Choroideremia (also known as choroidal sclerosis or progressive tapetochoroidal dystrophy) is a genetic condition that causes affected persons to slowly lose their vision. It affects males almost exclusively, and symptoms usually begin to appear during childhood. Most people who suffer from choroideremia lose their vision completely by the time they are middle-aged. Choroideremia is rare, affecting only one person out of every 50,000 to 100,000, but it is believed to be the cause of about 4 percent of all cases of blindness.
What Symptoms Will I Have with Choroideremia?
The first symptom of choroideremia is usually night blindness, which begins during childhood or adolescence; however, the age at which symptoms first appear and the speed at which they progress varies from one individual to the next. The child who is affected by choroideremia will begin to lose the ability to see well at night or in dim indoor lighting, becoming unable to distinguish objects that others can easily see.
Eventually the affected person will begin to experience a loss of peripheral vision and depth perception. As the loss of peripheral vision progresses, the choroideremia sufferer will find that they have tunnel vision (i.e., they are only able to see objects in the center of their field of vision). This tunnel vision worsens steadily but slowly—most sufferers retain their central vision until they are in their thirties or forties—but complete blindness is the end result in nearly all cases.
What Is Causing my Choroideremia?
Choroideremia is the result of a genetic mutation in the X chromosome. Since male children have only one X chromosome, only one of these chromosomes needs to carry the mutation for the disease to occur. Female children have two X chromosomes, however, and both of these chromosomes must carry the mutation for a girl to be born with this condition. To put it in simpler terms, the gene can only be passed to a girl if her mother is a carrier and her father has the disease as well.
The genetic mutation that causes choroideremia works by interfering with the production of the REP-1 protein, which enables cells to properly absorb nutrients. Other cells in the body are able to rely on different proteins in the absence of REP-1, but the retina and the choroid (a layer of tissue lying between the retina and the sclera—the white of the eye) are unable to make this adjustment. The retinal pigment epithelium (RPE), which lies beneath the retina and supports the work of the retina’s photoreceptors, is also affected. These three layers slowly degenerate over the years, causing the symptoms of choroideremia.
What Complications Arise from Choroideremia?
The complications associated with this condition are self-evident—sufferers must live with the knowledge that they will someday go blind, and that their vision will slowly worsen throughout their lives until they do. Most of them are never able to obtain driver licenses, and adolescent participation in sports is often complicated or impossible.
How Is Choroideremia Diagnosed?
Choroideremia is sometimes misdiagnosed; it can be confused with retinitis pigmentosa, which is also genetic and sometimeslinked to X-chromosome abnormalities, and which presents with similar symptoms. Fortunately, a genetic blood testcan be used to diagnose choroideremia. This test was conceived by a Canadian doctor named Ian MacDonald, and it is now widely available thanks to the eyeGENE project, a program administered by the National Eye Institute.
Can Choroideremia Be Treated?
Because there is no cure for this disorder, treatment has always been limited mostly to helping affected individuals learn to cope with their impending blindness, both practically and psychologically. Doctors generally recommend that certain nutritional needs be emphasized in the diet of affected individuals in order to slow the degeneration of the retina, choroid, and RPE. To this end they generally prescribe antioxidant supplements and a diet rich in fruits, vegetables, and omega-3 fatty acids (which can be obtained by eating plenty of fish). Glasses or contact lenses may be prescribed to help maximize visual acuity even as it fades.
There may be a ray of light at the end of this dark tunnel, however, in the form of gene therapy. Researchers in the UK have conducted studies of choroideremia sufferers in which healthy versions of the mutated gene were injected underneaththe parts of the retina that had not yet died. The principal aim of this study was to determine whether such injections were safe, but the researchers found that a few of the subjects demonstrated substantial improvements in their vision. This is not a cure by any stretch of the imagination, and further research will be necessary. Moreover, the procedure used in the study involved surgical detachment of the retina, making it unsuitable for patients whose retinas have already thinned too dramatically. Nevertheless, this type of gene therapy shows a great deal of promise, and may also help scientists to develop treatments for other progressive eye disorders, including macular degeneration.