Natural Course of Mild Graves Orbitopathy: Increase of Orbital Fat But Decrease of Muscle Volume With Increased Muscle Fatty Degeneration During a 4-Year Follow-Up

Purpose: To describe the natural course of orbital fat volume and extraocular muscle volume in mild Graves orbitopathy during a 4-year follow-up. To describe fatty changes within the extraocular muscles. Patients: Twenty-five patients with mild Graves orbitopathy, who did not require any therapeutic intervention other than supportive therapy, were followed for 4 years. Methods: CT scans were performed in all patients at baseline and follow-up. A validated technique using Mimics (Materialise) was used to calculate fat and muscle volumes. Outcomes were compared with previously collected data. The amount of intramuscular fat was assessed on CT scans in a semi-quantitative way by two blinded observers according to a four-point scale. Results: After a median follow-up of 4.3 years, the median fat to orbital volume ratio increased with 0.08 from 0.57 to 0.65 (p = 0.000), whereas the median muscle volume to orbital volume ratio decreased with 0.03 from 0.17 to 0.14 (p = 0.000). In this control group in patients between 49 and 54 years old, the changes were 0.01 and -0.002, respectively. The Clinical Activity Score decreased to zero (p = 0.000), and the median eyelid aperture decreased from 12 to 10 mm (p = 0.007). A significant increase of intramuscular fat was found in patients with Graves orbitopathy. Conclusions: The natural course of mild Graves orbitopathy, as observed over 4 years, is characterized by an increase of orbital fat volume, a decrease in muscle volume, and an increased intramuscular fatty degeneration.