Eye pressure results from a delicate balance between fluid - called the aqueous humor -- entering into and draining  out of it.   In some patients,  due to thickening or narrowing of the draining tissues,  fluid can build up leading to increased eye pressure.  This is known as glaucoma.   With time,  high pressures cause a slow atrophy of the optic nerve,  which in turn leads to loss of side and central vision.