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
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
Signs and Symptoms
The symptoms of colour blindness
are dependent on several factors, such as whether the problem is congenital,
acquired, partial, or complete.
The symptoms of more serious
inherited colour vision problems and some acquired types' problems may include:
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 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.