The build-up of pressure inside your eye leads to glaucoma. If this outflow of liquid is impaired at all, there is a build-up of pressure inside the eye that damages the optic nerve, which carries visual images to the brain. The result is a loss of peripheral vision. Thus, while glaucoma sufferers may be able to read the smallest line on the vision test, they may find it difficult to move around without bumping into things or to see moving objects to the side, such as cars. Some causes are known, others are not.
Causes differ depending on the type of glaucoma.
The exact cause of open-angle glaucoma, where the drainage channels for the aqueous appear to be open and clear, is not known. Closed-angle glaucoma can occur when the pupil dilates or gets bigger and bunches the iris up around its edge, blocking the drainage channel. An injury, infection or tumor in or around the eye can also cause internal eye pressure to rise either by blocking drainage or displacing tissues and liquid within the eye. A mature cataract also can push the iris forward to block the drainage 'angle' between the iris and the cornea. Glaucoma can occur secondarily to a number of other conditions, such as diabetes, or as a result of some medications for other conditions.