Red-Green Weakness: Description
Red-green deficiency (anomalous trichromasia) belongs to the color vision disorders of the eye. Affected persons recognize the colors red or green with different intensities and can distinguish them poorly or not at all. Colloquially, the term red-green blindness is often used. However, this is not correct, because in red-green deficiency, the vision for red and green is still present to a different degree. In true red-green blindness (a form of color blindness), on the other hand, affected individuals are actually blind to the corresponding color.
Two visual impairments are subsumed under the term red-green deficiency:
- Red visual impairment (protanomaly): Affected individuals see the color red more weakly and have difficulty distinguishing it from green.
- Green visual impairment (deuteranomaly): Affected individuals perceive the color green more poorly and have difficulty distinguishing it from red.
Both visual defects are genetic defects that affect the sensory cells for color vision.
Sensory cells and color vision
Color vision is an extremely complex process with essentially three important variables: Light, sensory cells, and brain.
Everything we see during the day reflects light of different wavelengths. This light hits three different light sensory cells in the retina (retina or inner lining of the eye):
- Green cone cells (G cones or M cones for “medium”, i.e. medium-wave light)
- Red cone cells (R cones or L cones for “long”, i.e. long-wave light)
They contain a pigment called rhodopsin, which is made up of the protein opsin and the smaller molecule 11-cis-retinal. However, opsin has a slightly different structure depending on the type of cone and is thus excited by different wavelengths of light – the basis for color vision: The opsin in the blue cones reacts particularly intensively to short-wave light (blue range), that of the green cones particularly to medium-wave light (green range), and that of the red cones mainly to long-wave light (red range).
Each cone cell thus covers a specific wavelength range, with the ranges overlapping. The blue cones are most sensitive at a wavelength around 430 nanometers, the green cones at 535 nanometers, and the red cones at 565 nanometers. This covers the entire color spectrum from red to orange, yellow, green, blue to violet back to red.
Millions of different color shades
Since the brain is able to distinguish about 200 color tones, about 26 saturation tones and about 500 brightness levels, people can perceive several million color tones – except when a cone cell does not work properly, as is the case with red-green deficiency.
Red-green deficiency: cone cells weaken
In red-green deficiency, the opsin of the green or red cones is not fully functional. The reason is a chemical change in its structure:
- Red-green deficiency: the opsin of the R cones is not most sensitive at 565 nanometers, but the maximum of its sensitivity has shifted towards green. Therefore, the red cones no longer cover the entire wavelength range for the color red and respond more strongly to green light. The more the sensitivity maximum is shifted towards that of the green cones, the fewer red hues can be detected and the more poorly red can be distinguished from green.
- Green vision deficiency: Here it is the other way round: The sensitivity maximum of the opsin of the G cones is shifted into the red wavelength range. Thus, fewer shades of green are perceived, and green can be distinguished more poorly from red.
Red-Green Impairment: Symptoms
Compared to people with normal vision, those with red-green deficiency perceive far fewer colors overall. Although they have normal vision for various shades of blue and yellow, they see red and green less clearly. Red-green deficiency always affects both eyes.
The extent to which those affected can still recognize colors depends on the severity of the red-green deficiency: If the wavelength range of the R cones, for example, is only slightly shifted to that of the G cones, those affected can see red and green relatively well, occasionally as well as a person with normal vision. However, the more the wavelength ranges of the G and R cones overlap, the less well the affected person recognizes the two colors: they are described in a wide variety of shades – from brownish-yellow to shades of gray.
Red-green deficiency: causes and risk factors
Red-green deficiency is genetic and thus always congenital:
Red-green deficiency affects more men than women
Both opsin genes are located on the X chromosome, which is why red-green deficiency occurs much more frequently in men than in women: men have only one X chromosome, whereas women have two. In the case of a genetic defect in one of the opsin genes, the male has no alternative, whereas the female can fall back on the intact gene of the second chromosome. However, if the second gene is also defective, the red-green vision defect also appears in the woman.
Figures prove that this is rarely the case: About 1.1 percent of men and 0.03 percent of women exhibit red-vision deficiency. Green-vision impairment affects about five percent of men and 0.5 percent of women.
Red-green deficiency: examinations and diagnosis
To diagnose red-green weakness, the ophthalmologist will first talk with you in detail (medical history). For example, he may ask the following questions:
- Do you know anyone in your family with red-green deficiency?
- Do you only see blues and yellows and shades of brown or gray?
- Have you ever seen red or green?
- Do you only see no red and green with one eye or are both eyes affected?
Color vision tests
The panels are placed in front of your eyes at a distance of about 75 centimeters. Now the doctor asks you to look at the depicted figures or numbers with both eyes or only with one eye. If you do not recognize a figure or number within the first three seconds, the result is “incorrect” or “uncertain”. The number of incorrect or uncertain answers indicates a red-green disorder.
The Color-Vision-Testing-Made-Easy-Test (CVTME-Test) is suitable for children from the age of three. It does not show numbers or complicated figures, but simple symbols such as circles, stars, squares or dogs.
There are also color tests such as the Farnsworth D15 test. Here, hats or chips of different colors have to be sorted.
Another way to diagnose red-vision deficiency or green-vision deficiency is with a special device called an anomaloscope. Here, the patient must look through a tube at a circle cut in half. The halves of the circle are different colors. With the help of rotating wheels, the patient must now try to match the colors and their intensity:
Red-Green Weakness: Treatment
There is currently no treatment for red-green deficiency. For people with only mild red-green weakness, glasses or contact lenses with color filters can be of help. On electronic devices (such as computers), someone with color vision deficiency can select colors in the control panel that they cannot easily mix up.
Red-green deficiency: course and prognosis
Red-green deficiency does not change throughout life – affected individuals have difficulty or no ability to distinguish red from green throughout their lives.