Choroideremia is a rare and inherited disorder that causes progressive loss of vision due to degeneration of the choroid and retina.
Choroideremia used to be referred to as, 'tapetochoroidal dystrophy,' and occurs almost entirely in males. During childhood, night blindness is the most common symptom people experience first. As the disease progresses, people experience a loss of their peripheral vision or, 'tunnel vision,' and later - a loss of their central vision. Progression of the disease continues throughout an affected person's life, although both the rate and the degree of the person's visual loss may vary, even within the same family.
An X-linked recessive retinal degenerative disease that leads to the degeneration of the choriocapillaris, the retinal pigment epithelium, and the photoreceptor of the eye. This is due to the degeneration of the choroid and areas of the eye which detect light, such as the retina. The choroid provides those areas with nourishment, including oxygen. When they degenerate, the eye is no longer able to detect light and transmit it to the brain. Choroideremia leads to total blindness by the time patients are in their 40s. It is genetically passed through families and mainly affects men and boys.
A person's vision loss due to choroideremia is caused by degeneration of several layers of cells that are essential to their sight. The layers, which line the inside of the back of the person's 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 person's retina and their sclera or the, 'white of their eye.'
Choroidal vessels provide the retinal pigment epithelium and photoreceptors with oxygen and nutrients that are necessary for usual functioning. The retinal pigment epithelium and the photoreceptors are parts of the person's retina. The epithelium is associated closely with the photoreceptors and is needed for usual function. The photoreceptors are responsible for converting light into the electrical impulses that transfer messages to the person's brain where, 'seeing,' actually happens.
The person's retinal pigment epithelium and their choroid initially deteriorate to cause choroideremia. Over a period of time, their photoreceptors break down as well. As the disease progresses, the clinical appearance of these cell layers changes in a characteristic manner and the person loses more of their vision.
Choroideremia and 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 2 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 usually are not affected by X-linked diseases such as juvenile retinoschisis. At times; however, when carrier females are examined - their retinas show minor signs of the disease.
Males have only 1 X chromosome, paired with 1 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 along to their sons. Female carriers have a 50% chance of passing the X-linked disease gene to their daughters, who then become carriers of the disease, and a 50% chance of passing the gene to their sons - who are then affected by the disease.
Medical science has discovered mutations on a gene on the X chromosome that causes choroideremia. New research, based on these findings, is pursuing the search for a treatment. Choroideremia is one of the few retinal degenerative diseases that may be detected prenatally in some instances. Female carriers of the disease might want to seek information concerning this testing from a medical geneticist or a genetic counselor. Every member of a family that is affected is encouraged to consult an ophthalmologist and to pursue genetic counseling. Ophthalmologists and genetic counselors can provide explanations of the disease and the recurrence risk for all family members and for future offspring.
While the disease is early in its course, choroideremia might be confused with X-linked retinitis pigmentosa. Both of these diseases have symptoms of night blindness and tunnel vision. Differences are clear; however, in a complete medical eye examination, particularly as the disease progresses. The disease most similar clinically to choroideremia is, 'gyrate atrophy.' Gyrate atrophy may 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.