WHAT IS GLAUCOMA? Glaucoma is a disease of the nerve for vision (optic nerve) caused by increased pressure in the eye. HOW DOES GLAUCOMA OCCUR? Glaucoma1The inside of the eye contains a fluid, which is being produced and drained out of the eye constantly. It is drained through a sieve like structure situated at the angle between the transparent cornea and the brown coloured iris. When the drainage mechanism becomes ineffective, the pressure in the eye (intra-ocular pressure) increases leading to optic nerve damage.