Optician India

The sooner a person knows that they have an inherited colour vision deficiency the better.

I check all children (male and female) who are younger than 18 years of age who are new to the practice or existing child patients whom I can’t find evidence of a previous test in the clinical records. By the time people reach adulthood they have tested positive for inherited colour vision deficiency or they have simply realised they have it by making frequent colour mistakes in everyday life.

It's very important to detect inherited colour vision deficiency early in childhood for two main reasons:

1) Once the presence of inherited colour vision deficiency is known children and their carers can be counselled about not being able to enter certain careers. People with inherited colour vision deficiency are excluded from certain parts of the miliary, police and from being a pilot and any other work that requires being able to accurately distinguish between colours. It’s probably safer if they don’t become electricians. It is far better for a family to know of a child’s inherited colour vision deficiency from an early age as possible in order to guide the child away from careers with a strict colour vision requirement.

2) The child’s carers should inform school teachers. There is often colour coding in maps for history and geography and colour changes in chemistry. Children with inherited colour vision deficiency are very likely to make mistakes with these types of school work and it’s important for the teachers to know that these mistakes are caused by difficulty distinguishing between colours and not learning ability. I will also advice carers that this is not treatable. It won’t get better but also it won’t get worse. They can sometimes be an awkward discussion when I diagnose inherited colour vision deficiency in a boy and the mother blames the father because he also has inherited colour vision deficiency. I explain to the mother that her son’s inheritedcolour vision deficiency sits on the X chromosome and her son only gets this chromosome from her. The father got his from his mother. The faulty genetic code for inherited colour vision deficiency is carried on the X chromosome and is a recessive genetic disorder. So, if there is an accompanying normal X chromosome the colour vision deficiency won’t manifest itself. Females have two X chromosomes so inherited colour vision deficiency is rare in females. The figure quoted in most reliable sources is 1 in 250, which to me isn’t that rare. The other thing I sometimes see in reliable sources is that the Ishihara colour vision test isn’t sensitive enough to detect inherited colour vision deficiency in females. For a female to inherit colour vision deficiency each of her X chromosomes has to have the faulty code for colour vision deficiency. One faulty X from the father (who will also have inherited colour vision deficiency) and the other X from the mother who may or may not have inherited colour vision deficiency. Males only have one X chromosome so colour vision deficiency is much more common in males because if they inherit a faulty X from the mother they can’t get a complete X to counteract the faulty one. One in 12 males (8%) have a colour vision deficiency.

It is easily tested in practice using the Ishihara test. Some attention needs to be given to the lighting used to illuminate the test plates. Tungsten lighting, although being phased out in many countries, is still common and may make the symbols which test red colour vision easier to see. It is better to conduct colour vision testing using fluorescent light.

I use the Ishihara test with both eyes open. I know the instructions for this suggest use the familiarisation plate and then to go through the screening plates and if some of these are failed to then go to the classification plates. However, my practice is to use the familiarisation plate so the child understands this test is about identifying numbers. If the child isn’t familiar with the names of double digit numbers it’s okay for them to say 1 and 2.

Using even a clean finger for tracing will eventually lead to dirty numbers which people with inherited colour vision deficiency will be able to identify.

Then I go straight to the classification plates and look for quickrecognition of the numbers on each plate. Again, if the child doesn’t know the names of double digit numbers they can call out the single digits.

Each plate is the same so the test can be limited to showing only one of these plates. I prefer to show both to reinforce a positive or negative result.

They are called classification plates because they help classify colour vision deficiency into protanopia/anomaly (red light detecting cone problem) and deuteranopia/anomaly (green light detecting cone problem). If a person doesn’t see the red digit(2 and/or 4) at all,the red pixels haveblended in with the pixels of the grey back ground and they can’t perceive it. This indicates protanopia. If they struggle to see the red digit but do eventually correctly identify it, they are protanomalous. See below for colours that a person with a protan-deficit will confuse.

If a person doesn’t see the lilacdigit(6 and/or 2) at all,the lilac pixels haveblended in with the pixels of the grey back ground and they can’t perceive it. This indicates deuteranopia. If they struggle to see the lilacdigit but do eventually correctly identify they are deuteranomalous. See below for colours that a person with a deuteran-deficit will confuse.The numbers are lilac but they are testing the sensitivity of retinal cones that detect green light.

When I identify a person with inherited colour vision deficiency I advise the carers that where possible children of the father’s and mother’s sisters (patient’s aunts) are tested for inherited colour vision deficiency.

Sometimes people are described as having red and green colour vision deficiency suggesting they have problems with red and green light detecting cones. They either have a problems with red or green detecting conesbut rarely with both.Nevertheless, protanopia/anomaly can cause problems with red and green colours and so can deuteranopia/anomaly.

Protans are more likely to confuse the following colours:

1. Black with many shades of red.

2. Dark brown with dark green, dark orange, dark red, dark blue/purple and black.

3. Some blues with some reds, purples and dark pinks.

4. Mid-greens with some oranges.

Deuterans are more likely to confuse the following colours:

1. Mid-reds with mid-greens.

2. Blue-greens with grey and mid-pinks.

3. Bright greens with yellows.

4. Pale pinks with light grey/white.

5. Mid-reds with mid-brown.

6. Light blues with lilac.

The 8% of males with inherited colour vision deficiency can be divided approximately into 1% deuteranopes (great difficulty with colours), 1% protanopes (great difficulty with colours), 1% protanomalous (some difficulty with colours) and 5% deuteranomalous (some difficulty with colours). Approximately half have a mild anomalous deficiency, the other 50% have moderate or severe anomalous conditions.

A case that comes to my mind from my own clinical practice is of a 16 year-old boy who wanted to enter the Navy in a role that required perfect colour vision. He had wanted to do this from a very young age. He had had his eyes examined around eight times previously in another practice from an early age. I assessed his colour vision and he had severe deficiency; deuteranopia. He was with his mother and when I told them, both cried. His mother asked why this had not been detected previously. It had not been detected because the optometrists who had tested the patient previously had not taken the time to test for it.

A lot of harm can be prevented by testing for inherited colour vision deficiency at an early age.