Quantification of orbital apex crowding for screening of dysthyroid optic neuropathy using multidetector CT

Abstract

Background and purpose: DON, a serious complication of GO, is frequently difficult to diagnose clinically in its early stages because of confounding signs and symptoms of congestive orbitopathy. We evaluated the ability of square area measurements of orbital apex crowding, calculated with MDCT, to detect DON.

Materials and methods: Fifty-six patients with GO were studied prospectively with complete neuro-ophthalmologic examination and MDCT scanning. Square measurements were taken from coronal sections 12 mm, 18 mm, and 24 mm from the interzygomatic line. The ratio between the extraocular muscle area and the orbital bone area was used as a CI. Intracranial fat prolapse through the superior orbital fissure was recorded as present or absent. Severity of optic nerve crowding was also subjectively graded on coronal images. Orbits were divided into 2 groups (with or without clinical evidence of DON) and compared.

Results: Ninety-five orbits (36 with and 59 without DON) were studied. The CIs at all 3 levels and the subjective crowding score were significantly greater in orbits with DON (P < .001). No significant difference was observed regarding intracranial fat prolapse (P = .105). The area under the ROC curves was 0.91, 0.93, and 0.87 for CIs at 12, 18, and 24 mm, respectively. The best performance was at 18 mm, where a cutoff value of 57.5% corresponded to 91.7% sensitivity, 89.8% specificity, and an odds ratio of 97.2 for detecting DON. A significant correlation (P < .001) between the CIs and VF defects was observed.

Conclusions: Orbital CIs based on area measurements were found to predict DON more reliably than subjective grading of orbital crowding or intracranial fat prolapse.

Trial registration: ClinicalTrials.gov NCT00665795.

Figures

Fig 1.

Flow chart demonstrating inclusions and exclusions of patients and orbits, with or without DON, in the study.

Fig 2.

Schematic representation of the CT study. Left, Axial scan with planning details of contiguous coronal sections. Right, Coronal images taken 12, 18, and 24 mm from the interzygomatic line.

Fig 3.

Left, Example of reformatted coronal section 18 mm from the interzygomatic line. A region of interest was drawn freehand around the bony orbital rim for automatic calculation of the total area of orbital content (bottom right). Area measurements of muscles and soft tissues were obtained based on a predefined range in HU (top right). The CI was calculated from the ratio between the soft tissues and the total orbital area. The range of thresholds in HU excluded bone tissue from analysis in all measurements.

Fig 4.

A, Axial CT of a patient with bilateral DON and lack of fat prolapse through the superior ophthalmic fissure. The positions of the superior ophthalmic fissures are shown by thin dashed lines. B, Axial CT of a patient without DON and showing bilateral fat prolapse through the ophthalmic fissure. The positions of the superior ophthalmic fissures are shown by thin dashed lines.

Fig 5.

ROC curves of the 3 CI parameters for the discrimination between orbits with and without DON.

Fig 6.

Scatterplots depicting the relationship between CI values measured at 18 mm from the interzygomatic rim and VF mean deviation expressed in decibels in patients with (closed circles) or without (open circles) DON.