Retinitis pigmentosa

Retinitis pigmentosa

What is retinitis pigmentosa?

Retinitis pigmentosa (RP) is the name for a group of eye diseases in which the sensory cells in the retina gradually degenerate. This leads to severe vision loss over the course of time. Reasons for this are mutations: permanent alterations of the genetic material. Such genetic defects force the retina to synthesise erroneous proteins, which lead to disturbances of the cellular metabolism and, in the end, to the death of those cells. Retinitis pigmentosa affects approximately 30,000 to 40,000 people in Germany alone; worldwide there are  approximately 3 million people living with this condition.
Scientists have identified more than 60 different genetic mutations that can cause retinitis pigmentosa. That is also the reason why the course and severity of the disease can vary strongly in different individuals. Commonly, retinitis pigmentosa commences with the loss of the rods in the periphery of the retina, which are responsible for our low-light and night vision. This loss of night vision is therefore a first indicator of the disease, which often starts in the early youth. With the disease progressing further the visual field gradually decreases, usually from the periphery inwards. In contrast to the rods, the cones that are primarily located in the centre of the retina are not affected until the later stages. Many affected patients therefore do not completely lose their vision. What remains in the end stage of the disease, however, is typically a very small visual field directly in the line of vision (tunnel vision), which is also called tubular vision, since it resembles the view through a very narrow tube. Eventually the ability to recognise contrast and colours decreases as well. 
Retinitis pigmentosa is a hereditary condition. It means that people who are affected can pass it on to their children. The disease can be triggered by a great variety of genetic defects and inheritance patterns, which in turn have a strong influence on its course and severity. Careful genetic counselling can help patients to understand the likelihood of passing on RP to their children.
 

 

 

 

 

 

Evaluations by Massof and Finkelstein in 1987 showed an exponentially
decreasing course of the visual field with an average time constant (half-life)
of approx. 4.5 years. This means that on average the remaining visual field
of an affected person is reduced by half during this time interval.

More recent research, however, points towards a half-life of 7 years (Grover et al. 1997).
 

What are the symptoms of retinitis pigmentosa?

Retinitis pigmentosa involves the loss of sensory cells in the retina, starting with the so-called rods, and followed later by the cones. This causes the increasing loss of retinal function, which leads to the typical vision impairment, including, for example:

  • Gradual narrowing of the visual field, ending in tunnel vision
  • Restricted ability to see in twilight or night conditions
  • Glare sensitivity
  • Impairments of contrast and colour vision

The type of symptoms and their sequence and severity depend on the areas of the retina that are affected by the loss of sensory cells.

How is retinitis pigmentosa diagnosed medically?

If there is a suspicion of retinitis pigmentosa, your eye doctor will in the consultation firstly enquire about typical symptoms. These include, for example, night blindness, restricted visual field up to tunnel vision, problems with colour perception, or pronounced glare sensitivity. The doctor will then use a variety of examination methods to assess the appearance and function of the retina for a reliable diagnosis.

The ophthalmoscope (ophthalmoscopy)

During a fundus examination the eye doctor will visually examine the retina and the rear part (fundus) of your eye using a light source and a magnifying lens. In most cases, the examination equipment does not touch the surface of the eye. The exam can therefore be carried out without local anaesthetic. In order to be able to see all areas of the retina, however, the doctor may dilate the pupil by applying a particular type of eye-drops. The doctor may be able to diagnose RP visually on the basis of characteristic pigmentations of the retina, narrowed retinal blood vessels or an unusually pale and waxen appearance of the optical nerve head.

Visual field examination (perimetry)

A visual field examination provides the doctor with important information for the diagnosis of retinitis pigmentosa. The visual field is the area we can see with one eye without moving it or changing the direction of our gaze. In people with retinitis pigmentosa, the visual field is restricted because of retinal damage. Consequently, these people fail to see certain objects in their surroundings, particularly in the area of peripheral vision. They may, for example, not see a curb or a car approaching from the side, run into obstacles, or miss an outstretched hand when being greeted. In the final stage, only a small visual field remains right in the centre, while the peripheral areas are completely obscured. Doctors refer to this condition as tunnel vision.

A restricted visual field can be documented with perimetry in the course of the diagnosis.

Examining twilight and night vision

The human eye has the ability to adapt to the prevailing light conditions, which normally allows us to sufficiently perceive our surroundings even at night or at dusk. For example, upon entering a dark room coming from a brightly lit area, we cannot see anything initially. The so-called dark adaptation occurs shortly afterwards as the rods and cones in the retina adjust their sensitivity to the incident light: The longer we stay in the dark, the more easily we can find our way around.
People suffering from retinitis pigmentosa frequently experience problems with insufficient twilight and night vision and the associated orientation, even at a young age. This night blindness is caused by the loss of the rods that are principally responsible for low-light vision. Patients therefore have to rely almost exclusively on the performance of the cones to see in the dark. As a consequence, the eyes can only adapt to darkness to a very limited extent and vision remains poor even after staying in the dark for an extended time.
A so-called adaptometer can be used to examine whether the dark adaptation of the eye is impaired and if night blindness is present. In this examination, the patient is asked to first look at a brightly lit surface for some time. Once the light has been turned off, the doctor uses a dimmable light source to measure in short intervals at which brightness the eye is able to perceive the light source. This identifies the time the eye requires for full dark adaptation. Whilst in a healthy eye this process takes about 35 minutes, in patients with night blindness it becomes apparent after approximately five minutes that dark adaptation is impaired.

Electroretinography (ERG)

When the retina detects light, the sensory cells, i.e. the rods and cones, convert the light stimuli to nerve signals. These signals can be measured with special electrodes that are placed directly onto the cornea. This enables the recognition of the impairment of the retina that is associated with retinitis pigmentosa at an early stage: Due to the damage of the sensory cells, here is a change in the electrical impulses typical for a healthy retina. This occurs long before the transformations characteristic for RP would become apparent in a fundus examination.
As the disease progresses, the signals become increasingly smaller and slower.

 

What is the course of retinitis pigmentosa?

Typical for RP is a slow, sometimes staged, but steady progression. Most people affected by RP realise this already at an early age, for example through night-blindness, in which the vision in low light and darkness are strongly impaired. Night-blindness is related to the early degeneration of the rods, the photoreceptor cells in the retina responsible for low-light vision.
Although in retinitis pigmentosa the rods are affected first, the disease will spread to the cones as it progresses further. These are responsible for colour vision and contrast distinction. When the cones start to become affected, this leads to impairment of colour vision of blue and yellow tones in the first instance. At the same time the contrast distinction is deteriorating with dark areas appearing overly bright. This contributes to an increased glare-sensitivity. In the later stages of the disease the affected people lose all remaining sight. 
 

This is (an illustration) how the progression of the disease might be perceived.

Sources:
Grafik: Adaptiert nach Massof et al. 1987
Lang GK et al. Augenheilkunde. 5. Auflage. Thieme, Stuttgart 2014
Grehn F. Augenheilkunde. 30. Auflage. Springer, Heidelberg 2008
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Sahni JN et al. Therapeutic Challenges to Retinitis Pigmentosa: From Neuroprotection to Gene Therapy. Current Genomics 2011; 12:276-284
Hamel C. Retinitis pigmentosa. Orphanet Journal of Rare Diseases 2006; 1:40
Fahim AT et al. Retinitis Pigmentosa Overview. Verfügbar unter www.ncbi.nlm.nih.gov/books/NBK1417/
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Daiger, S. P. et al.: Perspective on Genes and Mutations Causing Retinitis Pigmentosa. Archives of ophthalmology 125.2 (2007): 151–158. PMC
Onlineinformation der Universität Tübingen: www.medizin.uni-tuebingen.de/Presse_Aktuell/Einrichtungen+A+bis+Z/Kliniken/Augenheilkunde/Augenklinik/F%C3%BCr+Wissenschaftler/Elektrostimulation.html