 Introduction/Background Thoracic outlet syndrome (TOS) denotes the clinical entity that occurs with compression of the brachial plexus, subclavian artery, and/or subclavian vein at the superior thoracic outlet. There are three distinct spaces that can be implicated in TOS. Narrowing of each space results in slightly different presentations because of differing severity of impingement on the transiting neurovascular structures [1,2]. The costoclavicular triangle consists of the clavicle superiorly, the anterior scalene muscle posteriorly, and the first rib inferiorly. All three neurovascular structures pass through this space. Narrowing of this space tends to cause venous symptoms, frequently denoted venous TOS (vTOS), with varying degrees of additional symptoms due to arterial or brachial plexus compression. The interscalene triangle consists of the anterior scalene muscle, middle scalene muscle, and first rib. Trunks of the brachial plexus and the subclavian artery pass through this space; narrowing here causes neurological dominant TOS (nTOS), arterial dominant (aTOS), or combinations of both. The pectoralis minor space, defined by the pectoralis minor muscle anteriorly and chest wall posteriorly, is essentially an extension of the thoracic outlet and can result in varying degrees of compression similar to the costoclavicular space. During extreme shoulder abduction, the costoclavicular space is naturally narrowed. Anatomical variants such as a cervical rib can cause narrowing of the scalene triangle. Other possible sources of compression include anomalous first rib, C7 transverse process, or post-traumatic changes from prior clavicular or rib fractures. In patients who perform activities that require repetitive upper-extremity movement, such as swimming or throwing, or in patients who are not involved in excessive overhand motion but who have an anatomic predisposition to TOS, repetitive stress can lead to symptoms of TOS [3].
Thoracic Outlet Syndrome.  Introduction/Background Thoracic outlet syndrome (TOS) denotes the clinical entity that occurs with compression of the brachial plexus, subclavian artery, and/or subclavian vein at the superior thoracic outlet. There are three distinct spaces that can be implicated in TOS. Narrowing of each space results in slightly different presentations because of differing severity of impingement on the transiting neurovascular structures [1,2]. The costoclavicular triangle consists of the clavicle superiorly, the anterior scalene muscle posteriorly, and the first rib inferiorly. All three neurovascular structures pass through this space. Narrowing of this space tends to cause venous symptoms, frequently denoted venous TOS (vTOS), with varying degrees of additional symptoms due to arterial or brachial plexus compression. The interscalene triangle consists of the anterior scalene muscle, middle scalene muscle, and first rib. Trunks of the brachial plexus and the subclavian artery pass through this space; narrowing here causes neurological dominant TOS (nTOS), arterial dominant (aTOS), or combinations of both. The pectoralis minor space, defined by the pectoralis minor muscle anteriorly and chest wall posteriorly, is essentially an extension of the thoracic outlet and can result in varying degrees of compression similar to the costoclavicular space. During extreme shoulder abduction, the costoclavicular space is naturally narrowed. Anatomical variants such as a cervical rib can cause narrowing of the scalene triangle. Other possible sources of compression include anomalous first rib, C7 transverse process, or post-traumatic changes from prior clavicular or rib fractures. In patients who perform activities that require repetitive upper-extremity movement, such as swimming or throwing, or in patients who are not involved in excessive overhand motion but who have an anatomic predisposition to TOS, repetitive stress can lead to symptoms of TOS [3].