An introduction to normal tension glaucoma

The following article was originally written by Helen Doe for Insight magazine.

What is normal tension glaucoma?

For most people, glaucoma is associated with raised pressure in the eye. However, there is a type of glaucoma called normal tension glaucoma (NTG). This is when you have glaucoma even though your eye pressure isn’t very high. It may also be referred to as low tension glaucoma or low pressure glaucoma.

Rather than being a wholly different type of glaucoma, it is just a sub-group of the most common glaucoma in the UK, primary open angle glaucoma (POAG). The ‘normal’ range of eye pressure is between 10-21 mmHg (millimetres of mercury), and most people with glaucoma have a pressure above 21 mmHg. However, people with NTG have a pressure of 21 mmHg or below. Because the pressure is within the ‘normal’ range, it can be harder to pick up on during routine eye checks than other types of glaucoma.

How is normal tension glaucoma identified?

NTG is treated and monitored the same as POAG. Tests will include:

• a visual field test;
• a scan of the nerve fibre layers at the back of your eye;
• eye pressure measurements;
• measurement of the thickness of your cornea;
• a photo of your optic disc.

Sometimes, thinning in the layers of the retina and a haemorrhage or bleed around the optic nerve may be seen as an indication of NTG. If NTG is suspected, other tests may be done to rule out other health problems, which can affect the optic nerve. This may include a brain scan or 24-hour blood pressure monitoring.

Like POAG there are typically no symptoms until in the very advanced stages, but NTG can often affect the central vision earlier than other types of glaucoma.

What causes normal tension glaucoma?

We don’t quite know why in some people damage to the optic nerve occurs even though the eye pressure is not raised. However, there are risk factors which seem to make the development of NTG more common. You may be more at risk of NTG if you have one or more of the following things:

• a family history of glaucoma;
• low blood pressure leading to blood supply problems to the optic nerve at the back of your eye;
• defects in the mitochondria[1];
• weakened collagen[2], which can cause nerve fibres to become frail.

Research has shown that people who are affected by migraines, Raynaud’s disease or nocturnal low blood pressure are at increased risk of developing NTG.

How is normal tension glaucoma treated?

Treatments for NTG are the same as for other types of POAG, and include eye drops, laser treatment and surgery to lower eye pressure. However, NTG can need slightly more robust treatment than other types of POAG, because we need to get the pressures in your eye lower. This is to keep your eye pressure at a level to stop further damage happening to your optic nerve.

Eye drops and laser treatment are not always as effective in lowering the pressure with NTG, and often the target pressure in younger people needs to be lower still. Because NTG can be associated with nocturnal low blood pressure, beta blocker drops aren’t usually given to be taken in the evening, as these can lower your blood pressure further during the night. This may then affect the amount of blood supply to the back of your eye.

Is there anything I can do?

• If you have eye drops, always remember to put them in.
• Always attend your eye clinic appointments.
• Don’t smoke.
• Take regular daily exercise (walking is fine – you don’t have to join a gym!).
• Have a healthy diet, including green leafy vegetables such as broccoli, spinach or curly kale.
• Take vitamin B3 (always check with your eye specialist before taking this).

You can find out more about NTG by watching the recording of our digital glaucoma support group on the topic with Professor Gus Gazzard.

If you have any concerns about your normal tension glaucoma, don’t forget to contact our helpline by calling 01233 64 81 70 or emailing helpline@glaucoma.uk

[1] Mitochondria convert energy from our food into energy our cells can use.

[2] Collagen is a protein that holds your body together. It helps to give strength and structure.