Introduction to Retinitis Pigmentosa

Retinitis pigmentosa (RP) is a group of genetic ocular conditions that can affect an individual’s retina, resulting in visual loss over time.⁵ The visual loss in RP follows a specific pattern; an individual usually experiences a narrowing of their visual field (also known as ‘tunnel vision’) due to the loss of cells on the peripheral part of the retina. This article will explore RP and the condition's role in causing tunnel vision.   

Understanding the Retina and Visual Field

The retina, located at the back of the eye, captures light and converts it into signals that are sent to an individual’s brain to see an image.¹ 

There are two types of specialised cells known as photoreceptors found in the retina: rod cells and cone cells. Rod cells can be found in abundance. There are approximately 100 million rod cells within the retina.² Rod cells are found in the peripheral vision and work best in dim light conditions. Therefore they are responsible for an individual’s night vision. Whereas cone cells work well in bright light conditions and are found in the central part of the retina to detect colour and details for an individual’s central vision.² There are approximately 6 million cone cells that can be found in the retina.²       

Mechanism of RP

In RP, rod cells die due to genetic mutations.⁶ Rod cell death reduces oxygen consumption in the retina, thereby leaving excess oxygen in the surrounding retina. The excess oxygen increases the production of harmful molecules known as reactive oxygen species.^3,4 The imbalance between reactive oxygen species molecules and an individual’s ability to neutralise the molecules with antioxidants. This imbalance is known as oxidative stress.²⁵ Consequently, the oxidative stress damages cone cells, further contributing to visual function, including colour vision impairment and central vision loss.^3,4,25

Symptoms and Progression

RP usually affects both eyes, but there have been cases reported of RP impacting only one eye.^7,8

Early symptoms 

One of the earliest symptoms an individual experiences in RP is nyctalopia, which is when individuals have difficulty seeing in dim light conditions or darkness.⁷ This is due to the death of rod cells,  responsible for vision at night and dim light.^2,7 

Progression of visual loss

As RP progresses, an individual’s visual field narrows, creating a ‘tunnel vision’ effect because rod cells are responsible for peripheral vision, and their death would cause loss of the peripheral visual field.^2,7

Advanced stages 

At advanced stages of RP, an individual can experience severe visual loss. Cone cells responsible for central and colour vision can be affected by rod cell death, causing a decline in central and colour vision. In severe stages, due to the loss of peripheral and central vision, individuals can experience complete blindness.^2,7 Most individuals will keep some light perception even in severe cases of RP.⁷    

Genetic Causes and Risk Factors

Over 3,100 possible genetic mutations can impact photoreceptors' function and survival (rod and cone cells).⁹ 

As humans, a child inherits a copy of each gene from each of their biological parents. Males have one X and Y chromosome (XY), whereas females have two X chromosomes (XX).¹¹ RP can be inherited in a variety of ways:^10,11

• Autosomal dominant: If a parent has a mutation, there is a 50% chance that they will pass it on to their child. Autosomal dominant affects males and females equally. Approximately a quarter of RP cases are autosomal dominant
• Autosomal recessive - if a parent has only one copy of the mutation, they are a carrier but typically do not experience the condition. For an individual to be impacted, both parents must either be carriers or affected by the condition. For example, when both parents are carriers, there is a 25% chance their child will inherit the condition and a 50% chance their child will be a carrier. Approximately a fifth of RP cases are autosomal recessive 
• X-linked recessive - this is where mutations in genes are located on the X chromosome only. Males only carry one X chromosome and females carry two X chromosomes. This means males can only pass X-related genetic conditions on to their female children but not to their male children. However, if a mother is a carrier, her male child and her female child both have a 50% chance of inheriting the genetic mutation. Less than a fifth of RP cases are inherited via X-linked recessive means

Diagnosis and Detection

A full ophthalmological examination and genetic testing is recommended for individuals with RP or suspected RP.⁷ 

• Ocular examination

 An individual’s retina can be examined via fundoscopy to look for signs or changes to indicate a diagnosis or progression of RP.⁷

• Visual function tests
  • Visual acuity (VA) measures an individual’s ability to see details from a distance, this is usually tested with a letter chart each eye separately⁷ 
  • Colour vision tests detect issues with colour perception for an individual.⁷ This can help to confirm the level of cone cell involvement² 
  • Contrast sensitivity measures an individual’s ability to detect objects against backgrounds of a similar colour⁷ 

• Imaging tests
  • Fundus autofluorescence imaging helps to detect waste products from dying cells (i.e. photoreceptors in the retina).¹² This helps to look for early signs of RP as well track RP progression
  • Optical coherence tomography (OCT) imaging produces a cross section of the retinal layers to enable in depth clinical review. This enables it to detect any retinal abnormalities and track progress of these abnormalities (for example: retinal thinning or damage to the photoreceptor layer)⁷  
  • Optical Coherence Tomography Angiography (OCTA) imaging is useful in showing detailed images of tiny retinal blood vessels. OCTA can aid in detecting early retinal changes such as reduced blood flow and areas of the retina with fewer visible blood vessels.⁷ Some studies have shown that even individuals with RP with central vision 

• Visual field testing

Visual field testing is conducted to review an individual’s peripheral vision and can be repeated on visits to track RP progression as rod cells.^7,14  

• Fluorescein angiography

Fluorescein angiography is a test that uses a dye to highlight blood flow in the retina and can detect issues with blood flow and fluid build up. Individuals with RP typically present with macula edema (fluid build up), therefore fluorescein angiography is useful in detecting subtle retinal changes.^7,13  

• Electroretinography  

Electroretinography is a test that reviews the retina’s functional ability via electrical responses to light flashes. This can be helpful to detect issues with rod and cone cells before symptoms are present for an individual and can track the progression for RP.⁷  

Current Treatments and Research

Currently there is no cure for RP, but there are several treatments that are advised and future research advancements in progress.⁷

• Vitamin A therapy has been shown to to slow the progress of retinal decline thereby slowing down visual loss for an individual.¹⁵ However, other studies have shown no clear relationship between the intake of vitamin A, retinal deterioration and visual loss.¹⁶ Further research is required to conclusively confirm that vitamin A can support individuals with RP
• Gene therapy in animal studies has shown potential to restore visual loss by targeting faulty genes.^17,18 As there are over 3,100 genetic mutation across 80 different genes, identifying the specific gene responsible for RP in an individual is key to provide gene therapy treatment^7,9
• Stem cell therapy in animal studies has shown the possibility of visual restoration. Stem cells that were transplanted into animal eyes formed into photoreceptor cells improving vision^19,20 
• Neuroprotective therapies aim to slow down photoreceptor cell death in the retina. This can include substances such as; growth factors (substances that stimulate cell growth), antioxidants (substances that inhibit oxidation) and antiapoptotic agents (substances that inhibit planned cell death)²¹    
• Retinal implants have shown to have potential to restore vision in individuals with RP.²² Studies have shown that retinal implants in individuals with RP were successful in an increase in VA, colour vision, contrast sensitivity, and visual fields²³   
• Low visual aids can be used to support individuals who have difficulty in dim light, glare, reduced color vision and reduced contrast sensitivity. Everyday tasks such as cooking, walking and finding objects can be difficult for individuals with RP, but tools like penlights, magnifiers, and lighted reading aids can be helpful²⁴  

Support for those living with RP

RP can be emotionally challenging to live with, but support networks can provide key guidance for patients and their families and friends. Support can include online forums, peer-to-peer, professional counseling, occupational therapy assessment and social worker support.²⁶  

Summary 

In conclusion, RP is a genetic condition that causes a gradual narrowing of an individual’s visual fields, known as ‘tunnel vision’. This is due to the pattern of cell death in the retina. Although there is no cure for the condition, there is ongoing research being conducted to develop therapies such as gene therapy, stem cell therapy and retinal implants to support those diagnosed with RP. Early diagnosis, access to low-vision aids, and emotional support can greatly improve quality of life for individuals with RP.