Thursday, 6 December 2012

What is Retinitis Pigmentosa?

The best place to start would probably be explaining and describing my exact condition.

Retinitis Pigmentosa(RP) is a group of genetic degenerative conditions of the eye. The present in much the same way as macular degeneration because ultimately that what they are. The key to defining RP is the mechanism.

Everybody who sat GCSE science is familiar with the image of the double helix. The spiral ladder structure that represents our DNA. That structure contains all the information required to build you. It is effectively your blueprint. A huge proportion of the data held in your DNA is "junk" it doesn't code for any specific genes as far as we know, in some cases it's just redundant coding left from our evolutionary past. The rungs on the ladder are bonds that join two objects called nucleotides. In humans there are roughly three billion of these rungs joining six million nucleotides. 

Every time two people combine their DNA and make a child a number of coding errors occur. These are called mutations and they come in two forms. Deletions are where a nucleotide doesn't get copied leaving a gap. Or substations where a nucleotide is out in the wrong place. Mutations are normal and happen every generation most of the time they occur in areas of our DNA that don't code for anything and there are no physical symptoms as a result.

Here's the bit that blows my mind. At some point back in my family tree a deletion mutation occurred in one of the few areas of my DNA that codes for something. Of six billion nucleotides I am missing one. As a result my body is unable to produce sufficient quantities of an enzyme that would normally protect the retina of my eye. This condition is "X-Linked" which means I carry it in my X Chromosome. As I only have one X Chromosome(the other is a Y chromosome, XY is male, XX is female) all the data that codes me from that chromosome is used regardless of weather or not there are mutations. In women there is a certain degree of error checking, when their cells are dividing they identify that there is a mutation on the broken X chromosome so they use their other X chromosome. As a result women carry my variation of RP but don't develop any symptoms of it. If they have a child their is a fifty per cent chance they will pass on the mutation as each parent only passes one chromosome to their offspring. If they do pass on the mutated X and the child is a girl she will be a carrier, if the child is a boy he will be symptomatic with RP.

That's probably enough of the genetics you want to know what Retinitis Pigmentosa actually is. The retina of the eye is a disc covered with two types of photo sensitive cell. Light gets passed onto this disc through your eye and these cells act as your visual field, some control colour and fine detailed central vision, others control monochromatic night vision and peripheral vision. Due to the lack of the enzyme I mentioned previously my retina are being constantly damaged, the more vulnerable areas go first, the areas that effect my night vision and peripheral fields. As a result I have next to no vision in dim light and there are substantial gaps on my peripheral visual fields, and now one or two gaps in my central field. 

The human brain is an incredible thing, I am largely unaware of exactly where these gaps are because my brain fills them in with what it thinks should be there. If I am looking at a floor and there is a wet floor sign in my visual field gap my brain fills in the gap with empty floor because that's what's on every side of the gap, as a result I won't see the luminous yellow wet floor sign. The same principle applies to bollards, chairs and almost anything else at a worrying height to walk into.

I have also started noticing a steady deterioration in my colour vision. Add all this together and you can see why I had to leave clinical practice as a nurse. I think my medical background has helped me though. I genuinely find this condition fascinating. For example. Every time the parents procreate there is only a fifty per cent chance the mutation will carry(assuming only one parent has it) and I have traced progressive blindness back in my family tree to the mid 1600s. It probably goes back further but records get a little more ambiguous past that point. Every generation, every man and woman in that long chain leading up to me has passed on the mutation. 

I am sorry I could not relate the above information without going into so much scientific detail I know that will limit the audience who are interested in reading this considerably however I felt it was important when explaining what RP is to start with the foundations. I did simplify a large amount of the above information however I do not think that it has compromised the accuracy of the information. 

Should you desire more information I can recommend these sites. I will add to this list as I find sites I feel appropriate, should you know of any please feel free to comment. 

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