Primary open-angle glaucoma

• Summary
• Aetiology
• Clinical features
• Diagnosis
• Management
• Medications used for primary open angle glaucoma
• Surgery used for primary open angle glaucoma
• References
• Author(s)

Summary

Primary open angle glaucoma (POAG) is characterised by an open and normal appearing anterior chamber angle, with no other underlying disease. POAG is the most common form of glaucoma. It may be associated with raised intraocular pressure (IOP) but this is not always present. Some patients with POAG have normal pressure glaucoma.

Aetiology

The defining feature of primary open angle glaucoma is that there is no underlying disease attributable to the raised intraocular pressure. If there is an identifiable underlying cause for the raised intraocular pressure, this is termed secondary glaucoma.

Another form of glaucoma where the intraocular pressure is within normal limits is known as normal tension glaucoma.

In primary open angle glaucoma, there is damage of the optic nerve head due to loss of primary ganglion cell axons.

The risk factors for primary open angle glaucoma include:

• Age
• Family history
• African ethnicity
• Raised intraocular pressure
• Thin central cornea
• Hypertension
• Diabetes
• Myopia

Aqueous humour is produced by the ciliary epithelium in the posterior chamber of the eye. It passes through the pupil into the anterior chamber and drains in the anterior chamber angle, also known as the iridocorneal angle.

There are two main drainage pathways for aqueous humour:

• Canal of Schlemm: most of the aqueous humour drains via the trabecular meshwork into the canal of Schlemm.
• Uveoscleral pathway: a small portion of the aqueous humour drains via the ciliary musle into the suprachoroidal space and supraciliary space.

Clinical features

Both eyes are usually affected, but usually not equally.

Patients usually only become aware of the visual field loss when there is significant atrophy of the optic nerve. Thus, primary open angle glaucoma is asymptomatic until the late stages of the disease. Some patients may notice that they have difficulty going down the stairs or reading if the inferior portion of their visual field has been affected.

Diagnosis is mainly made through routine ophthalmic examinations at optical services.

Diagnosis

Gonioscopy: this allows assessment of the anterior chamber and the iridocorneal angle. In primary open angle glaucoma, an open angle that is not obstructed will be observed on gonioscopy.

Fundoscopy: there will be changes to the optic nerve visible on fundoscopy. These characteristic changes are known as optic disc cupping. This refers to an increase in the optic disc that is taken up by the central cup. Images may be taken of the fundus for follow-up monitoring.

Optic disc cupping. Image courtesy of Retina Rocks.

Visual field testing: patients may have visual field defects due to atrophy of the optic nerve.

These are the right and left visual fields of a 61 year-old patient, who was diagnosed with POAG 15 years prior to presentation. Image courtesy of Community Eye Health.

Intraocular pressure: normal intraocular pressure is 8-21 mmHg. In POAG, intraocular pressure tends to be raised and is usually higher in the eye with more optic nerve damage. The central corneal thickness may also be measured in order to interpret the intraocular pressure readings. However, not all patients with glaucoma will have a raised IOP.

Management

The management of primary open angle glaucoma involves measures to decrease the intraocular pressure in order to prevent visual loss. This may involve the use of medications and sometimes surgery.

Medications used for primary open angle glaucoma

Topical eye drops such as prostaglandin analogues such as latanoprost or tafluprost may be used. Beta-blockers such as timolol may also be used.

Other drugs include alpha-2 agonists, carbonic anhydrase inhibitors, topical sympathomimetics and topical miotics.

Surgery used for primary open angle glaucoma

Surgical procedures for primary open angle glaucoma includes laser trabeculoplasty or guarded filtration procedure. In patients with advanced disease, surgery with mitomycin C augmentation may be used.

References

1. ‘Primary Open-Angle Glaucoma - Eye Disorders’. MSD Manual Professional Edition, https://www.msdmanuals.com/en-gb/professional/eye-disorders/glaucoma/primary-open-angle-glaucoma. Accessed 21 Aug. 2022.
2. Primary Open-Angle Glaucoma - EyeWiki. https://eyewiki.aao.org/Primary_Open-Angle_Glaucoma. Accessed 21 Aug. 2022.

Author(s)

Dr Sara Memon

Sara is the Co-Founder of Ophtnotes. She is a doctor who graduated from UCL Medical School in London. She won the Allen Goldsmith Prize in Ophthalmology. Sara is also the co-founder of PAMSA: an organisation linking doctors and medical students of Pakistani origin. She’s especially passionate about teaching and education, having presented a workshop she designed herself at the 2019 Annual GMC Conference.