Where I live, there are a lot of blind people walking around. There are special road crossing signs for the blind. But because I wear very thick glasses, some people call me 'blind' as well! How do you define blindness?
Strictly speaking, blindness means the total absence of vision in either eye. A blind person cannot distinguish bright light from dark in that eye. So you have terms like, "You are blind in one eye", or "You are blind in both eyes".
Naturally, in our society, the term "blind" is thrown around freely to denote varying degrees of sightlessness. The saying, "I'm as blind as a bat" is actually wrong because most bats are far from blind and have excellent vision.
"Visually impaired" or "low vision" are two other terms which are commonly used.
Worldwide, around 400 million people (including me) are visually impaired. Out of this, 50 million people are completely blind.
People who don't see well without corrective glasses but have almost normal vision cannot be termed "blind", though there's a tendency for their spouses and parents to throw the word around.
Are there circumstances in which you are blind only in certain situations? I have a friend who can't drive in the dark because she says she has 'night blindness'.
There are indeed different types of blindness. Night blindness is a condition in which you have difficulty seeing in dim light, or in the dark. It has to do with having a low number of functioning rods on the sides of your retina, and may be either inherited or caused by certain diseases (lack of vitamin A, for one). It is not actual blindness as these patients function well in bright light.
Colour blindness is a sex chromosome-inherited condition that leads to the inability to perceive certain colours well. Patients who have colour blindness have normal vision otherwise.
Snow blindness is another condition which occurs after your eyes have been exposed to large amounts of ultraviolet light. In this, you can see some shapes and movement, and so you are not completely blind. It's caused by the swelling of your corneal cells. Thankfully, it's only a temporary condition.
Is there a measurement for blindness?
We use the Visual Acuity chart. You know the ones – they are displayed in the testing rooms of optometry centres and eye clinics everywhere.
Legal blindness is defined as having either:
- A central visual acuity of 20/200 or less in your better eye WITH corrective glasses, or
- Having a visual field defect in which your peripheral field (side of your vision) is contracted to such a degree that the widest diameter you can see is only within an angle of 20° or less in your better eye.
Yes, the latter is a bit confusing, but it implies a very narrow range of vision (or tunnel vision). When you have this, you are considered legally blind and are not allowed to drive.
What causes blindness?
In developed countries, the causes are usually diabetic eye disease (where new blood vessels sprout all over your retina as a result of vessel wall damage and poor sugar control), macular degeneration, and eye injuries. The most common infective cause is herpes simplex infection of the eye.
In poorer countries, where there are a lot of infections and there is no proper eye care for the masses, the leading causes of blindness are infections (trachoma, leprosy and river blindness), cataracts, glaucoma, eye injuries and the sheer inability of poor people to procure a pair of glasses.
Other causes include:
- Blood vessel disease involving your retina or optic nerve (a stroke)
- Vitamin A deficiency
- Eye tumours
- Congenital abnormalities of the eye
- Poisoning from toxins such as methanol
- Retinitis pigmentosa (where the rods and cones of your retina gradually become defective)
Can I prevent blindness?
Of course. Just look at the causes of blindness. If you have diabetes, make sure your blood sugar is well controlled and you get an eye check-up frequently with an ophthalmologist.
When you are performing dangerous jobs (such as working in a factory or chemical lab where you are exposed to the risk of injuries), make sure you wear protective eye gear.
Consume sufficient vitamin A to stave off night blindness (and subsequently actual blindness). Treat eye infections quickly when they occur.
And always remember, even if you are blind, you can have a full life.