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The main problem or pathology in glaucoma is caused by raised intraocular pressure. It is this raised pressure that compresses and damages the optic nerve. Once the optic nerve is damaged, it fails to carry visual information to the brain and this results in loss of vision.
The exact pathophysiology contributing to this is not fully understood. It is believed that the raised pressure on the retina causes the cells and nerve ganglions in the sensitive retina to die off (retinal ganglion apoptosis) and in addition the small blood vessels of the retina are also compressed depriving it of nutrients. This results in a clinically progressive loss of peripheral visual field and ultimately vision.
However, debunking this theory of high intraocular pressure alone that causes damage is the normal tension glaucoma. Here there is no rise of intraocular pressure. These patients are said to suffer from a problem in the blood vessels and perfusion and derangements of the immune system (autoimmune causes) that may lead to damage to the optic nerve.
Some studies show that optic nerve heads of these patients are particularly sensitive with damage occurring at much lower intraocualar pressures than in normal individuals. Thus they may also benefit from medications that can reduce intraocular pressure.
Normally the aqueous humor plays an important role in nutrient delivery and waste disposal for the cells. It is produced by the ciliary body epithelium and drains out through the trabecular meshwork at the anterior chamber angle. When this flow is disrupted the pressure within the eye builds up. This disruption can occur in two ways:-
Age is the most common reason for disruption to aqueous flow. With age the trabecular meshwork cells work less efficiently and this results in build up of aqueous humor within the anterior chamber of the eye.
In addition with age the lens also hardens and increases in size. This further narrows the anterior chamber of the eye and physically narrows the anterior chamber angle.