Visual impairment is when a person has sight loss that cannot be fully corrected using glasses or contact lenses.
Blindness as defined by national programme for control of blindness (NPCB) under following headings:
• Inability of a person to count fingers from a distance of 6 meters or 20 feet (technical definition)
•Vision 3/60 or less with the best possible spectacle correction
•Diminution of field vision to 20 feet or less in better eye
There are two main categories of visual impairment:
• Being partially sighted or sight impaired – where the level of sight loss is moderate
• Severe sight impairment (blindness) – where the level of sight loss is so severe that activities that rely on eyesight become impossible.
Types of Blindness:
Economic blindness: Inability of a person to count fingers from a distance of 6 meters or 20 feet technical Definition
Social blindness: Vision 3/60 or diminution of field of vision to 10°
Manifest blindness: Vision 1/60 to just perception of light
Absolute blindness: No perception of light
Curable blindness: That stage of blindness where the damage is reversible by prompt management e.g. cataract
Preventable blindness: The loss of blindness that could have been completely prevented by institution of effective preventive or prophylactic measures e.g. xerophthalmia, trachoma and glaucoma
Avoidable blindness: The sum total of preventable or curable blindness is often referred to as avoidable blindness.
Blindness can be caused due to numerous conditions, the leading causes are:
• Accidents or injuries to the surface of the eye (such as chemical burns or sports injuries)
• Macular degeneration
The type of partial vision loss may differ, depending on the cause:
• With cataracts, vision may be cloudy or fuzzy, and bright light may cause glare
• With diabetes, vision may be blurred, there may be shadows or missing areas of vision and difficulty seeing at night
• With glaucoma, there may be tunnel vision and missing areas of vision
• With macular degeneration, the side vision is normal but the central vision is slowly lost
Other causes of vision loss include:
• Blocked blood vessels
• Complications of eye surgery
• Tumors such as retinoblastoma and optic glioma
• Complications of premature birth
• Optic neuritis
• Retinitis pigmentosa
here are two main areas that are looked at when someone's vision is measured:
• Visual acuity – It is central vision and is used to look at objects in detail, such as reading a book or watching television.
• Visual field – It is the ability to see around the edge of vision while looking straight ahead.
Visual field testing
During visual field testing one will be instructed to look straight ahead at a device while lights are flashed on and off in peripheral vision. Person will be asked to press a button every time you see a light. This shows any gaps in his field of vision.
The Snellen test
A Snellen test measures visual acuity. It involves reading letters off a chart on which the letters become progressively smaller. This chart is used during a routine eye test. After the test, a score made up of two numbers is given. The first number represents how far away from the chart were able to successfully read the letters on the chart. The second number represents how far away a person with healthy vision should be able to read the chart.
So if you were given a visual acuity score of 6/60, it means you can only read 6 meters away what a person with healthy eyesight can read 60 meters away.
Partial sight impairement
Partial sight, or sight impairment, is usually defined as:
• Having very poor visual acuity (3/60 to 6/60) but having a full field of vision
• Having a combination of moderate visual acuity (up to 6/24) and a reduced field of vision or having blurriness or cloudiness in your central vision
• Having relatively good visual acuity (up to 6/18) but a lot of your field of vision is missing
Severe sight impairment (blindness)
The legal definition of severe sight impairment (blindness) is when ‘a person is so blind that they cannot do any work for which eyesight is essential.
This usually falls into one of three categories:
• Having extremely poor visual acuity (less than 3/60) but having a full field of vision
• Having poor visual acuity (between 3/60 and 6/60) and a severe reduction in your field of vision
• Having average visual acuity (6/60 or better) and an extremely reduced field of vision
The treatment of blindness depends on the cause of blindness.
Blindness due to nutritional insufficiency: It can be addressed by dietary changes.
Inflammatory and infectious causes of blindness can be treated with medication in the form of drops or pills.
Most of people are blind due to cataract: In these patients, cataract surgery would, in most cases, restore their sight.
An intraocular lens (IOL) is a lens implanted in the eye used to treat cataracts or myopia. The most common type of IOL for cataract treatment are known as pseudophakic IOLs which work by replacing the crystalline lens which has been clouded over by cataracts.
The second type of IOL, more commonly known as a phakic intraocular lens (PIOL), is a lens which is placed over the existing natural lens change the eye's optical power as a treatment for myopia or nearsightedness.
National Programme for Control of Blindness & Visual Impairment