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 difference 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 optic atrophy, and 13 had visual impairment with funduscopic evidence of optic 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 mm. The mean dimensions in the optic atrophy group were significantly less (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 atrophy group are statistically significant but due to individual variability and errors inherent in measuring small structures, these measurements may be difficult to apply clinically. The most clinically useful measurement is the chiasm width. A width < 13.5 mm correlates well with funduscopic evidence of optic atrophy.