Retinitis Pigmentosa or RP, is the most common type of inherited retinal disease. The retina is the lining at the back of the eye. Along with the optic nerve, it forms the ‘nerve’ of the eye. A healthy retina is essential for normal vision. At a cellular level, the retina has 10 layers and the most important of these are the photoreceptors called ‘Rods and Cones’ and the ‘Retinal Pigment Epithelium’. Rods generally help with night vision while cones help with day vision and colour vision. The central retina is called the macula; a healthy macula is essential to be able to read fine print, identify colours, faces and so on.
Genetic conditions like RP cause progressive loss of the photoreceptors, mainly rods to begin with. The affected person notices difficulty in night vision and progressive loss of side (peripheral) vision. Symptoms may start as early as in the first decade of life. As the disease progresses, the macula can get involved causing loss of central vision. As the disease progresses, the affected individual may find it difficult to drive and move around independently after dark. Many people in their 40s have severe difficulty with their day-to-day activities and maybe legally blind. However, the rate of progression of the disease varies from individual to individual.
While RP is rare world wide, with about 100,00 people affected in the US, this is fairly a common disease in India. According to studies, there are between 10-15 lakh affected with RP in India.
Like other genetic conditions, RP is caused by genetic ‘mutations’. The human body is made up of millions of cells. The genetic code within each cell is contained in the DNA, with the code being arranged as a set of spellings (Nucelotides). These nucleotides get together to form amino acids, which then form proteins. Proteins are essential for every single function within the human body. When an error occurs in these nucleotides (like the spelling mistakes within a word), a mutation occurs. A mutation that affects the normal functioning of the retina can lead to genetic conditions like RP.
RP itself is caused by mutations in dozens of genes. These can be transmitted from parents to offspring in different ways- autosomal recessive, autosomal dominant, or X-linked
In autosomal recessive RP, each parent carries one copy of the mutated gene (while the other gene is normal). They have no symptoms themselves and are unaffected carriers of the mutation. When such a couple has children, there is a 50% chance that the child inherits one affected copy from one of the parents (while the other copy is normal). This child then becomes an unaffected carrier. 25% of the children however can inherit both the abnormal copies, and they then develop the disease.
In autosomal dominant RP, one of the parents is already affected and carries a single copy of the mutation. 50% of the children may inherit the same and develop the disease. In X-linked RP, the mutation is carried on the X-chromosome. Females, who have two copies of the X-chromosome are mostly carriers while 50% males inherit the mutated copy from the mothers and therefore develop the disease.
Most of these mutations that cause RP affect the eye alone and these are referred to as ‘Nonsyndromic RP’. Some mutations affect other organs like the ears, kidneys and the brain. These are referred to as “Syndromic RP’. Examples include Usher Syndrome and Bardet Biedl Syndrome.
Genetic testing is available to help identify the mutation in a given individual/family. Identifying the causative mutation helps in several ways. The affected individual is able to understand the various treatment approaches under research, and the appropriate clinical trial. It may also help understand the severity/progression of the disease. The affected family will be able to understand the implications for future progeny. Appropriate genetic counseling and testing can reduce the risk of the disease being transmitted to the next generation.
Although RP is currently incurable, research into treatment has galloped world wide. There are several clinical trials that include gene therapy and cell replacement (stem cell) therapy.
Appropriate use of low vision aids and rehabilitation can help people with RP significantly.