Automated quantitation of the posterior scleral flattening and optic nerve protrusion by MRI in idiopathic intracranial hypertension

Abstract

Background and purpose: Subjective determination of the posterior sclera flattening and optic nerve protrusion in MRI is challenging because of the 3D nature of the globe morphology. This study aims to develop and compare quantitative measures of globe flattening and optic nerve protrusion with subjective rating, and assess relationships with papilledema grade and intraocular and CSF pressures.

Materials and methods: Data of 34 globes from 7 overweight female patients with idiopathic intracranial hypertension and 6 age- and weight-matched healthy female control subjects were assessed, as well as a subcohort of 4 of the patients with idiopathic intracranial hypertension who underwent follow-up MR imaging 2 weeks after lumbar puncture and initiation of treatment with acetazolamide. MR imaging examination included a 3D CISS sequence on 1.5T and 3T scanners with 0.6-mm isotropic resolution. Subjective ratings of globe flattening were obtained by experienced and inexperienced readers. Quantitative measures of globe flattening, nerve protrusion, and maximal deformation were derived by use of a 2D map of the distances from the globe center to the posterior wall.

Results: Contingency coefficients for globe flattening agreements with subjective rating by the experienced and inexperienced readers were 0.72 and 0.56, respectively. Mean values of the 3 deformation measures were significantly poorer in the idiopathic intracranial hypertension group, with nerve protrusion demonstrating the strongest difference (P = .0002). Nerve protrusion was most strongly associated with papilledema grade with a contingency coefficient of 0.74 (P = .01), whereas globe flattening was negatively correlated with intraocular pressure (R = -0.75, P < .0001). Maximal deformation was negatively associated with CSF opening pressure (R = -0.86, P = .0001). After treatment, only the changes in nerve protrusion and maximal deformation were significant.

Conclusions: Automated measures of globe deformation improve reliability over subjective rating. Of the 2 globe deformation measures, nerve protrusion had the strongest predictive value for papilledema grade and had the highest sensitivity for assessment of treatment efficacy in idiopathic intracranial hypertension.

Fig 1.

MR images of the globe reformatted in sagittal (A) and axial (B) planes. The coordinate system is defined by the center of mass of the lens and the globe (red and green points), respectively. The red contour marks the posterior sclera. Each point on the sclera is defined by the distance to the center of the orbit (dashed line), azimuth angle (θ), and the elevation angle (ϕ).

Fig 2.

A 2D color-coded distance map visualizes distances between the center of the globe and points on the posterior sclera. The map represents the globe shown in Fig 1. The inward protrusion of the papilla is visualized as an off-center blue patch (A). The central (red) and peripheral (yellow) ROIs used for the derivation of NP and GF are shown in B and C, respectively. The angular boundaries of the central (papillar), peripappilar, and peripheral ROIs are 0–10°, 10–18°, and 40–80°, respectively.

Fig 3.

Reformatted MR images shown in axial plane for a normal globe (A), a flattened globe with minimal optic nerve protrusion (B), and a globe with minimal flattening and extensive optic nerve protrusion (C). Respective distance maps are shown in the bottom. The corresponding GF and NP values are (A) 0.97 and 0.97, (B) 0.86 and 0.91, and (C) 0.91 and 0.88.

Fig 4.

Average left and right eyes, 2D-distance maps obtained from the control cohort (upper row) and the IIH cohort (lower row). The presence of nerve protrusion is clearly seen in the maps from the IIH cohort.

Fig 5.

Pretreatment (upper row) and posttreatment (lower row) average distance maps of the patients with IIH who had a follow-up MR imaging scan (n = 4). A significant reversal of the extent of the optic nerve protrusion is visualized in the posttreatment maps.

Fig 6.

Scatterplots of the relationships between GF and intraocular pressure (A) and between MD and CSF opening pressure (B).