The subclavius posticus muscle: an unusual cause of thoracic outlet syndrome.

[thoracic outlet syndrome]

We present the case of a 30 -year -old female , complaining of thoracic outlet compression symptoms caused by a supernumerary muscle , the subclavius posticus , accompanied by a caudally inserted middle scalenus muscle on the second rib . This rare anatomic variant was clearly shown on CT angiography and MRI images and surgical treatment was successful . As first described by Rosenmuller in 1800 , subclavius posticus is a supernumerary muscle originating from the cranial surface of the sternal end of the first rib , running laterodorsally beneath the clavicle , and inserting into the superior border of the scapula . Its role in thoracic outlet syndrome has been seldom demonstrated in living patients nor described in imaging , although it is theoretically easily recognizable on modern imaging modalities . It should be taken into account during workout of patients with thoracic outlet syndrome , since it can be potentially treated .