Retinitis pigmentosa (RP) is the name for a group of congenital eye diseases that lead to a destruction of the retina, i.e. of the tissue at the posterior end of the eye that is capable of transmitting visual impressions.
Worlwide about 3 Million persons –in Germany approx. 30 to 40 thousand– suffer from one of the different forms of RP. This – still incurable disease– is one of the most frequent causes of loss of sight in adult life. RP is a congenital disease that can be transmitted to one’s offspring. It has been estimated that every 80th person carries an “unfavourably modified” RP gene, i.e. a genetic information that can start the development of this retinal disease in RP-gene carriers or in their offspring.
The diseases of this type are characterized by the onset of night blindness in adolescence or in middle age, with a constriction of the visual field, contrast- and colour vision, but also visual acuity deteriorate, so that the ability to see gradually deteriorates, often until blindness is reached. The entire process is gradual, occurs in phases and over decades. This development leads to professional and private disadvantages and thus to heavy psychological stress.
At the bottom of these symptoms lies a gradual destruction of the retinal photoreceptors, usually beginning with the rods responsible for night and twilight vision, later continuing with the cones located at the center of the retina, important for reading and colour vision. Even though the disease has been described in the middle of the 19th century, with only very few exceptions there still exists no possibility , neither surgical nor pharmacological nor through a diet, to slow down or to halt the process of photoreceptor cell death. Ever since the disease became known, a whole spectrum of therapies has been tried out, from scientifically founded attempts to sheer quackery, all of which turned out to be ineffective.
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In order to explain the process of seeing, a comparison with photoraphy is useful. The light by which we are able to see falls into the eye through the pupil. The pupil corresponds to the aperture and widens or narrows according to the light intensity. The lens of the eye has the same function as the lens of the camera, i.e. to render an image sharp at a certain distance. The retina corresponds to the film. The retina constitutes the innermost of three layers of tissue that line the invisible back part of the eyebulb. Directly beneath the retina lies the choroid that supplies the retina with nutrients, and the outermost layer is made up of the protective sclera.
Whereas film has the same light sensitivity everywhere at its surface the retina has different sensitivity to light in different areas. The retina is made up of rods and cones. The rods enable us to see at night and during twilight; they cover the larger part of the retinal surface. The cones enable us to see colour and allow us to see images sharply. They are found mainly in the centre of the retina, in the so-called “yellow spot” or macula. From the rim of the macula outwards, the proportion of rods increases, that of cones decreases. The macula has a diameter of only 2 mm.
Our visual field extends to about 180°, but we are not able to see everywhere with the same acuity. The centre of the visual field, i.e. the area of our line of vision possesses the greatest visual acuity. We rely on this area for reading and for recognizing fine details. Towards the periphery, our visual acuity decreases. Already 5° laterally to the line of vision, visual acuity decreases to 30% (see Figure below). The peripheral zone of the retina allows spatial orientation. The retina contains millions of photoreceptors; these are stimulated by incident light, light is transformed into electric impulses transmitted via the optic nerve to the brain where the image that we see is finally composed.
Source: VISUS 1/95, Durch das Auge
Degenerative retinal diseases
The concept “degenerative retinal diseases” covers generally all those eye diseases that are based on the death of various retinal cells. Two main types are distinguished: Diffuse retinal degenerations (e.g. Retinitis pigmentosa, Usher Syndrome, Choroideremia) and localized retinal degenerations (e.g. age-related macula degeneration).
A series of congenital and non-congenital degenerative retinal diseases affect primarily the area of sharpest vision, the macula. The outer visual field and the capacity for orientation remain untouched. As a rule, night blindness does not occur, since the rods lying outside the central retina keep functioning normally. A few symptoms resulting from the damage to the macula correspond to those of advanced RP: Decrease of visual acuity, of the ability to read, of contrast sensitivity, of colour vision and an increase in sensitivity to glare. The affected person can no longer fixate an object. Thus it is possible to see a clock but not to recognize the time, to see a person without recognizing his or her features. The age of onset and the severity of the symptoms vary and depend upon the type of the disease.
In Germany 1-2 million persons suffer from one type of macula degeneration with the majority suffering from age-related maculare disease (AMD). The name “age-related” or “senile” macula degeneration (AMD) derives from the fact that the first symptoms appear only from the age of 45 to 50 years and the probability of being affected by AMD increases with age. The “juvenile” type macula degeneration can occur already in the 10th or 20th year and it subsumes various diseases with similar characteristics. All of them can lead to a progressive decrease of vision in the center of the retina.
Age-related macula degeneration (AMD)
Age-related macula degeneration can take two different courses.
The more frequent form (ca. 85%) is the “dry” type where central retinal cells slowly degenerate. This cell death leads slowly to a decrease in the ability to see. Occasionally, for a longer period of time, the process of degeneration comes to a standstill, so that some patients are able to read up into their old age by means of optic or electronic reading aids. Effective drugs or other therapeutical measures do not exist.
In the far more rare cases of “wet” macula degeneration, liquid collects beneath the macula, stemming usually from ingrown choroid vessels. The leakage of fluid from the blood vessels damages the light-sensitive cells of the macula. the image on the retina is distorted, so that the first symptom of this disease is the apparent contortion of straight lines into curved ones, later followed by spots in the center of the visual field.
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Cone-rod dystrophy is a disease which from its onset goes hand in hand with a heavy loss of visual acuity. It progresses with varying speed. Alone in Germany roughly 2000 persons are affected by this disease that in rare cases is connected with other organ diseases. Usually the disease manifests itself by age 20, sometimes, however, it appears later in life.
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Choroideremia constitutes a rare retinal-choridal dystrophy; according to its course it is a rod-cone dystrophy. In Germany about 1000 persons, usually men, are affected.
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Light-sensitive retinal photoreceptors.
There exist rods and three types of cones containing different photopigments for distinguishing various hues of colour
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“Visual acuity” is the eye’s capacity of recognizing different fine details; it depends above all on the resolution of the retina and lies in area of sharp vision around 1-2’.
With low brightness and at the periphery of the visual field visual acuity decreases markedly, because the concentration of photoreceptors is lower.
The optometrist or the ophthalmologist examines visual acuity with test images on which individual letters, numbers or pictures must be recognized at a certain distance.
A visual acuity of 100% is regarded as a good average value in normal-sighted patients. Beneath 20% is classified as “strong visual handicap” and beneath 3% is classified as “blind”, even though some remainin vision may still be present e.g. the ability to still distinguish darkness and light.
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a hearing implant, placed into the inner ear when the inner ear no longer functions but the acoustic nerve still is intact. In contrast to hearing aids, cochlear implants stimulate the acoustic nerve directly.