Orbital Volumetry in Graves' Orbitopathy: Muscle and Fat Involvement in relation to Dysthyroid Optic Neuropathy

Purpose. We wanted to investigate the relative significance of fat and muscle enlargement in the development of dysthyroid optic neuropathy (DON) in Graves’ orbitopathy (GO). Methods. Preoperative coronal CT scans of 13 patients with and without DON who subsequently underwent orbital decompression were retrospectively analyzed. Thirteen patients imaged for unilateral orbital fractures served as controls. Results. The retrobulbar muscle volume was 2.1 ± 0.5？cm3 (mean ± SD) in controls, 4.3 ± 1.5？cm3 in GO without DON, and 4.7 ± 1.7？cm3 in GO with DON. The retrobulbar fat volume was 5.4 ± 1.6？cm3 in controls, 8.7 ± 8.0？cm3 in GO without DON, and 9.4 ± 3.1？cm3 in GO with DON. The muscle and fat volumes were higher in patients with GO than in controls ( ), but the volumes in orbits with and without DON were not significantly different. The volume of the optic nerve were similar in the 3 groups. The number of apical, coronal 2？mm thick slices with no fat was 2.9 ± 0.9 in normal orbits, it was 4.1 ± 1.0 in GO orbits without DON and 5.3 ± 0.8 in GO orbits with DON ( ). Conclusion. Apical muscle enlargement may be more important than orbital fat enlargement in the development of DON. However, the fact that apical crowding and muscle enlargement also occur in orbits without DON suggests that other factors also play a role in the development of DON. 1. Introduction Graves’ disease is an autoimmune disease manifesting in the thyroid, in the dermis, and in orbital connective tissues and muscles [1]. Some 20–50% of Graves’ patients develop symptomatic Graves’ orbitopathy (GO) [2]. The time association between GO and thyroid disease varies; GO can occur before, during, or after the onset of hyperthyroidism and, less frequently, in euthyroid or hypothyroid cases. GO can occur as unilateral or bilateral disease [3]. The diagnosis of GO is made clinically assisted by biochemical thyroid evaluation and often magnetic resonance (MRI) or computed tomography (CT). The symptoms and signs of active GO include eye discomfort and watering eyes, double vision, eyelid retraction, proptosis, eyelid edema, strabismus, chemosis, disfigurement, corneal problems, and dysthyroid optic neuropathy (DON) [2, 4]. The latter is the most serious manifestation of GO. It is therefore of special interest to identify disease elements associated with the development of DON. DON occurs in 3.4–8% of patients and is thought to be due to optic nerve compression in the orbital apex by enlarged muscles [3–5]. Cigarette smoking, male gender, restrictive strabismus, and rapidly progressive disease are