Color Vision Deficiency

Causes

Mutations in the OPN1LW, OPN1MW, and OPN1SW genes cause the forms of color vision deficiency described above. The proteins produced from these genes play essential roles in color vision. They are found in the retina, which is the light-sensitive tissue at the back of the eye. The retina contains two types of light receptor cells, called rods and cones, that transmit visual signals from the eye to the brain. Rods provide vision in low light.

Cones provide vision in bright light, including color vision. There are three types of cones, each containing a specific pigment (a photopigment called an opsin) that is most sensitive to particular wavelengths of light. The brain combines input from all three types of cones to produce normal color vision.

The OPN1LW, OPN1MW, and OPN1SW genes provide instructions for making the three opsin pigments in cones. The opsin made from the OPN1LW gene is more sensitive to light in the yellow/orange part of the visible spectrum (long-wavelength light), and cones with this pigment are called long-wavelength-sensitive or L cones.

The opsin made from the OPN1MW gene is more sensitive to light in the middle of the visible spectrum (yellow/green light), and cones with this pigment are called middle-wavelength-sensitive or M cones. The opsin made from the OPN1SW gene is more sensitive to light in the blue/violet part of the visible spectrum (short-wavelength light), and cones with this pigment are called short-wavelength-sensitive or S cones.

Genetic changes involving the OPN1LW or OPN1MW gene cause red-green color vision defects. These changes lead to an absence of L or M cones or to the production of abnormal opsin pigments in these cones that affect red-green color vision. Blue-yellow color vision defects result from mutations in the OPN1SW gene.

These mutations lead to the premature destruction of S cones or the production of defective S cones. Impaired S cone function alters perception of the color blue, making it difficult or impossible to detect differences between shades of blue and green and causing problems with distinguishing dark blue from black.

Blue cone monochromacy occurs when genetic changes affecting the OPN1LW and OPN1MW genes prevent both L and M cones from functioning normally. In people with this condition, only S cones are functional, which leads to reduced visual acuity and poor color vision. The loss of L and M cone function also underlies the other vision problems in people with blue cone monochromacy.

Some problems with color vision are not caused by gene mutations. These nonhereditary conditions are described as acquired color vision deficiencies. They can be caused by other eye disorders, such as diseases involving the retina, the nerve that carries visual information from the eye to the brain (the optic nerve), or areas of the brain involved in processing visual information.

Acquired color vision deficiencies can also be side effects of certain drugs, such as chloroquine (which is used to treat malaria), or result from exposure to particular chemicals, such as organic solvents.

Inheritance

Red-green color vision defects and blue cone monochromacy are inherited in an X-linked recessive pattern. The OPN1LW and OPN1MW genes are located on the X chromosome, which is one of the two sex chromosomes. In males (who have only one X chromosome), one genetic change in each cell is sufficient to cause the condition.

Males are affected by X-linked recessive disorders much more frequently than females because in females (who have two X chromosomes), a genetic change would have to occur on both copies of the chromosome to cause the disorder. A characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons.

Blue-yellow color vision defects are inherited in an autosomal dominant pattern, which means one copy of the altered OPN1SW gene in each cell is sufficient to cause the condition. In many cases, an affected person inherits the condition from an affected parent.

Other Names for This Condition

• Color blindness

• Color vision defects

• Defective color vision

• Vision defect, color

Causes & risk factors

Usually, color deficiency is an inherited condition caused by a common X-linked recessive gene, which is passed from a mother to her son. But disease or injury that damages the optic nerve or retina can also cause loss of color recognition. Some diseases that can cause color deficits are:

• Diabetes.

• Glaucoma.

• Macular Degeneration.

• Alzheimer's disease.

• Parkinson's disease.

• Multiple Sclerosis.

• Chronic alcoholism.

• Leukemia.

• Sickle Cell Anemia.

Other causes for color vision deficiency include:

Medications : Drugs used to treat heart problems, high blood pressure, infections, nervous disorders and psychological problems can affect color vision.

Aging : The ability to see colors can gradually lessen with age.

Chemical exposure : Contact with certain chemicals—such as fertilizers and styrene—have been known to cause loss of color vision.

In many cases, genetics cause color deficiency. About 8% of white males are born with some degree of color deficiency. Women are typically just carriers of the color-deficient gene, though approximately 0.5% of women have color vision deficiency. The severity of inherited color vision deficiency generally remains constant throughout life and does not lead to additional vision loss or blindness.

Symptoms

A person could have poor color vision and not know it. Quite often, people with red-green deficiency aren't aware of their problem because they've learned to see the "right" color. For example, tree leaves are green, so they call the color they see green.  Also, parents may not suspect their children have the condition until a situation causes confusion or misunderstanding.

Early detection of color deficiency is vital since many learning materials rely heavily on color perception or color-coding. That is one reason the AOA recommends that all children have a comprehensive optometric examination before they begin school.

Diagnosis

Color deficiency can be diagnosed through a comprehensive eye examination. The patient is shown a series of specially designed pictures composed of colored dots, called pseudoisochromatic plates.

The patient is then asked to look for numbers among the various colored dots. Individuals with normal color vision see a number, while those with a deficiency do not see it. On some plates, a person with normal color vision sees one number, while a person with a deficiency sees a different number.

Pseudoisochromatic plate testing can determine if a color vision deficiency exists and the type of deficiency. However, additional testing may be needed to determine the exact nature and degree of color deficiency.

Treatment

There is no cure for inherited color deficiency. But if the cause is an illness or eye injury, treating these conditions may improve color vision. Using specially tinted eyeglasses or wearing a red-tinted contact lens on one eye can increase some people's ability to differentiate between colors, though nothing can make them truly see the deficient color.

Organizing and labeling clothing, furniture or other colored objects (with the help of friends or family) for ease of recognition. Remembering the order of things rather than their color. For example, a traffic light has red on top, yellow in the middle and green on the bottom.

Color vision deficiency can be frustrating and may limit participation in some occupations, but in most cases, it is not a serious threat to vision. With time, patience and practice, people can adapt. Although in the very early stages, several gene therapies that have restored color vision in animal models are being developed for humans.