Graves ophthalmopathy: the bony orbit in optic neuropathy, its apical angular capacity, and impact on prediction of risk

Abstract

Background and purpose: Optic neuropathy (ON), a serious complication of Graves ophthalmopathy, is often subclinical and masked by symptoms of orbitopathy. We examined herein bony and soft-tissue CT features associated with ON, including an angular assessment of orbital apex capacity, and their usefulness in the risk prediction of ON.

Materials and methods: The CT scans of 41 patients with Graves ophthalmopathy (17 men, 24 women; mean age, 49.1 years) clinically diagnosed with (19 patients, 32 orbits) or without ON were evaluated by 2 independent raters. Quantitative linear and angular measurements of the orbital structures and bony walls and categoric scores of apical crowding and intracranial fat prolapse were assessed on a clinical workstation. Inter- and intrarater variability of these features was determined. The CT features of the 2 patient groups were compared, and multivariate logistic regression analysis was performed to evaluate the predictive features of ON.

Results: Bony orbital angles (P < .005), length of the lateral orbital wall (P < .05), muscular diameters (P < .0005), muscular bulk of the medial rectus muscle relative to the bony orbit (P < .05), and apical crowding (P < .0005) were associated with clinical ON. Stepwise multivariate logistic regression analysis revealed the muscle diameter index and medial and lateral wall angles to be independent predictors. Combining these in a single multivariate equation yielded sensitivity, specificity, and positive and negative predictive values of 73%, 90%, 82%, and 85%, respectively.

Conclusions: Orbital wall angles, especially the medial wall, and muscular enlargement are independent risk predictors.

Fig 1.

A and B, Axial CT scans (bone window on left, soft tissue window on right) showing the orbital apex point indicated by the curved arrow (A) and the orbital rim angle (B). A, The apex point is defined as the anterolateral border of the groove in the sphenoid body formed by the intracavernous portion of the internal carotid artery (labeled 2) on the section just inferior to the anterior clinoid process. B, The orbital rim angle (43°) is measured at the level of the medial palpebral ligament (arrow). C−E, The same axial section containing the bulk of the medial and lateral rectus muscles shows the medial wall angle (C), the lateral wall angle (D), and the orbital apex angle (E). For the medial and lateral walls, the angle that best describes the widest bony point of the orbital wall around the muscular bellies is recorded. The angular change is recorded as positive if the resultant angle is wider than the orbital rim angle, and negative for narrower angles. F, The length of the lateral orbital wall is measured on the section just inferior to the anterior clinoid.

Fig 2.

Axial section at midglobe level showing the interzygomatic line (labeled 11) and maximum horizontal diameters of the right medial and lateral rectus muscles (measurements labeled 4 and 3, respectively). The distance from the midpoint of the maximum muscular diameter of the medial (measurement 8) and lateral rectus muscles (measurement 7) to the interzygomatic line is also recorded. Proptosis of the left globe relative to the interzygomatic line is labeled as measurement 6. Left optic nerve stretch (labeled 13) is measured from the retrobulbar optic nerve to the orbital apex point (labeled 1).

Fig 3.

Scatterplots of the MDI and orbital apex angle (A), MDI and medial wall angle (B), and MDI and lateral wall angle (C) in patients with and without ON. Greater muscular enlargement is accompanied by wider orbital angles with or without (w/o) ON. For identical MDI and orbital angles, the orbital angles are narrower and MDI greater in patients with ON, respectively. Within the borderline MDI range of 22–30 mm in B, 15 of 26 patients with zero or negative medial angles (57.7%) had ON (P = .07), compared with 1 of 6 patients with positive medial angles (16.7%).

Fig 4.

Scatterplot depicting the relationship between lengths of lateral orbital wall and orbital rim angles in patients with and without (w/o) ON.