It creeps up slowly, unnoticed, quietly stealing your sight a little at a time. The fluid must be constantly drained. It drains at the point where your iris and cornea meet through a meshwork, which acts as a valve to help regulate pressure within your eye. When drainage is blocked, fluid builds up, creating pressure. This progressively damages your optic nerve, which transmits electrical impulses from your eye to your brain.
Peripheral vision goes first.
Tunnel vision follows, and eventual total blindness is possible. For unknown reasons the meshwork becomes blocked. This slows fluid outflow and raises pressure in your inner eye. The pressure builds undetected for years, causing gradual permanent damage to vision. About 90 percent of glaucoma is open-angle glaucoma.