Abstract
PURPOSE: Exophthalmometry is a common examination in ophthalmology. For example it is relevant for diagnosis or follow-up of thyroid eye disease. However, exophthalmometry is affected by several factors such as ethnicity, sex and age. The purpose of this study was to determine the globe position by magnetic resonance imaging (MRI) and to investigate its correlates among the general Northeast German adult population. METHODS: A total of 3030 subjects aged between 20 and 89 from the population-based Study of Health in Pomerania (SHIP) underwent a standardised whole-body MRI. Axial length and globe position were determined in axial T1-weighted images of the orbit. The image had to include the corneal apex as well as the optic nerve head. Study participants were excluded from imaging analysis if there was no plane available that included both structures. Further exclusion criterion was a lateral deviation of the subject's viewing direction. Images with inadequate quality due to motion artefacts or other technical reasons were excluded as well. Globe position was defined as the perpendicular distance between the interzygomatic line and the posterior surface of the cornea (exophthalmometric value). The distance between the posterior surface of the cornea and the posterior pole of the eyeball, at the boundary with orbital fat, was defined as axial length. We used posterior surface of the cornea for our measurements, because it seemed to be less vulnerable for motion artefacts than the anterior one. Moreover body measurements including body mass index (BMI), waist and hip circumferences were determined. Associations between anthropometric measurements with exophthalmometric outcomes were analysed by linear regressions adjusted for age and stratified by sex. P-values <0.05 were considered as statistically significant. To assess intra-reader variability intra-class correlation coefficients (ICC) were computed for repeated measurements of the MRI scans of 25 subjects. RESULTS: After considering the exclusion criteria 1926 evaluable subjects remained. There was no significant difference between means of right and left eyes. The mean exophthalmometric value was significantly higher in men (16.5 +/- 2.2 mm) than in women (15.3 +/- 2.1 mm). The mean MRI-axial length was 23.4 +/- 0.8 mm for men and 22.8 +/- 0.9 mm for women. BMI, waist and hip circumferences were positively correlated with exophthalmometric value (p<0.001). Difference of mean MRI-based exophthalmometric value for obese subjects (BMI ≥30 kg/m2) and non-overweight (BMI <25 kg/m2) was 2.1 mm for men and 1.6 mm for women. ICC between 0.97 and 0.99 indicate excellent repeatability of our method. CONCLUSION: We conclude that MRI-based exophthalmometric values are positively correlated with BMI, waist- and hip-circumference among the general Northeast German adult population. This association is independent from age and axial length. Consequently bodyweight of patients should be regarded to interpret exophthalmometric values correctly. MRI-exophthalmometry seems to be a suitable method to determine globe position. Considering the large number of study participants, exophthalmometric values of our study could be used as comparative values for exophthalmometry of people of Western European descent in future.