Most Common Eye Conditions


Visual Impairment: An Overview

If your child has been diagnosed with a visual impairment, you may feel at first as though you are entering a world full of frightening possibilities described in unfamiliar language that you don’t understand. And, as you begin meeting with doctors and special educators, filing reports, and looking for information about your child’s eye condition, you may encounter many different terms used in reference to your child and to visual impairment. You may find it confusing to discover that many of the terms are used generally and do not have precise definitions, and that phrases such as “low vision”  and “legally blind” may no longer seem intimidating and mysterious.

What is a Visual Impairment, Anyway?

“Visual impairment” is a broad term that is used to refer to any degree of vision loss that affects a person’s ability to perform the usual activities of daily life. It does not refer to the kind of vision many of us have who need to wear eyeglasses or contact lenses to read a book comfortably or see highway signs when we’re driving a car. Instead, visual impairment refers to a loss of vision that cannot be corrected to normal vision, even when the person is wearing eyeglasses or contact lenses. Because it is so broad a term, “visual impairment” usually includes blindness as well.

Most visually impaired people have some usable vision. Only a small percentage of people with visual impairments are “blind”-that is, they have very little if any, vision. Some people who are blind may have light perception, or the ability to tell whether or not a light is on in a darkened room. But most will not be able to perceive light. It’s equally important to know that there is a great range of ability and disability among visually impaired people. Visual impairments range from mild to severe. And no two people see exactly alike. Even among people who have the same diagnosis, for example retinopathy of prematurity (ROP) or retinitis pigmentosa, people may have very different visual abilities. In addition, a person’s vision can be affected by daily factors and variables, such as fatigue and environmental conditions like lighting. Therefore, someone’s vision can be different from day to day.

Measurement of Visual Impairment

Although vision is variable, and influenced by day-to-day factors as well as the progression of a given disease or eye condition, measuring someone’s vision medically and clinically is done for many important purposes. For example, in order to prescribe eyeglasses or other optical devices for a person, his or her vision needs to be measured in a formal way. Two basic factors –visual acuity and visual fields-are often determined through this kind of measurement.

Visual Acuity and Visual Fields

Visual acuity refers to the clarity with which one sees an object, and to the sharpness of someone’s ability to see detail. Usually, someone’s near acuity –ability to see within 16 inches– and distance acuity-beyond 16 inches-are both measured. In the United States, acuity is most typically measured at 20 feet, so you’ll hear references to measurements such as 20/20, 20/70 or 20/200. The first number, 20, is the distance your child is from the eye chart being shown. (In the eye care specialists office, mirrors are sometimes used to trick the brain into thinking it is 20 feet from the chart.) The second number, 70, for example, is the distance where a person with perfect or normal vision would stand and be able to see the chart. So someone with 20/70 vision can see at 20 feet what someone with “normal” vision can see from 70 feet away.

The larger the second number, the more reduced the distance acuity a person has. People with acuities of 20/200, 20/400 or 20/600 have some vision but they don’t see with the same degree of clarity that a person with an acuity of 20/20 or 20/50 sees. When a person has 20/200 distance acuity, during an eye exam he or she is usually only able to see the big “E” on the eye chart in the eye care specialist’s office.

Near and distance acuity can vary for an individual. For example, your child may have near normal visual acuity for near tasks. Some children have poor near visual acuity and better distance acuity. For most children with visual impairments, visual acuity is less than “normal” both at near and at distance.

Visual field refers to what one can see to the sides, above, and below when looking straight ahead and without moving one’s head. For a “normally” sighted person, the visual field is approximately 190 degrees. It is possible for your child to have a “normal” visual acuity and reducesd or decreased visual fields. It is also possible for his visual fields to be normal and his acuity to be decreased.

Legal Blindness

Legal blindness is a confusing term because most people who are legally blind are not blind. Their vision may be reduced, but they are able to use it to some extent to function. The definition of legal blindness was developed in 1934 by a group of ophthalmologists. The United States government had called the group together to come up with a definition that could be applied to both children and adults, and it is used by some agencies in various states as a requirement for qualifying services. The definition of legal blindness is:  Visual acuity of 20/200 or less in the better eye after correction (that is, while wearing prescribed eyeglasses or contact lenses), or a visual field of 20 degrees or less with best correction.