Evaluation of reti̇nal microvascular changes in patients with thyroid orbitopathy

Objective: The aim of this thesis was to evaluate microvascular changes in the retina and optic nerve head using optical coherence tomography angiography (OCTA) in patients diagnosed with Thyroid orbitopathy (TO); and to compare the superficial and deep capillary plexus vessel density (VD), peripapillary vascular parameters, foveal avascular zone (FAZ) area and FAZ perimeter among patients with TO, individuals with hyperthyroidism without orbitopathy and healthy controls. Materials and Methods: A total of 180 eyes were evaluated: 60 eyes with TO, 60 eyes from hyperthyroid patients and 60 eyes from healthy controls. OCTA data were retrospectively analyzed. VD measurements from superficial and deep capillary plexuses in foveal, parafoveal, superior, inferior, nasal and temporal regions were assessed. Additionally, peripapillary vascular density, FAZ area and FAZ perimeter were measured and statistically compared between groups. Results: Significant differences were detected in retinal VD among the groups. At the superficial capillary plexus level, foveal VD showed no significant differences between groups (p=0.793). However significant VD reductions were observed in the TO group in the parafoveal inferior (p=0.011), inferior hemi (p=0.013), parafoveal inferior hemi (p=0.026) and nasal (p=0.027) regions, suggesting that orbital inflammation and mechanical pressure predominantly affect the lower retinal segments. A significant VD decrease was also noted in the superior hemi region in the TO group (p=0.01), whereas parafoveal superior hemi (p=0.057) and parafoveal temporal (p=0.107) differences did not reach statistical significance. The whole image superficial capillary plexus VD was significantly lower in the TO group (p=0.007), indicating a widespread microvascular effect. At the deep capillary plexus level, no significant differences were observed in the foveal region (p=0.402). However significant reductions in VD were noted in the TO and hyperthyroidism groups compared to controls in the parafoveal inferior, inferior hemi, superior hemi, parafoveal superior hemi, nasal, temporal and whole image regions (all p < 0.05), indicating that deep retinal microcirculation is affected by both inflammatory and thyroid dysfunction mechanisms. Among peripapillary regions, only the inferior nasal segment showed a statistically significant difference between groups (p=0.014), with the lowest VD in the TO group and the highest in the hyperthyroidism group. Other segments showed lower VD in the TO group but without statistical significance. FAZ evaluation revealed no significant differences among the groups in either FAZ area (p=0.456) or FAZ perimeter (p=0.628). Conclusion: The findings of this study suggest that TO is not confined solely to orbital tissues but may also exert significant effects on retinal and peripapillary microvascular structures. The VD loss observed particularly in the inferior retinal segments can be explained not only by inflammatory processes involved in the pathophysiology of the disease but also by the impairment of microcirculation due to mechanical compression associated with increased orbital tissue volume. This segmental involvement further indicates that certain retinal regions may be more vulnerable to orbital pressure depending on their anatomical location. Moreover, the VD reduction detected in the hyperthyroidism group compared to healthy controls demonstrates that subclinical microvascular alterations may begin prior to the clinical onset of orbitopathy. On the other hand, the absence of significant differences in FAZ area and perimeter measurements could be attributed to the predominance of patients in the inactive phase of the disease, during which the inflammatory burden remains limited. Overall, these results highlight the value of OCTA as a non-invasive, rapid, and high-resolution imaging modality capable of detecting subclinical microvascular changes in TO before overt clinical manifestations develop. Keywords: Thyroid orbitopathy, optical coherence tomography angiography, retinal microvasculature, vessel density, FAZ, peripapillary vessels.