Glaucoma

Glaucoma is the name given to a group of eye disorders that are characterised by a combination of:

• Changes to the head of the optical nerve
• Visual field deficit

In a minority of cases, glaucoma can lead to visual impairment. Some of the main risk factors include age and elevated eye pressure. Glaucoma is often chronic and the damage to the optical nerve is irreversible. However, this damage can be slowed down. With mild (to moderate) forms of glaucoma, you do not notice anything at all. This is why early detection is quite important. Regular check-ups and timely treatment prevent further damage to the optical nerve, thereby preventing additional visual symptoms. Glaucoma usually presents in both eyes simultaneously.

Glaucoma is often (though not always) caused by excessive eye pressure.

• Eye pressure between 10 and 21 mmHg (mercury pressure) is usually considered normal.
• If it is above 21 mmHg, the eye pressure is too high.

Fluid (aqueous humour) is produced in the eye to nourish the cornea and lens, and this is what allows the eye to maintain a pressurised state. The aqueous humour (not to be confused with the lacrimal fluid found on the outside of the eye) leaves the eye via a duct (the trabecular system), which is found on the inner side of the eye in the corner formed between the cornea (the clear window on the anterior side of the eye) and the iris (the coloured portion of the eye).

If this duct works less efficiently, it becomes more difficult to remove the aqueous humour and the interior eye pressure starts to increase.

Eye pressure is best measured using applanation. To do this, the ophthalmologist first numbs the eye and uses a device pressed against the eye to measure the eye pressure. This is a painless procedure.

Light that reaches the eye is transformed into electrical signals, which then travel over approximately one million special nerve fibres. All these nerve fibres together form what we call the ‘optical nerve’. The site where the optical nerve exits the eye is a soft spot. If the eye pressure is too high, this 'soft spot' becomes deformed. The nerve fibres that make up the optical nerve then become pinched and damaged.

The pinching eventually causes the nerve fibres to die off. If the optical nerve dies, the ophthalmologist can see changes using a light and a lens that looks inside the eye. These changes can also be seen with a photo or scan (OCT) of the eye. Subsequent scans can detect detailed changes of up to a thousandth of a millimetre. Detection of such changes — even if they are seemingly very small — can be a sign of future visual field deficit.

When the optical nerve is pinched and dies, as described above, the connection between the eye and brain is damaged slowly, but persistently, over time. This damage leads to initially small, and then subsequently larger, pieces disappearing from the visual field. The visual field is the complete picture that you see when you look at a single point.

Eventual damage of the visual field can be determined and controlled with the aid of a visual field test. This test is painless and is performed by one of the specially trained nurses. A person with glaucoma will regularly undergo this visual field test. The test is performed more frequently in the beginning in order to detect visual field deficits quickly. Afterwards, the number of visual field tests depends on your individual situation.

The patient can rarely perceive the start of a visual field deficit because, among other things, the brain itself fills in the missing portions of the visual field. The patient is only able to notice disturbances in his or her regular vision once the visual field deficits are more extensive. Glaucoma usually only affects the central visual acuity in its late stage and can finally lead to blurry vision or even visual impairment.

• Open-angle glaucoma: With open-angle glaucoma, the duct remains accessible for the aqueous humour but is internally blocked up, which leads to increased eye pressure. This is the case with the majority of glaucoma patients.
• Angle-closure, or closed-angle, glaucoma: With closed-angle glaucoma, the duct is healthy but not accessible because the iris closes it by letting in too much light. This type of glaucoma presents primarily in adults who are far-sighted (who use extra-strong prescription glasses).
• Normal pressure glaucoma: This develops when there is damage to the optical nerve, even when eye pressure is normal.

There are many other causes of elevated eye pressure, such as:

• Ocular trauma or eye surgery
• Medicine use
• Congenital defect

The only treatment that has proven effective in slowing the development of glaucoma is the decrease of eye pressure. It is important to consistently lower eye pressure. The degree to which the eye pressure must be lowered depends on your individual situation.

Lowering eye pressure can be done in three ways (or by using a combination of these three):

• With medication (usually eye drops, although tablets may sometimes be prescribed)
• Laser treatment
• Surgery

Latest publication date: 26/12/2023
Supervising author: Dr Vanwynsberghe David