Chronic open-angle glaucoma is a condition which causes damage to the optic nerve at the back of your eye and which can affect your vision. It is usually caused by an increase in pressure within your eye. If it is not treated, glaucoma can lead to visual loss and even to total loss of vision (severe sight impairment.) Treatment can slow down glaucoma and help to prevent this. All adults aged over 35-40 should have a regular eye check which includes measurement of their eye pressure.
The eye and aqueous humour
When you look at an object, light from the object passes first through the cornea of your eye, then the lens and then hits the retina at the back of your eye. The cornea and the lens both help to focus the light on to your retina. Nerve messages pass from the 'seeing cells' (rods and cones) in your retina, down nerve fibres in your optic nerve to your brain. The messages are interpreted by your brain, which enables you to see.
The black area in the middle of the eye (the pupil) helps to regulate the amount of light that gets into the eye. It does this by becoming smaller (constricted) when the light is brighter and larger (dilated) in dark conditions. The muscles of your iris (which gives colour to your eye) control the size of your pupil.
Your eye also needs to keep its shape so that it can work properly and so that light rays are focused accurately on to the retina. So, most of your eye is filled with a clear jelly-like substance called the vitreous humour (humour meaning fluid). The front of your eye is filled with a clear watery fluid called aqueous humour.
The part of your eye behind the lens is called the posterior chamber and is filled with vitreous humour. The part of the eye in front of the lens is called the anterior chamber and is filled with aqueous humour.
Aqueous humour is made continuously by the cells of the ciliary body (which is at the back of the iris at its base). It passes through the pupil from the posterior chamber to the anterior chamber. From there it drains away through a sieve-like area called the trabecular meshwork (which is in front ot the iris, also near its base.) So, there is constant production and drainage of aqueous humour fluid. This keeps the fluid levels balanced.
What are glaucoma and chronic open angle glaucoma (COAG)?
There are different types of glaucoma. The type usually referred to just as 'glaucoma' is the most common type. Technically it is called chronic open-angle glaucoma (COAG), chronic glaucoma or primary open-angle glaucoma.
COAG develops slowly and painlessly, so that any damage to the optic nerve and loss of sight are gradual and you may not notice anything has changed until the condition is very advanced. The term 'open-angle' refers to the angle formed in the eye between the outer edge of the iris and sclera. In COAG this is normal, whereas in acute angle-closure glaucoma (AACG), which is a much less common condition, it is narrowed or blocked off. See separate leaflet called Acute Angle-closure Glaucoma for more details.
The rest of this leaflet deals only with COAG, referred to from this point as glaucoma.
What happens in glaucoma?
In glaucoma there is a partial blockage within the trabecular meshwork through which the aqueous humour drains. This restricts the drainage and results in a build-up of pressure. The reason why the trabecular meshwork becomes blocked and does not drain well is not always fully understood. The aqueous humour builds up if the drainage is faulty and this increases the pressure within your eye.
The increased pressure in the eye can damage nerve fibres. These run from the retina and damage can occur at the point where they meet (converge) to become the optic nerve (known as the optic nerve head or optic disc). The optic nerve is the main nerve of sight. These damaged fibres result in permanent patches of visual loss. In some cases this can eventually lead to total loss of vision (severe sight impairment).
Glaucoma can affect both eyes. However, it can often progress more quickly in one eye than in the other.
What's the difference between increased eye pressure and glaucoma?
Most people with glaucoma have increased pressure in the eye (intraocular pressure) and signs of damage to the optic nerve. However, about 1 in 5 people with glaucoma have normal eye pressures. This is called normal pressure glaucoma. In this condition the optic nerve is damaged by relatively low eye pressures. It is possible that their optic nerves are sensitive even to modest pressure.
In contrast, some people have an increased eye pressure with no ill effect to the optic nerve and no visual loss. Raised eye pressure without glaucoma is called ocular hypertension. As a rule, if your eye pressure is high, you have a much increased risk of developing glaucoma and visual loss. If you are found to have high intraocular pressure, you should discuss with your doctor your individual risk of developing glaucoma.
The results from a large published study showed that if you have ocular hypertension without glaucoma you still may benefit from glaucoma treatment. The higher your individual risk of developing glaucoma, the more likely you are to benefit from having treatment to lower your eye pressure.
Who develops glaucoma?
Glaucoma is common. In the UK about 1 in 50 people aged over 40 have glaucoma. This rises to about 1 in 10 people over the age of 75. It is unusual in people under the age of 35. Glaucoma can affect anyone but it is more common if you:
- Have a family history of glaucoma.
- Have very short sight.
- Have diabetes.
- Are of African or Afro-Caribbean origin.
- Are older.
What are the symptoms of glaucoma?
There are usually no symptoms to begin with. 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 and when we look at the world most of us do so with two eyes. Areas that one eye does not see, the other eye will cover for; so we continue to see a complete picture until both eyes are badly affected. The brain is also very good at making up for, and not noticing, missing bits in the vision, particularly if they are round the edges. Although glaucoma usually affects both eyes, it often does not affect them equally. Central vision, used to focus on an object such as when we read, is not affected until relatively late in the disease. By then the nerve will be very damaged.
Some elderly people with glaucoma put their gradually 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 total loss of vision (severe sight impairment.) This can be prevented if glaucoma is diagnosed and treated early enough.
Because there are usually no symptoms at first screening for glaucoma is very important.
Who should be tested for glaucoma?
Everyone aged over 35 to 40 should have an eye check by an optician at least every five years. A check every two to three years is advised if you are aged over 50. Eye checks are particularly important if you are in any of the at-risk groups listed above, or if you are found to have raised pressure in your eyes. The eye check will detect early signs of glaucoma before any significant vision loss occurs. Most people with glaucoma have it detected at a routine eye check.
Certain people are entitled to free eye tests. For example, people aged over 40 with a first-degree relative (mother, father, brother or sister) with glaucoma should be tested without charge. If you have been found to have glaucoma, you should tell your close family members so that they can be tested too.
What does an eye test for glaucoma involve?
The eye test usually involves examining your eyes in detail using a special light and magnifier called a slit lamp. In particular, the back of your eye where the optic nerve leaves your eye (known as the optic disc) will be examined. There are specific changes that can be seen in this area in someone with glaucoma. The optic disc takes on a typical appearance and is said to be cupped. A photograph may be taken of your optic disc. This photograph can be used to refer back to in the future when your eyes are checked.
The pressure in your eyes (intraocular pressure) will also be measured.This is commonly done using a 'puff of air' test (tonometry).
The thickness of your cornea may also be measured. This is because the thickness of your cornea can affect your intraocular pressure reading.
A special lens may also be used to examine the drainage area (or trabecular meshwork area) of your eye. This examination is called gonioscopy.
Your field of vision will also be tested. This is essentially how much of the world you can see whilst you are looking directly forward. Glaucoma affects the outside (periphery) of your field of vision first.
What is the treatment for glaucoma?
The aim of treatment is to lower your eye pressure. If your eye pressure is lowered, further damage to the optic nerve is likely to be prevented or delayed. Sadly treatment cannot restore any sight that has already been lost.
The eye pressure to aim for varies from case to case. It partly depends on how high your original pressure is. Your eye specialist will advise. Eye pressure can be lowered in various ways.
A variety of eye drops can lower eye pressure. They may aim to:
- Reduce the amount of aqueous humour that you make (for example, beta-blockers)
- Increase the drainage of aqueous humour (for example, prostaglandin analogue drops).
Some drops work better in some people than in others. Some drops are not suitable for some people. For example, beta-blocker drops may not be suitable if you have asthma or heart disease. 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. Preservative-free eye drops are available if you find you are allergic to preservatives added to the drops.
It is vital to use your drops exactly as instructed. If you are unsure whether 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. How often you need to be followed up will depend on your particular situation.
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 now.
If eye drops are not helping to lower your eye pressure enough, laser treatment may be suggested. A laser can make tiny holes in the trabecular meshwork, which improves the drainage of the aqueous humour. This treatment only takes a few minutes and is done under local anaesthetic. A special contact lens is put on your eye to help the specialist focus the laser beam. You may feel a pricking sensation and notice some flashing lights but the procedure is usually well tolerated. Another technique is to use a laser to destroy parts of the ciliary body. This reduces the amount of aqueous humour that is made.
Eye drops are sometimes still needed after laser surgery.
If other treatments are not effective, an operation called trabeculectomy is an option. This involves creating a channel from just inside the front of your eye to just under your conjunctiva. By this route 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.
As 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.
Rarely, a tiny drainage tube may be inserted into your eye to drain the aqueous humour. This is usually only carried out if trabeculectomy has been tried a number of times and has been unsuccessful.
What is the outlook (prognosis)?
Most people treated for glaucoma will not go on to develop total loss of vision (severe sight impairment). However, in order to preserve your sight, it is very important that you follow the treatment plan outlined by your doctor. You should make sure that you follow the instructions and use your eye drops regularly.
Driving and glaucoma
Many people will be allowed to drive after glaucoma is diagnosed. Even if vision is reduced in one eye, you may still be allowed to drive if your vision is good enough in the other eye. However, you will need advice from your eye specialist. If you are a driver and have glaucoma causing loss of vision in both eyes, the law says that you must inform the Driver and Vehicle Licensing Agency (DVLA). The DVLA will usually contact your eye specialist and ask them for a report about your eye problems. The DVLA may also arrange an examination of your eyesight with an optician.
Further help & information
Dr Tim Kenny
Dr Mary Lowth
Dr Colin Tidy