Compression on the cervical plexus, and on the subclavian artery and vein at the entrance  in the upper limb due to the presence of supernumerary rib, close space of the scalene muscles and of the  costal clavicle space, etc.., which causes neurological symptom about  90% (pain and difficulty in arm movements, tingling, inability to hold objects), with prevalence of the venous thrombosis for 6% of humeral and axillary vein (pain, swelling and turgidity of the fingers, with superficial reticular venous system visible) or with 4% for arterial ischemia ( pain, sense of lack of power and risk of functional incapacity).
The only treatment is surgery by supraclavicular or transaxillary way with anterior and middle scalene muscle section, removal of the first rib and supernumerary rib, when present, and of the interscalenic bundles ligaments.