by David Twigg (freelance writer, Sheffield, England)
(Reprinted here with permission from https://www.edel-optics.co.uk/Retinitis-Pigmentosa.html)
Retinitis Pigmentosa covers a group of eye conditions affecting the retina, leading to sight problems and loss of vision. It is one of the conditions falling under the heading of Rod-Cone Dystrophy, genetic conditions in which the rod and cone cells in the eye deteriorate and die off. This means the retina does not react properly to the light that enters the eye and vision is impaired.
In order to understand how RP affects your eye and the consequences, it’s important to know a little about how the eye works, and in particular, the part the retina plays in the whole process.
How does your eye work?
The eye is constructed of many different parts, all working together to ensure you can see properly. The actual process of seeing is extremely complicated, but put simply, it all depends on light entering your eye and those light rays being sent to the brain, where the brain then processes the information it receives.
The surface of your eye is covered with a transparent layer called the cornea. The cornea protects the delicate mechanism of your eye and is also largely responsible for refracting (bending) the rays of light as they enter your eye so you can focus properly.
Light enters your eye through the pupil, the small dark hole in the centre of your eye. Muscles in the iris around the pupil allow it to expand and contract in response to the levels of light, controlling how much light enters your eye. For example, it becomes very small in bright sunlight, so that only a small amount of light is allowed in. When light levels are low, the pupil expands to allow more light in.
Behind that pupil lies the lens, a small transparent disc that channels the light rays onto the back of the eye. Muscles around the lens cause it to contract and relax, providing the rest of the refraction needed to help focus the light rays exactly on the back of your eyeball.
At the back of your eye is the retina, a ‘wall’ of light receptors.
How does the retina work?
The retina has three layers of nerve cells separated by two layers of synapses and is only around 0.5mm thick. It covers about two-thirds of the inner surface of your eyeball. The back layer of the retina is made up from two kinds of photosensitive cells: rods and cones.
Most of your retina is made up of rod cells. These are responsible for processing low levels of light and they also handle our peripheral visions (i.e. what we see on the edge of our vision when we are looking directly at something).
Cone cells are mainly located in one tiny spot, about the size of a pin-head, called the fovea or macula. This spot is directly in line with the pupil and the lens, and is responsible for our focused vision, such as when we’re reading, writing and recognising colour. Although it’s very small, several million cone cells are located here.
The retina and the optic nerve
The retina receives the light signals and converts the information into electrical signals. It then sends them to your brain via the optic nerve which is directly connected to the retina. The brain then processes what it receives and interprets it so you can ‘see’.
What is Retinitis Pigmentosa?
RP is an umbrella term for a number of inherited conditions which affect the retina, causing damage to this sensitive part of the eye. The cells in the retina die off and stop working, leading to impaired sight or blindness; this usually happens over a period of time.
RP affects about 1 in 3,500 – 4,000 individuals in the USA and Europe, making it one of the commonest genetic retina conditions. It generally presents in young adults between the ages of 20 and 40, but in some cases it’s found in newborns.
What causes RP?
RP is caused by a fault (mutation) in one of a specific group of genes which control the building and healthy maintenance of the retina. This kind of fault in the DNA genetic code is often described as being similar to a ‘spelling mistake’. The mutated gene can be inherited from one or both parents. RP and genetics
How is the genetic information passed between generations?
Each person is conceived from an egg and a sperm. Genes are formed in pairs, one from the mother and one from the father. If only one parent has the genetic fault, only one gene in each chromosome pair is affected, but if both parents have a faulty RP gene then both genes will carry the fault.
The exact genetic make-up of the condition depends on the precise type of RP involved.
What are the different genetic types?
There are three different genetic variations of RP.
Autosomal dominant inheritance
Both men and women can inherit this type, so there is often a history of impaired vision in the family. Around 15-25% of RP patients have the autosomal dominant inheritance form. It is generally less severe than other types (and in some rare cases, someone may have the RP gene but not have any symptoms). The first signs of RP are usually seen when the patient is around 30 years old.
In autosomal dominant inheritance only one parent has the mutated gene, so the person with RP has one ‘normal’ gene and one affected gene in each chromosome. With autosomal dominant inheritance, a parent has a 1 in 2 chance of passing RP to their child,
Autosomal recessive inheritance
This type of RP also affects men and women, but it’s less common so it does not always demonstrate a family history. Approximately 20 – 30% of those with RP have the autosomal recessive form. The impact on vision is more severe with autosomal recessive RP, and first appears when the patient is 30-40 years old.
Autosomal recessive RP is passed on through a defective gene from both parents that are carriers but don’t have RP. The chance of a child having RP is 1 in 4, and the likelihood of them being a carrier is 1 in 2. The chance of having a child completely free from the RP gene is also 1 in 4.
X-linked inheritance happens because of a mutated gene on the X-chromosome (from the father), so it’s harder to detect in a family history. Around 10-15% of RP cases are because of X-linked inheritance and this form can be passed on to multiple generations of the family.
X-linked inheritance can be carried by females but usually affects males. A male has one X and one Y chromosome, so if RP mutated gene is passed to a son, he will have RP.
Because the female has two X chromosomes, she is less likely to be affected as she may have only one chromosome with the mutated gene and one normal X chromosome. All the daughters of an affected male will be carriers but none of his sons. If a female is a carrier there is a 1 in 2 chance that her daughter will be a carrier and that her son will have RP.
If a female develops RP her vision is normally only mildly impaired, whereas in males, the sight is significantly impaired by the age of 40.
Sometimes it’s possible to identify female carriers of X-linked inheritance RP because their retina may have a golden reflex or slight changes in pigmentation.
No identifiable cause
It’s not always possible to identify exactly which type of RP is involved when someone develops symptoms. Around 50% of cases have no identifiable genetic cause.
This could be for several reasons: The mutation is the first to occur in that family Other family members could have the mutated gene but have not developed RP or it’s not yet been diagnosed The mutation could have been present for several generations but has not yet produced a person with RP. This is especially true of the autosomal recessive form, where a person will only develop RP if both parents are carriers and even then the odds are only 1 in 4
Which genes are related to RP?
Scientists have identified over 60 genes which can cause non-syndromic RP and it’s believed there could be many more. Over 20 of these genes relate to the autosomal dominant form of RP
Is RP related to any other syndromes?
In the great majority of cases, RP is unrelated to any other condition, but it is one of a group of symptoms in a number of recognised syndromes.
Usher syndrome causes loss of vision from RP and also congenital deafness. Some patients also suffer a deterioration in balance. There are three kinds of Usher syndrome, and they can be distinguished by the age of the onset, the degree to which the patient is affected and the speed of the progression of the disease. It’s believed that there could be further sub-groups of Usher Syndrome which could be revealed as research continues.
Usher Syndrome occurs in around 4 in 100,000 of the population, although recent research indicates that the ratio could be as high as 15 in 100,000. Statistics from specialist deaf school suggest that between 3% and 6% of the deaf population had Usher Syndrome, with more recent figures from other studies putting the incidence at 15-18%. Because congenital vision impairment and deafness are not commonly found together, patients who have both are considered quite likely to have Usher Syndrome.
Usher Syndrome is generally diagnosed via sight and hearing tests, strengthened by the presence of balance problems. At present there is no cure for Usher Syndrome, although it’s hoped that research will eventually find a solution.
Leber’s Congenital Amaurosis (LCA)
LCA develops in children and the loss of vision is usually severe. Vision stays stable in most people, but occasionally a patient’s vision may deteriorate slowly over time. LCA also causes other sight problems. The pupil in the eye may react slowly to changes in light levels, causing extreme sensitivity to light (photophobia). IN some cases the pupil may not react to changes in light levels at all. The cornea, which covers the front of the eyeball, may be thin and cone-shaped (kerataconus), and people with LCA my also have involuntary jerking eye movements, known as ‘nystagmus’ and extreme long-sightedness (hyperopia).
Another significant feature of LCA is that patients frequently rub, poke and press their eyes, and it’s believed this leads to the unique deep-set eyes seen in people who have LCA. This behaviour is known as ‘Franceschetti’s oculo-digital sign’.
13 different kinds of LCA have so far been identified, each with its own specific pattern of genetic cause, characteristics of vision loss, and other related eye symptoms.
Again, so far there is no cure for LCA, although research is continuing into possible treatments.
Bardet-Biedl Syndrome affects many parts of the body and the effects are not always consistent, meaning that even patients within the same family may display very different physical characteristics. However, vision loss is a significant symptom related to Bardet-Biedl syndrome.
With this condition, as in other RP types, the retina deteriorates over time.Typically, people with Bardet-Biedl Syndrome develop problems with night vision as children. Blind spots in the peripheral vision then occur, which gradually merge to produce tunnel vision that becomes increasingly blurred. Most patients with Bardet-Biedl Syndrome are registered as legally blind by their mid-teens.
Bardet-Biedl Syndrome also manifests itself in other physical characteristics. Patients become obese, making them vulnerable to all the related health problems such as high blood pressure, strokes and heart disease. Other physical symptoms associated with Bardet-Biedl Syndrome include: extra fingers or toes, short or webbed digits, learning disabilities, heart, liver, kidney and digestive problems.
RP is also one of several symptoms associated with Refsum Syndrome. As well as suffering from RP, patients with Refsum Syndrome also experience hearing loss, neurological problems, heart problems, dry or flaky skin, loss of sense of smell, and oddly shaped red blood cells.
Some cases of Refsum Syndrome respond well to a diet that eliminates or severely restricts phytanic acid.
What are the symptoms of RP?
The age at which people develop RP varies considerably, between early to mid-childhood, through to late adulthood. However, for the commonest types of RP, a formal diagnosis is normally given during the teens or early adulthood.
The exact nature of the early symptoms of RP depends on the specific type of RP. Generally, the overall quality of vision is not the issue at first. Early symptoms tend to cause a gradual deterioration of some vision functions rather than a wholesale sight loss.
One of the most common early symptoms is a loss of night vision, sometimes referred to as ‘night blindness’, making it difficult to see clearly in the dark. Healthy eyes normally take around 20 minutes to adjust fully to low light conditions. But those with RP may find it takes considerably longer for their eyes to adjust when moving from a light situation to a dark one e.g. leaving a lighted house and going out into a dark lane. In some cases, the eyes don’t adjust at all. The ability to drive at night or in low levels of light (e.g. early morning or at dusk) is also affected.
Loss of peripheral vision is also a common early symptom. People may bump into things which are not directly in their line of sight, or trip and fall over objects easily. Activities such as crossing a busy road may become more difficult. However, central vision takes longer to be affected, so activities like reading or the ability to recognise faces remain reasonable functional at first.
Some people will suffer a loss of central vision first, perhaps noticing it when they are reading or doing close-up work such as sewing. When the central vision is affected first, the ability to distinguish colour is also impaired.
Later symptoms of RP include the full development of tunnel vision caused by the deterioration of the rod cells in the retina. People often describe this sensation as being like ‘looking through a straw’. In time, even that central vision area may become blurred.
People whose central vision is affected first will find that their peripheral vision also deteriorates over time.
Because the cells in the retina aren’t functioning so efficiently, sensitivity to light may cause problems. Good sunglasses can help with this and they may need to be worn even in relatively low levels of light when the person is outside.
By the age of 50, most people with RP have their vision significantly affected. For example, they are unlikely to be able to read without using a magnifying glass and they will need to make lifestyle adjustments to help them cope with everyday life.
Some people with RP develop cataracts in later life. Naturally, if vision is already impaired, cataracts can have a devastating effect on any remaining vision. Cataracts can be removed easily under a local anaesthetic; the faulty lens can be replaced with an artificial one or prescription eyeglasses can be used to correct any vision faults.
How is RP diagnosed?
Diagnosing RP is a very complicated process that involves a variety of tests and examinations, so it can take some time before a definitive diagnosis can be made. Although RP is an inherited condition, there is often no obvious history of RP in a family. It’s also important to rule out any other eye conditions which could be causing vision problems, so diagnosing RP is likely to be a relatively long journey.
Another factor that makes diagnosis difficult is that problems with the vision tend to develop gradually. It’s easy to miss early symptoms and it’s only after some time, when the vision has become affected more significantly, that someone may realise there’s an issue.
Initially, it’s likely to be an optometrist that first investigates, perhaps because the patient is reporting difficulties with their vision or perhaps as part of a routine eye examination.
Eye testing and examinations
Most people have a sight test done by an optometrist on a regular basis. This could be to check the sharpness of their vision, prescribe eye glasses to correct long or short sightedness, and to check the general health of their eyes. If the optometrist feels that further investigation is necessary, the patient will be referred to an ophthalmologist (specialist eye doctor).
Physical eye examination
An optometrist can carry out a simple examination of the back of the eye. Although the retina is transparent, a healthy one appears orange because of the blood vessels behind it. Black spots (bone spicules) which indicate that cells in the retina are ‘dying off’ can be seen during a physical examination of the eye and a pale optic nerve can also indicate RP. Very often a physical examination is the first step towards diagnosing RP.
Visual acuity test
The optometrist can also carry out a visual acuity test. Many people are familiar with this test, which involves reading letter from a chart which become progressively smaller in size. This kind of test is done to determine the exact prescription for vision correcting glasses, but it can also reveal other more serious sight problems as well, e.g. deterioration in central vision.
People with ‘normal’ sight have 20/20 vision, whereas someone with 20/40 vision can only see at 20 feet what someone with 20/20 vision can see clearly at 40 feet.
People with RP who develop problems with central vision first are more likely to get an earlier diagnosis, as their symptoms will show up during this routine test.
This is also a common test in which the patient is fitted with different optical lenses to determine the exact function of each eye. This is normally done to create prescription lenses that are worn to correct sight defects such as short sightedness. Although prescription lenses themselves will not overcome the difficulties of vision loss from RP, they can help correct existing vision if it’s become blurred.
Visual field test
In this test, a light is progressively brought from the edge of a viewing screen towards the middle. The patient is asked to identify when they can see the light, which allows the range of peripheral vision to be mapped. Someone with relatively normal vision will be able to see the light across quite a wide area, but someone with peripheral vision loss will have vision limited to a small central spot.
Colour vision tests
There are several tests that can be used to measure levels of colour visions. The simplest involves looking at a book of patterns with different combinations of colours. More complicated tests, such as the Hardy-Rand-Rittler (H-R-R) and Ishihara tests, can also identify the exact type of colour blindness.
Electro-diagnostic tests measure electrical impulses to assess how well the retina is functioning. These tests include:
Electro retinogram (ERG)
To carry out an ERG, pain-numbing drops are put into the eye, which allow a tiny electrode (similar to a contact lens) to be placed painlessly on the surface of the eye. In a dark room, lights are flashed onto the retina and the electrode measures the retina’s electrical response to see if it’s functioning correctly. Some patients find this test a little uncomfortable, but it’s not painful.
An EOG measures electrical differences between the cornea and the retina, and it can also be used to track eye movements. For this test, electrodes are placed on the skin around the eye rather than on the surface of the eyeball.
Electro-diagnostic tests are so sensitive that they can show up problems with retina function even before the patient notices any physical symptoms. However, these specialist tests are not offered routinely at all eye health centres and are often done to confirm the exact nature and extent of vision problems already identified rather than a tool for initial diagnosis.
With a special camera, it’s possible to photograph the back of the eye (the fundus), to see whether there are tell-tale black pigmentation marks on the retina, which would indicate a diagnosis of RP. Drops are put into the eye to dilate the pupil fully, so the camera has a clear view of the fundus, and photos are then taken. This test only takes a few minutes, but the effect of the dilating eye-drops last for some hours, so patients are not able to drive immediately afterwards.
Optical Coherence Tomography
In this test, light is cast across the retina to test its function to obtain cross-section images of the retina to see if there are any abnormalities. It can also measure the thickness of the retina.
This test involves injecting a special dye through the arm. The dye travels through the whole body via the blood vessels, including the blood vessels at the back of the eye behind the transparent retina. This allows the ophthalmologist take photos of these blood vessels so that any problems can be identified. Narrowed blood vessels may indicate RP.
Like the fundus photograph, dilating eyedrops have to be used for this test to allow the camera a clear view of the retina.
It’s only when a combination of tests have been carried out that a definite diagnosis of RP can be made. Further genetic testing may reveal which particular type of RP is present, but in many cases the exact genetic cause can’t be identified.
What are the defining symptoms needed for RP diagnosis?
It’s generally agreed that these symptoms must be present before giving a diagnosis of RP: Both eyes are affected (although occasionally only one may be involved) Night vision and peripheral vision are impaired Electro-diagnostic tests reveal that the retina rods are not functioning correctly, leading to a slow or non-existent response time to lowered light levels Progressive deterioration in the function of the photoreceptors in the retina How is RP Treated? At present there is no complete ‘cure’ for RP, although some treatments have limited success in slowing the rate of deterioration. However, scientists are working hard on several fronts to find genetic treatments that will prevent or cure RP.
Many of the genes that are liable to mutate and cause RP have already been mapped but it’s believed there could be many more. The search for a genetic cure has so far been limited to the science laboratory and controlled testing, and are not yet generally available as approved therapies. But it’s hoped in time that this research will lead to a breakthrough in how RP is treated.
Research is being carried out on several fronts. The search for a cure Scientists are continuing to try and identify all the genes liable to mutation leading to RP. Most of this work is aimed at finding a genetic solution.
Searching for a genetic solution
The search for a genetic cure for RP is focusing on three main areas.
Gene therapy involves identifying the gene at fault and replacing it. This could be done by using a harmless virus as a carrier and injecting it directly into the affected retina. It’s hoped that this treatment could halt, or perhaps even reverse, damage to the retina. But for this treatment to work it would be necessary to identify the exact gene that has mutated, which can’t always be done.
Stem Cell therapy
Stem cell therapy relies on the ability of cells to divide and reproduce. Because stem cells have the ability to replicate other cells, it could be possible to inject them into the retina in the hope that the stem cells will differentiate (reproduce into another kind of cell) and replace the mutated retina cells.
But retina cells are very specialist, so much work needs to be done towards find stem cells which can differentiate into retina cells.
Using growth factors (chemicals) to try and stimulate the retina cells into growing and repairing themselves is a further option being explored. In this method the focus is on ‘kick-starting’ the existing cells into reproducing normally and perhaps even repairing damaged cells.
Some scientists are also undertaking research into molecular genetics. For example, the Center for Retinal and Macular Degeneration in central Florida is conducting research into the molecular genetics related to X-chromosomal retinal/macular dystrophies.
Some research into a cure for RP indicates that Vitamin A can help people with RP. There are many foods which are high in Vitamin A, including:
So far there has been much debate about the effectiveness of this therapy. Some professionals question its credibility and it also seems that the measurable benefits have been slight. This therapy is also potentially problematic because ingesting high levels of Vitamin A can actually be very harmful to health. So this is a therapy which should only be undertaken under strict medical supervision and few health practitioners are offering this as a form of treatment at present.
Other nutritional supplements
There are also investigations into the effectiveness of using combinations of nutritional supplements, especially anti-oxidants.
Nutrition and Refsum Syndrome
The only proven condition which can be managed by diet is Refsum Syndrome, where a diet eliminating or reducing phytanic acid has been shown to be beneficial for some patients. Phytanic acid is found in beef, lamb, dairy products and fish such as haddock, cod and tuna. Removing these from the diet under careful professional monitoring may be recommended for some Refsum Syndrome patients. What is the prognosis for RP patients? There is no cure for RP, although it’s hoped that in time a cure will be found. RP very rarely results in total blindness, but in many cases it does cause a significant loss of vision, with most patients being registered as legally blind eventually.
As well as helping those with RP come to terms with their progressive loss of vision, and enabling them to cope with the demands of their everyday lives, many people diagnosed as RP patients or carriers benefit from genetic counselling. This kind of counselling can help them to make informed decisions about the likelihood of having children affected by RP. Conclusion In some cases it’s possible to slow the progress of Retinitis Pigmentosa to give patients a longer period of time with better sight. But there is also a great focus on supporting those with RP, and their families, to make adjustments to their day-to-day living that will help them to successfully manage the gradual loss of their sight.
Further Resources on Retinitis Pigmentosa:
Hyper Physics – The Retina of the Human Eye – Copyright 2000 by Carl Rod Nave
Lighthouse International – Retinitis Pigmentosa – What Is Retinitis Pigmentosa (RP)?