Chronic Glaucoma Selectively Damages Large Optic Nerve Fibers

To determine whether glaucoma selectively injures a particular size of optic nerve fiber (and its cell body), we induced chronic experimental glaucoma in one eye each of ten monkeys. With automated image analysis, the number and diameter of optic nerve fibers were compared between each glaucomatous eye and its normal fellow eye. Fibers larger than the normal mean diameter atrophied more rapidly in glaucomatous eyes, though no fiber size was spared from damage. The mean fiber diameter for glaucomatous eyes was 0.74 micron, significantly lower than the mean for ten fellow eyes, 0.85 micron (P less than 0.005). There was preferentially greater atrophy of fibers of all sizes in the superior and inferior peripheral nerve sectors, as seen in human eyes with glaucoma. The more rapid atrophy of larger fibers appeared to result from two causes. The areas that suffer most rapid loss of fibers in experimental glaucoma normally contain a high proportion of larger diameter fibers. Furthermore, larger fibers were lost preferentially even in areas of the optic nerve with mild damage, indicating their inherent susceptibility to injury by glaucoma. The tendency for large fibers to be lost in glaucoma has implications for future improvements in testing for early glaucoma damage.