The subclavius posticus muscle: an unusual cause of 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.