Common Eye Conditions

Macular Degeneration

Briefly

This describes a condition where the central part of the vision is affected. It has two main types – dry and wet. It is often abbreviated to ARMD (Age Related Macula Degeneration)

Fuller explanation

The retina (the photosensitive layer at the back of the eye) is covered by photoreceptors just like a piece of photographic film. Unlike on Kodak though the receptors are not evenly spread across the retina. The rods (dealing with low light levels) are mainly situated away from the centre and the cones (used for high quality vision in good light) are most densely packed centrally. The geographic centre of the retina where this high density of cones is found is called the macula with a very small central peak of intense density called the fovea. Because the cones are so densely packed here the discrimination at this point is much higher than at all other areas on the retina.

One exposure of a film to light uses up all the photosensitive abilities of the film
and a replacement bit of film needs to be introduced before another exposure can be taken. The retina works differently in that it recharges its receptors so they can keep working. Imagine the cone to be a battery of discs of photosensitive pigment. Light lands on the cone triggering a nervous impulse which goes to the brain, the next light triggers the next disc and so on, gradually discharging the battery. This battery however is always on charge and recharges itself automatically. If you look at a very bright light directly you will notice that it takes time afterwards to recover your vision as the intense light has discharged your battery quicker than you can recharge it, this becomes more obvious as the eye ages as the eye cannot ‘recharge’ as quickly as it gets older.

The retina gets its nutrition from the layer below the retina (called the choroid); imagine that there was a sieve below these two layers. All the waste material from the receptors has to be taken away through this sieve and the replenishment needs to come up through it. In some eyes this sieve seems to become more clogged up as we age (just like the sieve we use to sieve flour – after a while you tip the lumps away to let the flour sift through more quickly again). Unfortunately we cannot empty the eye’s ‘sieve’ and so we have receptors that cannot discharge waste and replenish as fast and well as they once did and so the performance of the eye suffers.

As the receptors at the macula are incredibly more densely packed this problem affects the central retina rather than the peripheral area.
When we have this problem to an appreciable degree we call it Age Related Macula Degeneration or ARMD for short. There are two main types of ARMD – Dry and Wet.
The dry form is of the type described above and no help can be given other than advice on lighting (which can make a tremendous difference) and magnification if needed. It is thought that various dietary supplements can sometimes help but there is no definitive proof of any benefit.

The wet form of ARMD sadly can be much more disruptive and is due to the formation of new vessels forming underneath the macula area of the retina. This new network forms a barrier to the transfer of nutrition from the choroid to the retina and in fact the vessels often leak causing an even larger barrier.

The end result of this form of ARMD can be that a central area that is lost, the result the patient notices is as if they sent their film to the developers and just before they were given the prints the developer punched a central hole in the print.
The light that fell outside the central area will fall on good retina and will be processed normally and so good peripheral vision is still enjoyed.
This explains why some carers for ARMD sufferers become frustrated when the patient complains that they cannot see to read or check if the gas etc. is switched on and then will say Oh! You’ve dropped a coin over there, as that coin is seen with the peripheral vision but if they look directly at it the coin 'disappears'.

There is a lot of work being done to try to limit the effects of ARMD
generally and all the useful findings are in relation to the wet form, which is by far the less common form compared to the dry form.

Research that seems to be producing fruit at present seems mainly surrounding laser treatment of off-centre new vessel formation, use of photosensitive dyes that are used to block off errant vessels and even relocating the fovea away from the new vessels. Unfortunately other than the laser surgery the other options are not widely available as they are still regarded as experimental and not fully proven – particularly the relocating of the retina.
The fact that must never be forgotten is that this is a problem that only ever affects the central area of vision and never causes the peripheral vision to be affected. This condition will NEVER cause you to go blind but sadly can be very frustrating if the central vision is lost.

 

An example of dry ARMD

showing a collection of waste material around the central area of the retina (macula). This area will not respond as well to light but the surrounding area is unaffected and will respond well.

 

An example of wet ARMD showing a central area that is scarred

 

and will not respond to light that falls on it. The peripheral retina is unaffected and will respond normally to light giving a good peripheral vision.