Colour blindness may be a hereditary condition or caused by disease of the optic nerve or retina. Acquired colour vision problems only affect the eye with the disease and may become progressively worse over time. Patients with a colour vision defect caused by disease usually have trouble discriminating blues and yellows.
Inherited colour blindness is most common, affects both eyes, and does not worsen over time. This type is found in about 8% of males and 0.4% of females. These colour problems are linked to the X chromosome and are almost always passed from a mother to her son.
Colour blindness may be partial (affecting only some colours), or complete (affecting all colours). Complete colour blindness is very rare. Those who are completely colour blind often have other serious eye problems as well.
Photoreceptors called cones allow us to appreciate colour. These are concentrated in the very centre of the retina and contain three photosensitive pigments: red, green and blue. Those with defective colour vision have a deficiency or absence in one or more of these pigments. Those with normal colour vision are referred to as trichromats. People with a deficiency in one of the pigments are called anomalous trichromats (the most common type of colour vision problem.) A dichromat has a complete absence in one cone pigment.
The symptoms of colour blindness are dependent on several factors, such as whether the problem is congenital, acquired, partial, or complete.
Difficulty distinguishing reds and greens (most common)
Difficulty distinguishing blues and greens (less common)
The symptoms of more serious inherited colour vision problems and some acquired types' problems may include:
Objects appear as various shades of gray (this occurs with complete colour blindness and is very rare)
Colour vision deficiency is most commonly detected with special coloured charts called the Ishihara Test Plates. On each plate is a number composed of coloured dots. While holding the chart under good lighting, the patient is asked to identify the number. Once the colour defect is identified, more detailed colour vision tests may be performed.
There is no treatment or cure for colour blindness. Those with mild colour deficiencies learn to associate colours with certain objects and are usually able to identify colour as everyone else does. However, they are unable to appreciate colour in the same way as those with normal colour vision.
Illustrations by Mark Erickson
With acknowledgement to St. Lukes Eye Hospital.