Retinitis pigmentosa refers to a group of diseases that affect the retina. The retina is the delicate innermost layer of tissue that lines the eye. It contains layers of light receiving cells called photoreceptors that are connected to the brain by the optic nerve. If you think of the eye as a camera receiving images, then the retina is the film on which those images are recorded. Beneath the retina is the retinal pigmented epithelium (RIPE). The RIPE supports the function of photoreceptor cells in the retina.
There are two types of photoreceptor cells in the retina: cone cells and rod cells. Cone cells are concentrated in the center of the retina (called the macula), and are responsible for central and color vision. Rod cells are outside the macula and are required for peripheral vision and for night vision. Both cone and rod cells convert light into electrical impulses which travel through several types of nerve cells to the optic nerve, which then carries the signal to the brain, where “seeing” actually occurs.
With RP, photoreceptor cells begin to degenerate and eventually stop functioning. A diagram at the back of this booklet details the anatomy of the eye, and may help better explain the location of some of the parts of the eye that are described above.
What causes retinitis pigmentosa?
The basic cause of retinitis pigmentosa is thought to be genetic. The disease is programmed into your cells and not caused by injury, infection or any other external agent. Genetic defects, or mutations, give faulty messages to the retinal cells, leading to their progressive degeneration. This in turn, leads to vision loss.
How is RP inherited?
Genetics is a complex subject. There are so many different forms of RP, which fall within one of three standard inheritance patterns.
Each type of inheritance will create a different pattern of affected and unaffected family members. For example, unaffected parents could have affected children, and affected parents could have both affected and unaffected children. In some families, only males will be affected, while females will carry the trait but not typically exhibit it. There are also isolated cases in which one individual appears to be the only member of a family to have a retinal degeneration.
A gene for RP can be either autosomal dominant, autosomal recessive, or X-linked. Autosomal means that the gene is attached to one of the 22 pairs of chromosomes that are the same in males and females; X-linked (formerly referred to as sex linked) means that the gene is carried only on the X chromosome, which is one of the two chromosomes that help determine the sex of a child. If the gene is dominant, only one parent need have it for a child to be affected; if the gene is recessive, the disease can only show up in a child if both parents are carriers of the mutant gene; if it is X-linked, only males will be affected while females will be carriers of the gene but not typically experience serious vision loss themselves.
It is important to remember that because RP is an inherited disorder, it commonly affects other members of a family. If someone in your family is diagnosed with a retinal degeneration, it is strongly advised that all members of the family contact an eye care professional.