In the great majority of the cases, glaucoma occurs in susceptible individuals. This form of glaucoma is called “primary glaucoma” – it does not occur as a result of any other eye problem.
Glaucoma may also be caused by previous ocular trauma, associated systemic diseases, use of medications, consequence of complicated eye surgeries, and others. In these situations, the disease is called “secondary glaucoma”.
Nevertheless, in all cases, glaucoma occurs mainly due to a sufficiently high IOP leading to damage to the optic nerve. The critical IOP level in which optic nerve damage will occur varies among patients, and it depends upon many variables that will determine each patient´s susceptibility for the disease.
In other cases, IOP may be relatively normal, but glaucoma occurs anyway because of the inability of the eye to handle mechanical stress where the nerve fibers leave the eye or because of poor blood supply to these same nerve fibers.
So, glaucoma damage may occur within the normal IOP levels – normal levels for most of the individuals but not for all. In this situation, the glaucoma may be called “normal tension glaucoma”.
But as a main rule, the higher the IOP, the higher the risk for glaucoma development and progression.