Marked Enlargement of a Single Extraocular Muscle: Clinical and Imaging Patterns in the Prediction of Malignancy.

Purpose: To evaluate clinical presentation, imaging patterns, histopathology, and predictors of malignancy in patients with enlargement of a single extraocular muscle.

Methods: Retrospective review, with imaging categorized into 3 groups: single-muscle only (SMO), single excessively-enlarged muscle, and single-muscle with lacrimal gland enlargement (SMLG). Histopathology was classified as specific or nonspecific myositis, atypical thyroid eye disease, malignancy, or other diagnosis.

Results: Among 142 patients (54% female), 60/142 (43%) had SMO, 53 (37%) single excessively-enlarged muscle, and 29 (20%) single-muscle with lacrimal gland. The commonest diagnoses were myositis (43%), atypical thyroid eye disease (27%), and malignancy (27%). Malignancy-mainly lymphoma or metastases-was commonest in SMO (43%), while atypical thyroid eye disease predominated in single excessively-enlarged muscle (49%), and myositis in the single-muscle with lacrimal gland group (69%). From univariate analysis, predictors of malignancy included age ≥60 years (odds ratio [OR]: 2.6; p = 0.012), prior malignancy (OR: 15.6; p < 0.001), subjective visual impairment (OR: 3.5; p = 0.024), LogMAR ≥0.5 (OR: 9.0; p = 0.009), ≥3 mm relative exophthalmos (OR: 3.1; p = 0.008), SMO (OR: 4.8; p < 0.001), lateral rectus involvement (OR: 3.7; p = 0.008), the "amphora sign" (OR: 28.2; p < 0.001), and maximum muscle diameter ≥10 mm (OR: 35.5; p < 0.001). Multivariate analysis confirmed prior malignancy (OR: 27.7; p = 0.013) and muscle diameter ≥10 mm (OR: 24.8; p = 0.005) as independent predictive variables. The prevalence of pain or diplopia and symptom duration were not significantly different in patients with myositis or those with malignancy.

Conclusion: Excessive enlargement of a single extraocular muscle poses a diagnostic challenge due to the overlap of inflammatory and neoplastic features. Given the substantial proportion of malignancies, a high index of suspicion and early biopsy should be considered, particularly with SMO, a muscle diameter ≥10 mm, or prior history of cancer.