Some types of glaucoma happen when the fluid in the eye doesn't drain quickly enough or when a blockage stops the fluid from draining and pressure builds up in the eye. In some cases, the fluid drains properly, but there is still too much pressure on the optic nerve.
Because there are usually no symptoms, the only way to find out if someone has glaucoma is with a complete eye exam that includes dilating the pupils. There is no cure and vision loss cannot be reversed. But, if it is detected early, further damage can be avoided.