What is retinitis pigmentosa?
Retinitis pigmentosa (RP) is the name given to a group of genetic eye diseases that affect the retina, and that cause gradual, permanent loss of some vision. As the disease progresses, the retina develops a distinctive appearance that is caused by pigmentary clumping.
The retina is a thin sheet of interconnected nerve cells lining the inside of the eye. These specialized cells convert light into electrical signals that are sent to the brain to be interpreted as images. The structure and function of the retina is very complex, and the good health of all its cells is necessary for normal vision. In RP the cells responsible for dark and peripheral vision (rod cells) and the cells responsible for seeing colour, straight ahead and fine details (cone cells) degenerate and die, causing loss of vision.
It is generally agreed that defective genes initiate the process that causes the deterioration of the retinal cells. It is now believed that as many as 100 genes can each cause forms of RP. This helps to explain why rates of progression and the severity of the disease vary so much.
What are the symptoms of RP?
In some common forms of RP the rod cells are affected first. If this is the case, an individual may notice a gradual worsening of symptoms:
- difficulty seeing in dim light
- night blindness (nyctalopia)
- loss of peripheral (side) vision
- tunnel vision.
In other forms of RP the cone cells are affected first. An affected individual may experience:
- blurred central vision
- loss of colour perception
- eventual loss of peripheral vision.
Although the majority of people with RP do not suffer from associated disabilities, it does happen. Usher syndrome, which combines hearing loss or deafness with RP, is one of these conditions.
Who does RP affect?
RP is the most common inherited cause of blindness in people between the ages of 20 and 60, affecting:
- 1.5 million individuals worldwide
- men and women from many racial and ethnic backgrounds
- people of all ages, although symptoms are often first noticed in young adults
- children and occasionally infants (a condition called Leber congenital amaurosis).
How quickly does RP progress?
Most patients experience vision loss very gradually, often over many years or decades. Although different forms of RP advance at different rates, the progress of the disease may be similar within a family group.
Can the progress of RP be halted and is there a cure?
To date, there is no known way to halt the degeneration of the retina or cure the disease. Transplants of the retina are not possible at this time and there is no convincing scientific evidence to demonstrate benefit from any treatment methods currently available.
How is RP diagnosed?
Because RP can be a very slow, progressive disease, an individual may not notice any symptoms for many years. An ophthalmologist (medical eye doctor) may notice changes in the appearance of the retina during a routine eye examination. If RP is suspected, an ophthalmologist who specializes in diseases of the retina may be consulted. A number of eye tests may be performed to assess and diagnosis the specific type of RP.
These tests include:
- visual field
- dark adaptation
- electroretinogram (ERG)
- electro-oculogram (EOG)
- intravenous flourescein angiography
How is RP transmitted?
Approximately 50 percent of diagnosed individuals have no known family history of the disease. These cases are called sporadic or RP simplex. The disease is still genetic and may possibly be transmitted to future generations. The other 50 percent of affected individuals have a known family history.
There are three main ways of inheriting RP.
- Autosomal recessive forms occur when both parents, although unaffected by RP, are carriers of abnormalities (mutation) in the same gene. The risk that a child will inherit a defective gene from both parents and have RP is one in four with EACH pregnancy. The chance of having an unaffected child, who, like the parents is a carrier of the defective gene, is one in two. The chance of these parents having a child completely free of the RP gene is one in four.
- Autosomal dominant forms of RP are caused when the gene defect is inherited from one parent, or when the affected person has one normal and one defective gene. The risk is one in two that the gene will be passed on to a child with EACH pregnancy.
- X-linked (sex linked) recessive forms of RP can be transmitted to children in two ways. In these cases it is known that the gene defect is located on the X chromosome. If a father has X-linked RP, none of his sons will be affected and all of his daughters will be carriers. If a mother is a carrier of X-linked RP, there is a one in two risk with each pregnancy that a son will be affected, and a one in two chance that a daughter will be a carrier of the defective gene.
How is the specific type of RP determined?
An ophthalmologist will consider the following:
- appearance of the retina as seen during an eye examination
- age of the onset of symptoms
- mode of inheritance as determined by charting a family tree. By looking at the extended family and noting if there are any affected relatives, a pattern may emerge that suggests how the defective gene has been transmitted.
- when possible, blood testing to find the exact gene that is defective.
What action can be taken by those affected?
Individuals affected by RP should seek the best information available from ophthalmologists/optometrists about their particular condition. Depending on the condition, further action may be appropriate in the field of education, training and family planning.
Individuals concerned about the likelihood of transmitting an RP gene to their children may wish to consult a qualified genetic counsellor, a healthcare professional specially trained to look for patterns of inheritance.
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We would like to keep you updated with the latest news and updates, fill out the registration form to stay in touch.This information has been provided by:
Alex V. Levin, M.D., FRCSC
Ophthalmologist
The Hospital for Sick Children, Toronto
Medical Advisor, The Foundation Fighting Blindness - Canada
This information has been provided by The Foundation Fighting Blindness - Canada and has been reviewed by its medical advisor. It is not intended to replace the advice of a qualified vision care professional. The Foundation urges everyone affected by an eye disorder to seek the best healthcare possible.