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There are actually three spaces where compression can occur causing the symptoms of thoracic outlet syndrome (TOS), including:

• The thoracic outlet (also called the scalene triangle) - is located at the base of the neck above the first rib and behind the clavicle. This space lies closest to the neck. The boundaries of the thoracic outlet consist of:

  • anterior scalene muscle which forms the front of the thoracic outlet
  • middle scalene muscle which forms the back of the thoracic outlet
  • first rib which forms the bottom of the thoracic outlet.

  The subclavian artery and the brachial plexus are located between the first rib and the scalene muscle while the subclavian vein sits outside the scalene muscle.

• Costoclavicular space - this space is located adjacent to the scalene triangle and lies between the following structures:

  • clavicle
  • first rib
  • costoclavicular ligament
  • edge of the middle scalene muscle

  The subclavian artery, vein, and brachial plexus all pass through the costoclavicular space.

• Subcoracoid space which lies adjacent to the costoclavicular space and is closest to the arm. The location of this space is:

  • under the pectoralis muscle
  • under the coracoid process
  • in front of the ribs

If there are any anomalous bony structures (e.g., cervical rib) touching upon these small spaces, they have the potential to compress the neurovascular structures which pass through them. Approximately 0.17 to 0.74% of the general population has an anomalous rib and only 10% of people with such a cervical rib actually develop thoracic outlet syndrome. In the affected people, TOS typically occurs following trauma to the cervical spine.

It is thought that the most common site for compression of the blood vessels or brachial plexus occurs in the scalene triangle and subcoracoid space but it is difficult to confirm this.