## 4 Discussion  
TED accounts for 20% of orbital diseases in adults, clinically manifesting in exophthalmos, diplopia, restrictive myopathy, and optic neuropathy. The health-related quality of life of affected people is deeply compromised by these symptoms. The choice of treatment protocols varies according to the severity of the disease and whether it is accompanied by eye movement disorders and visual impairments [9]. Hence, evaluation measures are important to analyze functional changes in all stages of the disease. This study provides objective indicators using CT images of TED patients to reduce the subjectivity of clinical diagnosis of TED from images. Figure 6: Population distribution of the angle between the medial rectus and optic nerve. Figure 7: Population distribution of the angle between the lateral rectus and optic nerve. Previous studies have mainly emphasized the relationship between the volume of soft tissue and compressive optic neuropathy [10; 11; 12]. Nevertheless, little research has been done to explore the feasibility of analyzing soft tissue volume to assess TED classification. TED can be clinically categorized into two subtypes: (1) muscle-dominant TED with diplopia and visual defects and (2) non-muscle-dominant TED, which is often accompanied by protposis owing to orbital fat hyperplasia [13]. Furthermore, there have not been any clinical attempts to analyze retroorbital fat and extraocular muscle in patients with impaired ocular motility. Thus, we set out to evaluate changes in the orbital tissues, including extraocular muscle (EM) and retroorbital fat (RF) to determine whether CT parameters of orbital soft tissue could be used to detect muscle-dominant and non-muscle dominant TED. An increase in the volume of RF and EM is one of the obvious signs of TED, based on previous reports [14; 15]. These results may have value in discriminating between these two classifications. In this study, we found that EMV and EMV/OV in the orbits of the TED-IM group were larger than those in the TED-NM group. These results are consistent with prior studies and generally accepted, since the impairment is caused by enlarged EM tissue infiltrated with various inflammatory cytokines [16; 17]. Moreover, RFV and RFV/OV were found to be smaller in TED-IM patients. [PERSON] et al. found an increased total fat volume in orbits in all patients compared to normal, which suggests that it is universal for TED patients [18]. Our research indicated differences in RF tissue between different subgroups of TED patients, which verified a clinical sub-classification of predominant fat and muscle type in TED patients. EMV/OV \(\geq\)15.24% can be considered as a reference parameter in detecting TED-IM patients. In combination with clinical features of diplopia and exophthalmos, this parameter can also be a reference factor when evaluating whether orbital decompression surgery should be performed. Dysthyroid optic neuropathy (DON) is a serious TED complication that can cause varying degrees of visual impairment [19; 20]. Due to clinical features of congestive orbitopathy, it is sometimes difficult to diagnose in its initial stage [4]. CT images can directly demonstrate the extrusion of the optic nerve by EM tissue, which facilitates a precise clinical diagnosis of DON. EMV-OA/OAV can demonstrate the condition of apical crowding in a quantitative and visual way. Volumetric measurement of EM tissue has been used as an indicator of DON in many studies [21; 22]. In our study, we found that EMV and EMV/OV tended to be larger in DON than in non-DON, although the differences were not significant. [PERSON] et al. stressed the importance of using both EM enlargement and bony orbital structures to evaluate DON [23]. Therefore, we attempted a novel measurement by volumetric analysis to quantify the exact crowding condition of the orbital apex. EMV-OA/OAV was significantly larger in DON than non-DON patients.