Vision loss can be caused by many things. In young children, for instance, vision loss can be caused by damage to the eye itself, by the eye being shaped incorrectly, or by a problem in the brain. In adults, vision loss is often the result of age-related eye disease.
Approximately 14 million Americans aged 12 years and older have self-reported visual impairment, which is defined as distance visual acuity of 20/50 or worse. More than 11 million of these people could improve their vision to 20/40 or better with refractive correction (Centers for Disease Control and Prevention, September 28, 2008).
In the United States, vision loss is classified under three categories:
Low vision, which means visual acuity between 20/70 and 20/400 with the best possible correction, or a visual field of 20 degrees or less
Blindness, which means visual acuity worse than 20/400 with the best possible correction, or a visual field of 10 degrees or less
Legal blindness, which means visual acuity of 20/200 or worse with the best possible correction, or a visual field of 20 degrees or less
Causes of Vision Loss
Refractive errors are the most common eye problems in the United States. Refractive errors include myopia (near-sightedness), hyperopia (farsightedness), astigmatism (distorted vision at all distances), and presbyopia (loss of the ability to focus up close, often occurring after 40 years of age).
The leading causes of low vision and blindness in the United States are primarily age-related eye diseases. These include:
Age-related macular degeneration: This eye disorder affects the macula, or the central part of the retina, which is responsible for central vision. Central vision is needed for seeing fine details and for common daily tasks such as reading and driving. Affected persons have a woolly or cottony opacity that obscures their central view. Peripheral vision may be normal. The condition has two forms: wet and dry. It is termed “wet” when abnormal blood vessels behind the retina start to grow under the macula, ultimately leading to blood and fluid leakage. Dry macular degeneration is when the macula thins over time as part of the aging process, gradually blurring central vision. The dry form is more common, and generally affects both eyes.
Cataract: Cataracts can occur at any age, beginning at birth, but are more commonly associated with old age. The reason why cataracts develop is not entirely understood. Affected persons have clouding of the eye’s lens, which causes blurred vision. It is the leading cause of vision loss in the United States.
Diabetic retinopathy: This is a common complication of diabetes, and the most common cause of blindness in adults in the United States. It usually affects both eyes and is characterized by progressive damage to the blood vessels at the back of the eyes over the retina. Affected persons usually report seeing black spots or floating shapes in their field of vision.
Glaucoma: This is a group of diseases that can damage the eye’s optic nerve and result in vision loss or blindness. Glaucoma often occurs when there is high intraocular pressure — that is, when the normal fluid pressure inside the eyes slowly rises. However, glaucoma can also occur with normal eye pressure. There are two major categories of glaucoma: “open angle,” and “closed angle.” Open-angle glaucoma occurs over time, and victims seldom notice vision loss until the disease is very advanced. Closed-angle glaucoma can appear suddenly and is painful.
Other common causes of vision impairment include:
Amblyopia: Also known as “lazy eye,” amblyopia is the most common cause of vision impairment in children. In this condition, vision in one eye is reduced because of a miscommunication between the eye and the brain. Often the affected eye looks normal. Unless it is successfully treated in early childhood, amblyopia usually persists into adulthood.
Strabismus: Strabismus causes the eyes to wander or cross inward (esotropia) or outward (exotropia). This is because of a lack of coordination between the eyes, which prevents them from focusing simultaneously on a single point. As a result the eyes look in different directions. In most cases of strabismus in children, the cause is unknown. In more than half of these cases, the problem is present at or shortly after birth (this is known as congenital strabismus). Uncorrected strabismus can lead to permanent vision loss in one eye.
Diagnosing Vision Loss
You doctor will review your medical history and family history for possible causes of visual impairment and then perform a complete evaluation of your eyes. This includes an external examination of the eye (including eyelids, conjunctiva, cornea, iris, and lens), measurement of visual acuity to determine your prescription for corrective eyewear, measurement of your eye pressure, and an evaluation of the overall health of your eyes.
In addition to visual acuity testing, which is done using the Snellen’s chart (an eye chart with letters and numbers placed six feet away from the patient), your doctor may perform several different tests during the examination. Testing for vision impairment may include eye muscle testing; refraction assessment; a visual field test, which determines a person’s range of vision and peripheral vision; tonometry, which is used to screen for glaucoma; retinal examination or funduscopy, which allows your doctor to see the back of your eye; color vision testing; and slit-lamp examination.
The findings from these tests will determine whether any further testing is necessary or referral to a specialist is warranted.
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