Chronic open-angle glaucoma

How the eye works

You can learn more about how the eye works and the structure of the eye in the separate leaflet called Anatomy of the eye. Glaucoma is mainly to do with the fluid in the eye, called aqueous humour, not being able to drain away properly.

What are glaucoma and chronic open-angle glaucoma?

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, which is a much less common condition, it is narrowed or blocked off.

Acute angle-closure glaucoma is a medical emergency. It usually leads to severe eye pain and blurred vision. You will often feel generally very unwell, with nausea (feeling sick) or vomiting. If you get any of these symptoms, you must seek emergency medical help. See the separate leaflet called Acute Angle-closure Glaucoma for more details.

The rest of this leaflet deals only with chronic open-angle glaucoma, referred to from this point as glaucoma.

What happens in glaucoma?

Normally fluid (also called 'aqueous humour') in the eye drains away through a mesh as more fluid is made: that way, the pressure in the eye stays the same. In glaucoma, the mesh (which is technically called the 'trabecular meshwork') gets slightly blocked and the fluid can't drain away properly. This results in a build-up of pressure. The reason why the trabecular meshwork becomes blocked isn't always apparent. Often it is simply age-related. 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 an 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.

If you have ocular hypertension without glaucoma you still may benefit from glaucoma treatment. Depending on how high the pressure inside your eye is, you may be offered laser surgery - you can find more details about this below.

If tests show you are not currently at risk of developing eyesight problems in your lifetime, your specialist will not recommend treatment. However, it is very important that you continue to have regular eye checks, to make sure your condition has not progressed.

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 fairly common and usually affects elderly people. Family doctors are familiar with it and often prescribe the eye drops that are used to treat glaucoma. Glaucoma is rare at the age of 40, only affecting about 1 in 300 people. By the age of 70, about 1 in 33 people in the UK have glaucoma. Glaucoma 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 (if a parent, brother or sister has glaucoma, you are 2-4 times more likely than other people to develop it yourself).

• 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 around 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. Getting tested is simple and painless: every high-street optician should be able to check the pressure in each eye (what is called the 'intraocular pressure', or IOP) with a piece of equipment.

Who should be tested for glaucoma?

If you have the risk factors of glaucoma, as shown above, then you should have a glaucoma test at the age of 40. It is very unlikely you will develop glaucoma before that age.

Your local optician should be able to do this for you. If your eye pressure is normal, consider having a check every one to three years until the age of 54; then every one or two years until the age of 64; and every six months to a year after that.

Certain people are entitled to free eye tests. For example, in the UK people aged over 40 with a first-degree relative (parent, brother or sister) with glaucoma should be tested without charge once a year. If you have been found to have glaucoma, you should tell your close family members so that they can be tested too.

If you do not have any risk factors for glaucoma, there are no firm guidelines on whether you should get tested. If you are worried about glaucoma, your local optician can do a quick and simple test to check your eye pressure although you will have to pay for that check.

What does an eye test for glaucoma involve?

The test for the pressure in your eye is quick and easy. It sometimes involves the optician blowing a puff of air at your eye with a special piece of equipment. This is called puff tonometry. The other, slightly more accurate, way is by gently touching your eye with a pressure-measurer called a Goldmann's tonometer. Either way, it doesn't hurt and only takes a few seconds.

Most high-street opticians are also able to measure your field of vision. This is essentially how much of the world you can see whilst you are looking directly forwards. Glaucoma affects the outside (periphery) of your field of vision first.

To look at the back of your eye, at the optic disc and retina, is slightly more in depth. Some opticians will use a special camera to take a picture of that area. It takes about 30 seconds; hospital eye specialists, called ophthalmologists, will use a special machine called a 'slit lamp' and a little lens that they hold in their hand. Most are adept at examining what the back of your eye looks like in a few minutes.

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. This is quite specialised and is done by hospital eye doctors.

What is the treatment for glaucoma?

The main aim of treatment for glaucoma 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 target eye pressure 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.

Laser treatments

If you have ocular hypertension or glaucoma, you will usually be offered a form of laser surgery called 360° selective laser trabeculoplasty (SLT).

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.

Having this surgery can delay the need for eye drops. In some but not all cases, it can mean you will not need to use regular eye drops in the future. Sometimes a second laser procedure will be needed later if your pressure starts to increase again. Complications from this surgery are uncommon, but your surgeon will explain these to you before you consent to treatment.

Eye drops

A variety of eye drops can lower eye pressure. They may aim to:

• Increase the drainage of aqueous humour (for example, prostaglandin analogue drops).

• Reduce the amount of aqueous humour that you make (for example, beta-blockers).

You may be offered eye drops:

• Instead of laser surgery if you don't want surgery or if you aren't suitable for surgery.

• While you are waiting for surgery.

• In addition to surgery if you have severe glaucoma.

• If the surgery doesn't reduce the pressure inside your eyes enough.

The treatment you will usually be offered first is a type of prostaglandin analogue drops. Your doctor will show you how to put the drops in properly, and watch you putting them in, the first time you use them.

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

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.

Surgery

If other glaucoma 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.

A type of surgery called peripheral iridotomy (usually used to treat acute glaucoma) has been trialled in people with open-angle glaucoma. The aim of the trial was to see whether this type of procedure was effective in patients with chronic glaucoma at preventing acute glaucoma.

The study found that this surgery did cut the risk of acute glaucoma. However, fewer than 1 in 120 people in the untreated group developed acute glaucoma in the six years they were followed for. This means more than 250 people would need to be treated with surgery to stop one person developing acute glaucoma. For this reason, the experts carrying out the study say it should not be offered as routine treatment.

What is the outlook?

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. In the UK 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.

Dr Mary Lowth is an author or the original author of this leaflet.