If you have any concerns about your eye health, call us or drop in at the practice. An early professional examination offers you peace-of-mind and can ensure the most effective treatment if needed.
Figures quoted for the incidence of glaucoma are normally given at age 40+. The incidence increases with age. In the UK population as a whole there is a 2% incidence of glaucoma. Amongst diabetics this rises to approximately 5%. If there is a family history (parents, siblings or children) of glaucoma the figure quoted is 10% and Afro-Caribbean races have an incidence of roughly 8%. If you have circulatory problems, smoke, are highly short sighted and are in these categories the risk is higher.
Being in a higher risk category is not a reason for worry but concern is a very healthy attitude. Early treatment of glaucoma is generally very successful.
There are two main types of glaucoma:
- Acute (closed angle) glaucoma where there are normally symptoms of uncomfortable to very painful eyes and fairly rapid deterioration in vision during an attack – this is not the commonest or typically hereditary form of glaucoma.
- Chronic (simple) glaucoma is normally a very gradual deterioration and has no symptoms discernible to the patient until late stages of the condition when there has often been marked damage to the visual system. This is the most common form of glaucoma and we consider the hereditary connection to be greatest with this form.
To understand what glaucoma is it is best to understand the basics of how we see. We have a visual system that works in a very similar way to how we achieve a photograph with a camera. There are three main elements:
- The eyes act as two cameras taking pictures (we hope they are correctly focused but if not this can be achieved with spectacles or contact lenses)
- The back part of the brain functions as the developing house, creating images from the information sent from the eyes.
- The optic nerve (actually a bundle of over 1 million nerves) goes from the eye to the brain delivering the negatives in the form of nerve impulses. The nerves are “fed” by a blood supply, which brings them nutrition and oxygen. Without an adequate supply the nerve suffers.
Glaucoma is the name given to a condition when the nerve gets an inadequate blood supply. This is most often caused by an increase in pressure within the eye (the intra-ocular pressure, or tension, abbreviated to IOP). Treatment for glaucoma (or its prevention) is normally to try to increase the blood supply to the nerves by minimising any obstruction to the flow, particularly by lowering the IOP. There must be some pressure in the eye to keep it inflated but when it becomes too high, for that person, then problems start. There is a “normal” range of pressures when most people’s eyes will not suffer and above which the risk of problems are higher. Some people however develop glaucoma with a lower pressure than this (low tension glaucoma) and some people seem not to have problems with an IOP higher than “normal”.
Another way of understanding this concept is to think of the optic nerve as a fleet of over 1 million Royal Mail vans delivering the negatives to the brain. The vans need petrol to keep them working well. Imagine that when they go to the petrol pump someone stands on the petrol hose with sufficient weight to restrict the flow, the end result will be a van that does not have enough fuel to function properly and so a restricted delivery service is brought about. If the petrol hose has faults within it to limit the flow (maybe narrow calibre) then there is already some limitation to delivery of the fuel and even a lesser pressure on the hose will create an unacceptable restriction of the flow. It could be that the mail vans are inefficient, using more petrol than normal – so what would be normal for others is inadequate for them.
The blood that feeds the optic nerves is delivered by blood vessels, which are like flexible hosepipes full of blood. If there is a restriction to the flow of blood through theses vessels for reasons similar to the petrol hose then a malnourished optic nerve is found. Glaucoma is the condition when an inadequate blood supply is fed to the nerve.
Glaucoma treatment mainly centres itself on trying to increase the flow through the vessels by lowering the pressure on the vessels (it tells people to stop standing on the petrol hose – like a man with a loud hailer would tell people) to maximise the flow although some treatments try to help with flow within the vessels. This explains why glaucoma treatment in the form of drops to lower the pressure is a long-term course, as if you stop the course of drops prescribed then it is as if you remove the man with the loud hailer, the people climb back on the hosepipe and restrict the flow again with harmful effects following.
How do we check if there is glaucoma? And if there is, how do we monitor its control?
Measurement of the intra-ocular pressure is a quick and simple way of getting a very important benchmark. This is done in hospital departments and some optometrists by resting a probe against the locally anaesthetised eye but most optometrists use an instrument that blows a puff of air at the eye. As mentioned earlier the absolute value at which problems occur varies from person to person and so although this is an extremely valuable reading it is not diagnostic in itself.
The eyes see to the side of straight ahead and our visual field (the area we can see) can be measured. This test is very subjective (relies on the subject giving responses) and as the field tests become more detailed some patients find them more difficult and there is sometimes thought to be a lack of accuracy developing as the tests become more involved. If there is loss of nerve function due to glaucoma (or other causes) there will be a restriction to the fields. These tests can be very accurate but sometimes their subjectiveness causes less dependable results and they can be very time consuming.
Optic Disc Assessment
When a doctor or optometrist looks inside your eye they view the optic disc at the back of the eye where the optic nerve(s) leave the eye. To understand how viewing this helps, think of the optic disc as a plug hole and the nerves are water flowing out of a sink. When there is a lot of water flowing out the plughole will appear full but as the amount of water leaving the sink lessens then a central hole or depression forms. This hole keeps enlarging until, when the sink is nearly empty and there is only a small outflow we are left with a large hole and only a trickle of water around the edges.
When there are a large number of nerves leaving the eye there is a small “cup” (or depression) in the middle of the disc and the nerves have a healthy reddish tinge showing a good perfusion of blood within the nerve. With glaucoma, as the blood supply drops, the nerves change colour slightly and a greater ‘cup’ forms. In late stage glaucoma there is a marked loss of colour (pallor) of the nerve and a large cup as there are only a few malnourished nerves left.
A great skill is required to assess the colour and appearance of the disc/cup and the doctor or optometrist will write on the notes their assessment of this to compare against the next visit. This assessment is made more difficult as people can have quite different optic disc sizes. If there were a large disc then one would expect a greater amount of cup in a normal eye and if there were a small disc it would still give a ‘full’ appearance with fewer nerves.
Some optometrists are now taking photographs of the back of the eye, including the optic disc, so that they have an absolute record of how the disc looked and so that they can compare year on year without having to rely on just a written observation of someone’s (not necessarily themselves) interpretation at a previous exam. We also have an instrument called an OCT (Ocular Coherence Tomographer) that can measure, and mathematically compare from year to year, the nerves leaving the eye. This offers an enhanced ability to assess the chance of developing glaucoma or monitoring it’s progress. Please view our video on OCT to help understand this more.