Aqueous humour is made by cells that line the ciliary body. This fluid fills the front of the eye and gives a little outward pressure to maintain the shape of the eye. The fluid contains oxygen and sugars to nourish parts of the eye. The fluid drains from the eye into the bloodstream through the sieve-like trabecular meshwork. This is near the base of the iris. So, there is constant production and drainage of aqueous humour fluid.
What happens in primary open angle glaucoma?
In primary open angle glaucoma (just called 'glaucoma' from now on) there is a partial blockage within the trabecular meshwork. This restricts the drainage of aqueous humour. The reason why the trabecular meshwork becomes blocked and does not drain well is not fully understood. The pressure of the aqueous humour builds up if the drainage is faulty. But, this also increases the pressure on the back of the eye.
The increased pressure in the eye can damage the optic nerve (the main nerve of sight) and the nerve fibres running towards it from the retina. The retina contains the 'seeing' cells at the back of the eye. The damaged parts of the nerve and retina lead to permanent patches of vision loss. In some cases this can eventually lead to total blindness.
What's the difference between increased eye pressure & glaucoma?
Glaucoma means that part of the optic nerve is damaged, usually caused by increased eye pressure. But, about 1 in 5 people with glaucoma have eye pressures in the normal range. This is called 'normal pressure glaucoma'. In this condition the optic nerve is damaged by relatively low eye pressures. Other factors such as a poor blood supply may make the optic nerve sensitive even to modest pressure. In contrast, some people have an increased eye pressure with no ill effect to the optic nerve.
However, as a rule, if your eye pressure is high you have a much increased risk of developing glaucoma and visual loss.
Who gets primary open angle glaucoma?
In the UK, about 1 in 50 people over 40 have glaucoma. It is unusual in people under the age of 35. It becomes more common with increasing age. Glaucoma can affect anyone, but it is more common if you:
- have a family history of glaucoma
- have very short sight
- have diabetes
- are from African or Afro-Caribbean origin.
What are the symptoms of primary open angle glaucoma?
At first there are usually no symptoms. There is no pain or redness in the eye. Most people with glaucoma do not notice problems until quite a bit of visual loss has occurred. This is because the first part of the vision to 'go' is the outer (peripheral) field of vision. Central vision, used to focus on an object such as when we read, is spared until relatively late in the disease. Also, although glaucoma usually affects both eyes, it may not affect them equally. The better eye may 'fill in' for a while if the other eye starts to lose patches of visual field.
Some elderly people with glaucoma put their gradual failing vision down to 'just getting old'. They might not have had their eyes checked for many years and may needlessly lose their sight. Untreated glaucoma is one of the world's leading causes of blindness.
But, blindness can be prevented if glaucoma is diagnosed and treated early enough.
Who should be tested for glaucoma?
Everyone aged over 35-40 should have an eye check by an optometrist at least every five years. A check every 2-3 years is advised if you are over 50. Eye checks are particularly important if you are in any of the 'at risk' groups listed above. The eye check will detect early signs of glaucoma before any significant vision loss occurs. The eye test normally includes:
- measuring the eye pressure
- a look at the back of the eye with a special torch
- checking the field of vision
Certain people are entitiled to free eye tests. For example, people aged over 40 with a first-degree relative (mother, father, brother, or sister) with glaucoma.
What is the treatment for primary open angle glaucoma?
The aim of treatment is to lower the eye pressure. If the eye pressure is lowered, further damage to the optic nerve is likely to be prevented or delayed. The eye pressure to 'aim for' varies from case to case. It partly depends on how high the original pressure is. Your eye specialist will advise. Eye pressure can be lowered in various ways.
Eye drops
A variety of eye drops can lower eye pressure. They work either to:
- reduce the amount of aqueous humour that you make, OR
- increase the drainage of aqueous humour.
Your eye specialist will advise. Some drops work better in some people than in others. Some drops are not suitable if you have asthma. Also, the possible side-effects vary between the different types of drops. So, if the first does not work so well, or does not suit, another may work fine. In some cases, two different types of drops are needed to keep the eye pressure low.
It is vital to use your drops exactly as instructed. If you are unsure that you are using your drops correctly, ask for advice from your doctor or practice nurse. An eye specialist will keep a regular check on your eye pressures, optic nerves, and field of vision.
Tablets
Tablets work by reducing the amount of aqueous humour that you make. However, side-effects can be troublesome and so tablets are not commonly used these days.
Surgery
An operation called trabeculectomy is an option. This involves creating a channel from just inside front of the eye to just under the conjunctiva. So, the aqueous humour can bypass the blocked trabecular meshwork. In effect, it is like forming a small 'safety-valve' for the aqueous humour. Surgery may be advised if a trial of eye drops has failed to achieve target eye pressures, especially in younger people, or if you have very high eye pressures.
Like with all operations, there is a small risk of complications. Also, the operation may have to be repeated in some cases. This is usually because some scar tissue forms at the site of the channel and prevents it working to drain the aqueous humour.
Laser treatments
A laser can 'burn' the trabecular meshwork which improves the drainage of the aqueous humour. Another technique is to destroy parts of the ciliary body which reduces the amount of aqueous humour that is made. However, the reduction in eye pressure after laser treatments often only lasts a short time. So, laser treatments are not commonly done.
Driving and glaucoma
If you are a driver and have glaucoma in both eyes, the law says that you must inform the Driver and Vehicle Licensing Authority (DVLA). You will need to have a special eye test to check on how severely your vision is affected. However, in most cases, vision is not affected too severely and after assessment most people will still be allowed to drive.
Further information and help
International Glaucoma Association
Woodcote House, 15 Highpoint Business Village, Henwood, Ashford, Kent TN24 8DH
Tel: 0870 609 1870
Web: www.glaucoma-association.com
References
© EMIS and PiP 2007 Updated: 4 May 2007
