Primary Open Angle Glaucoma (POAG) is a disease in which the cells that produce sight in the retina are destroyed by a slow rise in the internal fluid pressure in the eye.
Normally this fluid, known as the aqueous, is constantly produced inside the eyes to feed the lens and the cornea. It is drained from the eye back into blood vessels and the production and drainage rates are balanced. In POAG it is a disturbance of drainage which leads to an accumulation of this fluid and hence the pressure rise. The sight gradually diminishes and, as the fluid pressure increases, the eye enlarges and the lens may dislocate. Eventually the dog becomes blind.