Retinitis Pigmentosa (RP)
What is the retina?
- The retina in your eye serves a similar purpose to a film inside a
camera. Light is focused by a lens at the front of the camera onto a
light-sensitive film at the back, to form a picture.
- In a similar way, light entering your eye is focused onto a
light-sensitive tissue which lines the inside of the eye at the back.
This tissue is the retina.
- The retina consists of two main layers, a thin one called the
pigment epithelium and a thicker one, made up of many layers of cells,
called the neural retina.
- One particular layer in this neural retina contains many millions of
cells called photoreceptors, which are able to respond to light. A few
million cells called cones are concentrated in the central portion of
the retina.
- These allow a person to see fine detail and color. Away from the central
portion of the retina are about 120 million cells, which are mostly
rod cells. They enable a person to see when light is dim and provide
peripheral vision outside of the main line of sight.
- When light is focused onto these rods or cones, a small electrical
charge is generated (the amount depends on the amount of light) and
this charge passes down the optic nerve to the brain.
- As each of these cells receives a slightly different amount of light
and sends a different electrical pulse, the brain is able to assemble
a picture.
What causes RP?
- It is now known that there are many different inherited defects
causing RP. In all RP-related conditions however, the ability of the
retina to respond to light is affected.
- The problem can be in many parts of the retina such as the rod or
cone cells, or in the connections between the cells of the retina.
What are the symptoms of RP?
- The most common first symptom is difficulty in seeing in poor light,
for example outdoors at dusk, or in a dimly lit room. A second symptom
is reduction of the visual field, in which sight is lost from the
sides, or from above and below.
- This is often referred to as tunnel vision and means that the rod
cells, and some of the outer cone cells, have been affected first.
- In some RP-related conditions central vision is lost first. The
first signs of this are difficulty in reading print or carrying out
detailed work. All RP conditions are progressive, but the speed at
which deterioration takes place varies from one person to another.
- In many types of RP, glare from bright lights is an increasing
problem, although some people do not experience this until the more
advanced stages.
How is RP inherited?
- There are three ways in which RP can be inherited:
- Autosomal dominant inheritance. This is the pattern of
inheritance where RP is known to exist in a family, affecting both
males and females. The probability of RP being passed from an
affected parent to a child is exactly 50 percent.
- Autosomal recessive inheritance. There will usually be no
known history of RP in the family but if two carriers who show no
obvious symptoms have a child, there is a 25 percent chance that
he or she will have RP.
- X-linked inheritance. This is a pattern of inheritance
where only males develop the disease, but female members of a
family are carriers. Some carriers can develop a mild form of RP.
- For example, if a man has X-linked RP, his sons will not develop
RP, but all of his daughters will become carriers.
- These daughters will each have a 50 percent chance of producing
an affected son and a 50 percent chance of daughters who will be
carriers.
- This inheritance pattern is sometimes difficult to identify in a
family where there have been no sons for several generations, as
the faulty gene could have passed down a line of female carriers
and then suddenly affect a male child.
When does RP develop?
- There is no hard and fast rule, but in most cases the early symptoms
of RP develop between the ages of 10 and 30.
Can it be detected by an eye test?
- The types of RP which cause loss of central vision become apparent
through an inability to read a test card, but peripheral loss, or loss
of side vision, is not so obvious and a person may be able to read an
optician's test card for many years.
- The condition is best detected by an examination of the inside of
the eye by a doctor, using an ophthalmoscope. Normally he or she would
see an orange-colored area called the fundus.
- When the patient has RP, the orange surface is broken by black or
brown clumps of pigment.
- Other tests are available which measure the area of visual field
which is still useable and the ability to adapt to low light levels.
Is RP just an eye condition?
- Yes, but there are other conditions which people with RP can
inherit. One example of this is Usher syndrome, where people develop
the dual disability of hearing loss and Retinitis
Pigmentosa.
Can RP be treated?
- Currently, no treatment is available to cure RP or arrest its
progress. It is the result of incorrect instructions being passed to
the body's chemistry by faulty inherited genes.
Last Revised: August 17, 2010 |
