Glaucoma is the second most common cause of blindness in Australians aged 55 and older. Patients with glaucoma develop blind spots in their vision which, if left untreated, can spread throughout the entire visual field. An important way that ophthalmologists and optometrists diagnose and monitor glaucoma is by assessing optic disc cupping, but what is this and why is it important?
To understand optic disc cupping, we first have to understand how the body transmits information from the eye to the brain. Each eye is connected to the brain by an optic nerve, which on average is 3-4 mm wide. Inside this nerve there are around 1.2 million individual nerve cells, each of which is responsible for carrying visual information from a small and specific area of the visual field. If these nerve cells are damaged, for instance by glaucoma, then they can no longer carry information to the brain which results in blind spots.
When these nerve cells travel through the small hole in the back of the eye they have to be so densely packed that the end of the optic nerve inside the eye isn’t flat but slightly raised. This means when you look through the pupil at the head of the optic nerve, also called the optic disc, it forms a cup shape with raised edges and a central depression. A healthy optic nerve with all of its nerve cells is more densely packed so has thicker edges and a smaller central cup. Glaucoma is caused by high pressure in the eye damaging the optic nerve, which results in loss of individual nerve cells. This causes a subsequent increase in the size of the cup, also called cupping.
As a general rule, the cup should not make up more than three tenths or 30% of the total area of the optic nerve. However, every patient is unique and some people have naturally very large or very small cups. Assessment of optic disc cupping is therefore just one tool used by ophthalmologists and optometrists to diagnose and monitor glaucoma.