Color blindness

Color blindness means having trouble telling certain colors apart, or seeing them differently from most people. The name is a little misleading: most people with it do see color but mix up some shades, which is why doctors usually call it color vision deficiency. The retina has three kinds of color sensing cells, called cones, each responding mainly to red, green or blue light. When one type is missing or works differently, the brain gets incomplete color information. Seeing only shades of gray is very rare and usually comes with reduced sharpness.

Red green color blindness is by far the most common type. People with it may struggle to tell red, green, brown and orange apart, or see some of these shades as duller. It is usually mild and often found by chance. Blue yellow deficiency is rarer and makes it hard to tell blue from green and yellow from red. The type can hint at the cause, but telling inherited from acquired takes a full exam.

Most color blindness is inherited. The genes for the red and green cones sit on the X chromosome. Men have one X chromosome, so one gene change is enough to cause it; women have two, and the working copy usually compensates. In populations of Northern European ancestry, red green deficiency is estimated to affect about 1 in 12 men and about 1 in 200 women, and rates may differ in other populations. The typical pattern runs from a carrier mother to her son. A father does not pass it to his sons, but all his daughters will be carriers. Blue yellow deficiency is inherited differently and affects both sexes similarly.

Red green color blindness in women is possible but uncommon, because both X chromosomes must carry the change. It mainly happens when the father is affected and the mother is a carrier or affected. Many women are carriers without knowing it, since their color vision is usually normal. Genetic counseling can help families understand the chance for their children.

The best known test is the Ishihara test: plates of colored dots hiding a number or shape that is easy to see with normal color vision and harder with a red green deficiency. It is quick and painless, and versions with shapes or paths exist for young children. The Ishihara test mainly detects red green deficiency and is not suited to blue yellow problems. Other tests, such as arranging colored caps by shade, can help define the type and severity. Online tests cannot replace an eye exam, since screen colors vary.

There is currently no treatment that restores normal color vision in inherited color blindness. Special tinted glasses or contact lenses may increase contrast between some colors for some people, but they do not create normal color vision. Phone apps that name colors through the camera can help day to day. Most people adapt well, using cues such as brightness, position and labels. Let a child's school know, so color based materials do not cause needless difficulty. Some jobs, for example in aviation and transport, have color vision requirements, so check with the relevant authority in advance.

Have a child checked if they struggle to name colors, use unexpected colors when drawing, or have color blindness in the family. It is especially important to be examined if an adult notices a new change, such as colors looking washed out or a difference between the two eyes. Such a change is not inherited and may come from optic nerve or retinal disease, general conditions such as diabetes, or side effects of certain medicines such as hydroxychloroquine. A sudden change in color vision, especially with vision loss or pain on eye movement, needs urgent assessment.