The optic nerve of the eye resembles a television cable, the fibres of which are nerve cells that transmit a picture from the eye backwards, to the brain. Glaucoma is a condition in which continuing stress of the optic nerve fibres at the point where they leave the eye, causes progressive damage and loss of vision. The commonest stress factor and the only one that can be treated directly is elevated pressure inside the eye.

Early nerve damage may be detected before there is any measurable effect on vision. Progressive nerve damage causes a faded or blank patch in the field of vision, usually off-centre. Without treatment this gradually spreads and becomes more pronounced and it may progress to blindness. The rate at which this can happen varies amongst patients. Central vision and the ability to see detail are usually maintained until glaucoma reaches a late stage. Patients may have little or no awareness of peripheral vision being poor if they have gradually adapted to the changes.

Optic nerve damage is irreversible and so it is important to detect glaucoma at an early stage when treatment can prevent loss of vision. With better detection, close monitoring and modern treatments, glaucoma can usually be well controlled, with lifelong good vision for the great majority of patients.