There are several types of glaucoma including:
Open-angle glaucoma accounts for 90 per cent of all cases in Canada (with similar numbers in the U.S.);
Primary acute closed-angle glaucoma occurs very suddenly and is treated on an emergency basis, and “results from a buildup of fluid in the eye because the distance between the iris and the drainage system has been closed, stopping fluid from draining from the eye” (CNIB); and
Secondary Glaucoma results from other conditions such as injury to the eye, inflammation, complications related to eye surgery, diabetes and use of certain medications.
Referring to open-angle glaucoma, if it goes untreated, a person will slowly start to lose their peripheral vision and experience “tunnel” vision. This vision loss will progress until everything is black.
The most important preventive measure is regular visual examinations. Ask your eye care professional if their examination includes testing for glaucoma.
https://www.empowher.com/glaucoma/content/what-glaucoma