Color Blindness
From WebHealth
Contents |
[edit] Introduction
Color vision is made possible by the retina having three different types of cones, which are color sensitive for long (red L cones), medium (green M cones) or short (blue S cones) wavelengths. These are photo cells imbedded into the retina and connected through nerve fibers with the brain that automatically analyzes what we see. We also have light sensitive rods that provide night vision and that are 100-fold more light sensitive than the cone cells. For color vision we need at least two different types of cones that will report different intensity of electrical signals to the brain. Perfect color vision is achieved by the presence of all three cones.
[edit] Genetics
The genes of the red and green cones (L and M cones) are located on the X-chromosomes. Women are relatively protected from color blindness by having two X chromosomes. In case one of them is defective for the red/green pigment cell type (let’s say the M cones), the other normal X chromosome will provide the missing genetic information. She will have normal color vision while a male with a genetic defect in the only X chromosome he has will not be so lucky and have red/green color blindness. Most cases of color blindness are not “blind” at all as there are different shades of inheritance patterns and color is replaced by various shades of grey tones. The male affected by this can mostly lead a normal life. About 4 to 7% of men are born with some kind of color vision problem. About half of them are due to the red or green color blindness (protanopia or deuteranopia).
[edit] Terminology
The terminology can be confusing. Here is a brief primer.
| Medical term: | Explanation: |
| Protanopia | Total red color blindness |
| Deuteranopia | Total green color blindness |
| Protanomaly | Anomalous color perception |
| Monochromacy | True color blindness, only sees black, white, grey |
| Dichromacy | Less severe defect of color recognition |
| Hereditary tritanopia | Has blue/yellow defect, but can see red and green |
| Anomalous trichromacy | Color vision better than dichromats, ranges from mild to severe |
| Cerebral dyschromatopsia | Color blindness with occipital stroke |
| Achromatopsia | Color blind, poor acuity, nystagmus and photophobia |
[edit] Diagnostic tests
For initial screening Ishihara Test Plates are used, which consist of color charts that have dots on them with different color shades. http://waynesword.palomar.edu/colorbl1.htm#color Only people with normal cones in the retina will be able to distinguish all of the numbers displayed. In babies born into families with known color blindness an electroretinogram (=ERG) can be done, which is an electrophysiological test offered in the ophthalmology department of a children’s hospital or another specialized lab. http://www.e-advisor.us/electrophysiology/erg.html
This test measures the electrical impulses produced by the retina in response to flashes of light. The visual evoked potential test (=VEP) is related to this, but measures the electrical signal originating from the occipital lobe of the brain where visual perceptions are processed.
[edit] Treatment
There is no treatment for this condition as the genetic defect cannot be changed. However, most patients function reasonably well as most of them do not have a complete color blindness, but have a certain degree of residual red-green hue discrimination.
| Home page | Eye diseases |
