Types of glaucoma

The most common type of glaucoma is chronic open angle glaucoma or primary open angle glaucoma, also known as POAG for short. This is the chronic glaucoma that develops slowly over several years as a result of the pressure in the eyeball becoming elevated with age. The underlying cause is the gradual blockage of tiny drainage channels that allows fluid to pass out of the eye to control eye pressure.

Three other distinct but less common types of glaucoma are also recognised:

• Secondary glaucoma: this develops because of a specific cause, for example another eye disease, such as inflammatory uveitis, or an eye injury.
• Primary closed angle glaucoma: this can also be called acute glaucoma or closed angle glaucoma. This develops more rapidly than open angle glaucoma because the drainage of fluid from the eye suddenly stops completely, causing eye pressure to rise dramatically.
• Congenital glaucoma: glaucoma diagnosed in a newborn.

Chronic open angle glaucoma

The cause of chronic open angle glaucoma is raised eye pressure. At first, the eye pressure may be only slightly above normal, but as it rises, the increased pressure starts to damage the optic nerve. This eventually causes loss of peripheral vision, leading ultimately to tunnel vision and blindness if it is not treated.

Chronic open angle glaucoma is essentially a preventable disease but because raised eye pressure can develop over a long period of time and cause no symptoms, it is not always detected early enough. Older people expect their sight to deteriorate slightly and it is also possible for one eye to be affected more seriously than the other.

The sight in the good eye makes up for the reduced sight in the bad eye, and even substantial loss of peripheral vision can go unnoticed.

Chronic glaucoma and eye tests

The pressure in the eyeball is usually between 16 and 21 mmHg. Pressure can increase without any symptoms to over 40 mmHg. At that stage, it is common to develop severe eye pain that is worse on waking up and can be unbearable.

Having regular check-ups with an optician usually involves having a quick and painless test called tonometry to measure your eye pressure. If you have been asked to sit in front of a piece of equipment that blows a sharp blast of air into your eye, then you have had your eye pressure readings taken.

If you are over 35, you should ideally have an eye test and have your eye pressure measured every 5 years. Once you reach 50, you should aim to have the test every 2 years. If you have diabetes, a family history of glaucoma, you are very short-sighted, or you are of Afro-Caribbean or African descent, you may need to think about a test every year whatever your age.

It is worth noting that the tonometry equipment that uses a quick blast of air does tend to over-estimate eye pressure. If you have no risk factors and are below 50, a marginally high reading may not indicate that you are at risk of glaucoma. It may be worth seeing an ophthalmologist for further investigations to make sure.

This will involve having your eyes examined with a slit lamp and biomicroscope to check for damage to your optic nerve and a test to assess your peripheral vision. A photograph may be taken of your optic disc so that any future changes can be assessed.

Preventing chronic glaucoma

If your eye pressure is elevated, and this is confirmed by an ophthalmologist, you will then be given treatment to reduce your eye pressure and prevent damage to the optic nerve. This will be done even if the optic disc looks normal. If it has already started to take on the ‘cupped’ appearance that is typical of damage due to glaucoma, your treatment may be more aggressive.