DIMENSIONS OF THE OPTIC NERVES, CHIASM, AND TRACTS - MR QUANTITATIVE COMPARISON BETWEEN PATIENTS WITH OPTIC ATROPHY AND NORMALS

Objective: The dimensions of the optic nerves, chiasm, and tracts were determined in normal patients and in patients with clinical evidence of optic atrophy to establish whether there was a significant differen ce between the two groups. Materials and Methods: The authors analyzed thin section coronal MRI of 110 patients. Seventy-seven patients were normal, 20 had visual impairment without funduscopic evidence of opti c atrophy, and 13 had visual impairment with funduscopic evidence of o ptic atrophy. Results: The normal mean dimensions were (height x width ): optic tracts 2.8 x 5.1 mm; nerves 3.0 x 5.9 mm; chiasm 3.5 x 15.0 m m. The mean dimensions in the optic atrophy group were significantly l ess (p < 0.01): optic tract 2.1 x 4.7 mm; nerve 2.7 x 5.8 mm; chiasm 2 .6 x 12.6 mm. Conclusion: The reduced mean dimensions in the optic atr ophy group are statistically significant but due to individual variabi lity and errors inherent in measuring small structures, these measurem ents may be difficult to apply clinically. The most clinically useful measurement is the chiasm width. A width < 13.5 mm correlates well wit h funduscopic evidence of optic atrophy.