A Magnetic Resonance Imaging Investigation Into the Dynamic Changes of the Thoracic Outlet: Normal Reference Range of...

Thoracic outlet syndrome (TOS) is a complex condition caused by the compression of the neurovascular bundle within the thoracic outlet. This study aims to characterize the physiological degree of vascular compression on the asymptomatic side in patients with unilateral TOS using dynamic MRI, establish normative reference ranges for vascular narrowing in stress position, and evaluate the influence of body mass index (BMI) on compression severity. In this retrospective study, 114 patients were screened and 57 patients with clinically diagnosed unilateral TOS were included (mean age 35.5 $\pm$ 13.5 years; 72% female). The asymptomatic side was required to remain symptom-free for at least 6 months following MRI to ensure that the asymptomatic side represented a true physiological baseline. Subclavian artery and vein compression and costoclavicular distance (CCD) were measured in neutral and stress positions. Intra-observer reliability was assessed with a 6-month washout period. BMI was categorized according to the World Health Organization criteria. In the stress position, median venous compression on the asymptomatic side was 64.3% (IQR 52.5-74.5), with 75% of patients exhibiting > 50% stenosis. CCD decreased significantly in the stress position (p < 0.001) and differed significantly across BMI groups (p < 0.001), with the greatest reduction observed in the obese group. Arterial compression also differed significantly across BMI groups (p = 0.004), with the overweight group demonstrating a greater degree of compression compared with the combined normal and underweight group and the obese group (both p = 0.015). No significant differences in venous compression were observed across BMI groups (p = 0.245). Intra-observer reliability was excellent, with Intraclass Correlation Coefficients of 0.88 for vascular stenosis and 0.91 for CCD. Significant venous compression is a common physiological finding in asymptomatic individuals in the stress position. These results suggest that imaging evidence of narrowing should not be used as the sole criterion for the diagnosis of TOS. BMI influences arterial compression and costoclavicular narrowing, highlighting the importance of considering patient-specific factors when interpreting dynamic imaging. Establishing standardized reference ranges for dynamic imaging and accounting for clinical context such as BMI may improve diagnostic accuracy and reduce overdiagnosis.