The term 'glaucoma' covers several different conditions. The most common is chronic (primary open angle) glaucoma - this form usually affects both eyes and develops slowly so that loss of sight is gradual. There is no pain, redness of the eye or dramatic change in vision.

With acute (angle closure) glaucoma, there is a sudden increase in the pressure within one eye. The eye becomes red and painful. Often there is mistiness of vision and episodes of seeing haloes around lights.

Sometimes, other diseases of the eye cause a rise in the pressure within the eye - this group of conditions is called secondary glaucoma.

Congenital glaucoma (buphthalmos) is a condition where glaucoma is present from birth. An increase in the pressure within the eye causes it to enlarge.

The contents of the eyeball are nourished by a fluid called aqueous humour, which is different to tears. It is secreted within the eye by a tissue called the ciliary body, and leaves the eye via another tissue, called the trabecular meshwork. This 'meshwork' is situated at the back of the eye.

An increase in the pressure within the eye usually happens because there is an obstruction within the trabecular meshwork. The obstruction means that the aqueous within the eye is not drawn out, although it continues to be produced. There is a build up of fluid in the eye, which increases the pressure.

This increased pressure pushes on the back of the eye, which damages the optic nerve. The optic nerve carries signals to the brain from the eye, and when this is damaged, it causes irreparable sight loss.

Glaucoma is one of the world's leading causes of blindess. In the UK, 1 in 50 people over 40 have this condition. Glaucoma is not curable, but blindness is preventable if the glaucoma is diagnosed and treated early enough.

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