The Effect of Inferior Rectus Muscle Thickening on Intraocular Pressure in Thyroid-Associated Ophthalmopathy

OBJECTIVE: We aimed to evaluate the effect of inferior rectus muscle thickening on intraocular pressure (IOP) in patients with thyroid-associated ophthalmopathy (TAO). MATERIALS AND METHODS: We analyzed 33 patients with TAO (50 eyes) who presented with hypotropia in the primary position. There was significant eyeball movement restriction and inferior rectus muscle thickening was confirmed on computed tomography or magnetic resonance imaging. We measured IOP changes in patients with TAO and normal participants in the primary position and upgaze using Icare tonometer. Moreover, we measured the preoperative and postoperative IOP in 13 patients with hypotropia who underwent inferior rectus recession. RESULTS: The average inferior rectus thickness in the TAO and control groups was 0.71 ± 0.13 mm and 0.36 ± 0.12 mm, respectively. In the TAO group, the mean IOP was 16.8 ± 2.1 mm Hg in primary position (hypotropia), which increased by 8.9 ± 2.7 mm Hg to 25.7 ± 3.1 mm Hg in upgaze (horizontal vision). In the control group, the mean IOP in the primary position (horizontal vision) was 15.1 ± 1.9 mm Hg, which increased by 2.5 ± 1.4 mm Hg to 17.6 ± 2.1 mm Hg in upgaze. Compared with normal participants, patients with TAO who presented inferior rectus muscle thickening had a significantly greater increase in the IOP (P < 0.0001). In the patients with TAO who underwent inferior rectus muscle recession, there was a postoperative reduction in the IOP in the horizontal vision by 9.4 ± 5.2 mm Hg. CONCLUSION: Inferior rectus muscle thickening in patients with TAO causes a significant increase in the IOP upon upgaze compared to that in normal individuals. Given the ease of misdiagnosis as glaucoma, IOP measurement in patients with TAO should be performed in the primary position.