Our retina is made up of photo receptors called rods and cones. The rods, which are more in the periphery of our retina, are involved in night vision and the cones, which are located more in the center of the retina (macula), allow us to perceive color in daylight. Cones are sensitive to different wavelengths of light due to the amount and type of pigmentation that are genetically coded in them. The X chromosome carries the genes that produce the pigments in the cones that are needed to differentiate color. When the genes that code the cones level of pigment are coded incorrectly the pigments are wrong. The result is color deficiency or color blindness.
Causes of Color Blindness
Color blindness is seen more in males than females. In the U.S. about 7% of males and 0.4% of females can’t distinguish between red and green for example. Poor cone development will lead to poor perception of color, especially with the three main colors of red, green and blue.
Generally a patient typically falls into two major categories of color blindness:
- Deficiency in determining red or green
- Difficulty seeing the difference between blue and yellow
In protanopsia there is an absence of red photo receptors and red appears black. There is also a reduced perception of orange and yellow. This appears in 1 out 100 males. Deuteranopsia is the absence of green receptors. This also occurs in 1 out 100 males. Tritanopia is very rare and there are no blue receptors. Blue appears green to these people.
Other causes of color blindness can be due to trauma, age related disorders such as macular degeneration, diabetic retinopathy and vitamin A deficiency. Glaucoma and cataracts can also affect color sensitivity. Certain antibiotics, barbiturates and high blood pressure medications can also affect color perception. Trauma along the visual pathway leading to the brain can also be a factor.
Color blindness is typically diagnosed by the Ishihara color test. There is typically a number or figure embedded in a background filled with a different color. It is hard for a color deficient person to see the number figure embedded in the background. The Farnsworth test is where a patient has to line up a large number of colored caps in order changing hue. This is difficult for a color blind person.
There is no real treatment for color blindness but some improvements has been seen with a patient wearing one red contact lens. It helps in discrimination between colors a bit more but isn’t always reliable.