A Novel Method of CT Exophthalmometry in Patients With Thyroid Eye Disease

Abstract

Purpose: Conventional computed tomography (CT) exophthalmometry requires an intact lateral orbital wall and is therefore not feasible in patients who have undergone any form of lateral orbital wall surgery where the normal bony landmark may be lost or displaced. The purpose of our study is to validate an alternative method of CT exophthalmometry utilizing the posterior clinoid (PC) process as a new reference point that will allow for reproducible comparison of the anterior-posterior globe position in the preoperative and postoperative settings.

Design: Cohort study.

Methods: This is a retrospective study of 48 patients with clinically diagnosed thyroid eye disease who had undergone cross-sectional CT imaging in the pre- or postoperative settings. CT exophthalmometry was performed using both the conventional interzygomatic method and our proposed PC process method on all pre- and postoperative CT imaging by two independent observers. Interobserver variability analysis was performed with intraclass correlation coefficient. Correlation and agreement between the two methods were analyzed with Pearson correlation coefficient and linear regression method. All analyses were conducted at 5% level of significance with Stata MP V14.

Results: Interobserver variability analysis showed an intraclass correlation coefficient of >0.9 for both interzygomatic and PC methods. There is good correlation between the two different measurements observed in both the pre- and postoperative groups (r = 0.68 and r = 0.72, respectively, P < 0.001). Linear regression showed good agreement between the two different measurements with most of the points lying within the 95% limits.

Conclusions: Our new method agrees well with the conventional method and has the added benefit of being able to reliably assess the anterior-posterior globe position in patients who do not have intact lateral orbital walls after decompressive surgery.

FIGURE 1

Axial CT orbit at the level of the mid-globes. A, Conventional method of assessing for proptosis using the interzygomatic line (yellow dotted line) as a reference for drawing the perpendicular measuring line to the anterior corneal surface (green arrows). B, Proposed posterior clinoid method using a single measuring line (red arrows).

FIGURE 2

Linear regression analysis in pre-operative and post-operative patients. PC indicates posterior clinoid.

FIGURE 3

A, Pre-operative axial CT scan in a patient with bilateral proptosis and extraocular enlargement. Using the conventional IZ method of exophthalmometry, there is exophthalmos of 24 mm of both the right and left globes. B, Post-operative axial CT of the same patient who had undergone bilateral medial and lateral wall decompressions. Absence of the lateral rim (arrows) precludes exophthalmos assessment using the conventional method.

FIGURE 4

CT scan demonstrating the normal posterior clinoid process on the A, axial, B, coronal and C, sagittal planes (arrows).