OPTIC-NERVE FIBER COUNT AND DIAMETER OF THE RETROBULBAR OPTIC-NERVE IN NORMAL AND GLAUCOMATOUS EYES

Background: The caliber of the retrobulbar optic nerve and the count of optic nerve fibers vary considerably in normals. The diameter of the retrobulbar optic nerve also decreases with optic nerve atrophy. This study aimed to determine the relationship between the caliber of the optic nerve and the optic nerve fiber count. Methods: We counted the optic nerve fibers and measured the diameter of histological cross sections of the optic nerve for 56 normal subjects and 23 patients with absolute glaucoma. Results: The optic nerve fiber count increased significantly (P<0.0001) by 777 000 fibers for every millimeter increase in retrobulbar optic nerve diameter, starting at a baseline diameter of 1.89 mm. Conclusion: Using this linear regression equation, the optic nerve fiber count can be estimated in routine histology by measuring the optic nerve diameter. Taking into account a fixation-induced tissue shrinkage, this method may also give some indication of the optic nerve fiber count intravitally, when, for eyes with opaque optic media, the diameter of the retrobulbar optic nerve has been measured by imaging techniques. Greater retrobulbar optic nerve caliber may indicate greater structural reserve capacity.