Your eye constantly makes aqueous humor. When new aqueous flows into your eye, the same amount has to drain out through an area called the drainage angle. This keeps pressure in the eye stable. But if the drainage angle is not working properly, fluid starts building up. Pressure inside the eye rises in glaucoma, which causes damage of the optic nerve.
Optic nerve damage caused by glaucoma creates a visual field defect. During a routine eye exam, your ophthalmologist may perform visual field testing to determine horizontal and vertical ranges and sensitivity of your vision. The image shows different stages of glaucoma and how your vision is affected in each of these stages.
The drainage angle is the location in your eye where the coloured part of your eye (iris) and the white covering over the eye (sclera) meet. This is the location where fluid within the inner eye drains. Blockage of this angle can cause increased pressure in the eye. The increased pressure can damage the optic nerve and cause glaucoma.