By Samara Macrae
Published 12:33 PM EST, Tue August 17, 2021
Introduction
While commonly referred to using the term ‘colour blindness’, colour deficiency vision instead is the term used when an individual’s colour vision is impaired, and as such they may not be able to distinguish between different colours1. Colour blindness is only where the individual cannot see any form of colour, and their vision is exclusively in black and white (monochromacy). This condition is very rare, while colour deficiency vision (a form of colour blindness) can affect up to 1 in 12 men and 1 in 20 women. The most common forms of colour blindness are protanopia and deuteranopia.
2Monochromacy, or complete colour blindness, can be caused by the individual having two sets of cones (out of short wave, medium wave, and long wave) which either do not function correctly or are simply not present in the retina. This results in the individual not being able to see a full spectrum of colour as a person with normal vision (which is also known as trichromatic vision) would be able to. Achromatopsia is where there are no functional cone cells at all, and so vision is only in varying shades of black and white.
Variations of Colour Deficiency Vision
3Red-green colour blindness is the most common form of colour blindness and is divided into two types: protan colour blindness is reduced sensitivity to red light, and deuteranopia is sensitivity to green light. Colour vision is controlled by cones in the retina, a layer of the eye onto which light is focused on, and when some of these cones are ineffective or not present, this will affect the individual’s colour vision. Protanopia is the result of missing long-wavelength cones (L-cones) in the retina and affects 1.01% of men but only 0.02% of women. People with protanopia as a result of missing L-cones are ‘dichromats’, and they have cones which can only detect short and medium wavelengths. Red-green colour blindness can also occur due to L-cones being defective but still present (protanomaly) and means that individuals can have varying strengths of colour blindness – this is referred to as anomalous trichromats, as the individual can still detect short, medium, and long wavelengths using their cones. 4Deuteranopia is the second form of red-green colour blindness and is also called green-blind. In cases of deuteranopia, the medium wavelength sensitive cones are missing – and so the individual can only differentiate between 2 or 3 different shades (typically blue, yellow, and brown), while a person with normal vision can distinguish between the 7 hues of visible light. As with deuteranopia the specific cones are missing, people with this condition are dichromats. Anomalous trichromats are individuals with deuteranomaly (green-weak), which is where the green-sensitive cones are deficient. Deuteranomaly can be very mild and is any form of colour deficiency vision between (very close to) normal vision and deuteranopia. Deuteranomaly affects 5% of the global male population, but only 0.35% of the global female population. 5Tritanopia (blue-yellow colour blindness) is where the short-wavelength cones are missing or otherwise impaired. Tritanopia is where these cones are completely missing, and only long and medium-wavelength cones are in the retina – and individuals with tritanopia are dichromats. Tritanomaly is where the short-wavelength cones are deficient in some way, often due to a mutation.
Inheritance
Colour blindness is a sex-linked genetic disorder and is carried on the X-sex chromosome. This is why men are more likely to have colour blindness, as they only have one X-sex chromosome and so only require one recessive allele coding for colour blindness. Women have two X-sex chromosomes though, and so two recessive alleles are needed for colour blindness – meaning that if a woman only has the allele for colour blindness on only one X chromosome, she will be a carrier but not have the condition herself. As a result, a man with colour blindness can pass this condition onto his daughter, who will inherit an X-sex chromosome from him as well as the mother but cannot pass the condition onto his son – who will inherit an unaffected Y-sex chromosome from him. Unlike both protanopia and deuteranopia, tritanopia/tritanomaly is not a sex-linked genetic trait – and thus men and women are affected equally by it, though it is a rare form of colour deficient vision – as it is carried on the 7th chromosome instead of the 23rd (the X-sex chromosome). Additionally, though less commonly, colour blindness or colour deficiency vision can also be the result of damage to the eye or optic nerve – as so is not necessarily solely congenital.
Diagnosis
While people who have colour blindness/colour deficient vision may not ever realise they have the condition, tests to ascertain whether a person is colour blind are often widely accessible. An example of such a test is the Ishihara Test for Colour Blindness. This diagnostic test was created by Dr Shinobu Ishihara, an ophthalmologist, as he was asked by the Japanese Army (in which he served as a military doctor) to devise such a test to use on those conscripted for the Army. The Ishihara Test can be used to detect red-green colour blindness/deficiencies but not the rarer form of yellow-blue colour blindness6. As part of this test, an individual is shown a series of coloured circles consisting of multiple small circles to make up a larger one. Within this larger circle, some of the smaller circles are differently coloured to make the shape of a specific number (which is different for each image). Depending on whether or not a person is able to ascertain what the number is within each image can help to indicate whether or not they have normal vision or colour deficiency vision.
Correction
Colour blindness is incurable, although some forms of colour deficiency can be lessened using corrective lenses or glasses. 7Dr Ivan Schwab, Professor of Ophthalmology at the University of California, says that such glasses or lenses “[enhance] the distinction between red and green” for the person wearing them, although full colour vision is not achievable using them. He also states: “Colour blindness glasses are made with certain minerals to absorb and filter out some of the wavelengths between green and red that could confuse the brain”. This can result in fewer colours being detected by the person’s cones, and so can allow for easier distinction between them. However, these corrective lenses or glasses do not have any effect on the optic nerve, brain, or cone cells – and furthermore, these lenses/glasses are often expensive yet yield minimal to no results, and additionally can worsen vision at night due to the fact that they work by reducing the amount of light entering the eyes and being detected.
Difficulties and Lack of Accessibility
Although many individuals with colour blindness or colour deficiency vision, as well as charities such as Colour-Blind Awareness, are campaigning for these conditions to be classified as disabilities, they are not currently. Under the 2010 Equality Act (UK), a disability is defined as “a physical or mental impairment that has a ‘substantial’ and ‘long-term’ negative effect on your ability to do normal daily activities”8. A simple but common example of how colour blindness can affect activities of everyday life is a person with deficient colour vision not being able to differentiate between unripe and ripe fruit, or raw and cooked meat. In addition, children especially can struggle in education as a result of their colour deficiency vision – and exam papers especially may not be fully accessible to them – and later in life, a person with colour blindness cannot become a pilot nor enter the army. Colour vision deficiency can also limit other future career options – particularly jobs involving heavy machinery, a job in aviation or any job predominantly based around driving. The consequences of colour blindness can also be potentially fatal – such as problems with traffic lights leading to road accidents. A person with protanopia may not be able to distinguish between the red and green traffic lights – for a person with protanopia, the lights can all look white/pale yellow – and so is potentially more likely to be involved in road accidents as a result. In Australia, since 1994 individuals with either protanopia or protanomaly have not been able to obtain a driving licence due to the increased risk of accidents.
Conclusion
To conclude, colour blindness is a complex condition which can be frequently misunderstood due to the multiple variations of the condition. Furthermore, there are frequent misconceptions as to what ‘colour blindness’ actually entails – as it is not any form of difficulty to distinguish between colours, but rather no colour vision at all. Education systems should work to help diagnose more children who have colour deficiency vision or colour blindness, as it can impede their daily life and schoolwork if they are unaware of their condition – and thus education facilities are unable to make education/work more accessible to them without this knowledge. Improved testing and diagnosis earlier on – especially for children in early years – can additionally mean that they do not suddenly find themselves unable to pursue a specific career path or obtain a driving licence when they are older, as they were not aware of their colour deficiency vision/colour blindness beforehand. While not life-threatening, colour blindness and colour deficiency vision can have significant impacts on daily life, and simply diagnosing these conditions earlier can help improve accessibility for all aspects of life for these individuals.
Samara Macrae, Youth Medical Journal 2021
References:
1. Mayo Clinic: “Colour Blindness” – https://www.mayoclinic.org/diseases-conditions/poor-color-vision/symptoms-causes/syc-20354988
2. Cambridge Cognition: “Could Colour Blindness be Affecting the Results of your Study?” – https://www.cambridgecognition.com/blog/entry/could-colour-blindness-be-affecting-the-results-of-your-study#_cc-image3
3. Colblindor: “Protanopia” – https://www.color-blindness.com/protanopia-red-green-color-blindness/
4. Colblindor: “Deuteranopia” – https://www.color-blindness.com/deuteranopia-red-green-color-blindness/
5. Colblindor: “Tritanopia” – https://www.color-blindness.com/tritanopia-blue-yellow-color-blindness/
6. Eye Magazine: “Nine decades on, a Japanese army doctor’s invention is still being used to test colour vision” – http://www.eyemagazine.com/feature/article/ishihara
7. American Academy of Ophthalmology: “Do Colorblindness Glasses Really Work?” – https://www.aao.org/eye-health/tips-prevention/do-color-blindness-correcting-glasses-work
8. GOV.UK: “Definition of disability under the Equality Act 2010” – https://www.gov.uk/definition-of-disability-under-equality-act-2010
Youth Medical Journal 