Glaucoma
Glaucoma is a condition that WILL (not may) cause blindness if not treated.
Treatment for glaucoma will never repair any damage already caused but hopefully will limit any further damage happening. For this reason early diagnosis and treatment is very beneficial.
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:
(1) 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.
(2) 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:
1. 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)
2. The back part of the brain functions as the developing house,
creating images from the information sent from the eyes.
3. 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.
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) 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 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?
Intra Ocular Pressure
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. The instrument used is called a tonometer. 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.
Visual fields
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 vessels 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 (disc 1).
With glaucoma, as the blood supply drops, the nerves change colour slightly and a greater ‘cup’ forms (disc 2).
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 (disc 3).
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 (plug hole) 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.
Heidelberg Retina Tomograph II
There is now an instrument that takes the measurement and recording of the optic disc appearance to a much higher level, the Heidelberg Retina Tomograph II (HRTII). This machine scans the optic disc area with a perfectly safe laser (similar to the one used with a CD player) with incredible accuracy. This instrument does not touch the eye and is totally painless, it involves shining a light into the eye whilst the patient looks at a small green target. It takes readings in a similar way to a CAT scan and measures where there are and are not nerves in a layer formation with 16 layers every millimetre. From this the software can build up an incredibly accurate 3 dimensional plot/image of the appearance of nerve. At the next measurement, the new values can be compared exactly with the previous results and any deterioration can be exactly quantified. Although with age there will be some slight loss of nerves, tables have been produced to show what is classed as predicted at a certain age and what is possibly abnormal.
As the number of nerves lessens the effectiveness of the visual system deteriorates. The first sign of this that can be recorded otherwise is when there is some loss of the field of vision but research has shown that there can be 40% of the nerves lost before there is a recordable field loss. If this instrument can show absolutely there is a harmful loss of nerve health (not just a difference in interpretations of observations) then treatment may be thought advisable. Previously the only way to achieve this recordable change was by field loss (when there may already have been a large loss in nerve health).
We have one of the Heidelberg instruments and a Retinal Camera
and can offer a range of supplementary services to our patients at fixed fees. We feel these are of particular benefit to our measurement and assessment of glaucomatous aspects of the eye. The people to whom these services are aimed at are people in the higher risk categories as mentioned earlier or those who have a borderline intra ocular pressure or disc appearance.
1. Retinal photography, which gives an absolute record of the optic disc
and the rest of the central retina in a two dimensional form (we suggest this optional extra for all our patients over the age of thirty years at routine eye examinations) at a cost of £10.
2. Heidelberg Retina Tomography scan of the optic disc
to give a three-dimensional record of the nerve fibre layer at a cost of £50. Patients who are having a normal eye examination at the same time will have the retinal photography included at this price.
3. Visual field screening is normally done as part of our regular examination and included in the normal examination fee. More in depth field tests are available and these may be required if the screening tests show an abnormality, or if more detailed reports are required for referral or for monitoring of conditions under the supervision of a consultant ophthalmologist. For most of the fuller tests on the Humphrey instrument there is a fee of £40.
I do not consider the HRT II instrument a diagnostic tool in itself but a very useful source of information, which enables a better decision to be made about the changes that may or may not be taking place in an eye that is either at risk of, or is under treatment for glaucoma. The benefits of the system are therefore only gained when a series of results are compared, a one off reading is not of great use and I feel sure that the correct way to use the system is as an ongoing series of comparative measurements. At present our local hospitals do not have this technology readily available and if the consultant treating / monitoring a glaucomatous condition would like to have a follow on set of readings we can supply them at the above fees to yourself if you feel happy paying for this element, we are presently carrying this out for some private patients.
In closed angle (acute) glaucoma treatment normally involves creating a way of removing the blockage of the drainage channel. Using a laser to create a small hole/s in the iris to allow the aqueous fluid through often does this. When a ‘free’ path is created often no further treatment is needed but sometimes drops are required to keep the pressure lower.
In chronic (simple) glaucoma treatment is given to lower the pressure in the eye. To achieve this eye drops are the most common method. These drops work on a variety of ways but generally aim to lessen the production of aqueous fluid in the eye and/or increase the outflow of aqueous fluid from the eye. If there is not an adequate response with drops an operation called a trabeculectomy may be considered. This operation creates a new drainage channel taking the aqueous fluid from inside the eye to the outer layers where it drains away free from the previous obstruction.
In both forms of glaucoma unless surgery is used the medication (eye drops) used will need to be long term as they control rather than rectify the natural disposition of the glaucomatous eye to create too high an intra ocular pressure.