The eye is a very complex structure. There are many parts of it which can be affected by the development of eye conditions. Glaucoma, for example, can alter the structures which make up the retina, causing changes to the optic nerve.

What is the optic disc?

The optic disc is the area where specific nerve fibres exit the retina to form the optic nerve. It is also where blood vessels enter the eye. The optic disc is usually round or oval in shape and is made up of two main parts: the outer rim and the cup.

The outer rim tends to be orange or pink in colour and contains the nerve fibres. The cup (in the centre) is a pit where there are no nerve fibres — this pit is where blood vessels enter the eye.

In people with glaucoma, elevated pressure in the eye can affect the optic disc, specifically the nerve fibres found within it.

Here, we take a closer look at optic disc cupping, its link to glaucoma and how it can help detect and diagnose the condition.

What is optic disc cupping?

Everyone has an optic disc cup — it is a completely normal part of the structure of your eye and is usually quite small in comparison to the rest of the optic disc.

Optic disc cupping refers to the cup appearing to become larger over time, often due to fibres in the optic nerve dying. As the structural support for the optic disc is no longer there, the cup seems larger.

How does glaucoma cause optic disc cupping?

In the case of glaucoma, increased pressure in the eye can cut off the blood flow to the optic nerve, which can eventually lead to the ‘death’ of optic nerve fibres in the outer rim of the optic disc.

If this continues over a period of time, more nerve fibres ‘die’ in the rim, leading to less and less structural support to the cup. As the amount of healthy nerve fibres in the outer rim decreases, the central cup begins to appear larger in comparison, which is what we refer to as optic disc cupping.

The initial size of the optic cup varies from person to person and it is perfectly possible for someone to have a naturally large optic cup without ever developing glaucoma.¹ It is only when the cup size increases (i.e. optic disc cupping) that there could be a cause for concern.

How can you tell if you have optic disc cupping?

The cup-to-disc ratio (or CDR) is the measurement most commonly used by optometrists to assess the extent of optic disc cupping. It compares the diameter of the optic cup with the total diameter of the optic disc. A typical cup-to-disc ratio would often be about 0.3.²

A CDR higher than this does not necessarily mean that you have optic disc cupping, however. Having a larger optic cup can occur due to hereditary factors and a large optic disc can happen with or without optic nerve damage. Usually, people with an overall larger optic disc will have a larger optic cup. So, even if your CDR is higher than 0.3-0.5 this does not necessarily mean that you have optic disc cupping.

Your cup-to-disc ratio progression, as well as the results from other tests such as optical coherence tomography (OCT), tonometry and visual fields, is what would be used by your ophthalmologist to help diagnose glaucoma.

How is optic disc cupping progression used to diagnose glaucoma?

While cupping alone is not enough to positively diagnose glaucoma, tracking its progression can be a useful tool for glaucoma diagnosis.³ Over time, measuring changes in the eye’s cup-to-disc ratio, as well as now using OCT to even more effectively monitor the optic disc and cup with 3D analysis, can be used to track and diagnose the degeneration of optic nerve fibres caused by glaucoma.

During your normal eye check, your optometrist may use what is called a slit lamp, along with a special magnifying lens, to have a look at the inside of your eye. A slit lamp allows the optometrist to get of view of how we naturally see depth, enabling them to see whether there are signs indicating that the loss of optical nerve fibres is creating the appearance of a larger optic cup. As the optic disc and cup have height and depth, using examination techniques that allow for a 3D analysis of the eye is very important in accurately detecting changes.

Another way that your optometrist can check for progression in optic disc cupping is by comparing the optic cup of each eye. If one eye has a significantly larger CDR, this could indicate some potential loss of nerve fibres, which could be a sign of glaucoma. Your optometrist would then measure other aspects such as your intraocular (eye) pressure and visual field, in order to gain an overall picture of the health of your eye.

The optometrist may use an OCT scanner in order to further investigate for signs of optic disc cupping and, if present, will refer you on to an ophthalmologist who would confirm a glaucoma diagnosis. You can read more about optical coherence tomography here.

If, over time, the CDR continues to increase, this is usually enough to inform a glaucoma diagnosis. The larger the cup appears and the greater the CDR, the more at risk patients are of developing chronic open-angle glaucoma.

If you have regular eye tests, your optometrist will be able to tell if there are changes to your CDR, and monitor any possible glaucoma progression or visual changes.

How can optic disc cupping affect your vision?

As optic disc cupping progresses, the loss of optic nerve fibres can begin to impact your vision — primarily affecting your peripheral vision.⁵ Since the loss of vision is very slow and gradual, the person affected may not notice it until significant peripheral vision has been lost.

A visual field test conducted by your optometrist or ophthalmologist will help to determine the extent and pattern of vision loss. As such, it’s important to have an eye test at least every two years, or more frequently if advised by your optometrist, to help mitigate some of these risks and prevent further damage to the optic nerve.

For more information on understanding how glaucoma is diagnosed, its causes and treatment options, head across to our glaucoma resource where you can learn more about optic disc cupping, as well as other diagnostic methods.