Living With Sight Loss in Tennessee
Part II
How You See and What Can Impair Your Vision
How Do You See?
Vision depends on light, either from the sun or from an artificial source such as a light bulb. Light reflects from objects and enters your eyes through a clear covering or window called the cornea over the front of the eye. Behind the cornea is the lens that focuses light in the form of a picture on the back of the eye, the retina. The most light- sensitive point of the retina, near its center where the sharpest vision is achieved, is called the macula. The retina turns the picture into tiny electric impulses that are sent by way of the optic nerve to the brain where seeing actually occurs. This diagram of the eye shows where the various parts of the eye are located.
Central vision is what you see when you look straight at an object. Peripheral vision is what you see surrounding the object at which you are looking. In central vision, the picture is focused on the macula. You look at the word on a page with your central vision, and with your peripheral vision you can tell if the word is at the beginning or end of the line or at the top or bottom of the page. Even while you are reading with central vision, you can see someone enter a room with your peripheral vision. Peripheral vision also lets you walk or drive without bumping into or stumbling over objects. Central vision sometimes is called "seeing vision", and peripheral vision is called "traveling vision."
If your eyes are normal, you can find out what loss of central vision is like by closing one eye and holding a penny directly in front of the other eye. Now try to look "through" the penny at a word or distant sign. You cannot see straight ahead, but you can see the surrounding area. You can find out what loss of peripheral vision is like by closing one eye and then looking through a long tube of newspaper with the other eye. Under this circumstance, you can see straight ahead but cannot see the surrounding area. The illustrations in this book might help you understand how loss of peripheral or central vision would affect your total vision.
Central vision is measured by looking from a distance of 20 feet at a chart printed with numbers or letters of different sizes. If you can read the small letters near the bottom of the chart, your central vision is said to be 20/20, meaning that you can see at 20 feet what a normal eye sees at 20 feet. If you are 20 feet away from the chart and you can read only the top line, your central vision is said to be 20/200 or 20/400, depending on the chart. In this case, you are 20 feet away from the chart, and are able to read only those numbers that a normal eye can read at 400 (or 200) feet from the chart. In between, there are various graduations of vision: 20/30, 20/50, etc.
Peripheral vision is measured by focusing your eyes at a spot straight ahead and telling the examiner when you see a light or object which is slowly brought from above or below or from left or right into your field of vision. Normal peripheral vision (called the visual field) for one eye is approximately 150 degrees from side to side, and for both eyes is approximately 180 degrees. Depending upon the visual problem, all or only part of the visual field may be affected. The fewer the degrees of peripheral vision, the greater the loss of vision.
What is Legal Blindness?
The term legal blindness is used by the Internal Revenue Service and others to determine whether you are eligible for disability benefits or for certain other benefits or services. There are two definitions of legal blindness. You are legally blind if central vision in your better eye (with the best correction possible) is no better than 20/200, or if peripheral vision is no greater than 20 degrees diameter (10 degrees radius) in your better eye. You are not legally blind if you are only blind in one eye.
Legal blindness does not necessarily mean total blindness. For example, you may still be able to read with special magnifying lenses and move without any difficulty even though your central vision is poor. On the other hand, you may be legally blind with respect to peripheral vision but be able to read even though you are not able to move about easily, especially in dim illumination such as at dusk or nighttime.
Who’s Who in Eye and Vision Care
While most of us know the term "eye doctor," you’ll find that you won’t be able to locate one by looking in the yellow pages under that title. There are actually three different types of specialists whom you may hear about or see in taking care of vision problems.
Ophthalmologists- These people are doctors of medicine (M.D.) or osteopathy ( D.O.) who specialize in the comprehensive care of the eyes and visual system in the prevention of eye disease and injury. The ophthalmologist has completed four or more years of college premedical education, four or more years of medical school, one year of internship, and three or more years of medical and surgical training and experience in eye care. The ophthalmologist is a physician who is qualified by lengthy medical experience to diagnose, treat, and manage all eye and visual system problems. He is licensed by a state regulatory board to practice medicine and surgery. The ophthalmologist is the medically trained specialist who can deliver total eye care: primary, secondary, and tertiary care services (i.e., vision services, contact lenses, eye examinations, medical eye care and surgical eye care) and diagnose general disease of the body.
Optometrists- The optometrist is a health care professional trained and state licensed to provide primary eye care services. These services include: comprehensive eye health and vision examinations, diagnosis and treatment of eye diseases and vision disorders, the detection of general health problems, the prescribing of glasses and contact lenses, low vision rehabilitation, vision therapy and medications, the performing of certain medical procedures and the counseling of patients regarding their surgical alternatives, and vision needs related to their occupations, avocations and lifestyle. The optometrist has completed pre-professional undergraduate education in a college or university and four years of professional education at a college of optometry, leading to the doctor of optometry degree (O.D.) degree. Some optometrists complete a residency.
Opticians- Opticians are professionals in the field of designing, finishing, fitting and dispensing of eyeglasses and contact lenses, based on the eye doctor’s prescription. The optician may also dispense colored and specialty lenses for particular needs as well as low-vision aids and artificial eyes.
Some Common Problems Affecting Your Eyes and Vision
Many of the conditions affecting vision are progressive, however, early diagnosis and treatment are often of great benefit. Just because one condition is present in your eye does not mean that another, entirely different problem cannot occur in the same eye. If you have a visual impairment, regardless of the degree, you should have periodic examinations by your eye doctor.
Certain signs and symptoms are danger signals that indicate a need for an examination by an eye doctor. These are:
The chief causes of blindness in the U.S. are glaucoma, cataract, diabetic retinopathy and macular degeneration. These and other conditions commonly seen in people with impaired vision are described here. The diagram of the eye may help you understand these conditions.
Cataract
The lens of the eye focuses light onto the retina or back part of the eye. Normally your lens is clear, but it can become cloudy as part of the aging process or as a result of eye injury or disease. The cloudy lens, called a cataract, can be removed by surgery whenever vision loss interferes with your occupation or other activities. The time selected for cataract surgery will vary from person to person depending on the age, mental status, occupation, visual needs, whether the cataract is in one eye or both eyes, and upon many other factors. Just because you have a cataract does not mean that you must have it removed. Cataracts generally progress very slowly over a period of years and can be removed at any stage of development.
A cataract does not have to be completely opaque ("ripe") before it is removed, but it must be removed before it swells and becomes "overripe." Left too long, the cataract can cause glaucoma and serious inflammation within the eye.
A common misunderstanding is that the laser beam can remove cataracts. Laser beams are used to treat several eye conditions, but they cannot be used to remove a cataract. They can, however, be used to remove the supporting membrane that sometimes becomes cloudy after one type of cataract operation. A cataract must be removed surgically, and there are several methods in use. One method is not necessarily better than another. What is important is not how your cataract is removed, but how well you can see again after the eye is healed.
Cataract surgery is one of the most successful surgical procedures in medicine. Following surgery, the lens of your eye must be replaced in one of three ways: spectacle glasses in front of the eye, a contact lens on the eye, or a permanent plastic lens (a lens implant) inserted inside the eye usually at the time of surgery. The implant is now by far the most common means of correcting vision after cataract surgery.
Occasionally, a cataract may conceal the presence of retinal detachment, tumor or other ocular disorder. Therefore, if you have a cataract, regardless of whether or not you are planning to have it removed, you should be examined at intervals prescribed by your doctor to prevent permanent loss of sight from other causes.
Glaucoma
Generally, an abnormally high pressure inside your eye causes glaucoma. Pressure is measured by an instrument called a tonometer. However, evaluation of damage to vision by glaucoma can be made only by examining the inside of your eyes and by charting the visual field of your eye. Glaucoma "screenings" are useful to check the pressure in your eye from time to time, but you must not consider them complete eye examinations. It is possible to have "normal" pressure and still have glaucoma. A complete examination is the only way to rule out the presence of glaucoma.
Glaucoma is called the "sneak thief of sight", because it can be present for many years gradually destroying your sight without causing any symptoms. There are many forms of glaucoma, some of which are treated with eye drops and pills, and others which require surgery. More recently, lasers are being used to treat some forms of glaucoma. Some people at higher risk for glaucoma include those who:
If any of the above applies to you, be sure to have your eyes checked frequently. Vision lost to glaucoma cannot be recovered.
Diabetic Retinopathy
Long standing diabetes can damage blood vessels in the retina. This is called diabetic retinopathy. Not all persons who have diabetes develop impaired vision. Severity of the retinopathy and the accompanying loss of vision are more likely to depend on the length of time you have had diabetes and the kind of diabetes you have than on the severity of the diabetes.
If you have diabetes, you should have eye examinations at intervals prescribed by your doctor. We recommend annually. Treatment of retinopathy with lasers at the appropriate time may delay or stop progression.
Macular Degeneration
Macular degeneration is a deterioration of the macular, the central part of the retina that enables you to read and see straight ahead (central vision). The cause is unknown. Although macular degeneration can occur in children and young adults, it is more common in older persons. Fortunately, in macular degeneration, peripheral vision is not affected, and thus a person with this condition does not face total blindness. If you have macular degeneration, you are usually able to move about unaided, but may find reading, close work, or distinguishing distant objects difficult or even impossible.
Although there is no cure for macular degeneration, usual forms of the disease respond to laser treatment if discovered early enough. Many persons with macular degeneration are aided in reading and seeing at a distance by special magnifying lenses and telescopic devices. Check our section on low vision aids.
Amsler Grid Instructions
Hold the grid at comfortable reading distance (about 14 inches) away from your eyes.
If you wear glasses to read, keep them on for the test (with bifocal glasses, look through the bottom portion of the lens).
Test one eye at a time, covering the other eye.
Look at the dot in the center of the grid.
If the lines look wavy or disappear, see an ophthalmologist within one or two days. This could be a warning sign of macular degeneration.
Corneal Disease
The cornea, the window at the front of the eye, is normally clear. Injury, infection and aging changes can cause the cornea to lose its clarity and thereby reduce vision. Some diseases of the cornea are benefited by medical treatment, while others require corneal transplantation, a surgical procedure in which the cornea from a normal eye is used to replace the damaged cornea. In most cases, corneal transplantation greatly improves vision.
Retinitis Pigmentosa
Retinitis Pigmentosa is a hereditary condition in which various elements of the retina degenerate. Cells filled with pigment appear in damaged areas of the retina, hence the name of the disease. Symptoms of Retinitis usually begin in childhood with difficulty seeing in dim illumination such as in a darkened room or at night. The disease affects peripheral vision first. Some persons retain useful central vision for many years and sometimes throughout their entire lives. Fortunately, the course of Retinitis Pigmentosa is usually very slow, and not all patients with this disease become blind. At present there is no treatment, but special wide-angle lenses and other mechanical and electronic devices may help some patients.
Retinal Detachment
Sometimes the retina pulls loose or is torn from the back of the eye. Eye injuries, extreme nearsightedness or a family history of detachments increase a person’s risk of retinal detachment. Symptoms are floating spots in the field of vision. A tear in the retina, if neglected, can lead to retinal detachment. Treatment of a tear in the retina by laser or surgery is usually successful in preventing detachment. Once the retina has detached, it can usually be reattached surgically, but the ultimate visual result depends on how much of the retina has been detached and for how long.
Other
There are many other causes of loss of vision and blindness. These may result from disease of the structure of the eye itself, disease of the optic nerve which carries the picture from the eye to the back of the brain or problems within the brain such as tumors, injury and stroke.
What Family and Others Should Know
Courtesy and Etiquette for the Vision Impaired
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