To understand what causes glaucoma, it first helps to know how the eye works.
Glaucoma is a disease of the optic nerve. When some of the fibres that make up the optic nerve are damaged, the result is blind areas in your vision. If left untreated, more fibres become damaged, and your field of vision shrinks further, causing tunnel vision and eventually blindness.
Raised pressure inside the eye
In most cases (but not all), glaucoma is associated with raised pressure inside your eye, which damages the optic nerve.
The front part of your eye is filled with fluid. The eye stays within a normal pressure range by producing and draining this fluid at a constant rate. However, a problem with the drainage canal can cause an increase of pressure within the eye, which can damage the optic nerve.
There are two main types of glaucoma:
This is by far the most common form. It occurs gradually and is often difficult to spot, so regular check-ups are essential.
Acute closed-angle glaucoma
This can occur when there is a sudden blockage in the drainage canals in the eye. The pressure builds up rapidly and can cause sudden blurred vision, coloured halos around lights, headaches and severe pain. This is an emergency as the rapid increase in pressure can severely damage the optic nerve in just a few hours.