The damage to the main nerve in the eye (the optic nerve) in glaucoma is usually associated with excessive pressure within the eye. A certain level of pressure is needed for the eye to keep its shape and to work properly, but if the eye pressure gets too high, it squeezes the optic nerve and kills some of the fibres, which leads to sight loss.
There are some forms of glaucoma where the eye pressure is not raised, but the optic nerve still becomes damaged. This is known as normal (or low) tension glaucoma.
High eye pressure does not always cause glaucoma. A common condition is ocular hypertension, where the eye pressure is above the ‘normal’ level, but there is no detectable damage to the field of vision or optic nerve. This condition may be monitored without treatment or may be treated in the same way as glaucoma.