- Inability of a person to count fingers from a distance of 6 meters or 20 feet (technical definition)
- Vision 6/60 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:
The leading causes of chronic blindness include cataract, refractive errors, glaucoma, corneal opacities, age-related macular degeneration, trachoma, childhood blindness and diabetic retinopathy.
Cataract is the opacification of the lens in the eye and in advanced cases a whitish reflex can be seen in the eye. The most important symptom is painless progressive blurring of vision.
Refractive errors occur when the rays of light entering the eye do not focus onto the retina, the part of the eye on which the image forms. This is a very important cause of poor vision in children.
Glaucoma occurs when the pressure of the eye is increased and that causes progressive loss of sight. It is very important to identify and treat glaucoma as the vision lost is irreversible.
In corneal opacities the clear part of the eye in the front through which the light enters the eye becomes opaque and the patient cannot see.
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 peripheral (side) vision is normal but the central vision is slowly lost.
There 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.
Test for visual acuity
A Snellen chart is used to measure 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. The normal eyesight is 6/6.
Partial sight impairment
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 defificiency: It can be addressed by dietary changes.
-Visual impairment due to refractive error: It can be addressed by doing a refraction and providing appropriate spectacles.
-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. Usually an intraocular lens (IOL) is implanted in the eye after removal of the opaque natural lens.