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Vision Impairment
How Do You See and What Impairs Your Vision?
- What Is Legal Blindness?
- Central Vision
- Common Problems
- Cataract
- Glaucoma
- Diabetic Retinopathy
- Macular Degeneration
- Corneal Disease
- Retinitis Pigmentosa
- Retinal Detachment
- Amblyopia / Other
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,the cornea, over the front of the eye. The cornea does most of the focusing of light onto the retina. Behind the cornea is the lens which does some of the focusing and enables the eye to vary its focus. Thus an image is formed on the back of the eye, on the retina.The most light-sensitive point of the retina, near its center,where 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 shows where the various parts of the eye are located
Central Vision
Central vision is what you see when you look straight at an object. Peripheral (side) vision is what you see of the area surrounding the object at which you are looking. In central vision, the picture is focused on the macula. You look at a 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 the 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 or reading” 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 and trying to look “through” the penny at a word or at a 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.
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 much poorer and is 20/200. In this case you are 20 feet away from the chart and are able to read only those numbers which a normal eye can read at 200 feet from the chart. In between these levels there are various gradations of vision, corresponding to visual acuity of 20/30, 20/50, etc., following the same principle. Peripheral vision is measured by focusing your eyes at a spot straight ahead and then telling the examiner when you see a light or object which is slowly brought from above or below, 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, depending upon whether you have lost central or peripheral vision. You are legally blind if central vision in your better eye, with the best correction possible with contacts or glasses, 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 blind in only one eye. Legal blindness does not necessarily mean total blindness. For example, you may still be able to read with special magnifying lenses and devices. If your peripheral vision is good, you may also be able to move about without 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 night time.
Some Common Problems Affecting Your Eyes and Vision
Many of the conditions affecting vision are progressive; however, early diagnosis and treatment may prevent or slow deterioration. Some conditions are difficult to recognize early in their course, but when possible, early diagnosis and treatment are often of great benefit.The presence of one condition in your eye does not mean that another, entirely different problem cannot occurring the same eye. If you have visual impairment, regardless of the degree, you should have periodic examinations. Certain signs and symptoms are danger signals that indicate an urgent need for an examination by an ophthalmologist. They are:
flashes of light; halos around lights; a curtain or veil blocking vision; sudden loss of vision; double vision; dimming of vision that comes and goes; sudden onset of floating spots in front of eye; crossed eye; pain in the eye; redness of eye or surrounding area discharge or crusting of the eyelids; difference in the size of the two pupils; swelling of the eyes; protrusion of one or both eyes; persistent irritation, watering or dryness.
The chief causes of blindness in the United States are: glaucoma, diabetic retinopathy, and macular degeneration. These and other conditions commonly seen in people with impaired vision are described in the following sections.This diagram of an eye may help you understand these conditions.
Cataract
In order to see clearly, the cornea and lens must be transparent. A cataract is a cloudy or hazy lens which blurs the picture on the retina. This occurs most commonly as a part of the aging process. A cataract may be removed surgically when it has become so hazy that it significantly interferes with sight: when it makes one’s daily activities difficult or impossible. The time selected for cataract surgery will vary from person to person depending upon age, medical 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”) to be removed, but it must be removed before it swells and becomes “overripe.” “Overripe” cataracts can cause glaucoma and serious inflammation within the eye. A common misunderstanding is that cataracts can be removed by laser beam. Laser beams are used to treat several eye conditions, but they cannot be used to remove cataracts — although they can be used to remove the supporting membrane that sometimes becomes cloudy after one type of cataract operation. A cataract cannot be made to go away with medication. It 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, without discomfort, after the eye is healed. Cataract surgery is one of the most successful surgical procedures in medicine. At the time of surgical removal of the cataract, your eye's lens is usually replaced with a permanent artificial lens implant.(You will probably still need glasses for reading and driving.) Occasionally, vision is corrected with thick glasses or a contact lens. 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
Glaucoma is called the “sneak thief of sight” because it can be present for many years, gradually destroying your sight without causing any symptoms. Generally, glaucoma is caused by an abnormally high pressure inside your eye, which, if untreated, can result in slowly progressive loss of vision. Pressure is measured by an instrument called a tonometer which the doctor places on the surface of your eye. In most cases of glaucoma, the pressure increase does not cause pain or discomfort, so you may not be aware that you have glaucoma. Evaluation of damage caused to vision by glaucoma can be made only by examining the insides of your eyes and by charting the visual fields of your eyes. There are many forms of glaucoma, some of which are treated with eye drops and pills, and others which require surgery. Various types of lasers can be used to treat glaucoma under certain circumstances. Some people are at high risk for glaucoma. If you are African American, have diabetes, blood vessel disease, a family history of glaucoma, or a history of eye injury or inflammation, then your risk of having glaucoma will be higher. Regardless of risk category, you should be examined for glaucoma through out your life at intervals prescribed by your doctor. Sight loss due to glaucoma may be prevented by early treatment. Remember, however, that vision lost to glaucoma cannot be restored by medical or surgical treatment. By contrast, with decreased vision due to cataract, removal of the hazy lens will ordinarily restore sight. Check out our dates of Free Adult Vision Screening Clinics, for glaucoma and other causes of sight loss.
Diabetic Retinopathy
Long-standing diabetes can damage the 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 depend on several factors including the kind of diabetes you have, age of onset, length have of time you had diabetes, and how well your blood sugar levels have been controlled. If you have diabetes, you should have eye examinations, at intervals prescribed by your doctor, to determine whether diabetic retinopathy is present. Treatment of retinopathy with lasers at the appropriate time may delay or stop progression. If any retinopathy is found, then regular exams should be performed by an ophthalmologist to evaluate the degree of damage and advise treatment.
Macular Degeneration
Macular degeneration is a deterioration of the macula, 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, macular degeneration does not affect peripheral vision, and thus a person with macular degeneration 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. At present there is no cure for macular degeneration. However, the progression of some forms of the disease can be limited by laser treatment if discovered early. Many persons with macular degeneration are aided in reading and seeing at a distance by special magnifying lenses and telescopic devices (see Adaptive Aids). You can test yourself for possible signs of macular degeneration with an Amsler Grid test.To obtain a free copy, please contact us
Corneal Disease
The cornea, the window of the front of the eye, is normally clear. Injury, infection, and aging changes can cause your cornea to lose its clarity and thereby reduce vision. Some diseases of the cornea are benefited by medical treatment, while other diseases require corneal transplantation, a surgical procedure in which the cornea from a normal eye is used to replace the damaged cornea. In most cases, a corneal transplantation greatly improves vision.
Retinitis Pigmentosa
Retinitis pigmentosa is an hereditary condition in which various elements of your retina degenerate. Pigmented scars appear in damaged areas of the retina—hence the name of the disease. Symptoms of retinitis pigmentosa usually begin in childhood with difficulty seeing in dim illumination such as in a dark room or at night. The disease affects peripheral vision first. Some persons retain useful central vision for many years,and sometimes through their entire lives. Fortunately, the course of retinitis pigmentosa usually is very slow, and not all patients with this disease become blind. At present there is no curative treatment, but some patients may be helped by mechanical and electronic aids.
Retinal Detachment
Sometimes the retina pulls loose or is torn from the back of the eye. Eye injuries, extreme near sightedness,or a family history of detachments all increase a person’s risk of retinal detachment. Symptoms are the sudden onset of floating the spots in field of vision, light flashes, shadows,and decreased central peripheral or vision. A tear in your retina, if neglected, can lead to retinal detachment. If detected by an ophthalmologist before the retina detaches, treatment of a tear in the retina by a laser or surgery is usually successful in preventing retinal detachment. Once the retina has detached, it can usually be reattached surgically, but the ultimate visual result depends on how much of your retina has been detached and for how long.
Amblyopia/Other Causes
There are many other causes of loss of vision and blindness. These may result from disease of the structures 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. This link describes Amblyopia (Lazy Eye)