Thoracic outlet syndrome

Thoracic outlet syndrome (TOS) refers to a group of clinical syndromes caused by congenital or acquired compression of brachial plexus  (neurogenic TOS) or subclavian artery or subclavian vein as they pass through the thoracic inlet. 

Clinical presentation

Clinical presentation will depend on the structure compressed and the degree of compression. Compression of brachial plexus is much more common (90-95%) than compression of the subclavian vessels (5-10%). 

Brachial plexus compression results in pain, paraesthesia and/or numbness of the upper limb. 

Subclavian artery compression causes ischaemia with coolness, pallor, claudication, paraesthesia and decreased upper limb pulses. 

Subclavian vein compression causes upper limb swelling and pain and may result in venous thrombosis (Paget-Schroetter syndrome). 

Combined neurovascular symptoms and signs may be present. The findings are exacerbated by certain arm positions and maneuvers, particularly with the arms raised above the head.

Pathology

There are three common sites of compression:

1. scalene triangle - between scalenus anterior and scalenus medius muscles
2. costoclavicular space - between clavicle and 1^st rib
3. subpectoral space - between pectoralis minor and coracoid process

The scalene triangle is defined by the first rib and the anterior and middle scalene muscles and is the most medial compartment. The subclavian artery and branches of the brachial plexus pass through the borders of this triangle while the subclavian vein passes anterior to it.

Possible causes of compression of the brachial plexus or subclavian vessels include a congenital cervical rib, bone abnormality of the first rib or malunion of a clavicular fracture, aberrant insertion of the scalene muscle, abnormal fibrous bands, and hypertrophy or fibrosis of the surrounding musculature.

Causes

• scalenus anticus syndrome - abnormal insertion of scalenus anterior onto 1^st rib (commonest cause)
• congenital cervical rib
• bony abnormality of 1^st rib or clavicle (congenital anomaly, malunited fracture, callus, Pagets disease, tumour)
• muscle hypertrophy
• fibrous bands
• supraclavicular tumour or lymphadenopathy

Radiographic features

Plain radiography is indicated to exclude an underlying bone abnormality.

Imaging with ultrasound, contrast-enhanced CT, MRI or conventional angiography is useful for detecting vascular compression. Imaging is performed with the patient’s arms both in the raised and neutral position (for comparison). Imaging may show the vascular stenosis and complications such as thrombosis, false aneurysm and distal emboli. Imaging of the bones and soft tissues may reveal the cause.

MR angiography with dynamic positioning is particularly useful in the diagnosis of vascular subtypes of thoracic outlet syndrome.

Treatment and prognosis

Treatment of arterial thoracic outlet syndrome is surgical intervention. Treatment is required to treat or prevent acute thromboembolic events.

Treatment of venous thoracic outlet syndrome depends primarily on the presence and extent of associated venous thrombosis and may include anticoagulation, thrombolysis, or surgical decompression.

EtymologyHistory and etymology

The term "thoracic outlet syndrome" is thought to have been coined by R M Peet et.al in 1956 ⁸.