Echographic measurements of the retrobulbar optic nerve in normal and glaucomatous eyes

A study was designed to investigate whether measurements of the optic nerve diameter (OND) and cross sectional area (ONCSA), as measured by B-scan ultrasonography, are altered in glaucoma. The reproducibility and test-retest variability of echographic estimates of retrobulbar optic nerve dimensions was also tested.

One eye of 49 glaucoma patients and 90 control subjects underwent five repeated echographic measurements of the maximal interpial diameter and cross sectional area of the orbital optic nerve on two separate occasions. All measurements were taken by one experienced ultrasonographer.

Mean optic nerve diameter (SD) for the control group was 2.86 (0.46) mm, and was independent of height (multiple regression analysis: p = 0.21), axial length (p = 0.74), spherical equivalent (p = 0.97), sex (ANOVA: p = 0.36), or race (p = 0.14), but was inversely related to age (p = 0.01). Reproducibility of OND readings in control subjects was 0.149 mm (coefficient of repeatability). Test-retest variability of interpial diameter was -0.02 (0.29) mm. Mean interpial diameter of the optic nerve was significantly smaller among glaucomatous eyes (2.58 (0.501) mm) than controls (Mann-Whitney U test: p < 0.0001). Glaucomatous optic nerves also had a significantly smaller cross sectional area (6.68 (2.58) mm2) than those of healthy volunteers (8.25 (1.67) mm2) (p = 0.004).

Echographic measurements of the orbital optic nerve are highly reproducible and not subject to clinically meaningful test-retest variability. Optic nerve interpial diameter and cross sectional area are reduced in glaucomatous eyes, reflecting nerve fibre loss. This technique may be useful in distinguishing between normal and glaucomatous eyes where optic disc morphometry is inconclusive or impossible as a result of opaque media.