Retinitis pigmentosa is not a disease you’re likely to hear about often, yet it can cause significant visual impairment, to the point of legal blindness and disability.
The retina is the light-sensitive tissue lining the back of the eye. There are ten layers to the retina, one of these being the photoreceptor layer. Photoreceptors are the sensory cells that react to light entering the eye, converting this response into a neural signal that is then passed along to the brain. There are two types of photoreceptors – rods and cones, so named due to their shapes. Cones are found in greatest density at the centre of the retina, an area known as the macula. Cones are responsible for the discernment of fine detail and colour vision. When light levels are high, such as during daytime, our vision predominantly uses cones. Rods are located mainly in the peripheral retina and are responsible for our side vision and for sight during low light levels. Retinitis pigmentosa primarily affects the rod photoreceptors. So, what is retinitis pigmentosa?
What is Retinitis Pigmentosa?
Retinitis pigmentosa is a group of rare, inherited retinal eye diseases with a prevalence of around 1 in 4000 people globally. It is characterised by progressive damage and loss of the rod photoreceptors of the retina, eventually also leading to dysfunction of the cone photoreceptors. Retinitis pigmentosa may be an isolated eye disease or occur as part of a syndrome that affects other parts of the body, such as Usher’s syndrome.
The characteristic symptoms of retinitis pigmentosa are night blindness and progressive loss of peripheral vision, eventually leading to tunnel vision. Due to the progressive nature of retinitis pigmentosa symptoms, the disease may not always be caught in its very early stages. The onset of the condition can vary between different subtypes of retinitis pigmentosa, from as early as within the first decade of life up to the fifties. Most commonly, retinitis pigmentosa will be diagnosed around childhood to young adulthood.
As late-stage retinitis pigmentosa also affects the cone photoreceptors and is known to cause lesions at the macula, people with this condition may eventually begin to find their central vision deteriorates. While both eyes are typically affected to some degree, it is rare for people to completely lose vision in both eyes. Other eye findings commonly associated with retinitis pigmentosa are cataracts, short-sightedness (myopia) and astigmatism.
Taking a thorough history of symptoms will help your optometrist or ophthalmologist diagnose retinitis pigmentosa. Several tests are able to confirm the diagnosis, including:
- Fundoscopy – having a clear view of the retina will identify characteristic signs, such as pigmentary changes, narrowed blood vessels, and a pale optic nerve
- Visual field testing – this is useful to determine the extent of peripheral vision loss and monitor progression
- Electroretinogram – electrodiagnostic testing measures the electrical activity of photoreceptors as an indication of their function
What is Retinitis Pigmentosa Caused By?
There are over 50 genes involved in retinitis pigmentosa. A mutation in one of these genes can affect a photoreceptor’s ability to produce a protein, whether a complete inability to manufacture a required protein or the production of a dysfunctional protein. Other types of genetic mutations may cause the production of a protein that is toxic to the photoreceptor. The inheritance pattern of retinitis pigmentosa may be X-linked recessive, autosomal dominant, or autosomal recessive.
Genetic testing, whether as part of diagnosing retinitis pigmentosa or after the diagnosis has been made, may be useful to determine the likelihood of passing on a genetic mutation to your children.
Management and Retinal Surgery
There is currently no known cure for retinitis pigmentosa, including retinal surgery. Low vision training, such as learning how to use a mobility cane or a guide dog, magnifying devices, or making modifications to everyday items, is an important part of living with retinitis pigmentosa.
Some studies have indicated that a high dose of vitamin A supplementation taken daily can help to slow the progression of vision loss in adults. However, as there are several different subtypes of retinitis pigmentosa, this option may not be effective for most. It is not fully understood how vitamin A acts to slow the sight deterioration in this disease.
There has been exploration into the use of gene therapy as a treatment option for retinitis pigmentosa, predominantly for conditions associated with a mutation of the RPE65 gene. However, as there are so many genes associated with retinitis pigmentosa, gene therapy currently has limited use in this disease.
Up until relatively recently, retinal surgery was not a viable option for treating retinitis pigmentosa. A device called the ARGUS II implant (no longer in production), was shown to have the potential to restore some sight to patients with late-stage retinitis pigmentosa. An electrode chip is implanted into the eye via retinal surgery, which stimulates the retina electrically after receiving signals from a camera mounted onto spectacles. More recently, there has been focus on the invention of an implant inserted directly into the brain, known as the Orion system. The device bypasses the damaged retina entirely, instead transmitting visual signals from the glasses-mounted camera directly to the brain’s visual centres.
As these devices are still being developed, the mainstay of retinitis pigmentosa management remains as low vision training.
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