Understanding Colour Vision Deficiency

Colour vision deficiency is an eye condition that affects your ability to detect different colours and shades and can cause issues with telling the difference between some colours. It is normally caused by a fault in one or more of the cone cells in your retina. The severity of the colour vision deficiency can differ depending on individual cases, and you may not notice that you have the condition until your eyesight and colour vision has been tested.

Types of colour vision deficiency  

Colour vision deficiency can come in different forms, depending on the number of colour sensitive ‘cones’ you have in your eyes and how many of them are fully functional. Trichromacy is when all three types of cone cells (long or red, medium or green, and short or blue) work properly, giving you normal colour vision.

Anomalous trichromacy

Anomalous Trichromacy is when light can be perceived by all three types of cone, but one type doesn’t work correctly, meaning you have reduced sensitivity to a particular colour depending on which cone is not working:

• Deuteranomaly – This is when the issue is with your medium cones and means you are less sensitive to green light. Deuteranomaly is the most common type of colour vision deficiency. 
• Protanomaly – This is when your long cones are affected, reducing your sensitivity to red light.
• Tritanomaly – This is when your short cones don’t function properly, leaving you with reduced sensitivity to blue light. Tritanomaly is the rarest form of colour vision deficiency.

The light that red and green cones perceive can overlap, therefore people with dysfunctional red or green cones often experience similar colour vision problems. Deuteranomaly and protanomaly are known as red-green colour deficiencies and affect your ability to tell the difference between shades of red, green, yellow, orange, and brown. As the ability to see red light is normally affected, it may also be difficult to distinguish between different shades of purple.

People with tritanomaly are known as blue-yellow colour deficient, and experience difficulty with shades of blue, yellow, violet, red, and green. 

Dichromacy 

Dichromacy means that only two of your three colour-sensitive cones can perceive light, and whichever cone doesn’t work determines what form of dichromacy you have:

• Deuteranopia – This is when your medium cones are lost and don’t work at all, therefore you are unable to perceive green light. You may confuse different shades of red, green, and yellow.
• Protanopia – This is when your long cones don’t function, meaning you are unable to perceive red light.
• Tritanopia – This is when you lose your short cones, leaving you unable to perceive blue light.

Monochromacy (Achromatopsia)  

Monochromacy causes people to see no colours at all, only different shades of grey. It is very rare, however, the symptoms can be burdensome.

Signs & symptoms

Finding it difficult or being unable to recognise different colours and shades can be an indicator of colour vision deficiency.

Depending on what type of colour vision deficiency you have, you may experience different symptoms:

Red-green colour deficiencies

• Reds, oranges, yellows, browns, and greens can seem similar or duller than normal, and you may only be able to differentiate between them through distinctions in brightness.
• It may also be difficult to tell the difference between shades of purple, or between black and some shades of red.

Blue-yellow colour deficiency 

• Yellow may seem to be light grey or purple.
• Blues and greens may look similar, with green sometimes looking blue.

Complications of Colour Vision Deficiency

Colour vision deficiency shouldn’t cause any other health problems in the long term, however, there are certain careers that require colour vision to be accurate, therefore you may be unable to perform these jobs:

• Electrician
• Pharmacist
• Technician in a Hospital Laboratory 
• Customs Officer
• Fire Services
• Position in the Armed Forces
• Pilot and Air Traffic Controller
• Paint, Textile, or Paper Manufacturer
• Train Driver and Railway Maintenance 

Colour vision deficiency can also affect a child’s ability to learn, therefore you should let your child’s teachers know if your child is colour vision deficient, so they are able to adjust learning materials appropriately.

What causes colour vision deficiency?

The condition is normally passed on genetically, however, there are certain medications and medical conditions that can cause colour vision deficiency to develop. It can vary in levels of severity, but the condition shouldn’t deteriorate due to age – it may get worse if you have another medical condition or are taking any medicines that affect your colour vision.

How your eye works: Detecting colour

Colour is detected in the eye as light passes through the lens and reaches the retina, which contains three different types of cone cells that each detect light at different wavelengths. The long cone cells detect light from the red part of the spectrum, while medium cones detect light from the green part and short cones detect light from the blue part.

As the cone cells receive light, they send signals to be processed by neural cells in the retina before being sent to the brain, where our perception of colour is reproduced. The cone cells respond to light at different levels depending on the colour – if something changes colour from green to red, the medium cone cells react less than the long cone cells. 

When cones function normally, it’s possible to tell the difference between hundreds of colours, but with colour vision deficiency colours will appear differently, or not at all if the colour vision deficiency is severe. 

Genetic inheritance

Colour vision deficiency is usually a genetic condition you inherit from your parents. Genetic conditions are passed on through chromosomes, which carry genes that determine how your cells will develop.

When you have colour vision deficiency, it means there is an issue with the photopigment development in at least one of your cone cells.

Red-green colour vision deficiency 

If a red-green colour deficiency is inherited, it can mean that the long or medium cones are either not there, lacking a particular photopigment, have a reduced sensitivity to light or that the pathway from the cone cells to the brain hasn’t developed properly. 
The condition is caused by a faulty gene found on part of the 23rd chromosome, which is also called the sex chromosome as it decides if the baby will be a girl or boy. The 23rd chromosome is different in men and women – in men, it consists of an X and Y chromosome, while in women it consists of two X chromosomes. 

Red-green colour deficiency is found on the X chromosome, therefore it affects men and women in different numbers:

• For males to have a red-green colour deficiency, the faulty gene only needs to be on the single X chromosome, but for females to have the condition, both X chromosomes must have the faulty gene.
• A female who has just one red-green colour deficiency gene is a carrier of the condition without actually having it.
• If a female carrier has a baby, one of her X chromosomes will be passed on: 
  • If she has a daughter who receives the faulty X chromosome, she will also be a carrier.
  • If she has a son and he receives the faulty X chromosome, he will have red-green colour vision deficiency. If he receives the non-faulty X chromosome, he won’t have the condition.
• Males can only pass their X chromosomes on to a daughter and not a son, therefore males cannot get red-green colour deficiency from their father even if they have the faulty X chromosome.
• If the male has a daughter, she will only be red-green colour vision deficient if she inherits a faulty X chromosome each from her father and her mother.
• However, the daughter can be a carrier if she inherits one faulty X chromosome from either her father or her mother.

This is why 1 in 12 men are affected by red-green colour vision deficiency but only 1 in 200 women. 

Blue-yellow colour vision deficiency 

Blue-yellow colour vision deficiency is carried on a different chromosome, therefore there is an equal chance of men and women getting it. 

Other conditions

While colour vision deficiency is normally inherited, it can also be a result of another illness or medical condition. This often results in difficulties with seeing blue and yellow, and the condition may affect one eye more than the other. The following conditions can affect your colour vision:

• Optic Neuritis – a condition that causes the optic nerve to become inflamed.
• Glaucoma –a condition that can affect intraocular pressure.
• Age-Related Macular Degeneration – a condition that can damage the retina.
• Diabetes – a condition that affects your blood sugar levels.
• Sickle Cell Anaemia – a genetic condition that causes red blood cells to become deformed.
• Alcohol misuse.

Your colour vision may be able to improve if it was caused by another medical condition that was treatable, however, this also means your colour vision becomes worse if the condition deteriorates.

Medication

Colour vision deficiencies can also happen as a result of taking some medications. If this is the case the symptoms should normally stop once the course has finished, and if you notice your colour vision becoming worse after medication you should tell your GP who may prescribe a different course.

The following medications can cause issues with colour vision:

• Digoxin
• Ethambutol
• Chloroquine
• Hydroxychloroquine 
• Phenytoin 
• Sildenafil (Viagra)

Chemicals

Working with chemicals such as carbon disulphide and styrene can increase the risk of developing issues with your colour vision.

Ageing 

As you get older you may develop colour vision deficiency, however, this is a normal part of ageing. If the symptoms develop quickly or become severe you should talk to your GP or eye care specialist. 

How is colour vision deficiency diagnosed?

Some people may not be conscious of a colour vision deficiency until they have a problem naming colours, such as on a map or diagram. Colour vision deficiencies should be recognised early as they can affect the learning process in young children. With an early diagnosis, their education can be adjusted so it is more suitable to their vision.

Colour Vision Tests 

The standard NHS eye test doesn’t include a colour vision examination, and most opticians don’t provide them unless you ask.
Some common colour vision deficiency tests are:

Ishihara Test

The Ishihara test is the most common exam for colour vision deficiency. It involves looking at plates consisting of multi-coloured dots, with a number in the centre made from dots of a different colour, to see if you can identify the number correctly. Difficulties in recognising the numbers can indicate that you have colour vision deficiency.

Arrangement Test

Another exam involves arranging objects by their shade, and a certain error pattern can be an indicator of colour vision deficiency.

How Do You Treat Colour Vision Deficiency? 

No cure currently exists for colour vision deficiency as the faulty cone cells that cause the condition cannot yet be replaced, and as it won’t normally cause health issues the treatment isn’t essential. However, there are things you can do to reduce the inconvenience caused by the condition.

Adapting your Lifestyle 

There are ways to adapt your lifestyle to your colour vision – 

• You can ask friends and family to assist in selecting clothes and items where colour can be important.
• You can install lighting that can help you better distinguish between different colours.
• You can inform your child’s school if they are colour vision deficient so they can adjust their learning materials and methods.

Contact Lenses

Tinted contact lenses can help some people distinguish between colours, however they don’t always work for everyone. They are normally used in just one eye, and can affect your judgement of distance and depth as well as the intensity of lights.

Other Eye Conditions 

If your colour vision deficiency is caused by another medical condition, having it treated may improve your colour vision.