Red Green weakness

Dr. rer. nat. Daniela Oesterle is a molecular biologist, human geneticist and trained medical editor. As a freelance journalist, she writes texts on health topics for experts and laypeople and edits specialist scientific articles by doctors in German and English. She is responsible for the publication of certified advanced training courses for medical professionals for a renowned publishing house.

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Red-green weakness is a genetic visual impairment of the eye. Affected people see red or green more weakly and thus have difficulty distinguishing the two colors from one another. They perceive the world to be less colorful than people with normal sight. Read more about red-green visual impairment and why it should not be confused with red-green blindness.

ICD codes for this disease: ICD codes are internationally recognized codes for medical diagnoses. They can be found, for example, in doctor's letters or on certificates of incapacity for work. H53

Red-green weakness: description

Red-green weakness (abnormal trichromatism) is one of the color vision disorders of the eye. Affected people recognize the colors red or green with different intensities and can hardly or not at all differentiate between them. Colloquially, the term red-green blindness is often used for this. However, this is not correct, since with red-green weakness the eyesight for red and green is still present to different degrees. In the case of true red-green blindness (a form of color blindness), on the other hand, those affected are actually blind to the corresponding color.

Two visual impairments are summarized under the term red-green weakness:

• Red weak eyesight (protanomaly): Those affected see the color red less often and can hardly distinguish it from green.
• Green weak eyesight (deuteranomaly): Those affected perceive the color green less well and can hardly distinguish it from red.

Both visual impairments 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 parameters: light, sensory cells and the 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 eye skin):

• Blue cone cells (B cones or S cones for "short", i.e. short-wave light)
• 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, the opsin has a slightly different structure depending on the type of cone and is therefore stimulated by different wavelengths of light - the basis for color vision: the opsin in the blue cones reacts particularly intensively to short-wave light (blue area), that of the Green cones especially on medium-wave light (green area) and that of the red cones mainly on long-wave light (red area).

Each cone cell thus covers a certain wavelength range, whereby the ranges overlap. The blue cones are most sensitive at a wavelength of around 430 nanometers, the green cones at 535 nanometers and the red cones at 565 nanometers. This covers the entire color spectrum from red through orange, yellow, green, blue to purple back to red.

Millions of different shades of color

If light of the appropriate wavelength hits the opsin of the B, G and R cones, the 11-cis retinal changes its chemical structure and activates a series of steps within the cell and ultimately neighboring nerve cells. These in turn pass the light impulses on to the brain, where they are sorted, compared and interpreted.

Since the brain can distinguish around 200 color tones, around 26 saturation tones and around 500 levels of brightness, people can perceive several million color tones - except when a cone cell is not working properly, as is the case with the red-green weakness.

Red-green weakness: cone cells weaken

In the case of red-green weakness, the opsin of the green or red cones is not fully functional. The reason is a chemical change in its structure:

• Red visual impairment: The opsin of the R cones is not most sensitive at 565 nanometers, but the maximum of its sensitivity has shifted towards green. The red cones therefore no longer cover the entire wavelength range for the color red and react more strongly to green light. The more the sensitivity maximum is shifted in the direction of that of the green cones, the fewer red hues can be recognized and the more difficult it is to distinguish red from green.
• Green visual impairment: Here it is the other way round: the maximum sensitivity of the opsin of the G-cones is shifted into the red wavelength range. This means that fewer green tones are perceived and green is more difficult to distinguish from red.

The red-green weakness should therefore not be confused with the real red-green blindness, in which the function of the red or green cones is completely lost. Red-green blind are completely blind to red or green.

Red-green weakness: symptoms

Compared with people with normal vision, people with impaired vision perceive significantly fewer colors overall: Although they are normally sighted for various shades of blue and yellow, they see red and green more weakly. The red-green weakness always affects both eyes.

The way in which those affected can still recognize the colors depends on the severity of the red-green weakness: If the wavelength range, for example, of the R cones is only slightly shifted into that of the G cones, those affected can see red and green relatively well, occasionally as good as a normal sighted person. However, the more the wavelength ranges of the G and R cones overlap, the less well those affected recognize the two colors: They are described in a wide variety of nuances - from brownish-yellow to shades of gray.

Red-green weakness: causes and risk factors

The red-green weakness is genetic and therefore always innate:

The genetic error lies in the gene for opsin of the green cone (for green eyesight) or on the opsin gene for the red cone (for red eyesight). The defect occurs during the first cell division of the fertilized egg, when the paternal and maternal genes mix. During this process (called "crossover") the genes can be damaged in different ways. In all cases, however, they lose gene sequences. The extent of the red-green weakness depends on which gene areas are lost, because some areas are more important for the function or the maximum sensitivity than others.

Red-green weakness affects more men than women

Both opsin genes are located on the X chromosome, which is why the red-green weakness occurs much more frequently in men than in women: men only have one X chromosome, while women have two. If there is a genetic defect in one of the opsin genes, men have no alternative, whereas women can fall back on the intact gene on the second chromosome. If, however, the second gene is also defective, the red-green visual impairment also shows in the woman.

Figures show that this is seldom the case: around 1.1 percent of men and 0.03 percent of women have red vision deficiency. About five percent of men and 0.5 percent of women are affected by green visual impairment.

Red-green weakness: examinations and diagnosis

In order to determine whether there is a red-green weakness, the ophthalmologist will first talk to you in detail (anamnesis). For example, he can ask the following questions:

• Do you know anyone in your family with red-green weakness?
• Do you only see blue and yellow and brown or gray tones?
• 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

In order to detect red-green weakness verifiably, the ophthalmologist asks you to look at so-called pseudoisochromatic tables such as the Ishihara tables. These consist of many small circles that represent numbers or figures. The background colors and the colors of the figures only differ in hue, but not in terms of brightness and saturation. Therefore, only a healthy, normal sighted person can see the figures, a person with red-green weakness cannot. This is how this color vision test works:

The boards are placed in front of your eyes approximately 75 centimeters away. Now the doctor asks you to look at the figures or figures shown with both eyes or only with one eye. If you don't recognize a figure or number within the first three seconds, the result is "wrong" or "unsure". 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 deposit tests such as the Farnsworth D15 test. Here, cones or chips of different colors have to be sorted.

A special device, the so-called anomaloscope, offers another way of diagnosing red or green weak eyesight. The patient has to look through a tube at a halved circle. The halves of the circle are different colored. With the help of rotary wheels, the patient should now try to match the colors and their intensity to one another:

A visually healthy person can adjust both the hue and the intensity, while a visually impaired person can only adjust the intensity. In addition, a person with impaired vision will mix in too much red, a person with impaired vision will mix in too much green.

Red-green weakness: treatment

There is currently no therapy for red-green weakness. For people with only a slight red-green weakness, glasses or contact lenses with a color filter can be of help. On electronic devices (such as computers), someone with color impairment can use the control panel to choose colors that they cannot easily mix up.

Red-green weakness: course and prognosis

The red-green weakness does not change in the course of life - those affected find it difficult or impossible to differentiate between red and green throughout their lives.

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