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Retinitis pigmentosa (dominant, RHO)
Dominant Retinitis Pigmentosa, often associated with changes in the RHO gene, is a progressive eye condition affecting the light-sensing cells in the retina. It typically causes night blindness and tunnel vision, gradually worsening over time.
Overview
Retinitis pigmentosa (RP) is a collective term for a group of inherited eye disorders characterised by the progressive degeneration of the retina [PMID:33606622]. The retina, located at the back of the eye, contains specialised cells called photoreceptors that detect light and convert it into electrical signals sent to the brain. In RP, these photoreceptor cells, primarily the rods which are responsible for vision in dim light, gradually break down. Over time, cone cells, which are responsible for colour vision and sharp central vision, can also be affected. This progressive loss of photoreceptors leads to a decline in vision.
Dominant RP, including types linked to the RHO gene, is one of the ways this condition can be inherited. The exact prevalence of RHO-related RP is not precisely known, but RP as a whole affects approximately 1 in 4,000 people in the general population. The condition can manifest differently among individuals, even within the same family, in terms of onset and severity of vision loss.
Symptoms & clinical features
The initial symptoms of Retinitis Pigmentosa often begin in childhood or early adulthood, although onset can vary [PMID:33606622]. One of the earliest and most common symptoms is nyctalopia, or night blindness, which is difficulty seeing in low light conditions. This occurs because rod photoreceptors, which are critical for dim-light vision, are usually affected first.
As the condition progresses, individuals may experience a gradual narrowing of their visual field, often described as 'tunnel vision'. This means their peripheral (side) vision becomes restricted, making it difficult to navigate in unfamiliar environments or see objects outside their direct line of sight. Central vision, which is important for tasks like reading and recognising faces, typically remains relatively preserved until later stages of the disease. Other symptoms can include photophobia (sensitivity to light) and difficulty with colour perception as cone cells become more significantly affected. The rate of progression varies considerably between individuals.
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Affected organs
Retinitis Pigmentosa primarily affects the eyes, specifically the retina. The retina is a thin layer of tissue at the back of the eye that is crucial for vision. It contains millions of photoreceptor cells: rods, which are abundant in the periphery and responsible for night and peripheral vision, and cones, concentrated in the macula for detailed central and colour vision.
In RP, the degeneration typically starts with the rod photoreceptors. This leads to the characteristic symptoms of night blindness and loss of peripheral vision. As the disease advances, cone photoreceptors can also be affected, leading to a decline in central vision and colour discrimination. While RP is primarily an ocular condition, some forms of RP can be part of broader syndromes affecting other body systems; however, RHO-related dominant RP is generally considered a non-syndromic form, meaning it only affects the eyes.
Risks & severity
The severity and progression of RHO-related dominant Retinitis Pigmentosa can vary significantly, even among individuals with the same genetic change. The age of onset for symptoms, such as night blindness, typically ranges from childhood to early adulthood, but some individuals might not notice symptoms until later in life. Vision loss is progressive, meaning it worsens over time, but the rate of decline is unpredictable. While central vision can be preserved for many years, most individuals will experience some degree of visual impairment, and in advanced stages, significant vision loss may occur [PMID:33606622].
There are currently no known ways to prevent the onset of RHO-related RP. However, management strategies focus on slowing progression where possible and supporting individuals with their visual impairment. Regular eye examinations are important to monitor the condition. Genetic counselling is also crucial for individuals and families to understand the inheritance pattern and implications for family planning.
Genetic causes
Retinitis pigmentosa can be caused by changes in many different genes. In the dominant form of RP discussed here, changes in the RHO gene are a common cause. RHO stands for rhodopsin. The RHO gene provides instructions for making the rhodopsin protein, which is found in the rod photoreceptor cells of the retina [PMID:28779001]. Rhodopsin plays a critical role in phototransduction, the process by which light is converted into electrical signals that the brain can interpret as vision.
When light strikes the rhodopsin protein, it undergoes a change in shape, initiating a cascade of biochemical events that ultimately lead to the generation of a nerve impulse. Pathogenic variants (sometimes called mutations) in the RHO gene can lead to a dysfunctional rhodopsin protein. These altered proteins can either be unstable, not properly made, or accumulate in a way that is toxic to the rod photoreceptor cells. This cellular stress and dysfunction contribute to the progressive degeneration of the rods and, eventually, cones, leading to the symptoms of RP.
- RHO rhodopsin
Inheritance pattern
RHO-related Retinitis Pigmentosa is inherited in an autosomal dominant pattern. This means that a person only needs to inherit one copy of the altered RHO gene from a parent to develop the condition. Each child of an affected individual has a 50% (1 in 2) chance of inheriting the altered gene and therefore developing the condition, regardless of their sex. The condition can affect multiple generations within a family.
sometimes a new, spontaneous genetic change in the RHO gene can occur in an individual who has no family history of the condition. In such cases, the affected individual would still have a 50% chance of passing the altered gene to each of their children. Genetic counselling is available to help families understand the inheritance pattern, discuss the risks for other family members, and explore options for family planning.
Each child has a 50% chance of inheriting the pathogenic variant, regardless of sex.
Diagnosis & testing
The diagnosis of Retinitis Pigmentosa typically begins with a comprehensive eye examination by an ophthalmologist, who may suspect the condition based on symptoms like night blindness and characteristic changes seen during an ophthalmoscopy (examination of the back of the eye) [PMID:33606622]. Specific tests used to assess retinal function include electroretinography (ERG), which measures the electrical responses of the retina to light, and visual field testing, which maps the extent of a person's peripheral vision. Optical coherence tomography (OCT) imaging can also provide detailed cross-sectional images of the retinal layers.
A definitive diagnosis for RHO-related RP is confirmed through genetic testing. This involves taking a blood or saliva sample to analyse the RHO gene for pathogenic variants. Genetic testing for inherited retinal disorders, including RP, is available through the NHS Genomic Medicine Service (GMS) pathways. A clinical geneticist or an ophthalmologist specialising in inherited eye conditions would typically refer individuals for genetic testing, which is covered by NHS R-codes such as R440 (inherited retinal disease). Genetic counselling is offered before and after testing to discuss the implications of the results.
Management & lifestyle
Currently, there is no cure for RHO-related Retinitis Pigmentosa, but management focuses on monitoring progression, maximising remaining vision, and exploring potential future therapies. Regular follow-up appointments with an ophthalmologist are crucial to monitor the health of the retina and track any changes in vision. Individuals may benefit from using low-vision aids, such as magnifiers or telescopes, to assist with daily tasks, and learning to navigate with impaired peripheral vision.
Research suggests that some vitamins, such as Vitamin A palmitate, may slow the progression of some forms of RP, though this should only be taken under medical supervision due to potential side effects [PMID:22108740]. Antioxidants are also sometimes discussed, but their benefit is not consistently proven for all forms of RP. The NHS provides care pathways for individuals with inherited retinal disorders, which include access to ophthalmology services, low vision clinics, and genetic counselling. Occupational therapists can also provide advice on adapting living environments. Ongoing research into gene therapies and other treatments offers hope for future interventions.
UK care pathway
In the UK, individuals suspected of having an inherited retinal condition like Retinitis Pigmentosa are typically referred by their GP or optometrist to an ophthalmologist. If an inherited condition is suspected, the ophthalmologist may then refer the individual to a clinical genetics service or an inherited eye disease specialist centre. These centres can offer comprehensive assessments, including detailed eye examinations and genetic testing through the NHS Genomic Medicine Service (GMS) [PMID:32252705].
Genetic testing for inherited retinal dystrophies, including RHO-related RP, falls under specific NHS R-codes (e.g., R440) within the GMS. Genetic counsellors play a vital role in these pathways, providing information about the genetic basis of the condition, inheritance patterns, implications for family members, and support in making informed decisions about genetic testing and family planning. They can also signpost to patient support organisations.
Frequently asked questions
What is 'night blindness' and how does it relate to Retinitis Pigmentosa?
Night blindness, or nyctalopia, is difficulty seeing in dim light or at night. It is often one of the first symptoms of Retinitis Pigmentosa because the condition typically affects the rod photoreceptor cells first, which are responsible for vision in low-light conditions. This means affected individuals find it harder to adjust to darkness.
Can Retinitis Pigmentosa be cured?
Currently, there is no cure for RHO-related Retinitis Pigmentosa. However, research into new treatments, including gene therapies, is ongoing and shows promise. Management focuses on monitoring the condition, using low-vision aids to maximise remaining vision, and providing support.
How is RHO-related Retinitis Pigmentosa inherited?
It is inherited in an autosomal dominant pattern. This means a person needs only one copy of the altered RHO gene to develop the condition. If a parent has the condition, each of their children has a 50% chance of inheriting the altered gene.
What kind of specialist will manage my care if I have RP?
Your care will typically be managed by an ophthalmologist, especially one specialising in inherited retinal diseases. You may also be referred to a clinical geneticist and a genetic counsellor to discuss the genetic aspects of the condition and family implications.
Will I lose all my vision if I have RHO-related RP?
The progression of vision loss in RHO-related RP varies greatly. While the condition is progressive and can lead to significant visual impairment, many individuals retain some central vision for many years. Complete blindness is not inevitable for everyone with RP.