Glaucoma means injury to the optic nerve in the back of the eye usually caused by elevated eye pressure. This optic nerve damage causes diminished peripheral vision over time. Because the  changes are gradual and initially subtle, many individuals do not realize that they have glaucoma until they have experienced significant abd permanent loss of vision. It is therefore, important to be examined by an opthtalmologist on a regular basis to be checked for glaucoma as well as other conditions that can be treated successfuly if caught early.

The artical below provides a nice summary of the different types of glaucoma as well as their diagnoses and treatments.