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There are about 120 million rods in each eye and they are more numerous towards the outer edge of the retina. The cone cells are used in colour vision and in close precision work like reading. There are not as many cones and they are more concentrated in the centre of the retina (the Macula). RP may be caused by a breakdown in the function of the rods or the cones in some part of the retina. The retina is so complex that breakdowns may occur in a variety of ways and so RP is not a single disorder but a great number of disorders. The breakdown of cone function may be called Macular Degeneration.
There are other disorders similar to RP like Gyrate Atrophy, Choroideremia etc. The common feature is the degeneration of the retina for one reason or another. Researchers are trying to find particular causes of retinal disorders. One early symptom of Retinitis Pigmentosa is difficulty seeing at night or in poor illumination. With loss of rod function there is a narrowing of the field of vision and as the degeneration gradually progresses it seems as if you are looking down a tunnel.This is why RP is sometimes called "tunnel vision".
Other symptoms includes low light to dark adaption times and poor contrast sensitivity. This inability to differentiate weak contrast affects children in the classroom -writing on the blackboard and on the overhead projector is difficult to read. This also accounts for difficulties in negotiating concrete or carpeted stairs without edge markings.
Usher Syndrome is Retinitis Pigmentosa accompanied by hearing loss. The hearing loss is congenital, stable and usually quite severe, although severity can vary in different individuals. Hearing can usually be assisted by modern electronic aids, hearing-assisted telephones, TV aids, etc.
Usher Syndrome is a
recessively inherited condition. One of the greatest difficulties experienced
by sufferers is increased isolation as the restrictions on both spoken
and visual communication increases.
The word "macula" comes from the Latin
it is the centre portion of the retina that makes central vision, the
vision directly in front of.you, possible. The macula is very small:
only about three by five millimetres (about the size of a ladybug)
covering about 10 percent of the retina.
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