Glaucoma is a disease in which the optic nerve becomes damaged, leading to progressive loss of visual field and eventually to visual impairment and blindness. There are several types of glaucoma, which differ mainly by cause and speed of symptom development (acute or chronic; NHS Choices 2014).

The damage to the optic nerve can occur because of raised pressure inside the eye (intraocular pressure, IOP). This can happen if the drainage tubes within the eye (called the trabecular meshwork) become blocked, so the fluid (aqueous humour) cannot drain out of the eye properly (NHS Choices 2014). IOP is normally about 15 mmHg. If it is greater than 21 mmHg on 2 or more occasions in 1 or both eyes (College of Optometrists 2012), this is classed as ocular hypertension (OHT). Although glaucoma is most commonly related to prolonged OHT, it can also occur when IOP is not raised (referred to as normal tension glaucoma).

OHT affects 5% of people aged over 40 years in the UK. This equates to around 1 million people (Burr et al. 2012). Risk factors for OHT include increasing age, short‑sightedness (myopia), diabetes, vascular disease, and a family history of OHT (International Glaucoma Association 2015; NHS Choices 2014) as well as a previous eye injury or existing eye condition.

The most common type of glaucoma, accounting for over 90% of all cases, is chronic open‑angle glaucoma (COAG), which is a slow‑developing glaucoma without an obvious cause (such as ocular injury or inflammation, or pharmacological treatment; European Glaucoma Society 2008; NICE guideline on glaucoma). COAG is usually asymptomatic until irreversible vision loss has occurred. The peripheral vision is typically lost first before the damage progresses towards the centre of the visual field. About 10% of UK blindness registrations are related to glaucoma and around 2% of white European people over the age of 40 years and 10% of those aged over 75 years have COAG. The prevalence may be higher in people of African or African‑Caribbean family origin or in those who have a family history of glaucoma. Based on these estimates, about 480,000 people have COAG in England (see the NICE guideline on glaucoma for more information).

People with OHT, or who have or are suspected of having COAG, need regular monitoring of IOP. This, together with other assessment tests (see NICE guideline on glaucoma), is used to identify changes that may indicate treatment is needed.