Demonstration of systematic variation in human intraorbital optic nerve size by quantitative magnetic resonance imaging and histology

PURPOSE. To use magnetic resonance imaging (MRI) to measure the diameter along the course of the intraorbital optic nerve in living subjects and cadaveric specimens, and to validate measurements histologically in the same specimens. METHODS. Measurements of the intraorbital optic nerve were made in 23 living human subjects and in three formalin-fixed orbits using high-resolution, surface coil MRI in the coronal plane. Fixed orbits were then serially sectioned in the same plane, and stained by Masson's trichrome for digital morphometry of nerve diameter and densitometry of connective tissue constituents. RESULTS. In cadaveric specimens, MRI and histologic measurements of optic nerve dimensions were in close quantitative agreement, showing significant decrease in the average optic nerve diameter along its retrobulbar course. This finding was confirmed by MRI in living subjects, with average optic nerve diameter declining from 3.99 +/- 0.04 mm (SEM) just posterior to the globe, to 3.50 +/- 0.04 mm at 10 mm further to the posterior (P < 0.0001). Color densitometry demonstrated a consistent cross-sectional area of nervous tissue, but decreasing amounts of collagen posteriorly. CONCLUSIONS. There is a significant decrease in normal optic nerve diameter along its length in normal subjects, reflecting reduction in connective tissue. High-resolution MRI is a valid and sensitive method of detecting subtle changes in retrobulbar optic nerve size and can be useful in the investigation of structural optic nerve lesions. Optic nerve diameters must be measured, however, at similar distances posterior to the globe to allow meaningful comparisons.