What Causes Glaucoma?

Whatever the cause, the risk is that because there are often no symptoms, the nerve damage can go unnoticed until a significant proportion of sight has been lost.  Glaucoma is often called the "silent thief of sight" because it develops without obvious symptoms. Therefore, the person with glaucoma is usually unaware they have glaucoma until serious loss of vision has occurred.  In fact, half of those suffering damage from glaucoma are unaware they have the disease.  As the damage from glaucoma cannot be reversed, early detection, diagnosis, and treatment by your eye doctor (ophthalmologist) is vital to preserve your vision.

Intraocular Pressure (IOP)

The outer coating of the eye is "floppy" like a beach ball. A clear watery fluid, aqueous, is produced inside the eye.  This fluid provides the necessary pressure to help maintain the shape of the eye.  We call this pressure the intraocular pressure (IOP). This fluid also nourishes the cornea and the lens, providing them with oxygen and vital nutrients.

The IOP is critical to the normal functioning of your eye.  If the pressure is too low, the eyeball deflates, and folds form in the back of the eye (retina), leading to a blurring of the vision.  If the pressure is too high the optic nerve fibres become compressed and eventually die. The death of these cells results in permanent visual loss.

A circular pump inside the eye, called the ciliary body, produces the clear fluid which is called aqueous.  The aqueous circulates inside the eye and returns to the blood stream through a lattice work called the Trabecular Meshwork.  This is situated in a ring extending right around the junction of the iris and cornea (called "the angle").  The meshwork provides some resistance to the flow of aqueous, so the pressure in the eye is maintained.  If the resistance to flow increases in the meshwork, the IOP builds up.  Aqueous fluid also drains out of the eye through the ciliary body and sclera (uveo-scleral outflow). The IOP is measured as millimetres of mercury (mm Hg). The normal eye usually has a reading between 12 and 21 mm Hg.

A number of things can happen to interfere with the normal flow of the aqueous. This can result in a build up of pressure. Understanding what kind of obstruction is occurring in a particular eye allows the ophthalmologist to identify which mechanism is causing the raised pressure and, if needed, to plan a particular course of treatment.

The most common cause of raised IOP is a malfunction in the Trabecular Meshwork - it simply does not drain the aqueous as efficiently as it should. The IOP rises slowly, but may not cause damage to the actual eye structure for many years. What can happen, though, is that the increased IOP begins to interfere with the delicate nerve cells at the back of the eye; these cells convert light energy into nervous impulses, and transmit them to the part of the brain which is responsible for sight.