chronic open angle glaucoma (COAG)

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Chronic simple glaucoma (chronic open angle glaucoma (COAG)) occurs when the intra-ocular pressure is raised over several months. The eye remains white and painless but there is insidious damage to the retina and optic fibres.

COAG is glaucoma without evident secondary cause, which follows a chronic time course and occurs in the presence of an open anterior chamber angle (the trabecular meshwork is visible on gonioscopy)

  • COAG is categorised as early, moderate and advanced
    • definitions are based on the Hodapp classification of visual field loss for the stages of glaucoma
      • in terms of mean defect (MD):
        • early, MD greater than -6 dB;
        • moderate, MD -6 dB to greater than -12 dB;
        • advanced, MD -12 dB to greater than -20 dB
        • severe visual impairment (blindness) is defined as MD -20 dB or worse

It affects approximately 2% of the population over the age of 40 years

  • approximately 10% of UK blindness registrations are attributed to glaucoma
    • around 2% of people older than 40 years have chronic open angle glaucoma (COAG), rising to almost 10% in people older than 75 years in white Europeans
      • prevalence may be higher in people of African/Caribbean descent or who have a family history of glaucoma
    • estimated that 480,000 people are currently affected by COAG in England

The majority of patients found to have increased intraocular pressures on screening do not have glaucoma when their peripheral fields and fundi are checked. Patients with raised intraocular pressure who do not have glaucoma require life-long follow-up in case of subsequent development of glaucoma.


  • the term suspected COAG is used when there are changes in the optic nerve head suggestive of COAG but the visual field appears normal or vice versa.
  • people with a diagnosis of ocular hypertension (OHT), suspected COAG or COAG should be monitored and treated by a trained healthcare professional who has all of the following:
    • a specialist qualification (when not working under the supervision of a consultant ophthalmologist)
    • relevant experience
    • ability to detect a change in clinical status
  • knowledge of corneal thickness is no longer needed to decide whether or not to treat OHT and a single threshold of 24 mmHg is now recommended for both onward referral and treatment.


Last reviewed 01/2018