Optic Nerve Stretch Is Unlikely to Be a Significant Causative Factor in Dysthyroid Optic Neuropathy.

PURPOSE: Although it is a commonly held belief that stretching of the optic nerve is important in dysthyroid optic neuropathy (DON), it is probably mainly due to microvascular ischemia arising from high pressure at the orbital apex. If stretch is important in DON, one would expect longer optic nerves and reduced optic nerve compliance in these patients.

METHODS

Retrospective comparative case series for 75 patients with DON, and for age- and sex-matched control patients with thyroid eye disease (but no DON). Masked assessments of medial wall length, lateral wall length, optic nerve limits (ONLs), globe dimension, and globe projection were made from orbital CT images, and, from these measurements, various derivatives were estimated. The optic nerve shape was also judged as "straight," "mildly curved," or "markedly curved." Estimates of optic nerve compliance were compared in the 2 groups and examined particularly to look for evidence of reduced compliance in orbits with DON.

RESULTS

Paired right-left comparisons suggested a small systematic bias in the estimates of orbital dimensions, possibly due to software performance; this does not complicate the study, as only ipsilateral orbits were compared in the 2 groups. In 115 paired orbits, there was no difference in the proportions with various optic nerve configuration (p = 1.0), but the mean ONL in DON was 32.9 mm, compared with 31.5 mm in thyroid eye disease without DON (p = 0.011). In 5 patients with bilateral thyroid eye disease but unilateral DON, when corrected for side-to-side bias, the ONL on the DON side was greater than the non-DON side (DON, 36.7 mm; non-DON, 34.7 mm; p = 0.066). Compliance estimates for the 115 paired orbits are almost identical (p = 0.99).

CONCLUSIONS

The almost identical compliance estimates for DON and non-DON orbits suggest that ONL elongation in DON, albeit significant, is not extending optic nerve to its elastic limits and is, therefore, unlikely to be causing metabolic stress to the optic nerve. The 1.4 mm extra ONL in DON orbits probably just represents more retrobulbar tissues-with more advanced disease-and optic nerve stretch is probably not significant in the pathogenesis of DON.