Vision impairment is defined as a limitation of one or more functions of the eye (or visual system).
The most common vision impairments affect:
- The sharpness or clarity of vision (visual acuity)
- The normal range of what you can see (visual fields)
Legal blindness in Australia means that someone with vision impairment, even with glasses or contact lenses, can see an object at 6 metres that someone without vision impairment could see from 60 metres. This is called 6/60 vision.
Normal vision is 6/6 vision (or 20/20 in imperial measures).
Causes of vision impairment can include:
- Genetic conditions
- Maternal infections experienced during pregnancy (e.g., rubella, cytomegalovirus, venereal diseases, toxoplasmosis)
- Consequences of disease (e.g., diabetes, glaucoma, trachoma)
- Complications associated with extreme prematurity
- Birth complications
- Trauma, poisoning, and tumours
- Diabetic retinopathy
- Ageing and age-related conditions such as macular degeneration, cataracts and optic nerve atrophy
Among the major causes of vision impairment worldwide are cataract, trachoma, Vitamin A deficiency and river blindness. Many of these are found in Africa, the Middle East and Asia.
Helping people with vision impairment to move safely and independently through any environment is usually known as 'orientation and mobility'.
Orientation and mobility training generally begins as soon as possible.
Preschool children learn how to:
- Travel around their school building and playground.
Primary school children learn more complex concepts like:
- Topography and textures
- Positions (eg in front of, at the back of)
Secondary school students learn how to:
- Cross streets at busy intersections
- Ride buses
- Use compass directions
- Plan a route of travel
- Shop and travel independently in unfamiliar areas.
There is a range of ways that people with vision impairment can travel safely, including:
- A sighted guide
- A cane, or for young children, a pushable mobility device, usually on rollers or casters
- A Guide Dog
- An electronic travel aid (ETA), which uses ultrasound or infrared light to detect when a physical object comes near.