What causes retinitis pigmentosa?
Retinitis Pigmentosa is the name given to a group of degenerative and inherited (passed down through families) eye disorders that cause serious visual impairments. It is a very rare condition and affects about 1 in every 4000 people. Some types of retinitis pigmentosa lead to sight loss (which is generally slow and gradual), whilst others lead to complete blindness. It usually affects both eyes, although the severity of the condition can vary between the eyes. It is highly uncommon for retinitis pigmentosa to occur in just one eye.
The condition produces harmful mutations that can be found in any of around 100 genes that exist, to our current knowledge. These genes carry instructions for producing proteins, crucial for cells within the retina. These cells are called photoreceptors and carry vital instructions to make proteins that are required for the retina to function healthily. Some of these mutations are so severe, that the gene fails to produce enough protein, limiting the cell’s function. In some cases, they can also produce proteins that are toxic to the cell.
What are photoreceptors?
Photoreceptors – more commonly referred to as rods and cones - are cells within the retina that observe and convert light into electrical signals. These signals travel through the optic nerve and into the brain, where they are processed into the images that we ultimately see.
When light enters the eye, it focuses on the central part of the retina called the macula, which is about the size of a pinhead. The macula contains a few million specialised photoreceptors - the cone cells. Cones allow us to see colour and perceive fine visual details.
Rods help us to see in darker light setting and are located in the outer regions of the retina – often referred to as the peripheral retina. The human retina has approximately 6 million cones and 120 million rods.
How does retinitis pigmentosa affect vision?
The first stages of retinitis pigmentosa affect the rods more severely than the cones. As the rods dilapidate, individuals will experience reduced visibility in darker light settings and at night, and a progressive loss of their immediate visual field. Following the breakdown of the rods, the cones soon follow and in the later stages on retinitis pigmentosa, the cones die, and eyes lose more of their visual field and develop tunnel vision. Because of this, people will have difficulty performing simply daily tasks like driving, reading, or recognising faces – even the faces of people they know - and objects.
How is RP inherited?
Retinitis pigmentosa is genetically inherited from one generation to the next. The condition is inherited in different ways, and identifying which process will tell you who in your family has had the condition, roughly how severely your vision will be altered and the chances of the condition being passed onto your offspring.
There are three different ways in which retinitis pigmentosa is inherited:
Autosomal dominant inheritance
Autosomal dominant retinitis pigmentosa affects men and women at the same rate and there will be a recognised history of the condition within the family.
This type of retinitis pigmentosa is less severe than the other types. The initial signs and symptoms tend to appear in an individual around the age of 30.
Only one copy of the gene with a disorder causing mutation can produce the condition. When a parent has a dominant gene mutation, there is a 1-in-2 chance that any children will inherit this mutation and the disorder.
Autosomal recessive inheritance
Similarly to dominant inheritance, recessive inheritance also affects men and women at the same rate. The major difference is that there may be little or no history of the condition in the family’s history. The condition usually begins around the age of 30, but for some people can develop a little later and can start when the person is around 40.
X-linked inheritance
With this type of retinitis pigmentosa, females are the main carriers of the faulty gene, but this pattern of inheritance predominantly affects men. Despite being the main carriers, women rarely develop the condition. This type of retinitis pigmentosa severely affects vision and can result in very poor vision by the age of 40.
In some cases, there is no known relative in the family with the condition and it may not be possible to trace which of the three types of inheritance is causing the condition. Genetic counselling will help you understand which type of retinitis pigmentosa you have and how it may affect you and your family. This is a free service on the NHS and you can ask your GP or ophthalmologist to refer you to your local genetic counselling service.
How is retinitis pigmentosa treated?
Unfortunately, there is no outright cure for retinitis pigmentosa and treatments that are supposed to delay its progression are still under development and testing.