Understanding Glaucoma

What is glaucoma?

Glaucoma is the name for a group of eye conditions in which the optic nerve is damaged at the point where it leaves the eye. The optic nerve carries information from the light sensitive layer in your eye, called the retina, to the brain.

Your eye needs a certain amount of pressure inside it to maintain its shape. Eye pressure is not related to blood pressure. In some people glaucoma damage is caused by raised pressure in the eye. In others the eye has normal pressure but there is a weakness in the optic nerve.

What controls pressure in the eye?

The watery fluid inside the eye, called aqueous, is produced just behind the coloured part of the eye, called the iris. This fluid passes forward through the central hole in the iris, called the pupil. It then passes to the front angle of the eye between the iris and the cornea, the clear window at the front of the eye. In the angle of the eye lie tiny drainage channels. The fluid passes through these channels, leaving the eye and joining the blood stream.

Normally all the fluid produced, drains out of the eye easily. If however, the drainage channels become partially blocked, fluid cannot drain freely from the eye and the pressure inside the eye rises. Occasionally there is an increase in production of fluid above that which the drainage channels can handle. The result is the same, raised pressure in the eye.

What problems are caused by increased eye pressure?

If the optic nerve comes under too much pressure, it may be damaged. The amount of damage depends on how high the pressure is, how long it has been high for, whether there is poor blood supply to the optic nerve and whether or not there are other weaknesses in the nerve.

Really high pressure will damage the optic nerve immediately. A lower level of pressure causes damage more slowly.

What are the different types of glaucoma?

There are four main types of glaucoma:

Chronic glaucoma

Chronic means slow. In chronic glaucoma, the angle of the eye is open (chronic open angle glaucoma) so the fluid can reach the drainage channels, but these are partially blocked. The eye pressure rises very slowly. There is no pain to show there is a problem, but the field of vision gradually becomes impaired. Click here for more information on Chronic Glaucoma.

Acute glaucoma

Acute means sudden. This is much less common than chronic glaucoma. The angle of the eye is narrow and suddenly blocks off. The pressure suddenly rises and the eye becomes painful. Click here for more information on Acute Glaucoma.

Secondary glaucoma

Raised eye pressure caused by any other eye condition is called secondary glaucoma.

Congenital glaucoma

Rarely, glaucoma can occur in babies. This is caused by a developmental malformation of the angle of the eye.

Ocular Hypertension

What is ocular hypertension?

This is when the pressure inside the eye is higher than what is considered “normal”. Unlike glaucoma however, there is no detectable damage to the optic nerve. Drops to reduce the pressure in the eye may be prescribed to reduce the likelihood of developing glaucoma in the future.

Will I be able to drive?

Most people with glaucoma will still be able to drive if the loss of visual field is not too advanced. The visual field test will help to determine if you are still able to drive safely.
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Fig 1. Cross section of right eye from above showing ‘open angle’ in chronic glaucoma.
Image - Chronic Glaucoma
Fig 2. Enlarged view of inset in Fig 1. showing how acute glaucoma is caused by closure
of a narrow angle.
Image - Acute Glaucoma


The information provided here is for general education only and should not be construed as individual medical advice. For advice relevant to your particular situation, please speak to Dr Hornsby.