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21 December 2010

Vision problems

Vision problems can include short-sightedness, far-sightedness, astigmatism, presbyopia, floaters and flashes, macular degeneration, and retinitis pigmentosa.

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The eye is like a big screen television. The eye muscles, cornea and lens are constantly adjusting in order to focus the light on the retina, the light-sensitive membrane at the back of the eyeball.

Specialised nerve cells in the retina, known as rods and cones, convert light energy into nerve impulses, and these are transmitted at lightning fast speed through the optic nerve to the back of the brain.

You will see a clear and sharp image if the light is focussed - through neat orchestration of the lens, cornea, eye muscles and pupil size (controlling the light intensity) - precisely on the retina. If not, either distant objects or close-up ones (or both) may look blurry.

With prescription glasses or contact lenses or surgery, most people with blurry vision are able to see clearly again.

Vision problems can include short-sightedness (myopia), far-sightedness (hypermetropia), astigmatism, presbyopia, floaters and flashes, macular degeneration, and retinitis pigmentosum.

Short-sightedness or myopia

A short-sighted person can see nearby objects clearly but distant objects are blurred. The light is focused in front of the retina because the eyeball or the cornea is too rounded, like a soccer ball squashed from the bottom and top.

Short-sightedness is a very common problem: nearly one in three people are short-sighted. It is often inherited, and generally develops before age 20 and then stabilises. In most cases it is first detected in school children, when the child has trouble seeing clearly on the chalkboard or a TV screen a few meters away.

You may need to wear glasses or contact lenses all the time, or only while watching a movie, or driving or when you need to look at distant objects. It is also possible to alter the shape of the cornea with laser surgery, but only after progression has stopped in the early or mid-twenties.

Check your prescription: if you are severe short-sighted and the first number is more than -4.00 D in either eye, you should see your ophthalmologist for a retinal examination at least once a year.

Far-sightedness or hypermetropia

In far-sightedness, nearby objects tend to be blurred, and distant objects can be seen clearly. The light is focused behind the retina because the eyeball is not round enough, but almost shaped like an upright rugby ball.

It tends to run in families. It is present from birth, but symptoms only appear later in life, when the eye loses some of its focusing power of accommodation, which initially compensates for far-sightedness.

Common vision screenings are ineffective to detect far-sightedness. A comprehensive optometric examination is necessary.

Glasses or contact lenses can restore clear vision.

Astigmatism

Astigmatism is a common problem where distant and nearby objects are equally blurred due to irregularities in the shape of the cornea. The uneven curves of the cornea, which should be spherical, prevent the proper focusing of light on the retina.

Most people have some degree of astigmatism. People with mild astigmatism may experience blurred vision at certain distances, eyestrain, fatigue or headache. Severe astigmatism causes blurred vision.

Almost all people with astigmatism can be sea clearly with prescribed glasses or contact lenses. In some cases laser surgery to alter the uneven curves of the cornea, may be an option.

Presbyopia

Presbyopia is what happens to your eyes when you grow older and need to hold a book or newspaper at arms length to read. It is a natural part of the ageing process. It may seem to occur suddenly when you are in your mid-forties. However, the focusing power of the eye gradually weakens with age as crystalline lens loses its flexibility over years.

Reading glasses, bifocals, multifocals or contact lenses mat solve the problem, but as the loss of focussing power continue with age, these need to be strengthened slightly every few years.

Macular Degeneration

The characteristic of macular degeneration is the loss of central vision. It is common for the cells of the macula – the central part of the retina – to stop functioning efficiently with age.

In most cases the disease is slowly progressive. The condition eventually stabilises. In some people macular degeneration – usually the cases due to formation of abnormal blood vessels under the retina – will progress more rapidly. You will notice a central grey spot in your field of vision.

Reading will become more difficult as well as recognising people’s faces. People over sixty, those very short-sighted, and those with a family history of this disease, are most at risk.

If diagnosed early, some cases of macular degeneration can be treated by laser surgery to seal leaky blood vessels and inhibit their growth, so preventing further deterioration of eyesight.

There are no eye drops, tablets, spectacles, operations or alternative therapies proven to restore the vision that has been lost. However, you will keep your peripheral vision and will not go blind.

Floaters, flashes of light and retinal tears

Floaters are small specks of spots, lines or strands that seem to float across your field of vision. They are caused by small particles of protein or other matter trapped in the clear jelly-like fluid inside your eyeball during the formation of your eye.

They move when your eyes move, and seem to dart away when you try to look at them directly. Floaters are more noticeable when one looks at a featureless surface like a piece of paper or a clear sky.

Most floaters are harmless. Very rarely floaters may be very troublesome or obstruct your view. In some cases, floaters can be indicative of more serious problems, like a retinal tear.

See your ophthalmologist within 48 hours if you experience a change or increase in floaters, or a sudden shower of dot-like floaters, or if it obstructs your view. Your ophthalmologist may be able to seal the tear with a laser and so prevent a retinal detachment from occurring.

Read more:
Laser eye surgery
Screensaver or eyesaver?
Contact lenses only occasionally?
Eyestrain
Not yet dark, but getting there

Useful resources:
South African Optometric Association
Tel: 011 805 4517
South African National Council for the Blind
Tel: 012 452 3811
Retina South Africa
Tel: 011 622 4904
Ophthalmological Society of South Africa

 
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