Choroideremia is a rare inherited disorder that causes progressive loss of vision due to degeneration of the choroid and retina.

Clinical Description

Choroideremia, formerly called tapetochoroidal dystrophy, occurs almost exclusively in males. In childhood, night blindness is the most common first symptom. As the disease progresses, there is loss of peripheral or side vision and later a loss of central vision. Progression of the disease continues throughout the individual's life, although both the rate and the degree of visual loss are variable among those affected, even within the same family.

Vision loss due to choroideremia is caused by degeneration of several layers of cells that are essential to sight. These layers, which line the inside of the back of the eye, are called the choroid, the retinal pigment epithelium and the retina. The choroid is a network of blood vessels located between the retina and the sclera, the "white of the eye." Choroidal vessels provide the retinal pigment epithelium and photoreceptor cells in the retina with oxygen and nutrients. The retinal pigment epithelium, directly beneath the retina, supports the function of photoreceptor cells. Photoreceptors are responsible for converting light into the electrical impulses that transfer messages to the brain where "seeing" actually occurs. In the early stages of choroideremia, the choroid and the retinal pigment epithelium initially deteriorate. Eventually, photoreceptor cells also degenerate. As a result, vision is lost.

Inheritance

Choroideremia is genetically passed through families by the X-linked pattern of inheritance. In this type of inheritance, the gene for the disease is located on the X chromosome. Females have two X chromosomes and can carry the disease gene on one of their X chromosomes. Because they have a healthy version of the gene on their other X chromosome, females typically are not affected by X-linked diseases such as choroideremia.

Males have only one X chromosome (paired with one Y chromosome) and are therefore genetically susceptible to X-linked diseases. Males cannot be carriers of X-linked diseases. Males affected with an X-linked disease always pass the gene on the X chromosome to their daughters, who then become carriers. Affected males never pass an X-linked disease gene to their sons because fathers pass the Y chromosome to their sons. Female carriers have a 50 percent chance (or 1 chance in 2) of passing the X-linked disease gene to their daughters, who become carriers, and a 50 percent chance of passing the gene to their sons, who are then affected by the disease

Treatment

Recently, scientists discovered the exact location of the gene on the X chromosome that causes choroideremia. New research based on these findings now drives the search for a treatment. However, at present there is no effective treatment or cure.

Choroideremia is one of the few retinal degenerative diseases that might be detected prenatally in some cases; female carriers may want to seek information about this testing from a medical geneticist or a genetic counselor. All members in affected families are encouraged to consult an ophthalmologist and to seek genetic counseling. These professionals can provide explanations of the disease and the inheritance risk for all family members and for future offspring.

Until a treatment is discovered, help is available through low-vision aids, including optical, electronic and computer-based devices. Personal, educational and vocational counseling, as well as adaptive training skills, job placement and income assistance, are available through community resources.

Related Diseases

Early in the course of the disease, choroideremia could be confused with X-linked retinitis pigmentosa. Both have symptoms of night blindness and tunnel vision. However, these diseases can be distinguished through complete eye examinations. Choroideremia can also resemble gyrate atrophy. However, it too can be distinguished.

The Foundation Fighting Blindness is a research foundation dedicated to finding the causes, treatments and cures for retinitis pigmentosa (RP), Usher syndrome, and macular degeneration.