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

Initially, there is no measurable effect on vision. Progressive nerve damage causes a blank patch in the field of vision, usually off-centre and unnoticed. Without treatment this gradually enlarges, becomes more definite and may progress to blindness. The rate at which this can happen varies amongst patients.

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 nowadays be well controlled with lifelong good vision for the great majority of patients.