Glaucomatous optic nerve damage involves the loss of axons. This nerve fiber loss is irreversible, and thus the blindness caused by glaucoma is irreversible. The nerve fibers most susceptible in glaucoma are those entering the optic nerve in the superior and inferior poles. Thus, one early sign of glaucoma is a thinning of the neuroretinal rim at the vertical poles, resulting in a vertical elongation of the cup.