Choroideremia Research Foundation Inc.
Choroideremia (CHM) is a rare inherited disorder that causes progressive loss of vision due to degeneration of the choroid and retina. It occurs almost exclusively in males. In childhood, night blindness is the most common first symptom. As the disease progresses, there is loss of vision, frequently starting as an irregular ring that gradually expands both in toward central vision and out toward the extreme periphery.
Progression of the disease continues throughout the individual's life. Both the rate of change and the degree of visual loss are variable among those affected, even within the same family.
The actual vision loss 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 (RPE), 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 oxygen and nutrients to both the RPE and the retina's photoreceptor cells.
The RPE, directly beneath the retina, supports the function of photoreceptor cells. Photoreceptors convert 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 egenerate. As a result, vision is lost.
Choroideremia is genetically passed through families by an 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, only one of which generally carries the disease gene. Because they have a healthy version of the gene on their other X chromosome, many females suffer some symptoms seen in the early stages of CHM, such as night vision loss and glare/light sensitivity. In rare cases, female carriers even suffer the full effects of CHM.
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.
Scientists discovered the exact location of the choroideremia gene on the X chromosome in the early 1990's. New research based on these findings now drives the search for a treatment. However, at present there is no effective treatment or cure. More recently, a diagnostic blood test has been developed to enable early and positive diagnosis of choroideremia.
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.
Thanks to the Foundation Fighting Blindness for much of the information in this narrative.