All four types are characterised by increased pressure within the eyeball, and therefore all of them can cause progressive damage to the optic nerve. Increased pressure occurs when the fluid within the eye (called aqueous humor), which is produced continuously,does not drain properly. The pressure pushes on the retina at the back of the eye. This reduces the blood supply to the nerves of the retina causing them to die. As the optic nerve deteriorates, blind spots and vision changes develop. Peripheral vision (side vision) is affected first followed by front or central vision. Without treatment, glaucoma can eventually cause blindness.

Acute glaucoma may occur with ageing as the lens of the eye gradually becomes larger, resulting in narrowing of the angle between the iris and the cornea (the anterior chamber angle). Eventually a blockage occurs to close off the exit of aqueous humor, and there is a sudden increase in pressure within the eye. Symptoms occur with a rapid onset in the elderly, especially those who are farsighted. Angle closure is associated with pupillary dilation and may be provoked by the use of drops that dilate the eyes. Attacks can develop without any warning symptoms and frequently occur in the evening when lights are dim and the pupils are dilated.

In chronic glaucoma, the drainage angle is not blocked by the iris as it is in acute glaucoma. The cause of the blocked outflow of the aqueous humor is not clearly established. The disease usually affects both eyes, and over a period of months the consistently elevated pressure slowly damages the optic nerve and retina. Chronic glaucoma has no early warning signs and the associated loss of peripheral vision occurs so gradually, it may go unnoticed until a substantial amount of damage and vision loss has occurred. The only way to diagnose glaucoma early is through routine eye examinations.

Secondary glaucoma is caused by other diseases including some eye diseases (uveitis) and systemic diseases (e.g. diabetes, thyroid disease), and by some drugs (corticosteroids).

Congenital glaucoma, present at birth, is the result of defective development of the structures of the eye. Surgery is required for correction. Congenital glaucoma is hereditary.

Risk factors depend on the type of glaucoma. For chronic glaucoma the risk factors include; age over 40, a family history of glaucoma, diabetes, and nearsightedness It is estimated that 1 to 2% of people over 40 have chronic glaucoma with about 25% of cases undetected.

The risk factors for acute glaucoma are older age, farsightedness, family history of acute glaucoma, stress, and the use of systemic anticholinergic medications such as atropine or eye dilation drops in a high-risk individual. Acute, congenital and secondary glaucoma are much less common than chronic glaucoma.

In most cases, there is no prevention for the development of glaucoma. If detected early, further vision loss and blindness may be prevented with treatment.

Careful use of dilating eye drops and systemic anticholinergic medications may prevent acute attacks in high-risk individuals.