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20 June 2011

Colour blindness

The term colour blindness is actually misleading, as it commonly refers to a condition where people are unable to distinguish between different shades of colour.

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Summary

  • The term colour blindness is actually misleading, as it commonly refers to a condition where people are unable to distinguish between different colours or shades of colour, not where they are unable to see colour at all. Colour vision deficiency would be a more accurate term for this condition.
  • Colour blindness is an inherited disorder. More men than women suffer from colour blindness.
  • There is no real treatment or cure for colour blindness, unless it is connected to some other eye disease.
  • Colour blindness only has a really detrimental effect on people who would like to follow professions requiring accurate eyesight, such as airline pilots or train drivers.

Definition

The term colour blindness is actually misleading, as it commonly refers to a condition where people are unable to distinguish between different shades of colour, not where they are unable to see colour at all. A more accurate term for this condition is colour vision deficiency. True colour blindness is a very rare condition, the sufferers of which are only able to distinguish black, white and different shades of grey.

Colour blindness actually describes a number of problems experienced in identifying different shades and colours. This can vary from an inability to distinguish between different shades of the same colour, through to an inability to see any colours at all.

Types of colour blindness

Defective colour vision may occur as a result of a genetic variation (often inherited) and is present from birth, affecting about 8% of men and only 1% of women. Other eye conditions which develop during the course of one’s life may cause acquired defective colour vision and are usually associated with poor vision. The most common type of colour blindness is genetic red-green colour blindness. Most people who have this type of colour blindness are unable to distinguish between red and green in dim light, and a small percentage have difficulty doing this even in brighter light.

How we see colour

Once light enters through the lens of your eye and passes through the transparent main body of your eye, light-sensitive cells called cones in the retina enable you to see colour. Chemicals in the cones of the retina distinguish between the different colours and send this information to your brain via the optic nerve. It is thought that there are three types of these cones, each of which is sensitive to a different range of colours (red, green and blue).

Genetic colour blindness

This is the most frequent cause of colour blindness, and is connected to the X-chromosome, of which men have one and women two in every cell. The reason why this condition affects women so much less frequently, is that it is highly unusual for the second X-chromosome in a woman to be defective as well, and all women have two. One healthy X-chromosome will override the defectiveness of the other one in women. Sons of women with a defective X-chromosome have a 50% chance of inheriting colour blindness.

The type of genetic colour blindness someone experiences, depends on which of these three types of cones are defective. The colour blindness experienced can be mild, moderate or severe, depending on the severity of the defect. The condition does not improve over time.

Types of genetic colour blindness are:

  • Protanopia : Red-purple appears grey.
  • Protanomalia: A milder form Protanopia, where red-purple appears as a red-purplish type of grey.
  • Deuteranopia: Green appears grey.
  • Deuteranomalia: A milder form Deuteranopia, where green appears as a greenish type of grey.
  • Tritanopia: Blue appears grey.
  • Total colour blindness (Monochromatism): Inability to distinguish any colour variations, sufferers are only able to distinguish black, white and different shades of grey.

Acquired colour blindness

The causes of acquired colour blindness are:

Medication

This is fairly rare, but certain medications, such as tamoxifen (which is prescribed to inhibit breast cancer) and viagra can alter colour vision.

Eye diseases

There are several diseases which can cause colour blindness. Certain degenerative diseases can affect the retina of the eye and limit your ability to see yellow and blue. Optic nerve disorders may also reduce your ability to distinguish between different colours. These can be caused by inflammation of the optic nerve or by a shortage of vitamin A. Cataracts can also cloud your vision, making it difficult to distinguish between different colours.

Who is at risk?

Mostly European Caucasian men, of which 8% suffer some form of colour blindness. Men whose mothers have one defective X-chromosome have a higher chance of inheriting colour blindness. Approximately 1% of women suffer from this inherited disorder. Others who are at risk include people with degenerative eye disease, such as cataracts, or inflammation of the optic nerve and those who suffer from a nutritional deficiency of Vitamin A. As these diseases worsen, so does the colour blindness.

Symptoms and signs 

People who are colour blind from birth are often unaware of it. It is usually picked up by people around them, who notice their inability to distinguish between different colours or between different shades of the same colour. Most people who are colour blind are only mildly so, therefore it can take years before it is actually noticed. It is not a serious condition and would only really have an impact on people who want to become train drivers, pilots, boatsmen, chemical engineers, and interior decorators.

Diagnosis and treatment 

If you or your ophthalmologist suspects that you might be colour blind, an easy dot-pattern test can be used to determine in a short space of time whether you are indeed colour blind, and if so, which type of colour blindness you have. These tests mostly consist of seeing whether you are able to recognise numbers concealed in a dot pattern. If you are unable to distinguish between red and green, you will not be able to see the green number or shape amongst the red dots. An instrument called an anomaloscope is also used by some ophthalmologists in determining the type and the severity of the colour blindness. This tests colour blindness by requiring the patient to mix and match changing colours.

There is no real treatment for colour blindness. It cannot be cured or prevented. The most a doctor can do is to check if there may be eye diseases causing the colour blindness, and to treat those. But this is very rarely the case.

When to see a doctor

Colour blindness is not really a condition that requires ongoing medical attention. If colour blindness is suspected in a child, it is always a good idea to have the child tested for colour vision and for visual acuity. The child can then be guided with respect to career choices later in life. It is also a good idea to make sure that there may not be eye diseases causing the colour blindness.

Previously reviewed by Dr L.C. Boezaart

Reviewed by Dr Clive Novis, Ophthalmologist, June 2011

 
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