Signs of Glaucoma

The signs of glaucoma are quite obvious to the Eyecare Practitioner when there is moderate or late stage disease. However, by this point the patient has already lost a lot of vision. We want to detect it in the very early stages so that we can prevent further damage and help you keep your vision.

So what signs of glaucoma are we looking for? There are a number of tests that, taken together, show a risk profile to detect those at greatest risk of developing full blown glaucoma.

1. Signs of glaucoma - Intraocular Pressure (IOP)

Approximately 1/3 of glaucomas present with abnormally raised IOP (above 23mmHg) or significant asymmetry in pressure levels between the two eyes.

IOP can be measured in a number of ways. Traditionally, it has been measured by placing a probe on the front surface of the eye and adjusting a dial on the tonometer to get the pressure reading. This method is called Contact Tonometry or Applanation Tonometry. It is still considered the gold standard and most accurate method.

Contact Tonometry requires topical anaesthetic so you don't feel the probe on your eye. A dye (sodium fluorescein) is also used to colour the tear film to be able to see the target in the instrument.

There are hand-held versions and desk-mounted versions of this instrument.

Non-Contact Tonometry

The most commonly used versions of this technique utilise a puff of air to indent the eye slightly, rather than using a probe. It doesn't require anaesthetic and can be a faster technique.

When first introduced, these machines were not as accurate as contact tonometry but have improved vastly over the years. They are now widely used for screening to search for the people with raised pressure. Contact tonometry is then used for greater accuracy on those at risk.

Both methods have to assume an average thickness of corneal tissue. Those with raised pressure will then have their corneal thickness measured by ultrasound. The measured IOP can then be adjusted to give their "true IOP" if their cornea is not "average" thickness.

Why does this make a difference? A thicker cornea is stiffer and will resist indentation - giving a false high reading.

2. Signs of glaucoma - Appearance of Optic Nerve

The physical appearance of the Optic Nerve can show signs of glaucoma. In glaucoma, neural tissue gradually dies off resulting in changes to the appearance of the top of the optic nerve. (Optic Nerve Head, ONH)

Your Optometrist checks and records the appearance of your optic nerves at each comprehensive eye examination.

This way, progressive changes can be seen through the records if you return to the same clinic each time. This is even easier if your Optometrist has a digital Fundus Camera to take photos of the back of your eyes.

3. Signs of glaucoma - Visual Field Analysis

Glaucoma affects the nerves responsible for vision in the mid-periphery first. This is the area about 20-30 degrees out from the centre.

A visual field test is a subjective test, meaning it relies on your responses. This can make it a less reliable test for some people.

For the test: you stare at a central fixation point and press a buzzer when you see a small light flash in your side vision. This directly measures the sensitivity of your side vision and the software compares your result to what is normal for your age group.

Your Optometrist will analyse the results looking for patterns typical of glaucoma.

4. Signs of glaucoma - GDx

This is the newest form of glaucoma assessment. It is an objective test that doesn't rely on your response. This makes it more reliable, more repeatable and it is easier to do.

It uses a type of scanning laser to measure the thickness of your retinal nerve fibre layer. Rather than measuring the resulting effect on your vision, this machine measures changes in your tissue that appears well before you can see a change in your vision. It has proved very useful in detecting glaucoma earlier than other methods. It is also very accurate in detecting small changes in tissue during treatment, to help guide the doctors in your management.

Glaucoma is a progressive disease of the optic nerve. For a diagnosis of glaucoma there has to be a worsening over time. If your measurements are stable or improving, then you do not have glaucoma.

Some people will be monitored closely if the doctor thinks you are highly likely to have glaucoma; watching for progression to prove it. However, if your risk profile puts you at very high risk, you and your doctor may decide to treat it to prevent loss of vision, without waiting for damage first.