DIAMETER OF THE OPTIC-NERVE IN IDIOPATHIC OPTIC NEURITIS AND IN ANTERIOR ISCHEMIC OPTIC NEUROPATHY

Purpose: There is considerable overlap in the clinical profile of pati ents with idiopathic optic neuritis (ON) and anterior ischemic optic n europathy (AION). We tested the hypothesis that the retrobulbar diamet er of the optic nerve may be a criterion for the differential diagnosi s between ON and AION. Methods: The diameter of the optic nerve was me asured by B-scan ultrasonography with the eye in an abducted position. Only patients with a unilateral optic neuropathy were included, 16 ON patients (mean age 24 years, 5 with and 11 without disc swelling) and 9 patients with AION (mean age 72 years). As controls for the ON pati ents 10 young normal subjects (mean age 25 years) and as controls for the AION patients 10 elderly subjects with eye problems not related to the optic nerve (mean age 76 years) were examined. Results: In the ON patients with disc swelling the diameter of the optic nerve was 5.4 /- 0.5 mm in the affected and 3.0 +/- 0.3 mm in the unaffected side. T his difference was significant (Wilcoxon-test, p = 0.043). In the ON p atients without disc swelling the diameter of the optic nerve was 4.4 +/- 0.4 mm in the affected and 3.0 +/- 0.3 mm in the unaffected side. This difference was significant (Wilcoxon-test, p = 0.003). In the AIO N patients the diameter of the optic nerve was 3.0 +/- 0.3 mm on the a ffected and 2.8 +/- 0.4 mm on the unaffected side. This difference was not significant (Wilcoxon-test, p = 0.093). Comparing the optic nerve s with ON and AION to those of the controls, the diameter was signific antly enlarged in the nerves with ON and normal in the nerves with AIO N (one factor repeated ANOVA). Conclusion: The diameter of the optic n erve is increased in ON without disc swelling and even more so in ON w ith disc swelling. The enlargement is probably due to edema of the ner ve itself, not the surrounding subarachnoidal space. In AION, the diam eter of the optic nerve is normal. Measuring the diameter of the optic nerve by B-scan ultrasonography is particularly useful in the differe ntial diagnosis between ON with disc swelling and AION.