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Common Eye Problems / General Interest
 
 
What is color blindness?

The human eye sees when light stimulates the photosensitive cells (photoreceptors) in the retina. The photoreceptors consist of rod and cone cells. There are 3 types of cones and each type is sensitive to either red, green or blue light. The eye sees colours when these cones sense different amounts of these basic colours. When one or more of these cone cells is absent or fails to function normally, the eye cannot see colours as normal.

A person with colour blindness has trouble seeing red, green, blue, or mixtures of these colours. Many people think that people who are “colour blind” only sees in black and white and shades of grey. This is a big misconception and is not true. It is extremely rare to be totally colour blind. Therefore the term “color vision defect” or “colour vision deficiency” is more accurate to describe this condition.

 

What causes color blindness?

Most colour vision defects are inherited (genetic) and are present at birth. It is estimated between 5-8% of men and 0.5% of women have colour vision defect making this a common condition. The commonest form of colour vision defect is the red-green colour vision defect. The gene for red-green colour vision defect is located in the X chromosome, one of the two sex chromosomes. In males (who have only one X chromosome), one defective copy of the gene is sufficient to cause the condition. In females (who have two X chromosomes), a mutation must be present in both copies of the gene to cause the disorder. Therefore males are affected more frequently than females. This pattern of inheritance is called X linked recessive pattern. One striking characteristic of this inheritance pattern is that fathers cannot pass X linked traits to their sons. 

Some forms of colour vision problems are acquired and can be caused by aging, eye disease or injury, disease of the optic nerve or side effects of some medicines.

 

What are the symptoms?

The symptoms of color vision defect vary with the severity of the condition. In some people, the condition is so mild that they may not be aware that they see colours differently from people with normal colour vision. Those who are moderately affected will demonstrate certain colour confusion. Others who are more severely affected may see only a few shades of colours. Very rarely a person cannot see color at all but sees only black, white, and gray (complete achromatopsia).

 

How is color blindness diagnosed?

Colour vision defect is commonly diagnosed using the pseudoisochromatic plate test (see figure 1). This test uses an arrangement of colored dots which forms a pattern (usually a number or a letter). People with colour vision deficiencies cannot identify certain patterns which are normally easily seen by a person with normal colour vision.

 

How is it treated?

There is no treatment for the inherited form of colour vision defect. Some acquired colour vision problems can be treated. For example cataract, which commonly becomes brown with time and can cause problem with color vision. Removal of such cataract may restore normal color vision.

Colour vision defect can have a significant impact on a person's life. It may affect learning abilities and reading development and may limit a person's career choices. However, most children and adults with colour vision defect learn to compensate for their inability to see colours and adapt remarkably well to their environment.

 
Pseudoisochromatic Plate Test

Figure 1: Pseudoisochromatic Plate Test. A- 25 (Sample plate); From top left clockwise - 25, 6, 8, 45, 29, 56

 
 
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