Clear fluid is secreted continually inside the eye, to nourish the structures inside. It is different from the tears. It seeps back into the bloodstream by passing through a sieve-like meshwork ring in the wall of the eye, around the periphery of the cornea. The production and drainage of fluid are normally balanced so that pressure in the eye is fairly constant, varying a bit during the day and increasing slowly over a lifetime.
If fluid is impeded leaving the eye, the pressure inside increases. The optic nerve may tolerate moderately elevated pressure for many years without signs of stress, especially in younger people. There is no discomfort or ache unless the pressure is very high. Raised pressure may eventually stress the nerve enough to damage it. Only when the appearance of the optic nerve changes from being normal to showing signs of damage is the eye is said to have glaucoma. Having raised eye pressure increases the risk of developing glaucoma but it is not inevitable.
Most people with glaucoma have increased eye pressure but there are many who develop the same pattern of optic nerve damage even though their eye pressure measures normal. For these, wide fluctuation of eye pressure during the day and structural weakness or poor blood supply of the optic nerve may allow glaucoma to develop or progress at lower levels of pressure or stress. Emerging evidence suggests indirectly, that the balance of pressure between the eye and the brain may influence the development of glaucoma – if eye pressure is high or pressure in the brain fluid that surrounds the optic nerve is low, it may distort the intervening membrane and stress the optic nerve fibres as they leave the eye.
Eye pressure determines the risk, rate and severity of visual loss in glaucoma. Lowering the eye pressure is the only course of action that has been shown to slow down or halt the progression of glaucoma. This applies to glaucoma in which the eye pressure is high and also in many cases in which it is normal.