Glaucoma is a group of eye conditions in which raised pressure inside the eye damages the optic nerve — the nerve that sends visual signals to the brain. It is diagnosed when there is evidence of this damage.
The most common type, primary open-angle glaucoma, occurs when the drainage channel between the iris and the cornea is open. In angle-closure glaucoma, the drainage channel becomes blocked when the iris moves forward against the cornea. This stops fluid from draining and raises the pressure, which can damage the optic nerve.
Some eyes naturally have a shallow front chamber and a narrow drainage angle because the iris and lens sit close to the cornea. This is more common in people with smaller eyes or those who are long-sighted.