Very few people are totally without sight. Most classified as blind today actually have some sight remaining and, thanks to developments in the field of low vision, can be helped to make good use of it.
Anyone with reduced vision is visually impaired and can have problems functioning, ranging from minor to severe difficulty. There are two general classifications of low vision in use today:
● partially sighted - visual acuity, that with the best correction is still between 20/70 and 20/200 (a person with 20/70 eyesight must be 20 feet away to see an object clearly that a person with 20/20 eyesight can see clearly at 70 feet)
● legal blindness - visual acuity that cannot be corrected to better than 20/200 with conventional lenses and/or the patient has a restricted field of vision of less than 20 degrees wide.
Low vision impairments take many forms and exist in varying degrees. It is important to understand that visual acuity alone is not a good predictor of the degree of the problem.
What causes low vision?
Several eye diseases may be responsible for low vision, including:
o Macular Degeneration - A disorder that affects the retina, the light-sensitive lining at the back of the eye where images are focused. The macula - the area on the retina responsible for sharp, central vision- deteriorates, gradually causing blurred vision, difficulty reading, and finally, a blind spot in the central area of vision. This is known as the "dry" form of macular degeneration, is age-related, and is the leading cause of blindness in people over 50. The exact cause is unknown, but may be related to smoking, and possibly long-term exposure to high levels of the sun's ultraviolet radiation and blue light. More rapid and severe vision loss comes from the "wet" form when abnormal blood vessels develop under the macula and leak fluid and blood. There are also juvenile forms which are hereditary.
o Diabetic Retinopathy - Diabetes can cause blood vessels that nourish the retina to leak, develop brush-like branches or enlarge. This can interfere with vision and, over time, may destroy the retina. Laser procedures and surgical treatments are used to reduce its progression.
o Retinitis Pigmentosa - Gradually destroys night vision, severely reduces peripheral vision, and may result in total blindness. An inherited disease usually produces its first symptom - night blindness - in childhood or adolescence.
o Retrolental Fibroplasia (retinopathy of prematurity) - Occurs in infants born prematurely and, in some cases, is caused by high oxygen levels in incubators during the first 10 days of life.
o Retinal Detachment - This can result in total blindness in the detached area of the affected eye. It involves the retina separating from its underlying layer. Causes are holes in the retina, eye trauma, infection, blood vessel disturbance or a tumour. Through early diagnosis, most detached retinas can be surgically reattached with partial restoration of vision.
o
Cataracts - A clouding of part of or all of the lens,
inside the eye. This prevents light from reaching the retina at the back of the
eye, resulting in a generalised loss of vision. Causes are ageing, long-term
exposure to the sun's ultraviolet radiation, injury, disease and inherited
disorders. If the eye is healthy, the cataract can be surgically removed and
vision restored, usually with intraocular lens implants. Cataract surgery has a
high success rate but a small number of those for whom it is not successful
will require low vision care.
Glaucoma - The internal pressure in the eye
builds up because of problems with the flow or drainage of fluid within the
eye, damaging the optic nerve and causing partial or total loss of sight. There
are no early symptoms in the most common form, but the first signs of damage
are peripheral vision defects. Early diagnosis and treatment with drugs or
sometimes surgery can minimise vision loss.
Vision can also be lost or damaged as a result of head injuries, brain damage or strokes.
What are the most common types of low vision?
o Loss of central vision - the centre of the person's view is blurred or blocked, but side (peripheral) vision remains intact. This makes it difficult to read or recognize faces and most details in the distance. Mobility, however, is usually unaffected because peripheral vision remains intact.
o Loss of peripheral vision - typified by an inability to distinguish anything to one side or both sides, or anything directly above and/or below eye level. Central vision remains, however, making it possible to see what is directly ahead. Typically, loss of side vision affects mobility and slows reading speed because the person sees only a few words at a time. Sometimes referred to as "tunnel vision."
o Blurred vision - objects both near and far appear out of focus, even with the best conventional spectacle correction possible and even when the target is very large.
o Generalised haze - the sensation of a film or glare that may extend over the entire viewing field and may produce various patterns or areas of relatively severe vision loss.
o Extreme light sensitivity - exists when standard levels of illumination overwhelm the visual system, producing a washed-out image and glare disability. People with extreme light sensitivity may actually suffer pain or discomfort from relatively normal levels of illumination.
o Night blindness - inability to see outside at night under starlight or moonlight, or in dimly lighted interior areas such as movie theatres or restaurants.