Glaucoma is a term applied to a group of eye diseases that cause a characteristic and recognizable deterioration of the optic nerve with associated loss of visual function. The optic nerve carries visual information from the eye to the brain, much like the cable from a computer screen to the computer. In the brain, the information gathered by the eyes is assembled into a visual image that we recognize as sight.

The simplest definition of glaucoma is a condition of the eye where the intraocular pressure (pressure inside the eye) is elevated, causing damage to the optic nerve. This damage prevents the optic nerve from functioning normally and can lead to vision loss. In most patients, a normal intraocular pressure is between 8 and 21 mmHg.
However, the above definition can easily be demonstrated as “simple” by two examples. First, there are some individuals who have “normal” pressures (usually defined as below 21 mmHg) who develop typical glaucomatous damage of the optic nerve head, characteristic visual field defects, and subsequent loss of vision. Conversely, there are other people with significant elevation in intraocular pressure who sustains no optic nerve damage, even after a protracted period of time. Thus, glaucoma defies a simple explanation and while elevated intraocular pressure is one of the primary risk factors, its presence or absence does not define whether a patient has glaucoma.
When the optic nerve is damaged or injured, transmission of visual information from the eye to the brain is limited, and hence the visual image is impaired. Fortunately, ophthalmologists can usually detect glaucomatous optic nerve damage before there is noticeable vision loss by examining the back of the eye. Often, damage to the nerve can be detected before there is a change in vision by specialized tests.