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Colour Defective Vision

The term colour blindness is a common but misleading term that implies total loss of colour vision. In most cases the defect is usually only partial and the term “colour defective vision” is more appropriate. The commonest defect is in the red/green part of the visual spectrum. Males are more commonly affected; about 8% of all males and 0.5% of females are affected to a varying degrees.

There are other defects due to failure in the blue receptors, a total failure of all receptors and a failure of the other retinal receptor, the rod. These are all rare and not suitable for discussion here.

Mechanism of defect:

All colours can be made by mixing the three primary colours; red, green and blue in varying amounts and the eye sees colours by detecting the amounts of these primary colours. This processing is done by special nerve endings in the retina of the eye. One type of nerve ending, the cone, has three special pigments which detect one colour, either red, green or blue. If one of these nerve types is impaired there is a colour defect.

Males are more affected because the genes that control the development of these colour sensitive nerve endings are carried on the X chromosome. Males have an X and a Y chromosome and women have two X chromosomes. Women have two copies of the genes for colour vision, one set on each X chromosome. Males have only one set of genes for colour vision and if they are not working properly, males do not have the back up of the other set of genes on the other X chromosome that women have. This is known as X-linked recessive inheritance.

Colour defects can also be acquired and a common cause in our community is cataracts. As the cataract becomes more dense it filters out all colours, but the bright reds, yellows, greens and blues are more affected. It has been said that the changing colour choices of many famous painters has been due to the affect cataracts have had on their colour vision. Other diseases affecting the optic nerve and the retina also affect colour vision but these are rare and often only affect one eye.

Type of defects:

The defects are named after the Greek words for the three primary colours: Protos for red, Deutros for green and Tritos for blue. Someone who has a complete red defect is said to have Protanopia and if he only has partial defect he is said to have Protanomaly. The commonest defect involves the green receptors and it accounts for over half of the defects; 4% of all males have a partial defect and 1% have a complete defect. As the loss of colour vision is in the middle of the visual spectrum this defect causes the least awareness. The problem is the inability to distinguish red and green but they are sensitive to red light.

The next most common defect is due to failure in the red receptors, and it affects 1% of males. These people also confuse red and green but are not sensitive to red light.

There are other defects due to failure in the blue receptors, a total failure of all receptors and a failure of the other retinal receptor, the rod. These are all rare and not suitable for discussion here.

Methods of detection:

A number of tests have been devised to detect colour vision defects. The most basic of these is to show the patient a number of coloured discs on a screen and ask him or her to identify the colours.

The commonest test and one of the most efficient is the use of special test plates called “pseudo isochromatic” plates or confusion plates. The Ishihara or H-R-R are the most commonly used ones in this country.

It can be seen that these plates are made up of a series of spots of varying colours and hues so that the central number or letter stands out from the background. Patients with normal colour sense can pick out the central figure from the background, but those with a defect are unable to make this distinction. These people either see no figure at all, or see a different figure due to different appreciation of the hues. By changing the colour and hues of both the figures and the background all basic types of defective colour vision can be recognised and classified.

For more subtle defects in colour vision more complicated tests are available. These are conducted by asking the patient to match a fixed colour with a mixture of other colours, or by trying to put coloured objects in a correct sequence. The lantern test is an old test that is used to check colour vision when applying for certain forms of work e.g. train engine drivers, signal men in the Navy, and pilots. This test mainly picks up red/green defects by asking the patient to pick and match seven colours on three discs.

Effects of Colour Defective Vision:

Most defective colour vision does not seriously handicap those suffering from it. This is because the patients have normal visual acuity and then they adapt, for example, they know that the red traffic light is at the top and the green at the bottom. They also see some colour but in different intensity and hues to those with normal colour vision.

There are restrictions on certain occupations such as train drivers, service personnel and pilots, as these occupations cannot afford to rely on associations to make correct decisions about colours of signals or signs. In fact, people with defective green receptors are very useful in the military services as they can very easily detect camouflage as they have the ability to pick up different hues.

There is no known treatment for defective colour vision and there is no test to determine who are carriers of the defect. Although 8% of all males suffer from some defect in colour vision, many of these are only minor defects and these people are unaware of the problem; it is only a minority of this group that have more severe manifestations and as was said before most of these people adapt without any serious inconvenience or problems.