Achromatopsia

Description

Achromatopsia is a disorder that causes problems with color vision. The retina, which is the light-sensitive tissue at the back of the eye, contains two types of light receptor cells called rods and cones. Rods provide vision in low light (night vision), while cones provide vision in bright light (daytime vision). Cones are also responsible for color vision.

There are two types of achromatopsia: complete and incomplete. People with complete achromatopsia cannot perceive any color; they see only black, white, and shades of gray. People with incomplete achromatopsia can see some color.

People with achromatopsia may have additional problems with their vision, including an increased sensitivity to light and glare (photophobia), involuntary back-and-forth eye movements (nystagmus), and significantly reduced sharpness of vision (low visual acuity). Affected individuals may also have a small visual blind spot (scotoma), farsightedness (hyperopia), or nearsightedness (myopia).

Achromatopsia is different from the more common forms of color vision deficiency (also called color blindness), in which people can perceive color but have difficulty distinguishing between certain colors, such as red and green. One form of color blindness, called blue cone monochromacy (BCM), is sometimes considered a form of incomplete achromatopsia.

Frequency

Achromatopsia affects approximately 1 in 30,000 people worldwide. Complete achromatopsia is more common than incomplete achromatopsia.

Complete achromatopsia occurs more frequently among Pingelapese islanders, who live in parts of Micronesia in the western Pacific Ocean. Between 4 and 10 percent of people in this population have a total absence of color vision.

Causes

Variants (also called mutations) in several different genes can cause achromatopsia. Most of the genes that are associated with achromatopsia are known to play important roles in the normal functioning of cones. Variants in the CNGA3 and CNGB3 genes are the most common causes of achromatopsia.

The CNGA3 and CNGB3 genes provide instructions for making different parts (subunits) of the cyclic nucleotide-gated (CNG) channel. These channels are found exclusively in cones and play an important role in a process called phototransduction. During phototransduction, rods and cones translate light into electrical signals. To help with this process, CNG channels transport positively charged atoms (ions) across the cell membrane and into cells. This movement of ions helps generate an electrical signal that is sent to the brain for interpretation.

Some of the variants in the CNGA3 and CNGB3 genes that are associated with achromatopsia cause cells to produce fewer CNG channel subunits, while other variants cause cells to produce subunits that do not function properly. These changes impair the function of the CNG channels, which disrupts the flow of ions across cell membranes and impairs the cell's ability to generate electrical signals.

Some people with achromatopsia do not have one of the gene variants known to cause the condition. In these individuals, the cause of the disorder is unknown.

Inheritance

This condition is inherited in an autosomal recessive pattern, which means both copies of the gene in each cell must have a variant to cause the disorder. The parents of an individual with an autosomal recessive condition each carry one copy of the altered gene, but they typically do not show signs and symptoms of the condition.

Autosomal recessive inheritance. Credit: U.S. National Library of Medicine.

Other Names for This Condition

• ACHM
• Achromatism
• Rod monochromatism

Where this page came from

This page was imported from MedlinePlus Genetics, National Library of Medicine. Courtesy of MedlinePlus from the National Library of Medicine; its genetics summaries and the illustrations credited to the library are in the public domain. Pictures credited to others are not copied.

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