What is the recommended initial imaging modality for diagnosing neurogenic Thoracic Outlet Syndrome (TOS)?

Imaging for Neurogenic Thoracic Outlet Syndrome

MRI of the chest without IV contrast is the recommended initial imaging modality for diagnosing neurogenic TOS, as it directly visualizes the brachial plexus and can demonstrate compression of neurovascular bundles in the costoclavicular, interscalene, and pectoralis minor spaces. 1

Initial Imaging Approach

Chest Radiography First

• Begin with plain chest radiography to identify osseous abnormalities such as cervical ribs or first rib anomalies, which account for 36% of neurogenic TOS cases 2, 3
• This serves as a screening tool before proceeding to advanced imaging 1

MRI as the Definitive Study

MRI without IV contrast is sufficient for neurogenic TOS diagnosis and should be performed after chest radiography 1, 2

Key advantages of MRI include:

• Direct visualization of the brachial plexus and cervical spine, which CT and ultrasound cannot adequately provide 1
• Superior soft tissue characterization compared to CT, allowing differentiation of neural structures, muscles, and fibromuscular anomalies 1
• Dynamic evaluation capability with imaging in both neutral and arm-abducted positions to demonstrate positional compression 1, 3

Essential MRI Protocol Components

The MRI protocol must include specific sequences and positioning:

• High-resolution T1-weighted and T2-weighted sequences in sagittal and axial planes to delineate the brachial plexus, muscular attachments, and compression sites 2
• Sagittal T1-weighted images obtained perpendicular to the longitudinal axis of the brachial plexus from spinal cord to the medial humerus in both neutral and abducted positions 3
• Turbo spin-echo T2-weighted or short tau inversion recovery sequences to exclude alternative diagnoses like brachial plexitis or spinal cord lesions 2
• Imaging must demonstrate effacement of fat adjacent to brachial plexus roots, trunks, or cords within the interscalene triangle or costoclavicular space 2

What MRI Reveals in Neurogenic TOS

MRI identifies the specific anatomical cause of compression:

• Congenital bone variations (36% of cases) including cervical ribs or first rib anomalies 3
• Congenital fibromuscular anomalies (11% of cases) 3
• Positional compression (53% of cases) 3
• The costoclavicular space is the most common site of neurovascular bundle compression, while the pectoralis minor space is rarely involved 1, 3

Modalities NOT Recommended for Initial Neurogenic TOS Imaging

CT Limitations

CT is limited by lack of resolution of neural structures, though it can show secondary indicators like space narrowing 1

• CT may be useful for quantifying costoclavicular or interscalene space changes with provocative maneuvers 1
• CT is primarily valuable for identifying bony abnormalities but cannot directly visualize the brachial plexus 1

Angiography Not Indicated

• Arteriography and venography are unable to directly image neurologic structures and are not considered first-line tests for neurogenic TOS 1
• CTA and CTV do not provide further evaluation of neurologic structures compared to standard chest CT 1

Ultrasound Considerations

High-resolution ultrasound may be used as a first imaging modality to identify accessory costocupular ligaments and stump ribs in suspected neurogenic TOS 4

• However, MRI has inherent advantages over ultrasound in delineating extravascular anatomy, particularly in anatomic sites with poor sonographic windows 1

Critical Diagnostic Pitfalls

Do not overlook concomitant cervical spine pathology (present in 14% of cases), which may mimic or exacerbate TOS symptoms 2, 3

• Severe cervical spondylosis can contribute to symptoms and must be evaluated 3
• In 5% of cases, unilateral brachial plexitis occurs in addition to neurovascular bundle compression 3

Avoid relying on CT or ultrasound alone for neurogenic TOS, as these modalities lack adequate resolution of neural structures 2

Post-Intervention Imaging

MRI without IV contrast may be performed postoperatively to evaluate interval changes in the thoracic outlet and assess adequate decompression 1

• Post-contrast imaging may be useful to assess soft tissues for inflammatory or neoplastic conditions 1