The definition of vision impairment by the Centers for Disease Control and Prevention (CDC) says a visually impaired person’s eyesight cannot be corrected to a “normal level”.
It may be said that visual impairment is the functional limitation of the eye or eyes or the vision system. This leads to (1-5) –
- Loss of visual acuity and inability of the person to see objects as clearly as a healthy person
- Loss of visual field meaning inability of an individual to see as wide an area as the average person without moving the eyes or turning the head.
- Photophobia – inability to look at light
- Diplopia – double vision
- Visual distortion or distortion of images
- Visual perceptual difficulties or difficulties of perception
- Or any combination of the above features
Low visual acuity
The CDC and the World Health Organization suggest that low visual acuity means vision between 20/70 and 20/400 with the best possible correction, or a visual field of 20 degrees or less.
Blindness
Blindness is defined as a visual acuity worse than 20/400 with the best possible correction, or a visual field of 10 degrees or less.
Legal blindness in the US means visual acuity of 20/200 or worse with the best possible correction, or a visual field of 20 degrees or less. (1)
Measuring visual acuity
Visual acuity is measured by use of a Snellen’s chart.
The chart consists of random letters and numbers of various sizes set at a distance of 6 feet.
The best possible vision if 6/6. Visual field is the width of the vision without moving or turning the head. It is measured in degrees.
Causes of vision impairment
Vision may be impaired due to multiple reasons. These could be due to eye damage, failure of the brain to receive and read the visual cues sent by the eyes etc.
Underlying diseases may also cause visual impairment. The commonest cause is diabetic retinopathy, age-related macular degeneration, formation of cataracts and raised pressure within the eyes leading to glaucoma.
Who does visual impairment affect?
Although impairment of vision can occur at any point in life, it is more common among the elderly.
Sometimes visual impairment may be inherited. It manifests at birth or in childhood in such cases.
Common conditions are retinitis pigmentosa, genetic or developmental abnormalities etc.
These children with partial or complete impairment are often developmentally delayed especially in gross and fine motor skills. Visually impaired adults struggle with gainful employment and day to day activities. (1-5)
How many people are visually impaired?
Some estimates suggest the approximately 10 million people in the United States are blind or visually impaired.
Some sources suggest that one million adults over the age of 40 are blind, and 2.4 million are visually impaired.
It is estimated that as the elderly population swells over the years, the number of adults with vision impairments is expected to double.
Figures also show that only 46% of working-age adults have vision impairments and 32% of legally blind adults of working age are gainfully employed. (1-5)
Further Reading
Gene therapy improves vision in patients with Bothnia dystrophy