Did you know that color blindness is pretty common? While it affects only a small percentage of women, it affects about one in twelve men. A genetic mutation causes most cases of color blindness. Acquired color vision loss can be caused by eye diseases like macular degeneration, cataracts, glaucoma, and diseases that affect the optic nerve. Genetic color blindness can’t be cured, but acquired color vision loss could be reversed or improved if the underlying disease is caught and treated early enough.
How Does Color Vision Work?
The retina lines the inside of the back of the eye. The cells that react to light are cones and rods. There are three types of cones: red, green, and blue. They are named for the color of light they perceive. The cones are concentrated in the macula, which is the part of the retina that gives us our best sight.
There is only one type of rod, and it signals when it receives any type of light. Rods perceive any wavelength of light as white. Cones require a lot of light to function and so they function best in daylight. Rods can signal even low levels of illumination, so they don’t need a lot of light to work correctly.
The color signals travel from the eye via the optic nerve and other nerve pathways to the vision center located in the back of the brain. Once there, the brain processes the information so that we perceive the different colors.
The Genetic Color Vision Disorders
The most common genetic color “blindness” is X-linked recessive and affects either the red or green cones. The mutated red or green cone genes are carried only on the X gene and not the Y gene. Women have two X chromosomes, and since the disorder is recessive, if women have one normal X gene, they will not have color blindness. Women have to have the mutation in both X genes to develop color blindness. Since men have only one X gene (the other being a Y gene), if the X gene has the color gene mutation, they will have color blindness. About 1 in 12 men has red-green color blindness while only about 0.5 percent of women suffer from this problem. The color vision defect may be mild, and some people can perceive bright reds and greens but have trouble only with grayed-out versions of these hues, like burgundy or dark evergreen. People with severe red-green color blindness may see red, green and mixes of the two colors as a muddy gray-brown.
Tritanomaly is a condition that affects the blue cones, and it is far more rare. It affects only about 1 in 10,000 people and affects men and women in equal percentages. The gene that codes the blue cones is carried on chromosome 7 rather than on the X gene. It is autosomal dominant, which means that if someone has one copy of the abnormal gene, she or he will have the disease. Patients with this condition have trouble perceiving blues and yellows.
Rod achromatopsia is the least common of the genetic color vision defects, affecting only about 1 in 50,000 people. It is autosomal recessive, meaning that a person needs two copies of the mutated gene to suffer from the disorder. There are a number of different genes that cause this condition, and the mutations can appear on more than one chromosome. People affected by this disease have few or no cones. Since only the rods function, these patients usually suffer from severe light sensitivity.
Acquired Color Vision Disorders
Diseases that affect the lens, optic nerve, retina, or the brain pathways leading from the eye to the vision center in the back of the brain can cause changes in color vision. Some of these conditions include the following:
- Macular degeneration
- Retinal detachment
- Diabetic retinopathy
- Multiple sclerosis
- Brain tumors, benign or malignant
- Optic neuritis
- Infections in the eye or brain
- Genetic retinal diseases that affect the macula or optic nerve
- Traumatic brain injuries
Cataracts can cause changes in color vision perception. As we get older, the lens starts to turn amber and then brown. This reduces the amount of blue light reaching the macula. Patients with severe cataracts often notice that things look dull, and these people may have trouble distinguishing purples, blues, and indigo.
Any damage to the macula can affect color vision. That is because the color vision receptors are concentrated in this part of the retina. Macular degeneration, retinal infections, diabetic eye disease, or any other condition that causes swelling or bleeding in the macula can cause color vision loss. Retinal detachments that happen at or near the macula can also cause varying degrees of color blindness. This loss can be mild or severe depending on how much of the macula is affected.
Since the optic nerve carries all light signals from the eye to the vision centers in the brain, anything that affects the optic nerve can cause changes in color vision. Glaucoma, which causes the nerve cells in the optic nerve to die off, can cause color blindness in the late stages. Optic neuritis and tumors that press on the optic nerve can cause decreased color vision. Multiple sclerosis (MS) sometimes affects color vision because of optic nerve swelling.
Diseases or injuries that affect the brain, particularly the parts of the brain that transmit vision information, can all cause loss of color vision. Strokes, traumatic brain injuries, brain tumors, meningitis, MS, and encephalitis can all alter how patients perceive color.
Genetic retinal diseases, especially those that affect the macula or optic nerve, can cause color blindness in varying degrees. Best’s disease, Stargardt’s disease, and Leber’s optic neuropathy are just a few of the many genetic eye diseases that affect color perception. Fortunately, many of these conditions are extremely rare.
How Do Doctors Diagnose Color Blindness?
Many people, especially children, don’t realize they have a color vision problem. Usually, this is because these patients have never seen color in any other way and don’t know what they’re missing. There are a number of color vision tests that optometrists use to diagnose color blindness. The most common one is the Ishihara color plate test, where patients have to identify a colored number in the center of a circle. This test is usually limited to testing for red and green color vision loss.
To test for tritanomaly or achromatopsia, doctors need to use tests that check all three cones. The Farnsworth D-15 and D-100 tests, as well as the RGB anomaloscope, can be utilized for this. Examples of these tests are located on various websites including the Colblindor site.
Everyone Should Be Tested for Color Blindness
Some doctors check only males or children for color blindness when in fact every person should be evaluated regularly for changes in color vision. One of my patients was a man in his 20s who was a journeyman electrician and had never been told he had red-green color blindness! No one had tested his color vision at any of his previous eye exams. Since his color vision defect was very mild, he had no difficulty performing his job unless he was in very low light settings. However, had he known about his mild color blindness, he might have chosen another occupation that didn’t involve differentiating between green and red wires.
Even if color vision problems are rare in women, they do occur, and many women have never been informed that they have color blindness. I have had six female patients in my career who have had genetic red-green color blindness. Only one had undergone a color vision test before I performed it with her. While it is rare, it is important for women to know if this is a gene they could pass on to their children. People with red-green color vision problems need to be aware that they could have children who either have color blindness or are carriers for the condition.
Anyone who notices altered color vision needs to see an eye doctor right away. Diseases like optic neuritis, retinal detachments, and retinal bleeding from macular degeneration or diabetic retinopathy need to be diagnosed as quickly as possible to prevent permanent damage. Diagnosing these diseases within a day or two of onset may mean the difference between temporary color vision loss and severe, permanent vision loss in extreme cases. Every patient also should be screened for genetic color blindness during routine eye exams so that he or she can be offered genetic counseling.
— Dr. Beth Carlock, O.D.