Use of high-resolution MRI of the optic nerve in Graves' ophthalmopathy.

The aim of this study was to test the hypothesis that optic neuropathy in Graves' disease is associated with measurable optic nerve compression. Magnetic resonance appearances of 32 normal subjects and 27 patients with Graves' disease were evaluated using T(1) weighted volume imaging with multiplanar reformats along the course of the optic nerve. The optic nerve diameter was measured at seven positions along its course. Patients with thyroid orbitopathy were evaluated clinically and categorised into those with (n = 6) and without (n = 48) optic neuropathy. The mean diameter of the optic nerve in normal subjects ranged from 2.2-5.2 mm. The average orbital nerve diameter decreased the further the distance from the globe within the orbit; however, it increased within the optic canal and in the pre-chiasmal region. Optic nerve diameter in patients with Graves' disease without neuritis was not significantly different from that of subjects with normal optic nerves. In patients with optic neuritis, the optic nerve was narrower throughout the length of the nerve but narrowing was most marked in the apex of the orbit (p<0.05) and in the pre-chiasmal intracranial optic nerve (p<0.05). The normal optic nerve has a variable but predictable diameter throughout its course. In Graves' optic neuropathy the diameter of the nerve is significantly reduced in the orbital apex and in the pre-chiasmal portion. The study supports the hypothesis and provides further evidence that the likely mechanism of Graves' ophthalmopathy is compression of the optic nerve at the apex.