Choroideremia is a rare inherited disorder that causes progressive loss of vision due to degeneration of the choroid and retina.
Choroideremia 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 vision 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 eyes, are called the choroid, 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.
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The retinal 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.
Choroideremia is passed to succeeding family generations through the X-linked inheritance pattern.
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Occasionally a woman who carries the X-linked choroideremia gene experiences some difficulty with night vision later in life. Only rarely do carriers lose peripheral vision.
Recently scientists discovered the exact identity 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 from a medical geneticist or a genetic counsellor. All members in affected families are encouraged to consult an ophthalmologist and to seek genetic counselling. These professionals can provides explanations of the disease and the recurrence risk for all family members and for future offspring.
Individuals with choroideremia may benefit from the use of low-vision aids, including electronic, computer-based and optical aids, as well as orientation and mobility training.
Early in the course of the disease, choroideremia could be confused with the X-linked retinitis pigmentosa. Both have symptoms of night blindness and tunnel vision. However, differences are clear in a complete eye examination, especially as the disease progresses. The disease most similar clinically to choroideremia is gyrate atrophy. It too can be distinguished base on its inheritance as an autosomal recessive disorder and based on its cause, known to be defect in an unrelated gene.
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Information about these conditions should be obtained by consulting an ophthalmologist, paediatrician and geneticist.