- Between the scalene anterior and scalene medial muscles is the brachial plexus, subclavian artery, and subclavian vein.  Compression of this region results in thoracic outlet syndrome.
- Thoracic outlet syndrome ultimately cause compression of either the brachial plexus or the subclavian artery.
- If you have an extra rib, it can cause thoracic outlet syndrome.

- Thoracic Outlet syndrome is caused by the embryological defect of having a cervical rib.

- The gap between the scalenus medius and scalenus anterior may be narrow and the tough tendon of scalenus anterior may compress the lower trunk of the brachial plexus and/or the subclavian artery, giving rise to one form of the thoracic outlet syndrome.

Reference:
1. Anatomy at a Glance. Omar Faiz and David Moffat.  Blackwell Science. 2002.