Introduction of the Ex/AxI ratio for optimized assessment of exophthalmos based on bulbar position parameters examined in a population-based MRI study.

Exophthalmometry is an essential component in the determination of changes in the position of the eyeball and is influenced by various factors. This paper analyzes the usefulness of the Ex/Axl ratio, meaning the ratio of anterior exophthalmos to bulbar axial length, in relation to the position of the eye in the orbit in order to take better account the anthropometric characteristics of individual exophthalmometry measurements. MRI scans of the head in the population-based cohorts SHIP-START-2 and SHIP-TREND-0 were used to perform anterior and posterior exophthalmometry as well as bulbar axial length measurements in 2083 subjects between 21 and 89 years of age. After excluding non-measurable images, 1926 subjects were included in the data analysis. We established sex- and age specific reference limits for the Ex/Axl ratio and the exophthalmometry parameters. If the Ex/Axl ratio was above 85.3% in men and above 79.7% in women, this indicated pathological exophthalmos in our study. Exophthalmometry should be considered on a sex-specific basis since there are clear differences in the measurements between men and women. MRI images of the head were used to determine separate reference values for men and women, which may be useful in clinical practice. Furthermore, our study highlights the clinical significance of measuring the individual bulbar axial length and anthropometric data for the personalized assessment of exophthalmos. In this context, the newly introduced Ex/Axl ratio may be a useful tool for a better differentiation between pseudoexophthalmos and clinically relevant exophthalmos.