Glaucoma is a disease of the optic nerve. High pressure in the eye causes loss of optic nerve tissue. The higher the pressure in the eye, the greater the likelihood that damage to the optic nerve will occur. High pressure cannot be felt, but is measured with a special instrument called a tonometer which should be a part of every routine eye exam.
Glaucoma rarely has symptoms before it affects vision. This is the reason that routine eye exams are critical. Early detection is the key to preventing vision loss or blindness from glaucoma.