Primary Open Angle Glaucoma
Glaucoma refers to a group of eye conditions that lead to damage to the optic nerve head with progressive loss of retinal ganglion cells and their axons. This leads to a progressive loss of visual field.
Simple (primary) open-angle glaucoma
Simple (primary) open-angle glaucoma (POAG) is a progressive, chronic condition characterised by:
- Adult onset.
- IOP at some point greater than 21 mm Hg (normal range: about 10-21 mm Hg).
- An open iridocorneal angle (between the iris and the cornea, where the aqueous flows out).
- Glaucomatous optic neuropathy.
- Visual field loss compatible with nerve fibre damage.
- Absence of an underlying cause.
- It is usually bilateral.
- Sex—Both sexes are involved equally.
Pathophysiology
The primary problem in glaucoma is disease of the optic nerve. The pathophysiology is not fully understood but there is a progressive loss of retinal ganglion cells and their axons. In its early stages it affects peripheral visual field only but as it advances it affects central vision and results in loss of visual acuity, which can lead to severe sight impairment and complete loss of vision.
In open-angle glaucoma, flow is reduced through the trabecular meshwork (whose role is absorbing aqueous humour). This is a chronic degenerative obstruction which occurs painlessly.
Risk factors
- Age - the incidence increases with age, most commonly presenting after the age of 65 (and rarely before the age of 40).
- Family history - there is a clear inherited component in many individuals (IOP, aqueous outflow facilities and disc size are inherited characteristics).
- Race - it is three to four times more common in Afro-Caribbean people in whom it tends to present earlier and is more severe.
- Ocular hypertension - this is a major risk factor for the development of glaucoma with about 9% of patients developing glaucoma over five years if left untreated.
Presentation
Unfortunately, in the vast majority of cases, patients are asymptomatic. Because initial vsual loss is to peripheral vision and the field of vision is covered by the other eye, patients do not notice visual loss until severe and permanent damage has occurred, often impacting on central (foveal) vision. By then, up to 90% of the optic nerve fibres may have been irreversibly damaged.