hopinet

An introduction to Colour Deficiency

4th July 2009

Colour deficiency is what people call ‘Colour blindness’ incorrectly. The reason it is called a deficiency and not blindness is because you can see the colours but just have problems distinguishing between some, basically colours that look different to people with normal vision are confused by people with a colour deficiency. There are two main types of deficiency, these are acquired and congenital defects. Acquired defects are as a result of a disease or drug induced where as a congenital defect is a hereditary defect that someone is born with and doesn’t get worse.

A quick look at the anatomy of the eye will help understand how we see colours and what can go wrong. I’m just gonna keep it really simple. The human retina is like the film in a camera, it is where the image that the eye ‘sees’ is formed. This image is then converted to messages and sent of to the brain to be interpreted. The retina is packed full of millions of light sensitive cells, these are called photoreceptors. There are two types of photoreceptors called Rods and Cones. Rods cells are used in low light levels, the clarity of vision these cells produce is worse than cones, they also don’t help with colour vision. The cones are what we need to look at when we are talking about colour vision. There are 3 types of cone photoreceptors that correspond to different wavelengths of light. Colour deficiencies are caused by problems with 1 or more of these types of photoreceptors.

When you visit your Optician they will be able to test your colour vision and if you do have a defect they will be able to tell you what colours you will have problems with. I use two main tests when I look at colour vision, the City University test and the Ishihara plate test. I mainly use the Ishihara plates to screen for congenital defects, it is the classic test where the patient has to read out the numbers they see on each plate (see picture for an example plate). I am now just going to have a brief look at Congenital defects as a question I often get asked when diagnosing a defect is “what colours will they confuse”.

There are two things that affect which colours are confused and how severe the defect is. Firstly how many cone pigments are present affects how severe the defect is. Some one who has no cone cells is called a typical or rod monchromat, there is also atypical (incomplete) monochromats that have 1 of the 3 types of cone cells and they have some colour vision. Both these are rare conditions and as well as their colour vision being affected their quality of vision is affect and in typical monochromats their vision is very poor. Dichromats have 2 of the 3 types of cone cells present (they are missing one), Anomalous Trichromats have all 3 cone pigments present but there is a problem with one of the three. A Dichromat will have a more sever deficiency than a Anomalous Trichromat, dichromats confuse brighter, fully saturated colours where as anomalous trichromates confuse pale, less saturated colours.

The colours that a person confuses is determined by which of the cone pigments is affected. Both Dichromats and Anomalous Trichromats have 3 different variations. Without going in to all the details about each colour vision problem below is a quick look at what colours are confused in the 3 variants.

Both Protan and Deutan defects will cause confusion between red, orange, yellow and green, between brown and green, between green and white and between red and white. Where Protan and Deutan differ is that Protan defects will also cause problems distinguishing between blue-green, grey and red-purple. They will also have confuse between red and black. Deutan defects will cause problems distinguishing between green, grey and blue-purple and also between green and black. The third defect is a Tritan defect people with this type of defect will confuse violet and yellow-green, between red and red-purple, between dark blue and black and also between yellow and white.

So we know now the terms defining how severe the defect is and what colours will be confused by putting these terms together we can classify colour deficiencies. Looking at the most common defect we can see how this works, someone who has slight problems with their green to black comparison is known as having Deutranomalous trichromatism and this affects approximately 5% of males. Overall Congenital colour deficiencies affect 8% in men, 0.4% in women. The prevalence is much higher in men due to the way the genetic information is passed from the adults to their siblings.

Colour vision is a huge topic and one I could write about for pages but hopefully you now know what colours people confuse with different defects and why some people have more problems than others.