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.
Inherited colour blindness is most common affecting both eyes and does not worsen over time. This type is found in about 8% of males and 0.4% of females. These color 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. People 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. People with defective colour vision have a deficiency or absence in one or more of these pigments. People 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.
Signs and Symptoms
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 types of acquired problems may include:
- Objects appear as various shades of grey (this occurs with complete colour blindness and is very rare)
- Reduced vision
Detection and Diagnosis
Colour vision deficiency is most commonly detected with special coloured charts called the Ishihara Test Plates. On each plate is a number composed of colored dots. While holding the chart under good lighting, the patient is asked to identify the number. Once the color defect is identified, more detailed colour vision tests may be performed.
There is no treatment or cure for colour blindness. People with mild colour deficiencies learn to associate colours with certain objects and are usually able to identify colour as everyone else. However, they are unable to appreciate colour in the same way as those with normal colour vision.
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