While there are over 20 types of glaucoma, the term is most often used to describe primary open-angle glaucoma (POAG), or chronic glaucoma, the most common form. In this condition, the aqueous humor - the nutrient fluid produced by the ciliary body (a small gland in the eye) - doesn’t drain properly from the eye and into the bloodstream. Pressure then builds up within the eye, resulting in damage to the optic nerve, which has the job of transmitting visual messages to the brain.

However, although hypertension in the eye’s blood vessels is a key risk factor for glaucoma, not all rises in eye pressure inevitably cause visual damage: in some people, the optic nerve is strong enough to withstand the increased pressure. Equally, those with particularly weak optic nerves can develop glaucoma even if their eye-pressure readings are normal.

Who are most at risk? Anyone can develop glaucoma, but the risk is significantly greater for those over 40, and doubles for those who are 75-80. It is suggested that age-related changes to various parts of the eye may be responsible for the loss of fluid regulation within the eye.

Race is another important factor. In a study funded by the US National Eye Institute, researchers at the Johns Hopkins University, in Maryland, found that glaucoma is three to four times more likely in people of Afro-Caribbean origin than in white Europeans, and it strikes at a younger age (Arch Ophthalmol, 2004; 122: 532-8).

Other high-risk groups include family members of those already diagnosed with the condition, and people who are extremely shortsighted, diabetic or suffering from high blood pressure.

Glaucoma can also be due to drugs (Drug Saf, 2003; 26: 749-67), including:
* corticosteroids (these mainly cause or worsen POAG)
* sulpha-based drugs
* antidepressants
* anticoagulants
* antihistamines/antacids (H1-/H2-receptor antagonists).

Ironically, drugs such as adrenergic agonists and cholinergics, which are often used to treat glaucoma, can also sometimes bring the condition on.

As prevention is always better than cure, those who belong to these high-risk groups should have their eyes regularly checked - that means going to your optician’s for that annual eye test - and their eye pressure monitored.