Basic Principles of Blindness and Visual Impairment
People often have misconceptions about blindness and visual impairments. We hope that this information can correct some of these and lead you to a clearer understanding.
- There are many causes of blindness:
- Very few people who are considered blind are in fact totally blind. There is a wide range of visual impairments:
Vision good enough to travel unaided
Minimal vision at close range
Each person’s visual impairment is unique. It is important to understand that each person functions as an individual according to his or her abilities and motivation.
- With vision loss, the other senses do not automatically become more acute; a person simply learns to use them more fully.
- Children who are legally blind have a best corrected central visual acuity in their better eye that does not exceed 20/200 and/or a visual field whose maximum angular diameter does not exceed 20 degrees.
- Children are functionally blind if they are totally blind or if the best corrected visual acuity in their better eye is not better than light perception.
- Children are visually impaired if they are not legally blind, but their best corrected central visual acuity does not exceed 20/70 or if the maximum angular diameter of their visual field does not exceed 30 degrees even if their best corrected visual acuity in their better eye exceeds 20/70.
- Children are totally blind if they are unable to visually detect light of any intensity. Children are said to have light perception if, with their better eye, they can detect light and determine from which direction the light is coming.
- The estimated incidence of blindness ranges from one to two percent of the population of those age birth to 20 years.
- Children who are blind or visually impaired may learn to read through the use of braille, large print, a magnifier, or recorded materials. They may learn to write in braille or in handwriting.
Common Causes of Visual Impairment and Blindness in Children
Cortical Visual Impairment: Cortical Visual Impairment (CVI) is a temporary or permanent visual impairment caused by the disturbance of the posterior visual pathways and/or the occipital lobes of the brain. The degree of vision impairment can range from mild to severe visual impairment. The degree of neurological damage and visual impairment depends upon the time of onset, as well as the location and intensity of the insult. It is a condition that indicates that the visual systems of the brain do not consistently understand or interpret what the eyes see. The presence of CVI is not an indicator of the child’s cognitive ability. The terms Cortical Visual Impairment, Neurological Visual Impairment, and Cerebral Visual Impairment, are sometimes used interchangeably.
Retinopathy of Prematurity: Retinopathy of Prematurity (ROP) is a condition associated with premature birth, in which the growth of normal blood vessels in the retina (a light-sensitive membrane in the back of the eye containing rods and cones that receive an image from the lens and send it to the brain through the optic nerve) stops, and abnormal blood vessels develop. As a result, the infant has an increased risk of detachment of the retina. ROP can lead to reduced vision or blindness.
Optic Nerve Hypoplasia: Optic Nerve Hypoplasia (ONH) refers to the underdevelopment of the optic nerve during pregnancy. The dying of optic nerve fibers as the child develops in utero is a natural process, and ONH may be an exaggeration of that process. ONH may occur infrequently in one eye (unilateral), but more commonly in both eyes (bilateral). ONH is not progressive, is not inherited, and cannot be cured. However, depending on the degree of visual impairment, a person with this condition may benefit from the use of devices for low vision. ONH is one of the three most common causes of visual impairment in children. The effects of ONH have a broad range, from little or no visual impairment to near-total blindness.
Albinism: Albinism is a congenital condition that affects the pigmentation and color of the eyes and may also affect the skin and hair. Most patients with albinism have a very high degree of vision and are some of the best candidates for low vision care. The main visual consequences of albinism are photophobia (sensitivity to bright light and glare), blurred sight, and nystagmus (uncontrollable shaking of the eyes). There is no surgical or medical treatment to correct albinism but, fortunately, it is a stable condition that does not lead to total blindness.
The lack of pigment in the eyes causes the sensitivity to bright light and glare (photophobia). The severity of photophobia is dependent on the amount of pigment in the eyes.
Patients with albinism tend to have much visual strength. They generally have excellent color vision, peripheral (side) vision, and night vision. Thus, they are often able to use their vision more effectively during the evening or on overcast days.
People with albinism may have pale pink skin and blond to white hair, but there are different types of albinism, and the amount of pigment varies. The irises of their eyes may be blue, violet, or even hazel in color.
Optic Nerve Atrophy: Optic Nerve Atrophy (ONA) is a permanent visual impairment caused by damage to the optic nerve. The optic nerve functions like a cable carrying information from the eye to be processed by the brain. The optic nerve is comprised of over a million small nerve fibers (axons). When some of these nerve fibers are damaged through disease, the brain doesn’t receive complete vision information and sight becomes blurred. A person’s ability to see clearly (visual acuity) is affected due to nerve damage that occurs in the central part of the retina responsible for detail and color vision (macula). These areas of the eye are more vulnerable to the effects of atrophy. ONA is the end result of damage to the optic nerve. It can affect one or both eyes. It may also be progressive, depending on the cause.