What is Choroideremia?
Choroideremia is a rare inherited disorder that causes progressive loss of vision due to degeneration of the choroid and retina.
What are the symptoms?
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 vision or “tunnel 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 can vary, 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 choroids, the retinal pigment epithelium and the photoreceptors. The choroid consists of several blood vessel layers that are located between the retina and the sclera (the “white of the eye”).
Choroidal vessels provide the retinal pigment epithelium and photoreceptors with oxygen and nutrients necessary for normal function. The retinal pigment epithelium and the photoreceptors are part of the retina. The epithelium is associated closely with the photoreceptors and is needed for normal function. The photoreceptors are responsible for converting light into the electrical impulses that transfer messages to the brain where “seeing” actually occurs.
The retinal pigment epithelium and the choroid initially deteriorate to cause choroideremia. Eventually, the photoreceptors break down as well. As the disease progresses, the clinical appearance of these cell layers changes in a characteristic manner and more vision is lost.
Is it an inherited disease?
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, carrier females typically are not affected by X-linked diseases such as juvenile retinoschisis. Sometimes, however, when carrier females are examined, the retina shows minor signs of the disease.
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.
What treatment is available?
Recently, scientists discovered mutations on a gene on the X chromosome that causes choroideremia. New research based on these findings now drives the search for a treatment. Investigators are now developing a gene therapy for evaluation in animal studies. With success in an animal model, they will move the treatment into clinical studies.
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 of an affected family are encouraged to consult an ophthalmologist and to seek genetic counseling. These professionals can provide explanations of the disease and the recurrence 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.
Are there any other 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, differences are clear in a complete medical eye examination, especially as the disease progresses. The disease most similar clinically to choroideremia is gyrate atrophy. It too can be distinguished based on its inheritance, as an autosomal recessive disorder, and based on its cause, known to be a defect in an unrelated gene.