Introduction to Thoracic Outlet Syndrome

What Causes Thoracic Outlet Syndrome?

Thoracic outlet syndrome (TOS) is a complex condition involving compression of the brachial plexus and/or subclavian arteries which can be caused by:

• Skeletal abnormalities
• Skeletal misalignment
• Muscular changes
• Pathological conditions

Skeletal Abnormalities

If there is a congenital abnormality it can cause a change in the shape of the thoracic outlet resulting in compression of the brachial plexus or blood vessels. Congenital anomalies may include:

• Extra cervical rib
• Abnormally long bony transverse process
• Band of fibrous tissue which arises off the spinal cord

Usually, the brachial plexus and the blood vessels adapt to the presence of these anomalies so that only a small percentage of people with these anomalies actually develop TOS.

Skeletal Misalignment

If the bones in the upper chest (e.g., cervical vertebrae, first rib, and collarbone) are not aligned properly, or have shifted or rotated out of their natural position, they may cause a narrowing in the opening of the thoracic outlet though which the brachial plexus and blood vessels pass, potentially causing them to be pinched or irritated.

Muscular Changes

The scalene muscles undergo change by shortening and tightening when posture is not erect and the patient stands with a head-forward slouch. This is followed by the muscles adhering to each other and to other structures as they seek support from the strain of misalignment. If the scalene muscles adhere to the facia (outer lining) of the brachial plexus, then every time the head moves or turns, the brachial plexus is irritated and this can be the source of considerable pain.

Long term abnormal posture can lead to:

• Increased pressure around the neurons at points of nerve entrapment
• Muscles remain in a shortened state and reset themselves at the new length (primarily found with the scalene, sternocleidomastoid, and pectoralis muscles)
• Shortening of some muscles causes others to be maintained in an abnormally lengthened state. Consequently, some muscles are underused and some overused leading ultimately to muscle imbalance. Overused muscles may include upper trapezius and levator scapulae
• Weakness in the middle and lower trapezius, as well as serratus anterior muscles

Another ramification of shortened scalene muscles is that the first ribs, to which they attach, are pulled upward resulting in a narrowing of the space where the brachial plexus passes between the first rib and collarbone, which causes compression. This, in turn, may cause other distal sites along that nerve (e.g., elbow and wrist) to be susceptible to compression injury as well. This condition is called double crush. There is no consensus regarding the theory of double crush (i.e., proximal and distal compression) but researchers point to the high incidence of carpal (wrist) and cubital (elbow) tunnel syndrome that is associated with TOS. The theory holds that any isolated compression at any of these points may not cause symptoms but the cumulative effect of several minor compressions may result in significant symptoms.

Pathological Conditions of the Thoracic Outlet

Possible structural changes within the thoracic outlet may be caused by:

• Overgrowth (hypertrophy) of the scalene muscle

• Swelling of adjacent tissue or ligaments which may be due to:

  • repetitive awkward movements
  • accelerated wear and tear with repeated activity
  • sudden straining injury

• Tumor

• Bone fragments secondary to a fracture