Achromatopsia

Achromatopsia is the inability to see color. In the most severe form of the disease, called complete achromatopsia, the patient cannot see any color at all. Incomplete achromatopsia is a milder form of the disease where the patient has impaired ability to see color rather than a complete loss of color vision.

Other vision problems may also occur, and these can vary in severity among patients, depending on whether the patient has complete achromatopsia or incomplete achromatopsia. These symptoms may include blurred vision, nystagmus (involuntary eye movements, such as shaking or wobbling), or photophobia, which is a heightened sensitivity to light that may cause discomfort or pain.

Achromatopsia is considered a non-progressive disorder, meaning symptoms do not become more severe over time.

Congenital achromatopsia is usually present from birth. Excessive quivering or squinting of the eyes may be signs that a child has achromatopsia.

Electroretinogram (ERG): A test called an electroretinogram may be used to measure the function of the cells in the eye. In this test, electrodes are placed on the cornea and near the skin of the eye, and the electrical activity in the eye cells is measured as a patient is shown different visual stimuli. This test can determine if the cone cells are functioning properly. This test may be used to confirm a diagnosis, especially in cases where a patient is too young to take other vision tests.

Vision tests: Achromatopsia may be diagnosed in older patients through the use of color vision tests.

Genetic testing: Genetic testing can be used to detect mutations in genes known to cause achromatopsia. These tests may be used to confirm a diagnosis if there is a family history of achromatopsia, or if symptoms of achromatopsia are present.

The vision problems experienced by patients with achromatopsia can be very disruptive to everyday activities, such as reading or driving. Also, simply being outside during the day can be difficult for patients due to the bright light. Patients with achromatopsia may make certain lifestyle decisions to increase their comfort. For example, some patients may avoid working, driving, or playing in daylight.

Congenital achromatopsia: Achromatopsia may result from a mutation, or error, within specific genes. This type of achromatopsia is inherited and is sometimes called congenital achromatopsia. Several genes have been linked to achromatopsia, and it has been shown that mutations in these genes may lead to the development of the disease. These genes include ACHM2 (CNGA3) and ACHM3 (CNGB3), which are ion channels found in cone cells (ion channels allow small electrically charged molecules, called ions, to enter or exit a cell), and ACHM4 (GNAT2), which is expressed in photoreceptors. All three of these genes are involved in transmitting the signal of light to the brain. Congenital achromatopsia is a recessive condition, meaning that both copies of a gene need to be defective for the disease to develop.

In congenital achromatopsia, the cone cells in the retina, which normally sense color, no longer function properly. In general, this defect results in an inability of the cone cells to properly respond to or detect light.

Acquired achromatopsia: Acquired achromatopsia is not inherited, but is due to damage in the brain, frequently in a region called the ventral occipital lobe. Individuals with acquired achromatopsia usually have properly functioning cone cells, but the color cannot be perceived by the brain.

The congenital form of achromatopsia is a recessive inherited genetic condition. Individuals have two copies of most genes (one inherited from the father and one from the mother). In a recessive genetic disorder, both copies of a certain gene need to be defective for the condition to manifest itself. Several genes have been linked to achromatopsia, and it has been shown that mutations in these genes may lead to development of the disease. People who only have one mutated gene are called "carriers." If only one parent is a carrier, none of the children will have achromatopsia, but 50% of the children will also be carriers; if both parents are carriers, then there is a 50% chance that a child will be a carrier and a 25% chance that a child will have achromatopsia.