Isolated superior rectus-levator complex enlargement: Inflammatory etiology, clinicoradiological features, and treatment outcomes.

PURPOSE: To analyze the etiology, clinicoradiological features, and treatment outcomes of orbital inflammation presenting with isolated superior rectus-levator complex (SR-LC) enlargement. METHODS: We retrospectively reviewed patients treated for orbital inflammation with isolated SR-LC enlargement, assessing the underlying etiologies, including thyroid eye disease (TED), idiopathic orbital myositis (IOM), and immunoglobulin G4-related ophthalmic disease (IgG4-ROD). The IOM group was further divided by the presence of upper lid ptosis. Clinicoradiological findings and outcomes were compared between the TED and IOM groups. RESULTS: Among the 58 patients, 39 (67.2%) had TED, 17 (29.3%) had IOM (10 without ptosis and seven with ptosis), and two (3.4%) had IgG4-ROD. Upper lid retraction was observed in both TED (79.5%) and IOM without ptosis (90.0%). SR tendon involvement was significantly less common in TED than in IOM (13.3% vs. 78.6%, p < 0.001). Infraduction limitation was more prevalent in TED (94.7%) and IOM with ptosis (100%) than in IOM without ptosis (25.0%), while supraduction limitation was noted in IOM without ptosis (75.0%). Compared to IOM without ptosis, IOM with ptosis had a shorter time to diagnosis (0.5 vs. 1.5 months, p = 0.088) and better clinical outcomes following steroid treatment (lid level asymmetry: 0% vs. 50.0%, p = 0.044). CONCLUSION: TED and IOM are the most common etiologies of orbital inflammation with isolated SR-LC enlargement. IOM can manifest as either ptosis or retraction, with the latter distinguishable from TED by clinicoradiological features. Early initiation of systemic steroids can be associated with favorable clinical responses, especially for suspected IOM cases; however, further prospective studies are warranted.