Colour vision deficiency - Causes of colour vision deficiency

Colour vision deficiency is usually inherited (passed on from your parents).

Sometimes it can be caused by a health condition or by certain types of medication.

Your condition can vary from mild to severe, but it won't get worse as you get older. Your colour deficiency will stay the same provided you don't develop any other conditions or take medication that affects your vision.

How the eye detects colour

When light enters your eye, it passes through the lens before reaching the colour-sensitive cells, called cones, of the retina. The retina is the light-sensitive layer of cells that line the inside of the back of the eye.

There are three different types of cone cells – blue, green and red, better known as short (S), medium (M) and long (L) wavelength cones – which are sensitive to different wavelengths of light.  

When different wavelengths of light fall on the three types of cone cells, the cone cells send signals about wavelength to the brain. The cone signals are processed by the neural cells of the retina and transmitted to the brain, which produces our perception of shades of colour.

For example, if a colour changes from red to green, the M-cones will respond more, but the L-cones less. This relative change signals the change in colour.

If your cone cells function normally, you'll be able to distinguish between hundreds of different colour combinations.

However, if you're missing one type of cone cells, or if the cones aren't functioning normally, you may see colours differently or, in the most severe cases, not at all.

Inherited colour vision deficiency

Colour vision deficiency is a common inherited condition that's passed down from your parents.

Red/green colour vision deficiency is passed from a mother to her son on the 23rd chromosome, which is known as the sex chromosome because it also determines a baby's gender.

Chromosomes are structures that carry the genes you inherit from your parents. Genes are single units of genetic material that contain the instructions for the development of cells, tissues and organs.

If you have colour vision deficiency, the instructions for the development of the photopigments in one of the cone cells in the retina are usually faulty.

The cone cells may be missing altogether or, in the case of the L- and M-cones, may be missing the affected photopigment or they may be less sensitive to light. Alternatively, the pathway from your cone cells to your brain may not have developed correctly.

The 23rd chromosome is made up of two parts – either two X chromosomes if you're female, or an X and a Y chromosome if you're male.

The faulty gene for red-green colour vision deficiency is only found on the X chromosome, so for a male to have the condition the faulty gene only has to appear on his X chromosome. For a female to have colour vision deficiency, the gene must be present on both her X chromosomes.

A woman with only one colour vision deficiency gene is a carrier, but she won't have colour vision deficiency. When she has a baby she'll pass one of her X chromosomes to them. If she passes the X chromosome with the faulty gene to her son, he will have colour vision deficiency.

However, if he receives the "good" chromosome, he won't have the condition. If she passes it to her daughter, her daughter will also be a carrier.

A boy with colour vision deficiency can't receive a colour deficiency gene from his father, even if his father is colour deficient himself. The father can only pass an X chromosome to his daughters.

Therefore, a colour-deficient daughter must have a father who is colour deficient and a mother who is a carrier who has passed the faulty gene to her.

If her father isn't colour deficient, the daughter of a carrier mother won't be colour deficient. A daughter can become a carrier in one of two ways – she can either acquire the gene from a carrier mother or from a colour-deficient father.

This is why red-green colour vision deficiency is far more common in men than women (1 in 12 men have a red-green colour vision deficiency compared with only 1 in 200 women).

Blue-yellow colour vision deficiency affects both men and women equally because it's carried on a non-sex chromosome. It's much rarer than red-green colour vision deficiency.

Other health conditions

Colour vision deficiency can sometimes be caused by an illness or pre-existing health condition. If you have colour deficiency as the result of a health condition, you'll usually have problems seeing blue and yellow colours. Your colour vision may also be worse in one eye compared with the other.

Conditions that can cause colour vision deficiency include:

• diabetes – a lifelong condition that causes your blood sugar level to become too high
• glaucoma – a group of eye conditions that affect vision
• optic neuritis – inflammation of the optic nerve; the condition is sometimes associated with multiple sclerosis
• age-related macular degeneration – a painless eye condition that affects the retina
• alcohol misuse
• sickle cell anaemia – an inherited blood disorder where red blood cells develop abnormally

If your condition is treatable, it may also be possible to improve your colour vision deficiency. However, if your condition gets worse, your colour vision deficiency may become more severe.

Medication

Some medications can cause abnormalities in colour vision. If the abnormality is caused by medication, your sight will usually correct itself when you stop taking the medication.

Speak to your GP if you find it difficult to distinguish colours after taking a medicine. They may be able to prescribe an alternative medication.

However, you shouldn't stop taking prescribed medication unless your GP specifically advises you to do so.

Medicines that can cause colour vision abnormalities include:

• digoxin
• ethambutol (a treatment for tuberculosis)
• chloroquine
• hydroxychloroquine
• phenytoin
•  (Viagra)

Chemicals

You may be at risk of developing colour vision deficiency if you're exposed to chemicals as part of your job. Chemicals known to cause problems with colour recognition include carbon disulfide and styrene.

Your health and safety should be protected in the workplace. If you develop a health condition such as colour vision deficiency, speak to your employer to ensure appropriate health and safety procedures are in place and that it's safe for you to continue working.

The Health and Safety Executive (HSE) provides more advice and information about health and safety at work.

Ageing

Most people's ability to distinguish colours deteriorates with age. This is a natural part of the ageing process and not something to be overly concerned about.

However, you should visit your GP if you have symptoms that start suddenly or if you're experiencing severe colour vision deficiency.