Colour blindness is a term many of us will have encountered at some point in our lives. We may know someone who has been labelled colour blind, because they confuse certain colours and see things differently, but it does not mean they can’t see colours at all. Such individuals are actually colour deficient, not colour blind. A true colour-blind individual sees the world in black and white, but such individuals are very rare.

Colour vision deficiency affects more males than females. In this article, we will explore the reason why colour vision deficiency occurs, the different types, and how it can be inherited.

Causes of Colour Vision Deficiency

Within the retina, there are specialized cells called photoreceptors. Photoreceptors enable us to see by absorbing light and sending that information to the brain for interpretation. There are two groups of photoreceptors, namely, rods and cones. Rods are responsible for low light vision, while cones are responsible for colour vision. These cones are all located at the macular, the area in the retina responsible for our central vision. Among the cones, 3 types exist, 1 for each primary colour – red, green and blue. Each of these categories are able to recognize these colours based on the wavelength of light.

In a non-colour deficient individual, each cone is present and has no abnormalities allowing them to absorb the wavelengths of light they are responsible for and sending them to the brain. However, when one type of cone is missing completely or is abnormal, then information being sent to the brain is altered or missing resulting in the confusion of colours. To reiterate, colour vision deficient individuals are still able to see colours, but they are unable to appreciate different shades of colour based on the type of colour vision deficiency they have.

Colour vision deficiency can be inherited or acquired due to trauma or underlying conditions such as diabetes, glaucoma, macular degenerations, Parkinson’s disease and many more.

Types of Colour Vision Deficiency

The types of Colour Vision Deficiency are as follows:

  1. Protanopia – usually confuse the difference between red/black, blue/purple
  2. Deuteranopia – usually confuse the difference between green/red, red/brown 
  3. Tritanopia – usually confuse the difference between blue/green, orange/red 

 Protanopia and Deuteranopia, also commonly referred to red-green colour deficiency, are commonly inherited at birth. Tritanopia, blue yellow colour deficiency, usually occurs due to another underlying condition or trauma at some point in life.

How is colour vision deficiency inherited?

A common misconception is that females cannot inherit CVD. Unfortunately, that belief is false, but it is very rare for it to occur. In this section, we will explore how genes can be passed down in a simplified breakdown. There are two chromosomes that pair up to determine our gender at birth, X and Y chromosomes. The XX pair determines the female gender while the XY pair determines the male gender. Red green deficiency is caused by a common X linked recessive gene. What this means is that the gene trait for the colour vision deficient gene is found in the X chromosome. Recessive means that in order for the mutation to manifest, both chromosomes must carry the deficient colour vision gene.

How is colour vision tested?

There are various methods available to test our colour vision. The most common method is through the use of pseudoisochromatic plates such as the Ishihara Colour Vision Test. Each pseudoisochromatic plate contains a circle consisting of random dots of various sizes, brightness and colours. They are designed in such a way that allows a person with normal colour vision to see a single digit or two digits within the circle. However, a person with colour vision deficiency may see no numbers or will see a different number as seen by an individual with normal colour vision.

Other colour vision tests that are available but are more commonly used in hospitals or specialized clinics may use a more robust test such as Farnsworth-Munsell 100 Hue Test. This test is much harder to do and takes a much longer time than the tests using pseudoisochromatic. In the 100 Hue test, there are a total of four trays, each consisting of many caps of varying hues. The individual being tested will have to arrange the caps in order by matching the cap with the closest hue starting from the reference cap. The closer the match between the tested sequence of coloured caps and the correct sequence, the more accurate the individual’s colour vision is.

Are there any treatments available for colour vision deficiency?

At present, there is no cure for inherited colour vision deficiency. There are specialized coloured prescription lenses in the market that can help a colour vision deficient individual to appreciate different shades of colours that they commonly confuse with. Unfortunately, they are subjective and may not work for everyone. 

For cases of acquired colour vision deficiency, there is a possibility of recovering and getting back normal colour vision if the underlying cause can be treated.

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