrain. Children who are farsighted may find reading difficult. Hyperopia is not a disease, nor does it mean that you have "bad eyes." It simply means that you have a variation in the shape of your eyeball. The degree of variation will determine whether or not you will need corrective lenses. What causes farsightedness? Hyperopia most commonly occurs because the eyeball is too short; that is, shorter from front to back than is normal. In some cases, hyperopia may be caused by the cornea having too little curvature. Exactly why eyeball shape varies is not known, but the tendency for farsightedness is inherited. Other factors may be involved too, but to a lesser degree than heredity. How does farsightedness affect sight? 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. If, as in farsightedness, the eyeball is too short, the "point of light" focuses on a location behind the retina, instead of on the correct area of the retina, known as the fovea. As a result, at the point on the retina where a fine point of light should be focused, there is a disk-shaped area of light. Since light is not focused when it hits the retina, vision is blurred. Convex lenses are prescribed to bend light rays more sharply and bring them to focus on the retina. How is it diagnosed and treated? Hyperopia is seldom diagnosed in school eye-screening tests, which typically test only the ability to see objects at a distance. A comprehensive eye examination that checks both near and far vision is necessary to diagnose farsightedness. In some cases it may be necessary for the practitioner to use drops during the examination to relax the eye muscles and ensure that the full degree of hyperopia is detected. This is necessary because the muscles which focus the eye are so accustomed to being used to compensate for the hyperopia that the muscles go into "spasm" and cannot relax without being forced to do so. Corrective convex lenses (positive powers) are usually prescribed. They bend light rays more sharply and bring them to focus on the retina. If you do not have other vision problems such as astigmatism, you may only need glasses for reading or other tasks performed at a close range. To determine the best avenue of treatment, questions about your lifestyle, occupation, daily activities and general health status may be asked. For instance, you may be asked whether or not you frequently need near vision. Providing candid, considered answers to the questions will help assure that your corrective lenses contribute to clear sight and general comfort. A comprehensive eye examination at the recommended intervals will ensure that minor changes in vision are diagnosed and treated so that your vision will remain as clear and comfortable as possible. If you experience a distortion or blurring of images at all distances— nearby as well as far—you may have astigmatism. Even if your vision is fairly sharp, headache, fatigue, squinting and eye discomfort or irritation may indicate a slight degree of astigmatism. A thorough eye examination, including tests of near vision, distant vision and vision clarity, can determine if astigmatism is present. Astigmatism is not a disease nor does it mean that you have "bad eyes." It simply means that you have a variation or disturbance in the shape of your cornea. Astigmatism is one of a group of eye conditions known as refractive errors. Refractive errors cause a disturbance in the way that light rays are focused within the eye. Astigmatism often occurs with nearsightedness and farsightedness, conditions also resulting from refractive errors. What causes astigmatism? Astigmatism usually occurs when the front surface of the eye, the cornea, has an irregular curvature. Normally the cornea is smooth and equally curved in all directions and light entering the cornea is focused equally on all planes, or in all directions. In astigmatism, the front surface of the cornea is curved more in one direction than in the other. With the cornea's shape more like that of an American football or rugby ball than a basketball, the light hitting the more curved surface comes to a focus before that which enters the eye through the less curved surface. Thus, the light is focused clearly along one plane, but is blurred along the other so only part of anything being looked at can be in focus at any time. This abnormality may result in vision that is much like looking into a distorted, wavy mirror. The distortion results because of an inability of the eye to focus light rays to a point. Why are corneas shaped differently? Not all corneas are perfectly curved, just as sets of teeth are seldom perfectly aligned. The degree of variation determines whether or not you will need corrective eyewear. If the corneal surface has a high degree of variation in its curvature, light refraction may be impaired to the degree that corrective lenses are needed to help focus light rays better. The exact reason for differences in corneal shape remains unknown, but the tendency to develop astigmatism is inherited. For that reason, some people are more prone to develop astigmatism than others. How does astigmatism affect sight? The clear cornea is situated at the very front surface of the eye and enables light to enter the eyeball. The cornea accomplishes about four-fifths of the refractive work needed for clear vision, bending light rays into a point. The crystalline lens, located behind the cornea, further refines the refractive work begun by the cornea and directs the point of light toward a precise location on the retina, known as the fovea. If light is not focused into a fine point on the fovea, the image that reaches the retina cannot be clearly transmitted to the brain and a blurred image is perceived. When astigmatism is present, the surface of the cornea is distorted instead of spherical. It is unable to focus light rays entering the eye into the fine point needed for clear vision. At any time, only a small proportion of the rays are focused and the remainder are not, so that the image formed is always blurred. Usually, astigmatism causes blurred vision at all distances. Who develops astigmatism? stigmatism is very common. Some experts believe that almost everyone has some degree of astigmatism, often from birth, which may remain the same throughout life. Of interest to parents and those who work with children, astigmatism may contribute to poor schoolwork but is often not detected during routine eye screening in schools. How is it diagnosed? Astigmatism is diagnosed in the course of a thorough eye examination. How is it treated? If the degree of astigmatism is slight and no other problems of refraction, such as nearsightedness or farsightedness, are present, corrective lenses may not be needed. If the degree of astigmatism is great enough to cause eyestrain, headache, or distortion of vision, prescription lenses will be needed for clear and comfortable vision. The corrective lenses needed when astigmatism is present are called "Toric" lenses and have an additional power element called a cylinder. They have greater light-bending power in one axis or direction than in the others. Precise tests will be made during your eye examination to determine the ideal lens prescription. Astigmatism may increase slowly over time. Regular eye examinations can help to ensure that proper vision is maintained. Hold the book up close and the words appear blurred. Push the book farther away, and the words snap back into sharp focus. That's how most of us first recognize a condition called Presbyopia, a name derived from Greek words meaning "old eye." Eye fatigue or headaches when doing close work, such as sewing, knitting or painting, are also common symptoms. Because it is associated with aging, Presbyopia is often met with a groan -- and the realization that reading glasses or bifocals are inevitable What causes Presbyopia? As we age, body tissues normally lose their elasticity. As skin ages, it becomes less elastic and we develop wrinkles. Similarly, as the lenses in our eyes lose some of their elasticity, they lose some of their ability to change focus for different distances. The loss is gradual. Long before we become aware that seeing close up is becoming more difficult, the lenses in our eyes have begun losing their ability to flatten and thicken. Only when the loss of elasticity impairs our vision to a noticeable degree do we recognize the change. Recent research suggests that Presbyopia occurs when the lens keeps growing as people get older and the ligaments become too slack for the muscles to work properly. This finding contrasts with the traditional view that aging cause the focusing muscles to become weaker and the lens to become more inflexible. How does the loss of elasticity affect sight? The crystalline lens plays a key role in focusing light on the retina. When we are young, the lens is flexible. With the help of tiny ciliary muscles, it changes shape, or accommodates, for both near and distant objects by bending or flattening out to help focus light rays. As we age, the lens becomes stiffer. Changing shape becomes more difficult. Not only does focusing on near objects become more difficult, the eye is also unable to adjust as quickly to rapid changes in focus on near and distant objects. The flexibility of the lens begins to decrease in youth. The age at which Presbyopia is first noticed varies, but it usually begins to interfere with near vision in the early 40's. Presbyopia affects everyone and there is no known prevention for it. How is the problem diagnosed and treated? An accurate, thorough description of symptoms and a comprehensive eye health examination, including a testing of the quality of your near vision, are necessary to diagnose Presbyopia. Usually, the treatment for Presbyopia is prescription eyeglasses to help the eye accommodate for close-up work. Prescription lenses (reading glasses) help refract light rays more effectively to compensate for the loss of near vision. If you do not have other vision problems, such as nearsightedness or astigmatism, you may only need glasses for reading or other tasks performed at a close range. If you have other refractive errors, such as nearsightedness, bifocal or progressive addition lenses (in which the power of the lens changes gradually towards the bottom to allow reading, without the reading portion of the bifocal lens being obviously visible) are often prescribed. Can I still wear contact lenses? Yes, you have three options with contact lenses: Bifocal contact lenses, monovision, and normal distance contact lenses with reading glasses. Generally, bifocal contact lenses are not as successful as the normal "single vision" ones. What lens option will work best for me? You will be asked a number of questions about your usual lifestyle or daily activities - to help determine the solution most suited to your needs. For instance, if you are a librarian, your needs will be significantly different from those of a truck driver or office worker. Presbyopia is a gradual change, happening over a number of years so your prescription will need to be updated periodically. Changes are best made at your regular eye examination rather than after the need for change starts to cause you difficulties. If you see two of whatever you are looking at, you may have a condition known as double vision, also referred to as diplopia. Double and blurred vision are often thought to be the same, but they are not. In blurred vision, a single image appears unclear. In double vision, two images are seen at the same time, creating understandable confusion for anyone who has it. What causes double vision? There are two possible causes: Strabismus—Failure of both eyes to point at the object being viewed, a condition referred to as "strabismus" or "squint". In normal vision, both eyes look at the same object. The images seen by the two eyes are fused into a single picture by the brain. If the eyes do not point at the same object, the image seen by each eye is different and cannot be fused. The result is double vision. Why might eyes not point in the same direction? Possibly because of a defect in the muscles which control the movement of the eyes or in the control of these muscles through the nerves and brain. Refractive—Light from an object is split into two images by a defect in the eye's optical system. Cataracts may cause such a defect. Strabismus is a more common cause of double vision than is refractive defect. What are its implications? Double vision can be extremely troubling. The brain acts to alleviate the discomfort by suppressing, or blanking out, one of the images. In young children, if this suppression persists over a continued length of time, it can lead to an impairment of the development of the visual system. The suppressed eye may get to the point where it is unable to see well, no matter how good the spectacle or contact lens correction. Doctors call this condition "amblyopia". Since it is a result of a defect in the interpretive mechanisms of the eye and brain, it is more difficult to treat than a refractive condition (one having to do with the eye's ability to bend light). How is it treated? Treatment of double vision consists of eye exercises, surgical straightening of the eye or a combination of the two. Therapy is aimed at re-aligning the squinting eye where possible without surgery and re-stimulating the part of the visual pathway to the brain that is not working correctly. If the double vision is due to the presence of cataracts, referral for possible cataract surgery will be undertaken. Do you occasionally see specks or threadlike strands drifting across your field of vision? Then, when you try to look at them, do they seem to dart away? If so, you're seeing what eye care practitioners call spots or floaters. While almost everyone sees a few spots at one time or another, they can occur more frequently and become more noticeable as you grow older. If you notice a sudden change in the number or size of spots, you should contact us right away so you can be sure they are not the result of a more serious problem. What are spots or floaters? Spots are small, semi-transparent or cloudy specks or particles within the eye that become noticeable when they fall within the line of sight. They may also appear with flashes of light. The inner part of your eye is made up of a clear, jelly-like fluid known as the vitreous. As time passes the jelly-like fluid gradually becomes more liquid in nature and cells and structural fibres detach and float around in this jelly, resulting in the floaters that we commonly observe. When flashes of light occur causing spots to become noticeable, it can be a result of the jelly-like vitreous shrinking and pulling on the retina. This tugging action stimulates the retinal receptor cells to "fire," causing the perception of light flashes. Can these spots cause blindness? Most spots are normal and rarely cause blindness. But spots can indicate more serious problems. If you notice a change in the number and size of spots, a comprehensive eye examination is in order to determine the cause. On rare occasions, vitreous detachment can cause small tears or holes in the retina. The damaged part of the retina subsequently does not work properly and a blind or blurred spot in vision results. If untreated, retinal tears or holes can continue to worsen and severe vision loss can result if the retina becomes detached. How are spots diagnosed? In a comprehensive eye examination, your eyes will be evaluated with special instruments that allow an examination of the health of the inside of your eyes and possible observation of the spots. This is often done after special drops are put in your eyes to make the pupils larger (called dilation) to allow a larger view of the inside of your eyes. These procedures provide the relevant information to detect spots. How are spots treated? While flashes and floaters are normally not serious or treatable, they can be symptoms or signs of either vitreous or retinal detachment. In either of these cases, treatment with lasers and/or surgical intervention may be necessary to preserve your vision. If you notice a sudden increase or change in the number and type of spots and floaters, contact us immediately. What is a cataract? A cataract is a clouding of the normally clear crystalline lens of the eye. This prevents the lens from properly focusing light on the retina at the back of the eye, resulting in a loss of vision. A cataract is not a film that grows over the surface of the eye, as is often commonly thought. Why are they
called cataracts?
While a comprehensive eye examination can determine for certain if you have a cataract forming, there are a number of signs and symptoms that may indicate a cataract. Among them are:
Who gets cataracts? Cataracts are most often found in persons over the age of 55, but they are also occasionally found in younger people, including newborns. What causes cataracts? It is known that a chemical change within the eye causes the lens to become cloudy. The change may be due to advancing age or it may be the result of heredity, an injury or a disease. Excessive exposure to ultraviolet or infrared radiation present in sunlight or from furnaces, cigarette smoking and/or the use of certain medications are also cataract risk factors. Cataracts usually develop in both eyes, often at different rates. Can cataracts be prevented and treated? Currently, there is no proven method to prevent cataracts from forming. If your cataract develops to a point that daily activities are affected, you will be referred to an eye surgeon who may recommend the surgical removal of the cataract. Prescription changes in your eyewear will help you see more clearly until surgery is necessary, but surgery is the only proven means of effectively treating cataracts. The surgery is relatively uncomplicated and has a very high success rate When will I need to have cataracts removed? Cataracts may develop slowly over many years or they may form rapidly in a matter of months. Some cataracts never progress to the point that they need to be removed. Usually, you will be ready to have the cataract removed when it is having a significant adverse effect on your lifestyle. Our office will arrange a consultation with a surgeon who will decide on the appropriate time for removal. Most people wait until the cataracts interfere with daily activities before having them removed. What happens after cataract surgery? You, along with your doctors, will decide on the type of post-cataract vision correction that you will use. Intraocular lens implants, inserted in your eye at the time of surgery, serve as a "new lens" and are the most frequent form of visual correction. In some cases, however, eyeglasses or contact lenses may also be needed to provide the most effective post-cataract vision. Cataract surgery has now developed to the point where most procedures are completed in a day and overnight stays in hospital are unnecessary. The results are usually excellent and patients are often able to appreciate a significant improvement in vision almost immediately following surgery. |
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300A
Faunce Corner Road, Suite 101, North Dartmouth, MA 02747 © 2006 Southcoast Eye Care, Inc. ~ Last Updated: August 8, 2008 |
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