What Is Retinitis Pigmentosa?
Retinitis Pigmentosa (RP )is the name that has historically been given to a group of many forms of inherited retinal dystrophy, or degeneration.
When it was identified in the 19th Century, the condition was identified as "retinitis" meaning inflammation of the retina, and "pigmentosa" meaning an associated discolouration of the retina, which becomes visible on examination.
For those people who find retinitis pigmentosa a difficult term to use, the shortened form "RP" serves as a simpler alternative.
What The Retina Is
The retina is located at the back of the eye and is connected to the brain. It is made up of many millions of light-sensitive cells known as photoreceptor cells. These photoreceptor cells have the vital function of transmitting electrical impulses to the brain to enable seeing to take place.
Retinal dystrophies are caused by the gradual breakdown of these photoreceptors. Therefore it is important to understand the structure of the eye.
EYE ANATOMY AND PHYSIOLOGY
Structure and Function of the Eye
The eye consists of several parts that resemble a camera (see diagram).
- sclera - the eye's white outer protective coat, normally seen as the "white of the eye".
- cornea - the transparent, curved structure at the front of the eye.
- iris - the coloured part of the eye - blue, brown, green, grey etc - that can be seen through the cornea.
- pupil - the black part of the eye in the middle of the iris. It constricts or dilates according to the amount of light passing through it.
- lens - the transparent disc (with both sides being convex) immediately behind the iris and pupil.
- aqueous humour - the transparent fluid (with consistency similar to water) that circulates behind the cornea and in front of the lens.
- vitreous humour - the material (like transparent jelly) that fills the eyeball between the lens and the retina.
- retina - the light-sensitive layer of millions of nerve cells that line the back of the eyeball. The cells consist of two main groups, called rods and cones due to their appearance under the microscope.
- rods - more numerous, spread out over the entire retina with more toward outer edge, respond to low levels of light.
- cones - far fewer, concentrated around the retina's centre, respond to colour and to details.
- macula - the small centre of the retina, responsible for reading vision.
- retinal pigment epithelium - This is a dark coloured layer of cells at the back of the retina responsible for providing oxygen and other nutrients to the rods and cones.
- choroid - a large network of blood vessels (behind the retina) that transport oxygen and other nutrients to the retinal pigment cells.
- optic disc - a small yellow oval structure in the retina, to which nerve cell connections travel from all the rods and cones.
- optic nerve and beyond - the "cord" of nerve cell connections that passes from the eyeball to destinations throughout the brain.
Function of the Eye
When you see an object, the light travels from that object to the cornea, then passes through the aqueous humour, pupil, lens and vitreous humour to reach the retina. During this passage, the light becomes focused onto the macula.
At the macula, the light causes chemical reactions in the cones, that consequently send electrical messages from the eye to the brain. The brain recognises these messages and indicates to you that this particular object has been seen. The cones are therefore responsible for you being able to recognise colours and to read.
The rods are essential for you to see in the dark, and to detect objects to the sides, above and below the object on which you are directly focused. This function prevents you from bumping into obstacles when moving around.
All the retinal cells (rods and cones) are provided with oxygen and other nutrients from the retinal pigment cells (epithelium), which are kept supplied by the rich network of blood vessels in the choroid.
What is responsible for RP
It is thought that one child is born with RP in approximately every 3,000 births in Australia. It is important to recognise that it is no one's fault and that RP can strike in a family with no known history of it. In fact, RP results from an imperfection in a tiny gene that causes an incorrect protein to be supplied to the retina. Over time this causes photoreceptor cells to die and progressive loss of vision results.