Selective extraocular muscle regression following tocilizumab in active thyroid associated ophthalmopathy: a 3D...

Thyroid-associated ophthalmopathy (TAO) is an autoimmune orbital disorder characterized by orbital inflammation and hyperplasia of extraocular muscles (EOMs) and adipose tissue. Tocilizumab (TCZ) has demonstrated anti-inflammatory effects in TAO, but its ability to induce structural regression of hypertrophic EOMs has not been objectively quantified using advanced three-dimensional (3D) imaging techniques. This retrospective study aimed to evaluate the clinical and structural effects of TCZ in active moderate-to-severe TAO using quantitative 3D MRI.

Twenty-four patients with active moderate-to-severe TAO who completed a standard four-dose TCZ regimen (8 mg/kg every four weeks) were enrolled. Clinical parameters (Clinical Activity Score (CAS), orbital tension, proptosis) and serum biomarkers (thyrotropin receptor antibody (TRAb), interleukin-6 (IL-6) were assessed at baseline (T1), post-treatment (T2), and at one-month follow-up (T3). Structural remodeling was quantified by measuring the cross-sectional areas (CSA) of individual EOMs and orbital fat volume (OFV) using coronal 3D modified Dixon quantitative T1-weighted MRI.

TCZ induced rapid inflammatory inactivation, with median CAS declining from 3 to 0 (P < 0.001). Proptosis showed modest reduction (20.00 mm to 19.75 mm, P = 0.020). Quantitative MRI revealed the most pronounced CSA reduction in the superior rectus (P < 0.001), followed by significant decreases in the inferior and lateral recti (P < 0.05), whereas no significant changes were observed in the medial rectus CSA (P = 0.170) or OFV (P = 0.179). Serum TRAb significantly decreased (4.95 to 4.62 IU/L, P = 0.007), while IL-6 showed a modest increase (1.50 to 2.98 pg/mL, P = 0.070). TCZ was well tolerated, with no severe adverse events observed. Most patients maintained clinical improvement at one-month follow-up without rebound.

TCZ is a rapid, potent, and safe intervention for active moderate-to-severe TAO. Quantitative 3D MRI provides objective evidence that TCZ selectively induces structural regression of hypertrophic EOMs, with the superior rectus showing the most pronounced response and the medial rectus exhibiting considerable resistance, which may reflect chronic structural remodeling or fibrosis-associated changes. The significant decline in serum TRAb levels further supports the systemic immunomodulatory effects of TCZ. These findings provide imaging-based evidence for the differential structural effects of TCZ on orbital tissues.