Figure 1: Arterial phase image with patient’s arms over her head demonstrates a focal stenosis in the left subclavian artery at the level of the mid-clavicle. Contrast material in the veins of the right arm reflect residual material resulting from a right antecubital venous injection.

Figure 2: Venous phase image with patient’s arms over her head demonstrates a focal stenosis in both the left subclavian artery and vein at approximately the same location as the vessels cross into the shoulder.

Figure 3: Arterial phase image with patient’s arms at her side demonstrates resolution to the focal stenosis in the left subclavian artery.

Figure 4: Venous phase image with patient’s arms at her side demonstrates resolution to the focal stenosis in both the left subclavian artery and vein.

Thoracic outlet syndrome.

The thoracic outlet or cervicothoracic-brachial junction is bounded by the first thoracic vertebra, the superior border of the manubrium sterni anteriorly, and the first rib and costal cartilage laterally. It can be further subdivided into three tunnels: the interscalene triangle, the costoclavicular space and the retropectoralis minor space. Thoracic outlet syndrome (TOS) refers to a diverse group of clinical syndromes caused by congenital or acquired compression of the brachial plexus and subclavian vessels as they pass through the thoracic outlet. Possible causes of compression of the neurovascular bundle include:

• Congenital cervical rib or fibrous extension of cervical rib
• Abnormal scalene muscle insertion
• Drooping of shoulder girdle resulting from generalized hypotonia or trauma
• Narrowed costoclavicular interval as a result of downward and backward pressure on shoulder (sometimes seen in individuals who carry heavy backpacks)
• Acute venous thrombosis with exercise (Effort thrombosis; Paget Schroetter syndrome)
• Bony abnormalities of first rib
• Abnormal fibromuscular bands
• Malunion of clavicle fracture
• Muscular hypertrophy or fibrosis (scalene muscles, subclavian muscle, pectoral muscles)

TOS can also be classified into three types based on the point of compression: (1) cervical rib and scalenus syndrome, in which abnormal scalene muscles or the presence of a cervical rib may cause compression; (2) costoclavicular syndrome, in which compression may occur under the clavicle; and (3) hyperabduction syndrome, in which compression may occur in the subcoracoid area. It can also be divided into three subtypes each with a distinct clinical presentation: neurogenic, arterial, and venous. The prevalence of TOS varies from source to source. TOS is rare under the age of 20 years and has a female predominance (female: male ratio of 3.5:1).

Symptoms and signs of thoracic outlet syndrome result from the compression or irritation of the neurovascular bundle at various levels of the thoracic outlet. Signs and symptoms are also related to the degree of involvement of each of the various structures in the neurovascular bundle and can be used to identify the structure(s) being compressed. Neurogenic TOS accounts for approximately 95-98% of all patients with TOS. Pain, numbness, and paresthesias, most frequently along the medial forearm and hand, are the most common presenting symptoms. Arterial TOS symptoms can range from Raynaud’s syndrome, including pallor, coldness, pain and paresthesias to abrupt severe ischemia. The venous subtype can cause arm cyanosis, aching, and swelling. True venous or arterial involvement is rare. Other symptoms include painful discomfort in the shoulder and arm especially with heavy lifting or carrying, shoulder stiffness and weakness, back pain, supraclavicular tenderness, a palpable thrill over the subclavian artery, diminished radial pulses, and a lowered brachial blood pressure. Symptoms may be produced or exacerbated by downward traction on the arm and sometimes by hyperabduction at the shoulder. The differential diagnosis for TOS includes carpel tunnel syndrome, cervical radiculopathy, brachial neuritis, ulnar nerve compression, reflex sympathetic dystrophy, and superior sulcus tumor.

MR imaging allows for multiplanar imaging and excellent soft-tissue depiction. Images obtained in the sagittal plane can depict the nervous or vascular structures in cross section as they pass through different spaces of the thoracic outlet. The sagittal plane is also suitable for evaluation of compression within these spaces. MR imaging is the imaging method of choice for evaluating the anatomy and pathology of the brachial plexus. Studies may require elevation of the arm or hyperabduction maneuvers to demonstrate compression of vascular or neurological structures. MR images can help to exclude cervical disease, spinal stenosis, or tumors. Gadolinium enhanced MR angiography allows for rapid evaluation and comparison of the vascular structures in both the neutral and abducted positions. It allows for identification of venous or arterial obstruction and the possible etiology of this obstruction, which may include arteriosclerotic stenosis, post-stenotic dilatation or frank aneurysm formation, and thrombus.

References:

1. Krinsky G and NM Rofsky. MR Angiography of the Aortic Arch Vessels and Upper Extremities. MRI Clinics of North America: Body MR Angiography. May 1998; 6(2): 269-292.
2. Higgins CB and A De Roos. Cardiovascular MRI & MRA. Philadelphia: Lippincott Williams & Wilkins, 2003. pp. 421-422.
3. Demondion X, Boutry N, et al. Thoracic Outlet: Anatomic Correlation with MR Imaging. American Journal of Radiology. 2000; 175: 417-422.
4. RA Cooke. Thoracic Outlet Syndrome – Aspects of Diagnosis in the Differential Diagnosis of Hand-Arm Vibration Syndrome. Occupational Medicine. 2003; 53: 331-336.
5. Demondion X, Bacqueville E, Paul C, et al. Thoracic Outlet: Assessment with MR Imaging in Asymptomatic and Symptomatic Populations. Radiology. 2003; 227: 461-468.